Club 80-90 Forums

Bus (2lt a/c) failed MOT on emissions today.
CO is fine but Hydrocarbons are well over the limit pointing to poor combustion on at least one cylinder.
This is backed up by a popping/missfire at fast idle which it has done for the last 2 years!
Got away with this on MOT's in the past as i had a twin tailpipe silencer fitted and the tester always put the gas analyser in the tailpipe fed by the good cylinders!
The question i suppose is what could cause the poor combustion?
I suppose i should check the state of the plugs first, but these were changed in august and the ones i took out didn't seem too bad then.
Any ideas anyone?

Keith.

Squash it.

Probably air leaks, vacuum pipes, knackered carbs.

Well... new plugs leads and dizzy cap n rotor would be a good start as would fresh oil and filter and air filter.. then the carbs might actually need adjusting.. or are worn.. you could also check the ignition timing and prob even reset the valve adjusters (if hydraulic) to 1/4 to 1/2 turn from just touching...

If its popping and farting then something is wrong and it needs fixing.. not hoping its going to fix itself..

Remember its a 25+ year old vehicle... it WILL need stuff doing....

airfilters not blocked is it?
just a thought but did the tester lift the revs to around 2.5 - 3 thousand for the test? cos he should have.
something to do with the technology of the day which allows them to lift the revs and clear it's tubes as not much of the old stuff passes at idle?

Cheers for the advise guys especially Simon's but i don't think its time for the crusher just yet Besides i like a good challenge.

Anyway as i said its popped n farted at fast idle and overrun for a few years even after changing plugs, leads, dizzy cap, rotor and vacume/balance pipes so i don't reckon the prob lies there.

So today i ran the motor at about 1800 - 2000 revs so it was popin like mad and pulled the leads off one at a time - result was it stopped popin with number 3 lead disconected. As one carb feeds 3 and 4 i think it would be safe to rule out knackered carbs.

Am i right in thinking its probably poor compression or a valve on number 3 contributing to the poor combustion/unburned fuel?

BTW its recently started pissing oil from no 3/4 rocker cover/pushrod tubes and out of the large crank oilseal (was replaced in last 10000 miles) so could be excess crankcase pressure caused by above.

Anyone got a head for sale or maybe a complete motor?

Keith.

Does sound like a valve problem. Have you checked valve clearances?
If all ok then its probably valve seating problems. May just need re-grinding if you're lucky!

Compression test, followed by a cylinder leakage test won't take too long.

[Mine failed for high HCs. Turned out one of the vacuum pipes had fallen off. Pushed on and passed