Changed carb, still having issues, diagnostics?

[benbloom]

hey all, i replaced my carburetor to try and solve backfiring through the carb and rough idle. with a new carb, and gave it a tune up , same issues keep happening. i suspect now that its something bigger and more serious going on inside the engine.

truck is burping black smoke out of the exhaust
backfiring through carb
idles very rough

anyone have any ideas? should i just have a mechanic look at it? i get this feeling like I'm going to be looking at a rebuild or crate engine

[GASoline71]

99.9% of backfires through the carb are timing related.

Could be a stretched chain. Harmonic balancer outer ring has slipped on the hub. Or just plain timed way off.

Do you have a timing light?

Gary

[geezer#99]

X2
All symptoms of a lack of timing and an idle cranked to high.
What's your initial timing?

[cpwhitey]

Could be too tight of a vavle or broke valve spring. If running rough locate cylinder causeing miss and do compression check.

[choctawchevyclassics]

Is it something that happened all of a sudden or has it been gradually starting to run bad. There's a lot of different components that can cause the very issues you are experiencing. intake, timing, carb, distributor, valve train, cam...A vacuum gauge can tell you almost anything with problems you might be having. If you don't have the info on vacuum, I'll post it when I can find the link.

[choctawchevyclassics]

Here's a link for Vacuum Gauge info...

http://www.secondchancegarage.com/public/186.cfm

Go through all of the scenarios on the page. Very interesting information.

[toolboxchev]

Second that on the vacuum check.

Also a vacuum gauge will not detect a leaking brake booster sometimes. Last time I went through this it was the Ignition Module in the HEI distributor and the brake booster.

[choctawchevyclassics]

Originally Posted by toolboxchev

Second that on the vacuum check.

Also a vacuum gauge will not detect a leaking brake booster sometimes. Last time I went through this it was the Ignition Module in the HEI distributor and the brake booster.

Not sure why it wouldn't detect a vacuum brake booster, unless it has something to do with the check valve.

[choctawchevyclassics]

OH!!! And by the way!!!Vacuum Sucks!!!

Just sayin!!!

[Mike C]

There's an old hot rodders adage that says 90% of carburetor problems are ignition. I'd go through that system completely then start back with carb.

Black smoke out the exhaust is typically too rich but a backfire out the carb is typically a lean response. Therefore my "internets diagnostics" says you have ignition problems.

If you haven't already, great time to upgrade to HEI and high performance spiral wrapped suppression plug wires.

[cleszkie]

Make sure your plug wires are routed correctly. Getting two mixed up will mimick a timing problem. Check the simple, obvious things first.