The Stovebolt Site Forums The Shop Area The HiPo Shop 57 with 350, exhaust crackle cause?

Now that the electric choke is working properly the 350* seems to be running very well, but may be a tad rich at this 5000' altitude.
The cast iron exhaust was cracked on one side, so I put on a set of block hugger headers, running true dual pipes with 4' long glass-packs that were already on the truck.
The exhaust ends about 2' behind the cab.
Hard to go longer with the axle and dump bed, but I might be able to add a foot or so with turn outs.

I'm pretty happy with the exhaust note, clearly V8 authoritative, not too loud, just a little lumpy at idle.

The only real annoyance is a crackle at higher RPM, the exhaust pipes are all new, there should not be any air leaks.
Is this crackle most likely due to the semi-short pipe length, an air leak, or the possibly slightly rich mixture?
I did wait until everything was fully warm to check it out.
Once I clear up the crackle the exhaust is done!

*(A 1969 factory 255 HP four barrel car engine, which appears to have been rebuilt, and which has a bit of cam lope, compression test indicates it may be 10-1 compression)

I would think that if you run the exaust further back the(crackle) won't be as noticable.

I assume you're experiencing this noise on deceleration, sort of a "BRRRRRRRAPPPPPP" as you're coasting down to a stop? If so, it might be ignition timing that's a little late, or a slightly lean mixture as you're coming to a stop. If the noise is happening at cruise speed, or on acceleration, pipe length or the lack of a crossover pipe ahead of the mufflers might be the problem. Try advancing the timing a little, and/or add a dashpot to the carburetor linkage to ease the throttle closed instead of chopping it off abruptly as you decelerate.
Jerry

Well, NUTS!

After going through all sorts of effort to rid myself of the crackle - popping through the carb I finally found time to try something that was suggested on another forum.

I did a compression check on this engine before I put any time or dollar$ in to it, compression was fine.

But when I ran it at idle with the drivers side valve cover off, I saw the #5 exhaust valve has only about half the lift it should. :mad:

So now I have to replace the cam and lifters.

Not sure what grind to use, it is a 3/4 ton truck with very low rear ratio.

Would like to get good mileage, kind of like a slightly lopey idle, want to be able to get enough RPM for freeway driving, will be doing some hauling and towing.
I know some of that is contradictory, just saying what I like.

Has to be flat tappet, roller cam is silly expensive.

It sounds like the "cam lope" you described in the original post was actually a problem with the cam profile! Bad news! Do you have any idea how much mileage the engine has since the rebuild? If it's pretty recent, less than 50K since the buildup, for instance, it would be worthwhile to install a new cam and lifters. More than that, and I'd suggest doing at least an in-the-frame rebuild, rings, rod bearings, valve job, etc., as well as the cam change.

I like the Comp Cams 260H cam for a little bump in performance without seriously affecting low speed torque, gas mileage, or towing ability. It's got a very slight idle lope, more like a slight miss in an engine with a factory-original cam. This smooths out at 1,500 RPM or so, and that's where I start doing dyno pulls. By 2,000 RPM or so, the torque comes on like a kick in the pants, and it pulls strong to 5,000 RPM or so. Good for street use, without adding a high-stall torque converter or tinkering with the rear end ratio. A 3/4 ton should have the right gears for that cam to come alive nicely at low to midrange throttle.
Jerry