The Stovebolt Site Forums The Shop Area Engine Shop Timing exhaust burps?

Need some advice...how much does timing effect deceleration exhaust burps
? I know that too lean a mixture is a cause of exhaust burps. I open the mixture screw and that helped maybe I need to go more ( I hate to have the screw fall out while on the road)... Should I also re-time the engine? Perhaps it's too advanced? I have the ball on the pointer but I can also time with a vacuum gauge?

Thanks in advance, Steve

Well if the ball is lined up with the BB(TDC)your not where you need to be yet. That setting is/was for the old gas. New gas formulations require an advance of 6-10* BTDC dependig on gas/dizzy/compression. that's where the Vac gauge comes in handy in determining where it's running best So IF your timed to the BB/Pointer your timing is retarded.

I found this link helped my understanding of how timing works. Dist. Advance

Might want to check for exhaust leaks around the manifold to head, or exhaust flange.

At the end of the exhaust manifold where the flange meets the exhaust pipe flange there is a donut like gasket between the two flanges. There is some space between the flange faces and the donut like gasket. Would this cause exhaust burps? Also we did richen up the mixture by moving the needle valve out. Can we drive until she is warmed up and then head for a steep downgrade and listen for the rumble then back off the mixture screw and try again...is this in any way a correct procedure?
Thanks in advance! Steve

Steve,

I had deceleration (back-off) burps in the exhaust line for a year until I diagnosed that I had too rich a mixture and also oxygen getting into the exhaust line, allowing "exhaust burps".

The rich mixture was due to a poor carburetor float-setting by the re-builder (from JC) and the oxygen in the exhaust was due to a poor donut seal (my poor attention to detail).

Your situation might be different, but I would check the exhaust seals (to assure no oxygen in the exhaust line) and reset your carburetor(s) mixture.

What is an exhaust burp? are you talking about the "BUM-BUM-BUM",coming down a steep grade,in "granny"-gear?? My 261's done it since the day I first laid eyes on it ,25 years ago. Sounds even better with the headers and dual stacks. All of the old farm tractors that I've been around,have done the same thing,while in low-gear,decending a steep grade. Even more so,when loaded. All of the old busses had the same decelleration burps,as well. It doesn't matter what the make is,either. All gasoline engines do it,even the 2-lung "Johnny-Poppers". I always hear it coming from the tail-pipe(s),not from the manifold area.

Yep those are the ones. I was told by my uncle that they are caused by a too lean mixture. So we have been backing off the mixture screw to get a richer mixture. It has helped BUT we think that we better time the engine with a vacuum gauge instead of a light. The truck has a Langdon HEI dizzy and he suggest 10 degrees advance. We are going to reset idle to 500 then mixture screw 2.5 turns out then turn dizzy to get max vacuum. While its running we will spray starting fluid around the manifold to see if there are any leaks. I hate the burp noise!!!! But have no idea if it hurts the engine no one has chimed in on that....if we are wrong with our plan please (anyone) chime in. As an ex-Army guy I take orders well!

I'm thinking you are trying to fix something that isn't broke.

My fire truck has done this as lone as I've had it and I've put 24k miles on it. I was told I needed a crossover pipe in the dual exhaust (haven't done it)...is your's dual exhaust?

Mike B

Hey,Mike! good news for ya,(I Think)! I used to have a '65 G.MC.,3/4-ton,with a 305E. First thing I did was took it to a muffler shop,and had duals added. I didn't ask for a cross-over,but it was added anyway. "She" still cackled like "she" had a "jake" brake added,during decelleration. It didn't bother me a bit. Trans. was a 2-speed hydramatic,which started out only in "Lo". Weird set-up. GREAT truck!! Did you check-out that "Well-Made Video",on "Youtube,yet? It was posted in the "Big-Bolt thread,about 3-weeks ago. '65 G.M.C. 4000/pulling Tandem/Dual pup-trailer,made from a '53 6500 chassis! 10 minutes of SWEETNESS!! 305 runs a GREAT sounding single-stack! At the end of the video,the "Jimmy" gets passed by his other truck,an A.D. 2-ton Chevy,pulling a tandem-axle "Pup"! Gotta LOVE it! ENJOY!!

HA........some people really like that noise( and my wife is not one of them). It's now a matter of me or them!

Yes, dual exhaust with old split exhaust manifold no cross over pipe. The good news is that I'm armed and ready for battle. Vacuum gauge, flat blade screw driver, tachometer and no fear, well maybe just a little. If I make it out alive maybe I'll have answers for those that are interested. Exhaust farts be dammed full speed ahead!

Thanks, Steve