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Marvelous1 · 10-21-2016, 09:03 AM

A 1989 Pace arrow 7.4L edelbrock quadrejet. Seems to run and pull well uphills, however, when coasting (even on flat grades) the driveline pulls the engine and keeps it at speed causing lots of popping and backfire sucking air and fuel even with the thottle closed.
Is there a release condition like in automobiles that will ease up on the coasting engine drag? Pretty sure it TH400

The other thing and it sounds terrible to hear it is when I ease into the throttle slightly to prevent backfiring popping, I get an awful sheetmetal shearing sound/snare drum rattle that is quite loud in the coach. I've checked the U joints in the drive shaft and they seem tight and quiet.
I was thinking maybe a flexplate in the bell housing but there are no other symptoms. Only noise when engine and vehicle are at matched no load speed.

Any experience or ideas on this?

nukerj · 10-21-2016, 02:29 PM

I would check the entire driveline with the wheels off the ground and in neutral, that way there is nothing to bind and you can possibly find the issue. Without any load on the shaft, its free to vibrate if something isn't tightened down or if something is worn out.

As for the backfiring, you need to get her tuned up. Unfortunately the Quadra-Junk is lovingly named so because if they are not kept in tip top shape, they act up horribly. I had one on my 69 Skylark GS400 and eventually got tired of it and swapped for a holley.

Get a good cleaning and tuning on the carb and all the jets properly adjusted and the off throttle behavior should be nice and smooth.

If the trans isn't unlocking the torque converter when you are off the gas and it is engine braking, you might have other issues going on as well in the tranny?

Marvelous1 · 10-22-2016, 05:06 AM

Not getting so much engine braking, I think 'cause it is pretty easy spin system, but I do notice the tach not changing a bit from throttle up or throttle back, and yeah that sort of made me suspect the TC not releasing.

they aren't real big backfires, just some popping in a couple cylinders, very much like the old ford large flatbeds I drove in the early 80's with 5 speed manuals

The carb setup has been rebuilt/tuned with a big cam not many miles ago and does run pretty good after warm up... The electric heat choke was riveted in place so now it's gonna be an issue to adjust, and overall I was very pleased with my mileage and performance on its shakedown trip. It gave me more than 8 mpg with a 2300# toad on a dolly.

After finding a very loose exhaust manifold gasket I started to think that maybe the noise could be the gasket vibrating in a static pressure condition.

I have the manifold off now and it needs welding, milling, and saw cut mods, before reassembly.

I will spin the wheels in neutral but I don't think it will give me the indications I need... This crazy noise would ONLY happen above 30-40 mph with a no load condition and only with the throttle open a bit to remove vacuum braking.

I will keep you posted... Might even try to record it if it comes back with the new gaskets on.

speed racer · 10-22-2016, 05:35 AM

The T400 does not have a release as you say. In a car you don't notice it because you don't have the weight pushing you as much.
The Qjet carb has a tendancy to leak fuel threw a plug in the center of the carb. or a air leak in the exhaust can make it backfire on decelloration.
A broken or cracked flex plate normaly sounds like a rod knocking under a no load condition.