Misfire / missing a cylinder, help!

Saul

Saul

Active Member
Hi guys,
My commer is running on only 3 cylinders (obvious when disconnecting the plug leads). It's the second cylinder back from the cab.
I have checked the lead and spark and found the spark was weak so have adjusted and got a better consistent spark now.
Still no luck getting it firing on that cylinder and was wondering where to go next with diagnosis?
The plug seems completely dry...
 
You checked the spark against the other plugs, need a good strong spark but a dry plug weird and points to no fuel which could mean compression, stuck valve? You checked the dizzy cap to make sure not worn might go be weak spark?
 
Does sound like a stuck open valve, as Jim says, not drawing in fuel - rocker cover off and check No.2 cylinder valve clearences.
 
Had the rocker cover off to inspect. Can't see anything out of the ordinary. The exhaust valve of number 2 appears to have been adjusted a lot less than all the others, which all seem the same.
Couple of questions:
Does the engine have to be really hot to check the clearances?
When turning the engine, can this be done manually easily? How?
Thanks!
 
When you say you adjusted to get a better spark , what did you actually adjust?
I would try a new plug, or at least swap the suspect one for another from a different cylinder, to see if the problem moves cylinders. Also try swapping no. 2 plug lead with one of the others for the same reason.
If the problem stays on no. 2 then do a compression test. That will tell you if there could be a stuck or badly adjusted valve.
Good luck, Mick
 
Spark plugs swapped over and also the leads and the issue remained at number 2.
Will take the rocker cover off again tomorrow and check gaps.
Another good opportunity to clean up the engine area :)
 
Agreed on the compression test.
My cylinder head is off at the moment and the inlet valve seats look only slightly pitted but the exhaust are quite heavily pitted and are taking some grinding to get smooth. If yours are anything like mine you could be experiencing VSR (valve seat regression), reducing valve clearances.
 
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Results of the compression test showed zero compression on number 2.
Everything looks good under the rocker cover so next job is to remove the head :-(
 
Hopefully it's just the valve and the seat will be OK after a good lapping in. The worst part of the job is getting the manifolds and thermostat housing off without snapping a stud - plenty of release oil on all fasteners for a couple of days before doing the job will help
 
Panky said:
Hopefully it's just the valve and the seat will be OK after a good lapping in. The worst part of the job is getting the manifolds and thermostat housing off without snapping a stud - plenty of release oil on all fasteners for a couple of days before doing the job will help
Click to expand...
Manifold bolts / nuts have undone straightaway, thermostat housing was removed and refitted only six months ago.
My next question was about these, the workshop manual says to lift the head out with manifold and carb attached and I assume the thermostat housing. I thought splitting at the manifold would be better? Can the thermostat housing be left on at all?
 
I just removed the head with manifolds on (removed the carb though) and thermostat housing left on. Dont forget to loosen the bypass hose:oops: Its an awkward lift and I tilted the seats forward and straddled it.
 
I was just typing the same reply:) I don't think the manual mentioned the by-pass hose.
 
Have stripped a lot off the head and will get the head off tomorrow night hopefully.
Will look at getting other jobs sorted at the same time (manifold cleaned, head skimmed.... etc) Any recommendations are welcome.
Especially anything that might give a few more horses :)
 
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