This has happened a few times over the last three months. I'll have the slant six at WOT, and running about 3200 or so on a steep grade and after about 30 seconds of WOT like that, it'll suddenly start missing on one hole. I let up and shifted, and about 10 seconds later, the miss vanished. It's happened every time on a steep hill, doing the same thing. And it goes away only after I let up. Not fuel starvation, I don't have hydraulic lifters, and certainly isn't anywhere near what the engine will rev to happily.

I've tried to catch it by pulling over and when I stop, the miss is gone.

Any ideas? I'm thinking a spark plug with some kind of issue... or a sticky valve? I don't hear any odd noises other than the miss.

This is the propane engine... rebuilt head with bronze guides, oversize stainless valves, 10.5 compression, 260 /248 .440/.437 cam... Ford EDIS crank trigger ignition / chrysler 3.3 coil

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

Are you sure it's not fuel starvation?

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Could very well be an exhaust valve sticking in the guide when it gets really hot. This same thing has been reported here before. Not enough stem to guide clearance

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Joshua

Also check for sneaky vacuum leaks at individual intake manifold runners.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

If it behaved like a vacuum leak, I'd buy it. But it doesn't. It only ever crops up at high speed and WOT. That's when there's the least vacuum and therefore hte least vacuum leaking.

The idea of a sticking valve due to being too tight of stem clearance... I suppose that seems pretty reasonable. How would I test this theory?

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

Vacuum leaks are not always as simple as they'd like you to believe they are. Sometimes they don't show up except under the right combination of harmonics, and then you can get all kinds of weirdness.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

IF this was something random, or uneven, I'd buy it. The dead hole suddenly happens, it stays completely dead, even if I change rpm, load, throttle, etc. Then, suddenly, it comes back, 100%. It's like someone pulls the wire off and then puts it back on.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

Given the combination of factors that bring it on, it will be tough to pin it down definitively. If the miss will stay for a few seconds under less throttle, then you MIGHT see as a slight pulse in a vacuum gauge reading ( an exhaust valve problem will affect vacuum less than intake valve). If you know anyone with a portable ignition scope, your bad cyl will show up easy on the secondary ignition waveform.

Otherwise, you might only find it by doing a teardown & inspection- looking for tight clearance or galling of the valve stem(s). (this used to be a BIG problem on small block Chebbies- it happened under identical conditions as yours)

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63 Valiant Wagon
225 - 4 bbl