Giving it a Miss

[IDave]

I have an intermittant miss that I am trying to track. F engine, standard exhaust. "NonUSA" Aisen carb and dizzy.

The miss seems to come about every 6-7 beats/cycles. In other words, the rate of miss is 1 miss per 7 sparks to a given cylinder. I detect it at the tailpipe easiest.

Each of the spark plug wires seems to spark regularly. I cannot determine that one or another cylinder is the one that is missing by removing spark plugs and holding them and grounding them to check the spark regularity. All are regular.

I checked the resistance on all of the wires, and they are symmetrical.

I don't see any damage to the rotor head or button or cap.

I DID notice that when using my induction timing light, I did not get a signal from the number 1 cylinder wire, and switched it for another wire that did give a signal. But that did not change the miss.

I played with the timing, but could not eliminate the miss that way, and I checked the dwell, which was 39 degrees: I guess that should be ok.

I have not yet pulled the plugs to inspect them, maybe tomorrow.

Any thoughts or recommendations?

 

[Romer]

Dave,
The wire you didn't get a signal from. Is it no longer hooked between the engine and dizzy, or did you just move the clip to another wire?

Is this off the 40 that has been running fine, or is it a problem you have been traking down with your new 45?

Dist. cap wires in what condition?

 

[IDave]

On the FJ40. Had been OK, except this miss, which is new since the headgasket blow/replacement.

I replaced the #1 cylinder wire with a spare wire from an old set I had laying around. Mind you, the old one (not that old) was sparking, and the resistance across it was the same as the similarly lengthed #6 wire. But for some reason I couldn't get an induced signal with the timing light on that wire, although I had used it previously without issue (whereas the others, and the older/replacement wire signaled fine. Who knows?)

The cap and rotor look fine, but I am going to replace them soon, anyway.

 

[Romer]

Has it been about the same since you replaced the headgasket or has it gotten worse? I assume you will check the gap when you check the plugs.

Since your not getting a timing signal on the #1 wire, but the wire appears to be ok and your getting spark could be telling you not a strong enough signal is getting to the wire. Rotor need cleaning, Dizzy, etc..

I am sure David Dearborn could be more helpful, I am just trying to work through it with you.

 

[IDave]

I'll check the gap, but the plugs are new since the head rebuild, and I gapped them pretty carefully. I replaced all of the plugs because one of the older plugs had a loose ceramic (only loose when torqued down.) I thought that would be the cause back then, but the miss is still there. And again, because it is intermittant, I am not sure which cylinder is actually missing.

 

[65swb45]

It may not be a miss at all Dave. It may just be a slow build up of backpressure and turbulance in the exhaust that 'burps' every so often.

Also, remember that the idle circuit in the carburetor does not meter gas into the carb in the same spray pattern as the primary venturi, and occasional sputtering in the idle circuit can create phantom symptoms.

 

[IDave]

Any simple way to discern a difference, then, Mark, between a burp and a miss?

 

[folsom50]

Just find some little shop that has a engine analizer (at least that's what I think you call them) A few minutes hooked up to that should tell you if there really is a miss. Beats the hell out of chasing invisible gremlins.

 

[theo]

Here's a completely irrelevant story out of the blue Dave, since I know you wouldn't expect any less from me.

I chased a similar symtpom in my '54 Chev pickup for years. I was convinced it was an ignition problem and so were the numerous onlookers and mechanics and internet troubleshooters I consulted. Annoying missing especially noticeable at the tail pipe, ran fine when you put the pedal down, intermittant timing light function.

Turns out the last time I had the carb off - which was maybe 5 years earlier but I don't drive it much anymore - I put the spacer on upside down. Well, the spacer has a hole that ports into the idle circuit. When I flipped it right side up the miss disappeared.

This was after replacing virtually all of the ignition parts, measuring valve and lifter travel, etc.

Your carb spacer probably doesn't have any ports in it I guess.

 

[IDave]

Yeah, Theo, there are no ports in the spacer, just two holes and a side port that goes to the PCV, which, other than the fact that the side port is bent (up or down depending on the way you put it on, but makes no functional difference).

Folsom, I don't think there is an analyzer for a hundred and 10 miles. But I'll ask around.

I did have an opportunity to experiment last night at 1 am, and made a 14 mile trip out and back. What I found is that my heatriser is sticking in the cold position. Not much, I can flip it with one finger. I haven't had enough opportunity to play with it, but I am wondering now if that could be my issue!

 

[sanjuancruiser]

Have you checked the valve adj.? Since you have had the head off you might have a valve slightly(and I mean ever so slightly) out of adjustment. You can get an irregular idle from maladjusted valves.--just a thought.