Idle Misfire (1 Viewer)

[mattressking]

I don’t disagree with you, I would like to have more vacuum but I’m not convinced there’s a big vacuum leak yet. I think you’d also be hard pressed to find original F engines out there routinely pulling 21-22 based on what I’ve read. Correct me if I’m wrong.

I fogged the manifold gaskets with carb cleaner and the idle didn’t change. The FSM says 16.5+ is in spec. I know I’m right at that threshold.

You positive it couldn’t be ignition? Do you have an F? What manifold gasket are you running? Remflex?

Also, when you say to check dwell are you saying I need to run a tighter points gap? What’s the dwell spec to shoot for?

Before you go nuts, take a breath. When you're new to diagnosing engine issues, people get flustered and want an easy fix...there might be multiple things at play or just one.

Engine diagnostics for carbs start with vacuum. I understand FSM says 16.5 + but for a smooth runner, you want to bump it up as it will cause issues. F motors can have higher vacuum, just depends on how well yours is maintained.

Next steps....

1. Hot compression check - verify even cylinders, note variation.
2. Dwell check - pull up in FSM, spec I don't have off-hand.
3. Pull spark plug lead off plug one at a time, make sure you hear engine misfire to confirm its firing. No change should be inspected.
4. Lean drop - learn how to do this. Vacuum issues can be masked a bit by playing with mixture. Simply - you can start at 690rpm with speed screw then use mixture screw to richen/lean. If idle goes up with mixture screw, reduce with speed screw, continue until no change and use mixture screw to reduce to 650. You can do a higher base idle such as 750 if it runs rough - which is an indicator of poor vacuum if spark is confirmed on all 6.
5. What carburetor?

I will still say vacuum is your issue and the downside is, most tune vacuum issues out with adding fuel, which isn't the correct way, but works.

 

[nachohouse]

Before you go nuts, take a breath. When you're new to diagnosing engine issues, people get flustered and want an easy fix...there might be multiple things at play or just one.

Engine diagnostics for carbs start with vacuum. I understand FSM says 16.5 + but for a smooth runner, you want to bump it up as it will cause issues. F motors can have higher vacuum, just depends on how well yours is maintained.

Next steps....

1. Hot compression check - verify even cylinders, note variation.
2. Dwell check - pull up in FSM, spec I don't have off-hand.
3. Pull spark plug lead off plug one at a time, make sure you hear engine misfire to confirm its firing. No change should be inspected.
4. Lean drop - learn how to do this. Vacuum issues can be masked a bit by playing with mixture. Simply - you can start at 690rpm with speed screw then use mixture screw to richen/lean. If idle goes up with mixture screw, reduce with speed screw, continue until no change and use mixture screw to reduce to 650. You can do a higher base idle such as 750 if it runs rough - which is an indicator of poor vacuum if spark is confirmed on all 6.
5. What carburetor?

I will still say vacuum is your issue and the downside is, most tune vacuum issues out with adding fuel, which isn't the correct way, but works.

I'm not going nuts just yet, but I don't rule it out.

I'll check dwell, get compression numbers and report back. I'll probably double check my valve adjustment before I do it.

The misfire is intermittent and not sustained, so it's doubtful that I'll be able to pull a plug wire to pinpoint the cylinder where it's happening, right?

I think the carb is original, albeit recently rebuilt. The diaphragm from the side has been deleted, is that a vac secondary? It looks like this one from the SOR site, minus that diaphragm. I can get a pic of the actual carb/my engine bay if it's helpful—then you guys can tell me about all the other broken stuff I've got under the hood that I haven't noticed yet.

 

[mattressking]

Post pics, easier to determine what you’re dealing with.

 

[nachohouse]

Update: Misfire at idle seemed to be the result of weak spark. I changed ignition to pertronix (ignitor 2 + flamethrower coil) and my old F runs better than ever. Starts better, runs better and exhaust even smells like I’m getting a more complete burn.

Vacuum still isn’t as high as I’d like it but after fiddling with everything I’m getting 19-20 in/hg at idle. Still haven’t done a compression test, it’s good enough for now.

Thanks for all the help.