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Misfire that JUST WON'T DIE!
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Author:  Plymouth_Ruster [ Sun Oct 18, 2026 4:48 pm ]
Post subject:  Re: Misfire that JUST WON'T DIE!
Quote:
Please try again with the microphone out of the exhaust blast. The "wind" noise is masking any misfires we're supposed to be hearing. Put the camera off to the side of the tailspout, not in line with it.

Then also bring the camera to the front of the car and let us see/hear what the engine looks and sounds like up there.

In both places, try to demonstrate the misfire.
The gaps in the wind noise are the misfires. Don't have time right now but I'll get more videos when I can.

In the meantime, this didn't actually solve anything. But it was fun to do and ran about as well.

Image
Author:  SlantSixDan [ Sun Oct 18, 2026 5:58 pm ]
Post subject:  Re: Misfire that JUST WON'T DIE!
Cool. How are you heat-sinking the module?
Author:  Plymouth_Ruster [ Sun Oct 18, 2026 10:44 pm ]
Post subject:  Re: Misfire that JUST WON'T DIE!
Quote:
Cool. How are you heat-sinking the module?
Didn't have time to do it right, but I was just gonna buy a heatsink off of amazon if that became a permanent thing. I used too thick of a wire gauge to make it easy to mount, and one of the connections is the wrong size, so it's relegated to the "maybe someday" pile along with three hundred other minor projects.

Since I'm here, I may as well throw out some more info on this motor, unhelpful though it may be. This was originally a hydraulic lifter motor, but when it was rebuilt it was swapped over to a solid lifter setup. It did not come with a distributor, so the ones I've been using have been from solid lifter engines; as far as I can tell there's no notable differences between them. I swapped the late-gen head out for a spark plug tube head that I ported myself after the previous head gasket blew; the problems I've been having were also present with the previous head on. I swapped back to a factory style steel shim gasket, which combined with the slightly decked head surface gives me about 130psi, on average. I run only premium gas, because Washington gas is godawful and my Jeep diesels horribly on regular despite being an 8.5:1 motor. Premium also seems to last longer and destroy carbs significantly less.
Author:  Plymouth_Ruster [ Mon Oct 19, 2026 7:49 pm ]
Post subject:  Re: Misfire that JUST WON'T DIE!
Alright, here we go. Again, the wind noise in the engine bay is DEAFENING thanks to the six blade fan, so... hard to convey what it's doing from there. I tried, though.

https://www.youtube.com/watch?v=qDK9ARQgueE
Author:  SlantSixDan [ Mon Oct 19, 2026 9:10 pm ]
Post subject:  Re: Misfire that JUST WON'T DIE!
Quote:
I run only premium gas, because Washington gas is godawful
No. For one thing, your cause/effect statement here is false; if Washington gas were godawful, running premium wouldn't exempt you from the godawfulness. Two, Washington gasoline is not categorically godawful in any real way. It is substantially the same as Oregon and Idaho gasoline, and neither of those is godawful in any real way, either. Whatever fairytales someone's been telling you about this…they're not true.
Quote:
my Jeep diesels horribly on regular despite being an 8.5:1 motor.
Premium gasoline's higher knock-resistance is masking the real cause. Find and fix it. The Jeep doesn't diesel because regular gasoline.
Quote:
Premium also seems to last longer and destroy carbs significantly less.
No, neither thing is true. The one and only thing premium gasoline does is better resist preignition, full stop. Burning it in a car that doesn't actually require the greater knock resistance is flatly nothing but a waste of money.

But it's your money, so if burning it gives you a warm fuzzy, I wouldn't stop you even if I could.
Author:  SlantSixDan [ Mon Oct 19, 2026 9:15 pm ]
Post subject:  Re: Misfire that JUST WON'T DIE!
Quote:
Alright, here we go.
Your misfre reminds me closely of ignition misfires I've had. But it also reminds me closely of carburetion (lean) misfires I've had, some of which really seemed like they had to be ignition-related.

So if this bug's defying squashing, it's time for some more diagnosis. Ideally that would be with an ignition sillyscope (oscilloscope) and exhaust analyzer, but you might not have those. I would first try to rule out a lean-out by hand-holding the engine at an RPM where it wants to misfire and using your other hand to gradually close the choke. Does the engine speed up and smooth out?
Author:  Plymouth_Ruster [ Mon Oct 19, 2026 10:12 pm ]
Post subject:  Re: Misfire that JUST WON'T DIE!
Quote:
Quote:
Alright, here we go.
Your misfre reminds me closely of ignition misfires I've had. But it also reminds me closely of carburetion (lean) misfires I've had, some of which really seemed like they had to be ignition-related.

So if this bug's defying squashing, it's time for some more diagnosis. Ideally that would be with an ignition sillyscope (oscilloscope) and exhaust analyzer, but you might not have those. I would first try to rule out a lean-out by hand-holding the engine at an RPM where it wants to misfire and using your other hand to gradually close the choke. Does the engine speed up and smooth out?
I was just about to break out the oscilloscope, 1/2 of a Tektronix 2235. It's probably massive overkill but it *does* work. I don't have an exhaust analyzer.

Closing the choke (or spraying brakleen down the throat) at high rpm kills the engine. Sending the metering rods as far down as they'll go doesn't do much of anything. It doesn't blow black smoke when revving after a sustained high RPM run, so I don't think it's going rich either. I'm not sure if it's clear in the video, but it also tends to misfire on deceleration as well. I'm wondering if the one older plug wire I have is somehow interfering with the signal from the pickup, given that it's pretty close to it; never had issues with the 455 it came off of, but after a week and a half of waiting my new wire set is in, so if that doesn't solve the issue I'll defer to the scope.
Author:  Plymouth_Ruster [ Mon Oct 19, 2026 10:21 pm ]
Post subject:  Re: Misfire that JUST WON'T DIE!
Quote:
Premium gasoline's higher knock-resistance is masking the real cause. Find and fix it. The Jeep doesn't diesel because regular gasoline.
The "real cause" here is that the FSJ hood is well-known for basically being a pent-roof oven with the carb at the top, and no real thought for venting heat anywhere. It boils the carb while you're driving, especially badly if you're in traffic, and on hot enough days it'll diesel. I probably have it worse because I run a Qjet, with its tiny fuel bowl. The solution is putting in hood vents, or vents anywhere else that air can escape, really. There's a reason the temp gauge shoots into the stratosphere if I run it and then let it sit for ten or fifteen minutes.
Author:  SlantSixDan [ Tue Oct 20, 2026 10:34 am ]
Post subject:  Re: Misfire that JUST WON'T DIE!
Quote:
Closing the choke (or spraying brakleen down the throat) at high rpm kills the engine.
Yeah, that'll happen. This isn't a matter of "close the choke", but gradually closing the choke, a little bit at a time.

Spraying Brakleen into an engine is hideously dangerous. Never do this; if the can of Brakleen you happen to grab is the chlorinated variety, it creates highly toxic phosgene gas when burned. Get a whiff of exhaust and you can very seriously damage your lungs or end your life. Carburetor cleaner only!
Quote:
it also tends to misfire on deceleration
Caused by wet-flash. Common with carbureted and TBI engines. Misguided nostalgia for this is why we are now cursed with dumb artificially-induced pop-pop-crackle-BANG! deceleration exhaust noise from new cars designed, priced, and purchased to advertise the buyer's excessive money and nonexistent manners.
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I'm wondering if the one older plug wire I have is somehow interfering with the signal from the pickup
EMI/RFI was where I was headed next. What kind of plug wires do you have now? And what kind did you order?
Author:  Plymouth_Ruster [ Tue Oct 20, 2026 11:15 am ]
Post subject:  Re: Misfire that JUST WON'T DIE!
Quote:
Yeah, that'll happen. This isn't a matter of "close the choke", but gradually closing the choke, a little bit at a time.

Spraying Brakleen into an engine is hideously dangerous. Never do this; if the can of Brakleen you happen to grab is the chlorinated variety, it creates highly toxic phosgene gas when burned. Get a whiff of exhaust and you can very seriously damage your lungs or end your life. Carburetor cleaner only!
I buy non-chlorinated by the case. No worries there.
Quote:
EMI/RFI was where I was headed next. What kind of plug wires do you have now? And what kind did you order?
Had Denso plus one AC, now using SMP wires. No change at all.

Gonna take me a few days to get to the oscilloscope; didn't have an inductive clamp for it.
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