Jerking / Juddering / num num num again!!!!!

[wells01]

Hi
I am at my wits end with this. When accelerating gently I get a juddering / jerking its like a num num num num sound. If I put my foot right down it improves. I can,t feel anything through the gearstick, and Ive pushed and pulled the engine and gearbox about so I am fairly sure its not the mountings. It seems to be connected with engine speed rather than wheel speed.
I have tightened the steady bar and checked the gearbox strap/cable. Rebuilt the distributor and checked the mechanical and vacuum advance. Checked the timing. Rebuilt and set the carburetor, topped up the dashpot and set the mixture. Checked the brakes are not binding.
The only thing I am not sure of is the piston in the carburetor lifts the first 1/8 of an inch very easily after which you can feel the oil damping. Is this correct? I am wondering if its the piston chattering at small throttle openings.
Any ideas would be greatfully received.
Many thanks
David Wells

[alex_holden]

The only thing I am not sure of is the piston in the carburetor lifts the first 1/8 of an inch very easily after which you can feel the oil damping. Is this correct?

Yes, that's normal.

[wells01]

Yes, that's normal.

Damn!!
Thanks for your reply
Its very strange.
I can feel the car jerking slightly and hear it, its like you were tapping your foot on the throttle 3 or 4 times a second while accelerating gently.
Regards
David Wells

[bmcecosse]

Piston free lift is correct - use 3 in 1 oil in the damper, and advance the timing till you get some pinking - then back it off very slightly till 'just not quite ' pinking. Would be interesting to see a compression check on the engine.

[wells01]

[quote="bmcecosse" Would be interesting to see a compression check on the engine.[/quote]

Mmmmmmm!!!
Timing was set as you suggested.
Compression.
152 129 150 147
Damn!!!!!

Why would poor compression cause this on small throttle openings?
Regards

[cadetchris]

i have exactly this problem with primrose. i solved it with a new coil, but its returned now with a passion

[MarkyB]

It's missing due to the poor compression on number 2, but you knew that when you saw the numbers

[wells01]

It's missing due to the poor compression on number 2, but you knew that when you saw the numbers

Hello
I agree the compression is a good indication of the possible cause of this. But why only on part throttle? Why not when i accelerate hard? Or at constant speed/throttle?
Regards

[hotrodder13]

i had the same problem but this was solved by a better needle in the carb the orignal was too weak so i put a richer one in and its perfect

[MarkyB]

I'd say it's to do with when it's most noticeable to the driver.
Also leaks can only flow so much, higher speeds mean there is less time for pressure to leak off.
If a different needle can fix this, I'm at a loss to know how, but I learn something every day and have an open mind.

[hotrodder13]

well mine did the same and it was set up on a co2 machine i then put a richer needle in got it set up again and it runs much better

[MarkyB]

Fair enough, I wouldn't claim to know everything by a long chalk.

[bmcecosse]

If that low compression is being caused by a poor inlet valve (or just badly adjusted inlet valve gap ) then the gas will be blowing into the inlet manifold and upsetting the whole engine at lowish speeds. At higher speeds - it won't be as noticeable. Check the valve gaps first - but I think it will be 'off with it's head'!

[wells01]

If that low compression is being caused by a poor inlet valve (or just badly adjusted inlet valve gap ) then the gas will be blowing into the inlet manifold and upsetting the whole engine at lowish speeds. At higher speeds - it won't be as noticeable. Check the valve gaps first - but I think it will be 'off with it's head'!

Thanks
I will recheck the clearances tomorrow but i suspect the worst as the CR improved when i poured some oil into the cyclinder.
I wonder if I will get away with just rings!!
Regards

[bmcecosse]

Live in hope!

[wells01]

Live in hope!

Hi
Ive checked the valve clearances this morning and all are set to 12 thou. I tested the compression again and nunber 2 is still low but comes up 20psi with the addition of some oil into the plug hole.
Rebuild time I think!!! The trouble is its my only car.
Now does anyone have a spare 1098 engine?
Many thanks for your help.
Regards
David Wells

[wells01]

Hello
I went a bit off track with this post. The problem does not feel like the engine is missing, it isnt a stumble. It is more like a fast drone, num num num is the best way I can think of, a couple of nums per second speeding up and slowing down with the engine. The engine isnt using any oil to speak of and and the exhaust doesnt smoke! Perhaps I should re-examine the engine mounts. Is there a know test for them?
Thanks
David Wells

[MarkyB]

Try running it without the fan belt and see if the noise is still there.
Visual inspection is about it for engine mounts.

[wells01]

Live in hope!

Right. So Ive removed the head and fitted a new exhaust valve to no. 2 cylinder, re ground all the valves and now have good compression on all cylinders. Fitted a new carb and a new vacuum unit. This has improved drivability no end, but its still there. I no longer think its in time with the engine speed, its in time with the wheels / gearbox and if I feather the throttle it does it at a constant speed. Could it be slop in the diff? Worn half shaft splines. Or could it be the shock absorber springs in the clutch plate? I once had a citroen which was fitted with a re-manufactured clutch which displayed similar symptoms.
Many thanks
wells01

[bmcecosse]

Could be just about anything! Start with prop shaft joints - oil in gearbox , oil in diff that sort of thing.