Mystery Missfire
 

On idle my P/U will not fire on no.4 cylinder .I checked the compressions on the engine cold . 1,2,3 are all spot on 60lbs . No 4 is 65 lbs which I guess could be that caused by unburnt residue in the cylinder/rings etc . I have changed the plugs around,changed the manifold gasket,checked all points gaps are correct and even .Changed the distributer cap . When I short out no 4 with a screwdriver I have a hefty spark the same as the others . I thought maybe an airleak in the manifold but no 3 shares that, it fires fine . On idle I have enriched the GAV but no change . What am I missing ??? I need a logical explanation !!! When at highway speed it runs on 4 and pulls fine maybe I worry too much !!! Keep on driving what the heck but still it bugs me .

John in cloudy mild rain on the way AGAIN Suffolk County England .

Re: Mystery Missfire
 

The distributor body likes to short between #3 and #4 inside the Bakelite, where it can't be seen. Try another one to see if the miss goes away.

Re: Mystery Missfire
 

Have you thought about that it may be also misfiring on #4 at highway speed, you just don't notice it because it's being masked by the other 3 cylinders?

Re: Mystery Missfire
 

I have changed the cap as I have had that short between 3 and 4 before and I binned that cap . I put a new one on also I have changed the carb . The truck runs fine at highway speed and the plug is dark grey not oily black so I think on load the engine is running normally . I think I would notice if I was 25% down on power also if I am passing a lot of unburnt fuel through the muffler there might be other symptons also my gas mileage is still 20 MPG or thereabouts .

John same place same weather.

Re: Mystery Missfire
 

How are you determining that #4 is not firing at idle?
I am also puzzled...

Re: Mystery Missfire
 

On idle I short out the plug connecter to the head by using a large insulated screwdriver. When I short out 1,2 and 3 the engine slows and is idling on two cylinders it is very evident .When I short no 4 absolutely no change . As an experiment I disconnected the plugs except no 4 and tried to start to see if it would fire with any combination of timing/throttle settings but not even a cough dead as a Dodo I remain mystified .

John still in the same place and weather ,maybe if I changed the motor this might cure it !!!

Re: Mystery Missfire
 

John,
I am still puzzled... You are verifying the condition exactly as I would
Given that you have spark when you short #4 plug to the head with a screwdriver it proves the energy is getting to the terminal. You changed the plugs around to rule out a bad plug. Try cleaning the base and threads of the plug hole to improve conductivity (long shot here)
Based on what you are describing, the only thing I can think of is because the compression is slightly higher in #4, the spark may not have enough energy to fire due to too much gap. Try slightly decreasing the gap at plug #4.
Double check that at the low idle, you still have a strong spark when you short the plug to the head....

Likely this is not the cause but it is easy to lessen the gap.
Keep posting what you try and someone will come in with the correct cause.
I'll keep thinking...
Good Day

Re: Mystery Missfire
 

Spark plugs will do this when the center electrode is worn to a point with rounded edges and no longer square on the end.

Re: Mystery Missfire
 

Swap #1 and #4 plugs then do the balance test once more. The problem might move with the plug. If your compression tester uses a hose, remove the schrader and idle the engine. Will be running on 3 but you can hear the compression on #4.listen for a change in the sound as weak valve spring or valve bent or sticking. This is similar to vacuum readings but specific to a cylinder connected rather than a tree in the forest.
.

Re: Mystery Missfire
 

Hi John,
I think I would check the intake manifold that feeds the 3-4 cylinders for a fine crack using a propane torch, at high vacuum and lean idle mixture it would not take to much to drop that cylinder, the crack if there is one would in the side of the runner nearest #4.

Re: Mystery Missfire
 

I have swapped the plugs about but no change .I installed a NOS Canadian 1 COM this has duall electrodes still no change . I have got to thinking that this a valve problem and will try 100th suggestion with the compression tester etc etc I will keep you informed this is a new one on me for sure .

John same place same weather .

Re: Mystery Missfire
 

Hi Brian T The Big Three swap meet at the Qualcom Stadium is the best that is where I bought this Budd cab pickup . I do not suspect the manifold as it feeds no 3 which runs fine but I will investigate .

John same details.

Re: Mystery Missfire
 

How does it pull from low rpm---- like when rounding a corner and leaving it in 3rd
On my car I was getting a weakness in one cylinder at idle, but it pulled evenly at low rpm, by shorting plugs it showed weaknetin #3, compression even, eventuality I checked valve clearance, that cyl was at .008, adjusted clearance and all better

Re: Mystery Missfire
 

John,
A slight intake valve leak in #3 cylinder could disrupt the fuel charge to #4 when #3 fires.
I think the sequence of valve movement is correct for this to happen. A leak down test would confirm the condition.
Good Day!

Re: Mystery Missfire
 

John,
My thinking here is, although that runner feeds 3-4 the column of air is somewhat divided so the leak would lean out that side of the column, I have seen similar situations with SU carbs where worn fuel needles have been refitted and the scored side of the needle is to one side making one cylinder leaner than the other making for uneven idle, as my old shop foreman would say "Keep looking Lad".

The Big Three Swap wasn't to big this year a lot of no shows, I came home with nothing, even the $ piles were sparse.

Re: Mystery Missfire
 

Might be a long shot, but maybe check the valve lash.

Re: Mystery Missfire
 

I think you are the winner. good choice for trouble shooting.

Re: Mystery Missfire
 

Intake leak

Re: Mystery Missfire
 

We just had this same thing with a broken valve spring. The spark was good the compression was good but as soon as it ran the valve wouldn't seat.

Changed the valve spring and all was good with the world.

I would check valve springs and valve lash.

Re: Mystery Missfire
 

Thank you all,the valve lash/broken spring seems to fit the bill so I will dive into the valve chest and investigate . I will keep you all informed .

John in heavy rain Suffolk County England .