AC Engine Project

Life for The Weller 2 Litre 6 Cylinder Engine


HINTS & TIPS - REVISITED Originally published Dec 1953


'Notes ON Maintenance AND Tuning The AC Engine by Dr. J.W.E. Fellows PART One OF THREE'

(all three parts included)


Stripping AND rebuilding
Before going into the question of tuning I propose to describe the process of stripping and rebuilding the engine in detail. (This may be superfluous for the majority of members, but I feel that it is worthy of inclusion for the benefit of those who have not yet seen their engines in bits!) I would stress that one of the most valuable aids to increased power and reliability is careful and accurate assembly, with every part fitting as it should - with correct clearances, etc. The main 'tuning' details of each component will be described as we go along. Before any serious tuning is undertaken I would strongly recommend that the engine is removed from the frame and stripped for examination.

After the engine is removed from the frame the cylinder head should be removed. Before the 14 nuts holding down the head are slackened, the camshaft sprocket should be taken off by undoing the centre bolt. The sprocket (which is spigoted to the camshaft by two offset dowels) can then be lifted off and placed on the bracket provided for that purpose. The head should then be removed and laid on one side. This exposes the pistons and liners. The latter can be removed in pairs by gently levering with two screwdrivers under the top flanges. Undue force should be avoided in this operation as it may damage the engine casing, or even crack the liner. Each pair, after removal, should be marked so that if they are used again they will be running with the same pistons.

CYLINDER Liners
Standard bore, 65mm. If the wear is more than .005 they should be scrapped and replaced by new liners; in my experience grinding the liners is never satisfactory. If grinding is then undertaken the liners should not be ground more than .030'' oversize, if the condition of the liners is satisfactory, and if it is felt desirable to raise the compression ratio by fitting different pistons, the slight wear ridge at the top of the liner should be carefully scraped away. If this ridge is left, the top ring of the new piston will be destroyed the first time the engine is turned over. When replacing the liners, always use new figure of eight copper asbestos gaskets. In refitting the liners it will be found that effort is necessary to get them finally 'home' in the engine case. If a block of wood is used on top of the liner it can be tapped down with a hammer. It will then be found that the top land is .020'' to .030'' proud of the top of the block. Pressure of the cylinder head will compress the gaskets; the liners will then take their proper position, which is approximately .005'' proud of the top of the engine case.

TIMING CHAIN
The next step in dismantling the engine is the removal of the crankshaft and connecting rods. Before the sump is taken off it is advisable to remove the timing chain. On the earlier models, fitted with the inverted tooth chain, this can be done by removing a small split-pin and detachable link. On the later pre-war models fitted with the double roller chain, some have a detachable link, while on others a rivet has to be removed.

SUMP removal
Having removed the timing chain and camshaft wheel, the oil pipe from sump to cylinder block should be disconnected and the sump removed by undoing the securing bolts. It should be noted that the oil pump drive shaft will come away when the sump is removed.

CRANKSHAFT
This exposes the crankshaft and main bearings. The big- ends should be removed by undoing the two bolts securing the big-end caps: a careful note should be kept of the order and way in which the connecting rods are removed. They are usually marked, but if no markings are to be found they should be centre-punched with the numbers of the journals from which they are removed. The oil-gallery pipe should now be removed.

Prior to the removal of the crankshaft, the fan-pulley (or, in the case of earlier models, the starting-handle dog) must be shifted. In the centre of the pulley (or dog) is a locking-bolt which has a right-hand thread in the earlier models; the post-war car has a left-hand thread. The fan- pulley may now be unscrewed from the nose of the crankshaft (right-hand thread). On the nose of the engine casing will be seen the thrust race cover, held on by three nuts which should be removed, together with the cover (do not lose the shims); the thrust race may now be pulled off. On the post-war cars the double ball thrust race is not fitted; the crankshaft thrust is taken on No. 3 main bearing.

Before removal of the main bearing caps, note should be made of the cap numbers to see if they correspond with the numbers stamped on the engine casing. The bearing caps may be removed by undoing the two securing nuts on each cap. The crankshaft may now be lifted out.

The crankshaft should be placed on a bench and the flywheel removed by undoing the six damper nuts. These may be recognised by the fact that they are large castellated nuts with a flat washer about the size of a penny (old money) under each one. (On cars manufactured prior to 1936 the nuts are on the gearbox side of the flywheel, whereas on later cars they are on the sump side). The flywheel may now be removed.

A return should now be made to the engine case. The dynamo should be removed by its three nuts; also the distributor drive cover. The dynamo rubber coupling should be examined, as a worn one can give rise to an annoying knock.

CROSS SHAFT
This drives the dynamo and distributor and does not often need attention. It can be removed by unscrewing the four set screws on the distributor side, when it will pull away.

It will be noticed that the shaft is supported by a brass bush on the dynamo side, fitted to the engine proper, while the distributor end is carried in a bush in the aluminium cross shaft casing. Engine thrust is absorbed by a double ball thrust bearing fitted to the distributor side of the gear. Slight end float is allowed, and is controlled by a spring fitted immediately behind the distributor driving flange. A steel plate inside the cross-shaft casing prevents any possible chance of wear on the aluminium case. In reassembling to the engine special care should be taken to see that the offset three holes in the cross-shaft case correspond with those in the engine case, and that the instructions given for dismantling are reversed in refitting.

Oil Pump
The oil pump is situated in the sump on the exhaust side of the engine. Access is possible by removing the two nuts on the underside of the sump. The pump will then pull away. On later models the two nuts on the sump release a cover which exposes four studs going through the main pump body. On slackening these back - not removing them - the pump will drop away.

The pump is held together by the front studs already mentioned. Access to the gears is possible on removal of these. If the car has done a large mileage the pump is generally worn. If the gear recesses in the pump body are badly scored, it should be scrapped and a new body obtained. This also applies to the end cover in every case it is advisable to replace the gears. Care should be taken to see that the driven gear is in its correct place.

It is most important to see that the oil pump is in really good condition. If the pump is not 100 per cent efficient and the engine is tuned, bearing failure will inevitably result.

We now have the engine completely stripped. All the parts should be thoroughly cleaned and laid out for inspection on a CLEAN bench. The word ''clean'' cannot be too strongly emphasised; a trace of dirt in the wrong place can ruin the finest machine.

The engine having been stripped, we will now consider the refitting and ''tuning'' of each part or assembly. We will reverse the usual order of things and start at the top.

Note- Oil Filter/strainer & oil pressure relief valve need to be stripped & cleaned.

The CYLINDER HEAD
A1l the component parts should be cleaned thoroughly and laid out on the bench for inspection. We will start with the head casting itself the first thing to ascertain is whether the gasket surface is true or not. This is best done with a surface plate. Should the head be distorted, it must be surface ground until it is true. Only the smallest possible amount should be taken off, otherwise the ''sealing'' grooves into which the gasket is compressed will be removed. It is not advisable to raise the compression ratio by machining the cylinder head, for two reasons, the first of which is, as stated above, that it will destroy the sealing grooves. The second is that it will vary the distance between the camshaft sprocket and the crankshaft sprocket. The compression ratio should only be raised by fitting pistons with longer compression centers; this will be described later.

The next step is to clean up the ports. They should be carefully filed with a riffle file until the passages present an easy entrance and exit for the gases. The corners should be rounded, and all rough surfaces smoothed. Time spent on this is not wasted, as it is one of the most important aids to an increased power output. When finished, the ports should present a smooth, polished appearance.

The next step is to line up the exhaust and inlet manifolds with the ports. It will be found that in the majority of cases the transition from manifold to port is not smooth, and that either the port overlaps the manifold slightly, or vice versa. Whilst the inlet manifolds are being lined up, the opportunity should be taken to smooth and polish them.


'NOTES ON MAINTENANCE and Tuning The AC ENGINE - PART TWO OF Three'


VALVE SEATS
The valve seats should be trued up and recut if necessary. The face angle is 45 degrees. On the majority of old engines the valves will be found to be "pocketed"; that is to say the valve has sunk into the head with repeated grinding The pocket thus formed impedes the flow of the gases and should be removed by beveling the sides. On older engines (pre-1935) the inlet valve seats and the entrance from the seat to the port should be enlarged to take the later 1 3/8" tulip valves. The inlet and exhaust valves are not interchangeable and have a stem diameter of 5/16" on the 16/70 engine, and 3/8" on the 16/80 engine. 'Head diameter inlet 1 3/8" exhaust l ΒΌ". Face angle is 45 degree with tulip heads on the 16/80. The 3/8" diameter valves can be fitted to the 16/70 engine by merely changing the valve guides. This is strongly recommended and, indeed is imperative if advanced tuning is to be undertaken or if it is intended to indulge in long periods of full throttle work. The 5/16" exhaust valves just will not stand up to it. *

Note - This is not necessarily current thinking

If it is intended to re-use the existing valves they should be trued and refaced to the correct angle of 45degree. Should the edges of the valves be at all "thin" after re- facing, they should be scrapped, as they will only burn quickly and can also give rise to pre-ignition due to the thin edge becoming hot enough to fire the mixture.

After refacing, the valves should be carefully ground in, if the seats have been recut, fine grinding paste is all that is necessary. Grinding should continue until an even seat all round the valve is obtained. A wide seat is neither necessary nor desirable. If seats are excessively pocketed, it is as well to have them built up, or new stellite seats inserted. *

Note - Many unleaded inserts are now available

Valve Guides
The valve guides are easily detachable from the cylinder head. They should be driven out from the combustion chamber side of the head by a brass drift. If the clearance between the valve stem and the guide exceeds .003" to .004" the guides should be replaced.

CAMSHAFT and ROCKER ASSEMBLY
From the point of view of power output there is little difference to pick between the .005" clearance camshaft and the .020" camshaft - the only difference is that of tappet noise. The .005 in. camshaft (fitted up to 1936) is extremely noisy, whereas the .020 in. camshaft is quiet, due to its eccentric based cams.

In practice it is seldom necessary to adjust the camshaft bearings in the cylinder head as they are well lubricated and consequently little wear takes place. The correct camshaft bearing clearance is .003 in. End float is controlled by a shoe in the rear camshaft bearing engaging with a groove in the camshaft. If the shoe is excessively worn it should be replaced.*

Note - New camshafts are now available

The rockers are carried on a hollow shaft in the top halves of the camshaft bearings. In the forked ends of the rockers are rollers which bear on the camshaft. Should the rollers be very ''sloppy'' on the pins, they should be replaced. The ends of the roller pins are merely riveted over. The part of the rocker which bears on the hardened thimbles of the valve stems should be carefully examined. In most engines a wear ''groove'' will be found on both rockers and thimbles. The rockers should be care- fully stoned until this groove is removed and the pad of the rocks is perfectly smooth. The valve thimbles should be scrapped and replaced if they are excessively worn. If, however, the wear is only slight they may be stoned. The removal of this groove in the rocker and the hollow in the thimble is very important otherwise it is impossible to set the tappet clearance correctly.

The rocker shaft should then be reassembled, taking care that distance pieces, which are in three different sides, are replaced so that the valve stem bears on the centre of the rocker. Do not forget the mouse-trap springs between each rocker. On the older engines the rocker shaft is in three parts. If they are very badly worn it is advisable to replace them with the later one-piece shaft. The method of fixing is obvious. *

Note - The original AC rocker shaft is too short & does not fully engage into the first pedestal which causes fretting wear, new longer shafts should be fitted ensuring oil supply to the first pedestal cam journal.

VALVE TIMING Vernier
One fault of the A.C. engine is the long timing chain, and I find in practice that the valve timing of an engine is appreciably altered by timing chain stretch. The standard method of attaching the camshaft sprocket is by three equally spaced holes, and does not permit any variation between the chain wheel and the camshaft being made, so that any very fine adjustment of valve timing to compensate for chain wear and stretch is impossible. The slight inaccuracy of valve timing due to the chain is not sufficient to be noticeable in the average car, but it does make a great deal of difference if you want to get the last ounce of power as the A.C. engine is very sensitive to errors in valve timing!

Fortunately a simple vernier can be made by drilling twenty equally spaced holes on the wheel carrier plate, and twenty-one equally spaced holes on the camshaft sprocket. These holes should only be drilled on a drilling machine equipped with a dividing head. The holes should be drilled on the same pitch circle diameter as the existing bolt holes. *

Note - Vernier camshaft sprockets are now available
Three bolts, as before, are used to bolt the sprocket to the carrier plate. The difference of one hole between the sprocket and its carrier enables extremely fine variations of valve timing to be made.

The valves may now be replaced in the head. New double Terry's Aero valve springs should be used. The camshaft and rocker assembly can be replaced. Do not forget to oil the valve stems, rockers and camshaft bearings before they are replaced. After tightening the camshaft bearings it is advisable to turn the camshaft by hand. If the shaft is very stiff to turn, slacken off the rear bearing - it is very easy to get the end float shoe out of alignment with the cap.

The cylinder head is now finished. *

Note - The AC valve spring design is not ideal& can bind on full cam excursion, also they are of too low resonance for maximum RPM operating, new springs are available which overcome the problems & make possible the fitting of standard stem seals.

Reminders
Do make a really good job of the ports and manifolds - it is well worth it. Mine took about a week to do! Be sure your valves are well ground in and that the seating is as perfect as you can make it. Be clean! - remember that dirt in an engine is a sure route to excessive wear. And hasten slowly.

Note - New upgraded Aluminium heads will shortly be available which have significant gas flow improvements & compression ratio of 8.23 to 1 with flat top pistons.

CRANKSHAFT
Has five main bearing journals. No. 5 main bearing is larger in diameter than the other 4. This is due to the fact that this bearing is part of the adaptor carrying the flywheel and timing sprocket. Normally this adaptor should never be removed except in the case of damage. The adaptor is held on to the crankshaft by a keyed taper and a large nut and tab washer; behind the tab washer is a paper gasket - this is most necessary to prevent oil leakage along the keyway from No. 4 bearing.

Journal diameter 1 to 4 - 1. 749'' No. 5 - 1.998"

Big end journals 1.624''

The journal should be checked for wear and ovality.

If this is found to exceed .004'' regrinding is necessary otherwise low oil pressure and consequent bearing failure will ensue. Maximum grinding limit .020''.

Crankshaft end float .002''. Prior to 1937 a double ball thrust race is fitted on the front of the crankshaft, shim adjustment is provided. After 1937 end float is taken on No. 3 main bearing.

Main BEARINGS
Pre-war cars die cast white metal Post war bronze backed white metal shells.

Both are doweled in block and caps.

The bearings should be carefully examined and if there are any signs of cracks new shells should be obtained.

If new shells are obtained they are supplied under-size to permit line boring and hand scraping. It cannot be emphasised too strongly that 100 per cent bearing fit on the A.C. is absolutely essential and that plain line boring is just not good enough - especially if an increased power output is desired. I recommended line boring or reaming to within .003" of finished size followed by hand scraping to 100% bearing area. The caps may be filed slightly to take up wear.

Note: Care should be taken to see that the bearings are bored in the centre of the shells, and that, when scraping, not too much is removed from the engine case side of the bearings as this would allow the cross shaft gears to run too deep in mesh.

Normal running clearance of bearing is .0015''.

Note - The existing thick walled main shells may be re-white metalled as new thick wall shells are not available.

Care must be taken to maintain the centre distance between the crankshaft & the cross shaft (critical dimension)

CL Engines require thin wall shells which are now difficult to source, some study is needed to find an equivalent.

As previously mentioned 100% bearing fit is essential and when making the final fittings it should be possible to turn the crankshaft by hand with only the slightest amount of drag when the bearing caps are fully tight If the engine is assembled with the bearings too tight a great deal of strain is placed on the dowel pins which retain the shells to the case or caps, and might result in the pegs shearing and allowing the shells to turn thus cutting off the oil supply to the adjacent big ends with dire results.

CONNECTING RODS
Steel, direct white - metalled big ends.

Examine carefully if there are any signs of cracks or if the bearing surface is rough have them re-metalled it is well worth it.

If the engine is being tuned for a very high power output or is to be supercharged lead bronze big ends and sports connecting rods are strongly recommended.

Big end bearing should be 100% fit. Running clearance and end float is .0015''.

The bearing cap may be filed to take up wear.*

Note - Filing of bearing joint faces is bad practice (not good engineering) as it can change the centerlines & cause uneven wall thicknesses of bearing material.

In fitting the bearing great tightness is not desirable. They should just fall with their own weight when the nuts are fully tightened. The radii must be a good fit or over- oiling of the bores will result.

Check the alignment of the gudgeon pin. Re-align if necessary. Little end bearings are bronze bushes and should almost always be renewed and reamed - the gudgeon pin should be a nice sliding fit.

For extreme engine smoothness the connecting rods should be weighed, and filed on the outside of the big end caps until their weights are equal.

After fitting the main and big end bearings the crank- shaft oil ways should be syringed out thoroughly with clean paraffin and the main bearing shells should be checked to see whether the oil holes are drilled and that they register with the gallery pipe unions on the caps.

The engine is now ready for assembly.

Note - Crankshafts & con rods should be crack tested as with the cylinder head, the crankshaft & con rods should be checked for straightness as well.

If new Crankshaft & con rods are considered one may look for standard shell bearings to fit the block & new rods. Main bearings may be selected to fit between the main cap studs which are narrower on CL engines compared with UMB engines on 1-4 mains.

ASSEMBLY
The engine case should be washed well in clean paraffin. The first step is to assemble the crankshaft and main bearings and on pre-1937 models adjust the crack- shaft end float to .002 by means of the shims provided on the first oil seal (on post-l937 cars the end float should have been adjusted when scraping the bearings). Tighten the main bearing caps and lock with tab washers, refit oil gallery pipe.

Note: Before assembling the bearings oil the journals liberally with clean engine oil.

The front oil seal on the pre-war cars can be improved by fitting a small disked cap which houses a felt washer - (this cap can be obtained from A.C. Cars Ltd.). Contact Brian Eacott

The flywheel may now be fitted. Always renew the rubber damper rings, two to each hole. In re-assembly always check that the friction surfaces of the steel plates behind the nuts are perfectly smooth and that the copper ring is clean and in good condition, also check that the marks 1 and 6 coincide with the piston positions, The damper nuts should be tightened using a large screwdriver placed in one of the slots of the hexagon. One hand only should be used and the nuts tightened as much as possible and then slacked back two serration's and split-pinned.

The damper is now correctly set.

To be continued

Note - If new Crankshaft & rods are to be fitted then serious consideration should be given to fitting a lip seal to the flywheel end of the crank, if this is adopted a new flywheel or a flywheel adapter will be desirable.

Post war engines have the opportunity to fit a modern crankshaft damper to the front of the crank.

Pre war engines are restricted due to mounting feet but advice should be sought!


'NOTES ON MAINTENANCE and Tuning THE AC Engine PART THREE OF THREE'


The postwar engine can be much improved by dispensing with the flywheel damper, which is then modified by replacing the rubber rings with steel distance pieces machined to fit the space normally occupied by the rubber exactly and the flywheel locked solid. A conventional torsional vibration damper is then fitted to the front of the crankshaft behind the fan pulley.

This modification would be difficult on the pre-war car owing to be restricted space between the front engine bearers.

The front mounted damper is a vast Improvement as it completely eliminates that ''A. C. period at about 60 MPH.

The fan pulley or starting dog should now be refitted.

The pistons should now be refitted to the connecting rods making sure the circlips are in their recesses.

The pistons and connecting rods should now be fitted to the sleeves - it is assumed that the piston rings have been set to their correct gap which is .005" (if worn sleeves are used the gap should be set at the bottom of the ring travel as the bore wears taper and if the gap was set at the top of the bore it would be too little at the bottom). The sleeves complete with pistons should be fitted to the engine case using new figure of eight copper and asbestos gaskets. Smear the crank journals with oil and couple up the big ends and split pins (Simmonds nuts on post war cars!) It is advisable always to renew the big end bolts and nuts.

The engine should now be turned by hand and there should not be undue stiffness.

The Cylinder head can now be fitted using a new gasket. Care should be taken that the large water port at the rear of the gasket coincides with the water port in the head - most gaskets are marked ''this side up'' which is to prevent the water port in the gasket from being put on the wrong way.

Note - New liners & matched piston sets are available, there are however two liner lengths now available the original 3& 15/16th'' between shoulders & 4'' between shoulders, one will need the appropriate figure of eight gaskets to suit!

Modern head gaskets are available & are on test however the copper asbestos type are still made to special order (by hand which leads to some inconsistencies) These gaskets are not as compliant as the original asbestos filled types.

The cylinder head nuts should be tightened from the centre outwards omitting the four long studs which should be tightened last. Care should be taken to tighten the head down evenly. Refit the camshaft and rocks gear and also the camshaft sprocket. The timing chain should be renewed if worn in fad it is always better to renew as the chain is rather prone to stretch. The valve timing may now be set.

In June last year we who were pledged, sent our money to Colin Dunn at Solent vintage Engineering, Lymington to start the manufacture of the worms and wheels. So that the centre of the worm wheel could be machined to suit my particular final drive it was necessary to strip my axle to discover whether mine had two, three or even a rumored four planets in its differential. This I did just be- fore Christmas and was pleased to discover that this time the bucket was almost free of detritus. (Last time I had filed off the sharpest knife-edge peaks of the worm wheel's teeth in the hope of preventing a repetition).

With the whole assembly now in pieces I resolved to do something to stop the incredibly noisy second gear that spoiled an otherwise wonderful drive. The noise varied widely with different howls coming in as the revs rose. The noise in some bands was bearable but in others much less so. In the end I decided to have a new laygear cluster a new laygear spindle and a new second gear manufactured by Kingsway Engineering Ltd. of Hanham, Bristol. These newly manufactured parts were ready for me to collect at the end of February.

In this type of gearbox the reverse idler gear because it is permanently meshed with the laygear, spins around all the time the engine is running with the clutch engaged. So it is not surprising to find that its bushing was pretty worn and loose. I took the opportunity while waiting for the other gears to arrive to turn a nice new phosphor bronze bush with better clearances.

Assembling all the new parts with new bearings was a pleasure and the gearbox and final drive rotated smoothly and sweetly. The nut at the rear end of the third motion shaft on my original fitting was a castellated nut and the shaft was drilled for a split pin. The pattern that Colin Dunn had had copied had a plain nut and no drilling for a split pin and of course now it is too hard to drill. We decided that the nut would be self-tightening. In any case I ground a small groove across the end of the thread into which I could peen the nut and assembled it with Locktite.

By fitting a piece of wood the height of the overflow holes on top of the lower part of the bulkhead between the gearbox and the final drives in the bottom of the axle casing, I was able to check that just under 2 pints of oil gives the correct.

Set the engine so that No. 1 cylinder is on the firing stroke, turn the flywheel until it is 20degrees before T.D.C. (use the protractor previously made), fit the distributor taking care that the Rotor Ann is pointing to No. 1 cylinder contact and the points are just breaking (remember to get the direction of rotation correct).

Before fitting the distributor I have assumed that it has been overhauled and the shaft bearings renewed if necessary and that the automatic advance and retard is functioning correctly. Worn distributor bearings often cause misfiring.

I have found the standard coil ignition system very satisfactory and no worthwhile advantage seems to be gained by fitting a high voltage coil.

If the engine is to be used at high R.P.M. a Scintilla Vertex Magneto is a definite advantage.

Prior to 1936 a single carburettor was fitted. On 1936 engines three 1 1/8" S.U. carburttors were fitted as standard and on the postwar car the centre carburettor was fitted with an auxiliary starting carburettor which fed a rich mixture into the balance pipe. This instrument is controlled by a solenoid which is switched off automatically by a thermostat in the cooling system.

The carburettors should be dismantled and thoroughly cleaned taking care not to mix the parts. The throttle spindles should be checked for wear and if they are a loose fit in the body they should be renewed. In extreme cases where the throttle bearings in the carburettor body are worn it may be necessary to renew the body. The jets and needles should be renewed if the car has covered a large mileage. A new set of corks for the jets should be obtained. The normal needles are D.W. but if extra power and acceleration are required 'L' type needles should be fitted.

The dashpots and pistons should be cleaned. The pistons should be placed in the dashpots and they should fall freely by their own weight. If they do not fall easily put a little metal polish on the piston (not the guide) and push it up and down the dashpot whilst spinning it.

Thoroughly clean the piston and dashpot in paraffin to remove all traces of metal polish. The needles should now be placed in the pistons and the shoulder of the needle should be flush with the bottom of the recess of the piston. Make sure the needle-holding set screw is really tight.

Reassemble the carburettors, but leave slack the large hexagon nut which holds the jet carrier to the body - this is necessary to allow the jet to centre itself and so avoid bending the needle. Replace the dashpot checking before tightening the screws that the piston slides up and down freely. Be careful - it is very very easy to bend the needle. The jet carrier nut should now be tightened - again lift the piston up and down whilst tightening the nut to centre the jet. If the jet is properly centered the piston should fall freely and should 'bottom' on the body; this can be detected by a distinct thud as it hits the body. If it does not do so slacked the jet and start again. It is often necessary to repeat this three or four times. It is most important to be sure that the needle is free for the full length of its travel.

Refit the carburettors, without the float chamber to the engine.

FLOAT chambers
Remove the needle valve and seating. If either are ridged, renew. The needle valve assembly does not have any washer between it and the float chamber top. Check the floats for leaks - renew if necessary. Soldering leaks is unsatisfactory as it alters the weight of the float. Refit the yokes to the float chamber top and insert a piece of 3/4" diameter rod across the float chamber top between the yoke and the top. Bend the yokes so that they just touch the rod when the needle valve is shut. This should ensure that the petrol levels are all equal. Mount the float clambers on the carburettor bodies and couple up the petrol supply. We are now ready for tuning the carburettors.

TUNING
Remove the dashpots and pistons (do not on any account slacken the jet carrier nut as this would allow the jet to become out of centre). Switch on the petrol supply - check for leaks. The jet-adjusting nuts should be screwed up as far as they will go; observe the petrol level in the jets - they should all be equal and should be about 1/8" below the top of the jet. If they are not equal remove the float chamber top of the offending carburettor(s) and bend the yoke slightly (up to raise the petrol level and down to lower it) until the levels are equal. It is of course necessary to empty the float chamber of petrol in between adjustments.

It is well worth while taking a little trouble over these levels as on this depends the ultimate even setting of the carburettors.

Refit the dashpots tightening the screws evenly and checking all the while that the pistons move freely. Unscrew the jet adjusting nuts about 10 flats each (on the post war cars the jet adjusting screws are under hexagon caps at the bottom of the jet carrier. These caps should be removed and the screws thus exposed unscrewed about 1 1/2 turns from the hard up position).

Slacken the clamping bolts on the universal-jointed throttle connectors. This will allow independent setting of the throttle stops on each carburettor. Unscrew the throttle adjusting screws until they will just hold a thin piece of paper inserted between the screw and the stop lug; then screw in a complete turn.

Connect all pipes that convey essential fluids to your masterpiece.

Add these fluids and satisfy yourself that drain plugs are tight and that no leaks are present.

Check that all the electrics are correctly connected.

When you are completely happy that all is well!

The engine should now be started. When it is thoroughly warmed up the idling speed may be adjusted by turning the throttle screws equal amounts in either direction - in for a faster speed and out for a slower. To check for exact synchronisation. listen to the intakes through a piece of rubber tubing inserted in one ear and held close to the in- take. The stop should be manipulated until the hiss at each carburettor is the same. Now retighten the throttle connectors.

The mixture should now be set by screwing the jet adjusting nuts up or down equal amounts until satisfactory running is obtained. If the engine speeds up, the throttle screws should be unscrewed equal amounts until the idling speed is satisfactory. The mixture may now be checked by lifting the pistons in turn, a slight amount with a pen knife - if the engine speeds up, carburettor is too rich; if it slows down it is too weak. Correct by the jet adjusting nut- screw up to weaken, unscrew to enrich.

If lifting the piston on one carburettor stops the engine and lifting that of another does not this indicates that the mixture on the first is weaker than the second.

When the mixture is correct lifting the pistons for a moment for a small amount should make no difference to the engine speed. The exhaust beat should be even and regular. If it is irregular with a splashy type of misfire and a colourless exhaust, the mixture is too weak. If there is a regular rhythmical type of misfire together with a blackish exhaust then the mixture is too rich. Spend a long time on mixture setting, it is very important.

MODIFICATIONS
A great deal of extra power can be obtained by fitting 1 1/4" S.U. Carburettors in place of the standard 1 1/8" instruments. This necessitates the making of special manifolds.

SUPERCHARGING
The engine may be supercharged, but I have found in practice that the extra power outputs is not worth the sacrifice in reliability and engine life. In any case the boost pressure should not exceed 3 lbs. per square inch.

The engine power output can be substantially increased by fitting a made-up exhaust manifold having a separate pipe for each port all the pipes streamlined into a larger pipe a la bunch of bananas.

All the bold typed notes are to offer updated thinking & should be regarded as prompters before you commit the work on the AC Engine, it is our contention that the original AC 'Weller' still has good potential & will over time be seen as an equally preferred choice for the traditional AC cars.