uneven running

Post by **grainger** » Mon Jan 12, 2004 5:19 pm

hi

ive had a few problems recently, my 1970 1098 traveller started running a bit rough for no apparent reason .... when i took the plugs out, the two front ones are black and sooty whereas numbers 3 and 4 are both white !! ... i presume carb or inlet bother, any ideas ? the distributor, cables and plugs all seem ok. i have had trouble with oil getting into number 2 cylinder if i thrash it.

i used a bottle of redex in it a few weeks ago which i think improved things for a while.

cheers
grainger

Post by **rayofleamington** » Mon Jan 12, 2004 5:57 pm

I would look at the electrics again. If the back 2 cylinders are running ok then the carb / manifold shouldn't be the problem.
Try swapping a sooty plug with a good one.
Also have a close look at the dizzy cap for cracks - in damp weather like this a cracked dizzy will make a shorts circuit.
The rotor arm and dizzy cap pick-ups should be good condition.
additionally the 2 leads could be shorting out to each other.

If everything is ok so far, then check the compression is ok on the front 2 cylinders!

Post by **Cam** » Tue Jan 13, 2004 11:10 am

If cylinders 1 & 2 are sooty then they are running rich, if 3 & 4 are white then they are running VERY lean. Because of the siamese'd head arrangement 1 & 2 are fed together and 3 & 4 are fed together.

I would say that all cylinders are firing as the combustion products are appearing. I would say that you have the mixture set rich and there is an air leak in the manifold or joint at the rear inlet port. Have a check to see. If you can't see anything then take the maifold off and have a look and possibly replace it with a spare. Then clean the plugs and run it for a while (few hundred miles) before removing the plugs and checking it again.

The ideal plug colour is a grey/brown mixture. Black = too rich, white = too lean.

Post by **rayofleamington** » Tue Jan 13, 2004 11:31 am

If cylinders 1 & 2 are sooty then they are running rich

i was thinking more along the lines that
A) they are not firing properly (this leads to a black crappy plug)
B) headgasket
The most common place for the headgasket failure is between 2 and 3 but the gasket can fail anywhere it likes.

have had trouble with oil getting into number 2 cylinder

C) some mechanical problem worse than a headgasket.

but yes, an airleak could cause lean on the other 2 if there was an air leak at the inlet port - if it was further into the manifold wouldn't they all run lean?

Post by **Cam** » Tue Jan 13, 2004 2:55 pm

Yes Ray, all possible but it was the white plug that rang alarm bells.

Those cylinders are running lean and the most common reason for this is an air leak. If the leak is near the head port then it is possible for those two to experience the leaning leak before the air has had time to back up (through particle inertia) and find it's way through to the other end of the manifold as the cylinders suck independantly and at high RPM the change in air direction takes time relative to the time taken for the Otto cycle to complete.

If the head gasket has blown and is letting air into cylinder 3 and 4 then it must also be letting air out so it should be really noticable as sounding like a tractor and not 'running a bit rough'.

It could be any of the reasons that you mentioned though, but it's unusual for a headgasket failure of that nature not to cause a drop off in power and that was not mentioned. It all needs checking though.

Post by **Matt** » Tue Jan 13, 2004 5:19 pm

isn't it far more common for a head gasket to go between one of the cylinders and the water ways? i had that and i drove through the center of southampton with a big white cloud coming out of the back! people kept back then!!!

Post by **grainger** » Tue Jan 13, 2004 7:19 pm

brilliant

thanks very much to everyone

i am pretty sure that the electrics are all good (dizzi cap is less than 6 months, cables all good, clean rotor arm, points etc and brand new plugs) soooo it looks like a leak on the inlet at the back then. no oil in the water so the head gasket is probly ok. but i have definately got some oil leaking into cylinder number 2 when i thrash it, last month it failed altogether, when i looked at the plug it was wet and had got lumps of dirt shorting it out.

would low compression (ie leaking rings) on one cylinder make it run lean compared to the others ?

right so i'll go out tomorrow and check the inlet

i'll get back to you then

cheers
grainger

Post by **Matt** » Tue Jan 13, 2004 7:26 pm

it would definately let oil in the bores! - try a compression test!