Misfire no.4 cylinder

[drydockdavies]

All four plugs were fouling so new choke seals were fitted but no.4 is running cooler and still fouling. Went for a run last week after adding a drop of Sea Foam and fresh E5, couldn't believe how well she ran! Fired her up today and she was a bit lumpy so took her for a spin but didn't improve. No.4 plug is fouled again, exhaust header cooler than others, swapped plugs same result.
Decided to do a compression test, engine had cooled a bit but still warm, 1,2,3 75psi, no.4 50psi with a closed throttle. Wide open throttle all around 125 psi with no.4 a few psi over.
Carbs ultrasonic cleaned twice, new coils, plugs, caps and points set. Valve clearance set last year, done less than thousand miles since. First thought sticking valve but not sure now!
She was definitely running on 3 cylinders today but was fine last time out,
Any ideas
TIA

[sean_rfi]

When I reassembled my carbs (with new choke seals) I noticed that one of the raised cylindrical bosses that the choke rubber seals against was quite rough from corrossion i.e.not a good sealing surface. I made up tool to recut/ dress the bosses to make sure the new choke plungers actually sealed. Might be worth checking these on you carbs?

[warren3200gt]

Double check the simple things first. New plugs of from known genuine supplier. No 4 plug maybe breaking down. Or swop the plugs and see if the issue moves. Check ht lead connection has clean core wire and tight connection to cap.

[Ultim8pc]

Double check the simple things first. New plugs of from known genuine supplier. No 4 plug maybe breaking down. Or swop the plugs and see if the issue moves. Check ht lead connection has clean core wire and tight connection to cap.

What Warren said.
Swap plugs, plug caps and coils over and see if the fault moves.

[drydockdavies]

luckily I can remove no.4 choke in situ, new seal now looks like the one I replaced!

[sean_rfi]

That looks fine to me - witness mark on the rubber surface suggests that it is sealing correctly

[drydockdavies]

Will have a look at valve stem seals, can't think what else it could be now.

[warren3200gt]

Valve stem seals tend to show after the bike has bee sitting ad oil has seeped past. Once you start it you get smoke until the oil burns off. Once that's happened they tend not to leak enough to show exhaust smoke.
Are you getting start up exhaust smoke?

[drydockdavies]

Valve stem seals tend to show after the bike has bee sitting ad oil has seeped past. Once you start it you get smoke until the oil burns off. Once that's happened they tend not to leak enough to show exhaust smoke.
Are you getting start up exhaust smoke?

Yes, smokes until the engine is hot and the choke off. I ran my race bike without seals on the exhaust valves and big puff on start up. After having the choke seals replaced because of the symptoms I have now she fired right up, high revs on choke, warmed up quickly and settled to a steady tickover. Happy days. After around 100 miles, a couple of ride outs, symptoms have returned, poor starting, smokey until hot, no.4 misfire and cool, sooty plug.
Next step maybe is carbs off again and swap parts around to see if I can move the problem to a different cylinder.
I've swapped and replaced all the obvious stuff.

[warren3200gt]

Valve stem seals tend to show after the bike has bee sitting ad oil has seeped past. Once you start it you get smoke until the oil burns off. Once that's happened they tend not to leak enough to show exhaust smoke.
Are you getting start up exhaust smoke?

Yes, smokes until the engine is hot and the choke off. I ran my race bike without seals on the exhaust valves and big puff on start up. After having the choke seals replaced because of the symptoms I have now she fired right up, high revs on choke, warmed up quickly and settled to a steady tickover. Happy days. After around 100 miles, a couple of ride outs, symptoms have returned, poor starting, smokey until hot, no.4 misfire and cool, sooty plug.
Next step maybe is carbs off again and swap parts around to see if I can move the problem to a different cylinder.
I've swapped and replaced all the obvious stuff.

Did you change the o ring seals at the top of the plungers where they screw in. Critical to get a good seal there.

[drydockdavies]

Yes, I sent the choke plungers away because I was convinced that was the problem. I'm losing the will to live taking these carbs on and off!

[warren3200gt]

No not the plungers I mean the o ring where the plunger screws into the carb body.
An easy check on plungers and choke seals is to smear grease around the plungers before fitting. It'll seal the enricher circuit off from the running circuits. If the issue still persists its not a enricher issue. The grease seal won't last long as when you use the choke it'll gradually get washed off by the fuel cleaning it off.

[drydockdavies]

Do you mean the seal on top of the 14mm half nut? If so, no.

[warren3200gt]

Yes.

[gray]

My tuppence worth - and I agree with a previous post. Do the simple/ easy things first- take 1 and 4 plugs out and swop them over to see if the cool cylinder moves. ( a plug might seem to be sparking in the open air, but fail to spark under compression pressures) If that doesn't help, tank off and swop 1 and 4 plug leads over ( hopefully they'll reach), tank on and fire it up again. That will check the electrical items that are specific to cylinder 4. A coil issue would affect cylinder 4, but also No. 1 so not likely to be a coil I think.
We armchair mechanics await your results with interest