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Hello all and thanks in advance for any advice or help...

I recently got a low mileage, basically rust free '76 bone stock that had been sitting for a year due to a blown fuse. Hilarious but...

It runs like a sewing machine, starts fine and is basically a great truck but... I have one minor thing that is bugging me and I cannot figure it out at all. When I'm driving (not idling, only when moving) and I push in the clutch to shift, about 70% of the time there is a slapping sound from what sounds like the area of the bell housing or transmission. By slapping sound I mean 1 to 3 distinct sounds like heavy leather being slapped against something metallic. I hope that description makes some sense. There is not hesitation in the clutch not slippage and the truck picks up just as it should... Basically there is no symptom other than the noise. I have been under there checking all the bolts and making sure everything is tight and I have found nothing out of place. I did find the bolts missing from the lower cover on the bell housing and thought sure that had been it but after replacing them... still the same noise.

Basically, I'm thinking a slight exhaust leak might be letting oxygen into the exhaust and when i let off the gas to push the clutch in it is momentarily running rich and thus the noise would be a very slight backfiring? The other possibility I thought of is possibly some play in the U-joint that when pressure is taken off the drive-line slaps against itself. Either of those sound reasonable? I cannot see anywhere the exhaust is leaking and the drive-line seems nice and tight but, who knows.

Sorry if this is long and I'm sorry to make my introduction with a problem but I do appreciate any help and hope I can be of help to someone here in the near future. Thank.

Does the truck still have all of the smog equipment? If so there is a check valve or a anti backfire valve that can cause the small backfire you think you may have.

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Certified pilots, looking down on people since 1903.

The truck is bone stock with all the smog equip attached. I bought it from the estate of an old farmer that used it to get into town.

Check valve, really? I'm not near my haynes manual to look and see but do you know where said valve is located? I have my truck with me and would love to check it out this afternoon. I'd assume it would be in the manifold or the header/down pipe? That makes perfect sense as, when I let off the gas to push in the clutch it would create a "vacuum" of sorts in the exhaust and thus possibly open the failing check valve.

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Marvel, at what was lost...

It sounds like a carb/tuning issue to me. The clutch should make the sound even sitting still, and a small pop or backfire in the exhaust when shifting between gears isn't that uncommon. I think you are on the right track in thinking it is slightly rich, and the AIR system (smog pump) is actually putting air/oxygen in the exhaust just like it should. In this case I think there is a drop in pressure in the exhaust when you let up to shift that lets the air/fuel pop off.

Things to check:

Coasting richer system- a set of switches that open/close a valve on the carb that lets a bit more fuel in when the clutch is not pushed in, the gas pedal is not pushed in, and the trans is in 3rd or 4th. If that valve is just open all the time or not closing when the clutch is pushed in that would create a slight rich condition. It is somewhat hard to test all these things working together, but you can test each switch pretty easy.

Timing, points, etc - general tune up stuff, anything that might make it not run 100% would leave some gas unburnt going out the pipe

AIR System/Smog pump - there is a vacuum switching valve and some other parts involved in the AIR system that could make it pump too much into the exhaust. Might be worth trying just bypassing it to test. Pull the belt, or better pull the hose off the AIR manifold that goes into the head just above the exhaust manifold. Plug the manifold off (you could pull the hose from the pump and plug it with a large bolt if its hard to find something to plug that large port on the manifold) so that air can't get in and exhaust won't leak out. If disabling this does cure the popping, it could still be fuel related and this just provides the air for the bang.

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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)

MEPR: Man, my 4x4 makes all other LUVs look good