Tight Valve clearances causing Misfire?

[Pikey]

Hi all
As the title says really, Ive sometimes got a intermittent misfire but only when the car is really hot, and this is normally accompanied by a small amount of running on when I turn off, it only fires on about once or twice on one cylinder by the sounds of it.

Its particulary noticable after Ive just done a fairly steep climb and then turn her off, ie getting into my housing estate. Or when Ive just travelled along a by-pass at 60mph and turn off. Basically when Ive just given the engine some decent work to do. At normal road speeds I dont experience any running on or misfiring.

Ive changed virtually everything on the ignition and Ive set the ignition timing to 5BTDC static, with a test bulb on the LT wire, so Im thinking it might be tight valve clearances (only happens when shes hot).

Im running on a standard leaded head with castrol valve master additive. With Champion Plugs.

Ive got a water temp gauge and that never reads anything higher than just below the Normal on a 82deg stat. And Ive got excellent oil pressure so it would seem that the engine isnt getting that hot.

Ive read threads on setting valve clearances to 12 for the inlets and 15 on the exhausts. Is this still the general consensus on valve settings.

Does it sound like valve clearances? When shes just been started from cold I do have some tappet noise from one or two tappets, but after shes been running for a period longer than about 10 minutes the engine is virtually silent and the tappet noise has gone. It sounds like a swiss watch when shes all warm.

Steve

[bmcecosse]

Try the clearances as you say - I recommend the slightly larger 15 thou for the exhausts because that valve does get hotter and expands more - especially now with hotter burning unleaded fuel. However -I suspect it's not the cause of your problem - but it does help to protect the valve and it's seat. I also suggest - try without the 'valve master' additive - it does very little and may be the cause. Running on is something glowing red inside the chamber - could be a rough edge, or the spark plug tip - or some carbon. Various remedies - including a full decoke and polish up the chambers removing all 'edges' - to dribbling some diesel fuel (or even just water) through the inlet while running the hot engine at high idle speed. Best done away from housing............ Also worth checking if your vacuum advance is working as it should - suck the pipe and watch to see if the base plate in the dizzy moves. And lastly - what colour are the plugs - and what grade are they ?

[Pikey]

Hi
Thanks I'll check the valve clearances first I think and open the exhaust valves up to 15. Ive checked some mini forums and I see they recommend 0.3mm is that the same size or not? I havent got my Zeus book on me at the moment.

The engine has only done 2200 miles since its restoration, so I doubt it'll need a decoke yet could it? The engine has had a 30thou o/s rebore, new bottom end bearings all round, new timing chain, oil pump and associated gaskets etc. The head I just decoked and lapped the valves in at the time, I couldnt afford a unleaded head and LRP was still available, so I didnt bother changing it.

The Spark plugs are N9YC's purchased from one of the minor specialists. The vacumn was replaced when I rebuilt the engine too and I checked it a couple of weeks ago and it appears to be working ok.

Spark plug colour is on the slightly rich side of tan brown, so a sort of dark brown really, so not in the slightest bit lean.

Steve

[bmcecosse]

N9Y are fine - I use N6Y, but my engine is a bit warmed up. Try leaning it off slightly. 0.3 mm = 12 thou!! ( 1mm ~= 40 thou) Use 15 on the exhausts - 'leaded' head is a waste of money unless thrashing the car hard on Mway.