What would cause every other cylinder not to fire on a big block chevy?

In my truck I have a 366 big block chevy motor and cylinders 2, 3, 5 and 8 are not getting spark and the firing order is 1,8,4,3,6,5,7,2. So I am not getting spark to every other cylinder I have an H.E.I distributor. I am thinking it could be the ignition module but am not sure any help is appreciated.

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petercd3 years ago

Sounds like the ignition module is set on 4cylinder firing instead of 8cyl, take a closer look at the module and how its being triggered.

vince 09 (author)  petercd3 years ago

The truck had been running fine and ignition is not my strong spot. The ignition module has been replaced but like I said the truck was running fine up until now. Do you think that the ignition module has gone bad again?

Thats where I'd be looking, it doesnt really sound like a mechanical problem, more like electronic related.

vince 09 (author)  petercd3 years ago

So I had the ignition module tested and it passed so still not sure what it is now. Is there a chance it could be the coil?

There is only 3 or four things to check, compression, fuel, spark and timing. It not likely to be compression, if you have a timing light you can check spark at each lead, or pull off a dead lead and put a spark plug in the end of it to check. which leaves fuel, its possible that one side of the carb is blocked, check the runners on the manifold do 2, 3, 5 and 8 all go to one side of the carb?

vince 09 (author)  liquidhandwash3 years ago

You are 100% correct on the carb. I have a dual plane intake and left side of the carb feeds 2,3,5,and 8 I am not sure what caused it but the left float was not lowering all the way and not getting fuel to that side I fixed that and it works fine. not going to lie I feel kinda dumb that I didn't notice that I thank you for the answer very much.

good to know it helped you out

Most cars have a single coil feeding the distributor, in the unlikely event you have dual coils, its possible one of them isnt firing.

Another thing I seem to recollect is dual points in the distributor of some V8's, so you could check to see if you have such a beast, it might need new points of have a condenser shorting out one set.

It should be a simple matter to trace out one of the offending cylinders spark plug leads to see which set of points is triggering it, and then sand/clean points or whatever is necessary.

vince 09 (author)  petercd3 years ago

So I feel a little dumb I figured out what was wrong and it had nothing to do with the distributor. I have a dual plane intake on my truck so the right side of the carb feeds cylinders 1,7,4 and 6 and the right side feeds cylinders 2, 3, 5 and 8. the left side float was messed up I fixed it and it works perfectly now. So what I thought was no spark was actually no fuel. I thank you for all of your advice

vince 09 (author)  petercd3 years ago

Ok thank you I will pull it off and have it tested. That is kind of what I was thinking it was.

Burf3 years ago

Pull the plug on the #1 cylinder and crank the engine until the piston is at TDC on the compression stroke. Pop the distributor cap and see if it is pointed directly (or nearly directly) at the number 1 contact on the distributor cap. If it isn't pull the rotor off and move to the correct position.
I'm a Ford man but if I recall correctly, I believe there are 13 positions the rotor can be set on a Chevy and only one of them is the right one.

vince 09 (author)  Burf3 years ago

On my distributor the cap and rotor only go on in one position and the distributor is turned to set timing. That may be true about diffrent position on some distributors but thank you for the answer.

iceng3 years ago

If this system uses contacts to trip the spark then the rotary switch cam could be worth looking into.

If it uses flux switching i don't know.

vince 09 (author)  iceng3 years ago

Ignition is not my strong spot. all I can tell you is that it is an MSD HEI distributor.

Burf3 years ago

It sounds to like you have the distributor cap rotated 180 degrees. Or, you've put the plug wires in the wrong holes.

vince 09 (author)  Burf3 years ago

Well the truck had been running fine all week and out of nowhere this happened so I am sure the cap and wires are right. Thanks for the answer tho

Sure sounds like a distributor problem. If it was the ignition module then the misfire would not be so uniform. Check the contacts in the distributor

vince 09 (author)  mpilchfamily3 years ago

I'm sorry I forgot to say that there is a new cap on it, that was the first thing I replaced when I got the misfire.