Misfire troubleshooting

[Rockiemosley]

I having a problem with rough running on my 1985 FJ60. It has been desmogged according to JimC's instructions. Ran well for quite a while, but now having trouble with rough running. Compression checks out okay with around 150psi on each cylinder. Carb was rebuilt by JimC but was quite a while ago, maybe 10-12 years ago, although not alot of mileage in the interim. I attached a smoke machine to the carb, thinking ive got a vacuum leak, and ive got smoke pouring out the accellerator pump hole. Accellerator pump squirts fuel when the throttle is operated. Little boot on accellerator pump has seen better days and maybe this is my problem. Additionally, ive got smoke pouring out the High side vacuum advance on the distributor, which probably means the vacuum advance diaphragm is shot and creating another vacuum leak. Anybody know if the accellerator pump should be sealed? or any other thoughts? thanks.

 

[bhsdriller]

grab a cheap vacuum gauge it will tell you alot

 

[Rockiemosley]

grab a cheap vacuum gauge it will tell you alot

Ive got a vacuum gauge, but not sure what to do with it to diagnose this problem.

 

[bhsdriller]

What's your vacuum at idle? A good spot to plug it in is the connection toward the firewall on the intake where the ac idle up gets the vacuum.

 

[Rockiemosley]

What's your vacuum at idle? A good spot to plug it in is the connection toward the firewall on the intake where the ac idle up gets the vacuum.

View attachment 4168557

Im getting about 17-18
inches of mercury vacuum at idle. ? Im guessing there is some kind of vent tube in the carb going to backside of accellerator pump plunger, where my smoke is coming through. not sure if this is really a vacuum leak, but if its pulling air, it must be. Accellerator pump boot is rotted out.

 

[bhsdriller]

That's too high not possible lol, if you can get a pic of the gauge it will help probably just reading it wrong

 

[Rockiemosley]

That's too high not possible lol, if you can get a pic of the gauge it will help probably just reading it wrong

Sorry, I just edited my original post, it was 17 to 18 inches of mercury.

 

[Pighead]

Im guessing there is some kind of vent tube in the carb going to backside of accellerator pump plunger, where my smoke is coming through. not sure if this is really a vacuum leak, but if its pulling air, it must be. Accellerator pump boot is rotted out.

The float bowl vents to atmosphere, the accelerator pump boot is to keep dust out, not meant to be air-tight. The boot is up too high to be a vacuum leak. 17 to 18"/Hg is good depending on elevation, timing and other stuff...

 

[CruiserTrash]

The accelerator pump is a plunger with a seal that slides up and down the bore. Seal is probably shot.

Possibilities for running rough and/or misfiring:
It could be letting too much fuel into the carb throat and causing you to run rich. The accelerator pump plumbing goes from the float bowl to a brass tube way up high in the carb throat, above the venturis, so that would not cause a loss of vacuum.
If one of your advancer diaphragms on the dizzy is shot, you could definitely be misfiring due to incorrect timing through the rpm range. Which diaphragm, though? The one closest to the dizzy or the one further away? Further away is the High Altitude Compensator advance (static 6 degrees I think). The important one is the diaphragm closer to the dizzy body. Remedies could be getting a new used dizzy from one of the part out guys or shops, ordering a non-US "medium" cap dizzy from the Middle East (still available new), or digging around for an alternative advancer diaphragm that could bolt to your existing dizzy and -via some homebrew DIY fab - work for your application.

My bets are on the dizzy.

N.B.: Jim C also changes the advance on distributors for desmogged trucks because I guess they run better that way. The non-US "medium cap" dizzy already has an advance curve appropriate for non-smogged trucks. The non-US dizzy only has one advancer diaphragm though, so if you have your HAC system intact (not likely if you're desmogged) just know that you will lose that functionality.

 

[Rockiemosley]

The accelerator pump is a plunger with a seal that slides up and down the bore. Seal is probably shot.

Possibilities for running rough and/or misfiring:
It could be letting too much fuel into the carb throat and causing you to run rich. The accelerator pump plumbing goes from the float bowl to a brass tube way up high in the carb throat, above the venturis, so that would not cause a loss of vacuum.
If one of your advancer diaphragms on the dizzy is shot, you could definitely be misfiring due to incorrect timing through the rpm range. Which diaphragm, though? The one closest to the dizzy or the one further away? Further away is the High Altitude Compensator advance (static 6 degrees I think). The important one is the diaphragm closer to the dizzy body. Remedies could be getting a new used dizzy from one of the part out guys or shops, ordering a non-US "medium" cap dizzy from the Middle East (still available new), or digging around for an alternative advancer diaphragm that could bolt to your existing dizzy and -via some homebrew DIY fab - work for your application.

My bets are on the dizzy.

N.B.: Jim C also changes the advance on distributors for desmogged trucks because I guess they run better that way. The non-US "medium cap" dizzy already has an advance curve appropriate for non-smogged trucks. The non-US dizzy only has one advancer diaphragm though, so if you have your HAC system intact (not likely if you're desmogged) just know that you will lose that functionality.

The accelerator pump still pumps fuel down the carb throat when the accelerator pedal is pressed so even though the boot is rotted out, I don't think that is my problem. For the dizzy, I've got the vacuum advance line going from ported vacuum on the back of the carb, going to the main distributor diaphragm. I had the HAC diaphragm unused and sitting there with its tubing connection open, and the smoke machine was pushing smoke from the carb to the main distributor vacuum advance diaphram, and then the smoke was coming out the HAC port. If I could get a used distributor from the Middle East, that would be great, do you know any sources? Im pretty sure I've got a leaky advance diagram causing a vacuum leak as well as no distributor advancing. Thank you!

 

[chptuls]

we are saying misfire, but when does it happen?
Undernload? Idling? Sitting in the driveway raving a little?

I’m asking cause I had a slight misfire sitting in the driveway reving/kept at 1500-2000 troubleshooting something and mine had a slight misfire.
Long story short, it t turned out to be the plunger that opens the bowl to the main circuit was open all of the time dumping gas into the carb when it wasn’t needed. That was also causing my plugs to foul too.

Fixed that and everyone was good.

 

[chptuls]

To be clear it was the power valve was stuck open.
It would have a slight misfire reving in driveway, but wouldn’t on the highway, under load.

 

[Rockiemosley]

Pretty sure ive found the problem here and posting for the benefit of the community. In the desmog mode, the outer distributor advance diaphram is not used... I left the port open. This is part of the HAC, high altitude compensator. I took the distributor cap off so I could see if the main advancer was working, it was not working with the HAC advancer open, but it does work with the HAC advancer closed. I suspect internally the HAC diaphram is shot. This was creating a vaccuum leak and not spark advancing. Fairly sure ive fixed the problem, but be aware, and advancer diaphram, or any of the various emissions valves that operate by way of a rubber diaphram, this rubber is 45 years old on my truck. Imagine using a 45 year old rubber! My main distributor advancer is still working, but maybe not for long.