FIRING ON 3 CYLINDERS

Post by **newmoggy** » Sat Jan 28, 2006 6:47 pm

My Mogs been in the garage for 3 months, ive just started it after fitting a new fuel pump. Its fine on idle but when you rev its rough & sounds more like a beetle. I pulled no. 1or 4 plug off. (plug nearest the radiator) & it made no difference, so i guess its only running on 3 cylinders. Fidd;ling with the HT lead makes no diference. What could be the issue other than the lead & plug?

Post by **Cam** » Sat Jan 28, 2006 8:54 pm

Dizzy cap and/or rotor arm might be suspect. It also might be damp and/or need a clean up.

Check (with another spark plug) if number 1 (nearest radiator) is actually firing so that you can narrow it down to ignition rather than engine problems.

Post by **newmoggy** » Sat Jan 28, 2006 10:40 pm

cheers cam, out of interest, what in the engine can go wrong to produce the same effect?

Post by **Willie** » Sat Jan 28, 2006 10:58 pm

If it was a problem in the engine you would need to do a compression check.
This would highlight any problems such as a broken valve etc when compared
to the other three readings.

Post by **Gareth** » Sun Jan 29, 2006 12:30 pm

For reference, the one nearest the radiator is plug no.1. I always had trouble remembering which was which.

It would be worthwhile swapping a known good plug (from no.2) to see if the plug has gone west. If no.1 works, but no.2 doesn't, then the plug is at fault. If no.1 still doesn't fire, then it could be ignition (no spark) or valves in the cylinder head.

Start with the electrical side, in the hope that it's something simple that a bit of logic might solve.

Post by **bmcecosse** » Sun Jan 29, 2006 1:22 pm

Could be loads of things - but if it was running ok 3 months ago it's likely to be just an oily plug. If it's not that - then start looking at HT leads/dizzy cap - and check the valve clearances - could be a valve has stuck open (most unusual) during the lay-up period.

Post by **Cam** » Mon Jan 30, 2006 11:30 am

newmoggy wrote:cheers cam, out of interest, what in the engine can go wrong to produce the same effect?

Well, if it's not firing on that cylinder then it either means that fuel is not getting in (or far too much is getting in), you have no spark or there is no compression.

I seriously doubt that it's fuel related as the inlet ports are connected together so if number 3 was duff then number 4 would be too.

So that leaves spark and compression.

Compression problems are usually valve related (can be head gasket/piston rings though) and can be burnt/chipped valves, sunken worn valves, over adjusted valves etc etc.

But a compression test will tell you if the cylinder is sealing on compression then if that's ok then it's just fault finding the electrics!

Post by **bmcecosse** » Tue Jan 31, 2006 8:09 pm

Simple compression test on a Minor is to turn it over with the handle - and feel if all the compressions are similar. If one is 'easy' and three are stronger, then it points to a valve problem.