Not enough revs.

[Phidi]

Good morning. Still having trouble with my 1956 four door, fitted with 1098 low compression engine.
Problem is it starts and runs beautifully and pulls very well but will not reach higher revs. I don’t know the actual revs but I get 10/12 mph in first, 15/20 second, 30/35 in third and 40/45 in top. At these speeds the engine splutters and misfires but will immediately start pulling again if I change up or drop speed. No load, the engine will briefly rev then drop back to a steady fast but not high rev.
I have replaced: new coil(high power type), new electronic ignition, overhauled carb with new float and valve, new fuel pump, new fuel line(with fuel tank emptied and cleaned), Top end refurb with all new gaskets, new plugs (25thou set), valve clearance (checked at 12thou), new fuel hose - pump to carb.
Any suggestions on how to cure this would be greatly appreciated. Thanks, Phil.

[geoberni]

Well that's a lot to go changing just 'on spec'.... How about some basics.
What gearbox and diff have you got?
How much does it rev in neutral?
If it revs OK off load, then I'd be looking elsewhere than the engine for 'not enough revs'

Can you expand on this...

No load, the engine will briefly rev then drop back to a steady fast but not high rev.

[Phidi]

Hi thanks for your response. It won’t rev to a point in neutral, that sounds as if it’s reaching the 4500/5000 rev level, I’ve had lots of A series engines and it’s no where near where I would expect to need to change gear. When I floor the accelerator on neutral, it revs freely to maybe 3500 revs, then drops back (still with the pedal floored) to maybe 2000/2500revs and won’t increase.

[Bill_qaz]

Have you checked you are getting full throttle at carb when pedal full pressed?
Are the advance weights in your distributor free moving, have you put a timing light on to see if your timing is not retarded and is advancing?

[59Elva]

Leaking or stuck vacuum advance? Stuck carb piston? Spark plug color might suggest whether a fuel or spark problem.

[Phidi]

Thanks for that. It’s something I hadn’t thought of- a stuck/sticking piston, will check that and also make sure the throttle is opening fully. I admit, having stripped and rebuilt the carb, I assumed it was fully functioning - possibly a touch of vanity on my part.

[Phidi]

Tried all the suggestions, still not getting there but when I got the car, the card needle (AN) had been fitted sticking out about 3mm rather then the shoulder being level with the piston face, which I guess would have made the mixture lean. Put it back to that position and quite an improvement but still not right. I noticed in one of the ‘repair books’ that the low compression engine (mine) may have been originally fitted with the lean EB needle, does that sound right?
Also, the timing marks on mine are beneath the pulley rather than on top, as all other ‘A series’ I’ve seen, again is this correct? Assuming the timing marks are in the right position, I’ve set the timing to 4deg btdc, yet again, sound about right?

[Bill_qaz]

A series in minor the timing mark are below mirror on floor or paint duplicate on top if you are having difficulty viewing lower marks with timing light. Your 4 degrees is static ok but check advancing with timing light.
Can't help with needle info but I'm sure someone will help.

[svenedin]

If it is an HS2 carburettor on a 1098cc then yes the weak needle is EB.

Before changing the needle though, it would be wise try to work out what is actually stopping the engine from reaching higher revs. You say it starts and seems to run well but at higher revs it splutters and misfires. What happens when it starts to misfire? Is the exhaust black (too rich)?

I would have a look at the carburettor. Check there is oil in the carburettor dashpot. Remove the air filter so that you can see the carburettor piston. Without the engine running, lift the dashpot piston up with your finger as far as it will go and let go. Does it drop back down with a "metallic clunk". If the piston is sticking there could be a number of different reasons: absent/weak piston spring, jet not centred properly, bent needle.

Assuming the carburettor piston passes the clunk test, start the engine leaving the air filter off. When the engine is up to normal temperature increase the revs using the throttle lever on the carb whilst watching what happens. Does the piston rise as you open the throttle? It should respond briskly.

Some other ideas. Get a set of neon spark testers and fit them to the spark plugs. Start the engine and directly observe whether the spark becomes weak as the revs rise and the engine starts to falter. If it does, it is an ignition problem. Why this would only affect higher revs though is guess work on my part. Bad rotor, distributor cap carbon contact worn out etc.

I had my car tuned on a rolling road and I watched the process. Several off the shelf needles were tried. The wrong carb needle was really pretty obvious even without the sensor equipment. A needle which has a profile which is too rich will idle well (because many of the needles have the same diameter at the idling segment) but as the revs rise, the needle diameter is too small and the mixture becomes too rich. So what is observed in this situation is that the exhaust belches black smoke at higher revs.

Stephen