The Stovebolt Site Forums The Shop Area Engine Shop Help diagnosing exhaust popping

Hi. I have been working on a 59 Chevy 2.5 ton and it is coming allong but something is not right somewhere. It starts up fine and idles good. But when it warms up popping noises start out of the exhaust. It occurs out out the right side exhaust more than the left. I believe it is carburetor related but I am not sure. I am posting a link to a short YouTube video. The exhaust starts popping at about 20 seconds into the video. It is a 650 cfm Holley carb mounted on a mid 70s 366 engine. I was having a problem with sludgy gas out of the tank so I am using brand new gas out of a 5 gallon can. I have played with the mixture screws and it runs best with the screws all the way in. Any advice would be appreciated.
Link:

Those mixture screws are for IDLE adjustment. Might check for exhaust manifold leaks at head and at manifold/pipe joint.

The idle screws being turned all the way indicates that the carb is running to rich.

Backfire in the exhaust indicates the carb is running to rich dumping in more fuel then can get completely burned before the exhaust stroke. All that unburned fuel is getting ignited in the engine pipe, muffler, tail pipe. That is causing the popping sound.

Edit to add check the timing as well.

1. Remove the Holley carburetor.
2. Do your best big league pitcher imitation and chuck it as far away as you can throw the thing!
3. Put the Quadrajet on it that came from the factory!

Problem solved!
Jerry

Hotrod Lincoln, your answer has been recommended. I actually have 3 other people who have said the same thing. I just have been unwilling to spend the money.

32vld, I Have those needle screws all the way in. Is that the "too rich" position for those? Do I need to run them out. One other person said the same thing that it was too rich. I thought running the needles in would reduce the fuel and richness. Maybe I have it backward.
Gene

The Holley has adjustable float levels, which are probably too high. The nut and big-headed screw on top of each float chamber controls the float level, and the plug in the side of the float chamber is at the correct level for the floats to be adjusted. Loosen the screw at the top of the adjustment assembly, and turn the nut CW to lower the fuel lever, or CCW to raise it. Remove the sight plug, and adjust the level until it's just at the bottom of the gauge hole. Have some rags handy- - - -there will be spillage! There's only one good use for a Holley- - - -wide open throttle racing.

The idle jets are full lean in the CW position. The fact that the engine runs at all with the idle adjustments bottomed out tells me you've got a severely too rich situation going on, either float levels too high or damaged needle valves.
Jerry

Listen to Mr. Wonderful and throw the Holley as far as you can. I do not recommend the big league pitcher method, as you may blow out your shoulder. I recommend throwing it like an Olympic discus champion. You will get better distance.
Carl

Hy Guys, the engine in this truck is a 366, that is a truck only engine, I have never seen a truck 366 or 427 truck engine with anything but a governor equipped Holley. Thousands of those trucks ran for years very well with that setup, possibly someone swapped a carb setup for a 427 or 454. Maybe we should try to identify what carb we are dealing with, before we suggest throwing it away.

This is a holley 1850 carb without a governor. The 366 engine replaced the original 283. This is a retired firetruck that has sat for several years. I was initially happy to get it running, now I want to iron out these kinks.

I'm with EdPruss on this one. In the video it sounds like you have a pretty good exhaust leak. The place where my 366 leaks is at the manifold to exhaust pipe. I think the popping is coming from fresh air being introduced into the exhaust system through a leak. You could check and see if the Holley has a blown/leaking power valve. Early Holleys did not have the PV backfire protector. You have the classic symptom for the over rich condition relating to the PV. Also as HRL said, check the float level first it's easier than tearing into the PV. BTW I don't think I have ever seen a QJet on an early 366. Does your manifold have a square bore to spread bore adapter under the carb?

SimS

Yes, after a bit more research, I find that the tall block 366 was equipped with a governed Holley 4 barrel. I doubt that the OEM carb is on this engine, however. There are a set of numbers stamped on the air horn near the choke valve, so if you can post those, I'm sure somebody can research the application. A Q-Jet manifold for a 396 or a 454 would require a set of spacer blocks between the manifold and the heads to fit a 366 engine.
Jerry

I can confirm without question the exhaust is leaking at the manifold where it connects to the pipes. I did not realize air entering there would would cause a problem. I will check the floats and then the power valve next. Thanks for the input. I guess I need to go get new exhausts made too.

The shop I worked at for Rider Truck was a shipping depot, we would get 5 or 6 366's a week and ship them out to other dealers. This was in the early 90's. GM tried to save money and removed the upper bushing in the distributor. The housings were wearing out causing a misfire on the back cylinders. Most of the cores coming through had holes blown through a piston or two.

You might check the play in the distributor shaft if you find nothing else wrong.

Joe

I will check the stamp on the carb body tomorrow. I lowered the floats and and i tightened the manifold to exhaust connection where it was loose. Not sure that fixed the leak there but it couldn't hurt. I then started the engine. It fired right up and ran pretty good when it was cold. I would rev the engine and the instant I backed of the throttle it would pop. I probably ran it at idle and reving it for 20 minutes. It seemed to be idling fast so I backed off the throttle idle screw and it continued to run ok. Once it was up to temp I turned it off let it sit a few minutes then tried to start it. It would not start. It would turn over but never would fire. Same thing that happened before. It seemed like it was not getting fuel. I looked down the throat of the carb while my dad turned it over and pressed the throttle. I could not see gas squirting out of the jets in the horn. I will check that passage tomorrow but the squirted holes are clear. How is it running when cold but not when it warms up?

Hy Gene_Spartan80, Is this carb a manual choke or automatic choke? It sounds like the carb should have a total rebuild, and a check of the power valve and all circuits. There could be many marginal conditions on this engine causing different symptoms, but we have to start to correct everything somewhere and the carb is as good a place as any to start, hope that helps.

The carb has a manual choke. I will check the power valve tomorrow.

You might want to do a compression test. Never know, it could be some burned valves.

I adjusted the floats and while I was at it I inspected the passages in the metering plate. It had sludge clogging one of the metering screw orifices. I also put a new power valve in it. The truck stated and ran fine with an ocassional exhaust pop. It was a little sluggish when pressing the throttle but that improved afterit warmed up. After a while we reved the engine it backfired and now I need another new power valve. I checked it by putting my finger over the front bowl vent and the engine died immediately. It still idles ok so will move to some other areas you guys have recommended. Thanks for all your input.

I felt bad for a while for concurring with HRL to throw the Holley as far as you can (he can be a bad influence).
But with your recent troubles, I still stand by it. Even though you have other issues with your engine, Carters and Q-jets, don't generally get blown up by backfires.
Carl

Gene;

I don't know what your long term goals are with this truck. But Holley makes a retrofit kit that you can install on older carbs that will protect the PV against back fire damage. I think they are $15-$20. Any FLAPS should have the kits. Good luck

SimS

I had to be away from the truck project over the weekend. My dad took the liberty of replacing the points, condensor, rotor, and distributor cap. He reports there are no more popping noises. I have not tested it myself but I hope he fixed the problem. Thanks, to everone for your input.