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GTO with new carb, rumbles in the exhaust more

I put a new carb on my GTO several weeks ago. It's the correct carb. The prior one was incorrect and starved the engine at high RPM.

Now, with this new carb, the exhaust rumbles alot more than before when I decelerate (manual trans). I've got turbo-flows and it never did this as much with the old carb. Somethings definitely different in the exhaust sound.

Also, perhaps unrelated, the engine has been running a bit hot and knocking a bit more. It even dieseled in turn-off for the first time ever.

Timing is set correctly, 8-10 BTC.

Any ideas? This is very strange.

Reply to
none
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Sounds like the new carb needs some adjustement. I think the mixture is off - probably too lean.

Tony

Reply to
Tony V.

I agree. If it's too lean on main metering (as indicating by running hot and pinging), then that's beyond "adjustment" and getting into "rejetting". If the carb is set up exactly as original, and your engine has been modified, that might be the problem.

Reply to
Joe

Is it possible that it sounds correct now & you just have a problem overheating? If the engine is overheating, this would cause the engine to diesel. What engine do you have & year?

Reply to
Noah Jennings

Reply to
Bryan Lee

Bryan Lee said

Engine:

69GTO, 400, 4spd 74 block, 69 heads/intake/exh, HEI, 69 carb (was 1974 carb)

I rebuilt the engine a few years ago from it's 1974 "base", it's run fine for over 10K miles. However, it always had this annoying habit of falling on it's face at ~3000RPM and 90MPH. I always assumed it was fuel and this new carb (correct for 69 automatic (mine is a stick)) confirmed it. The high RPM and high speed power is much improved. As is the rubber in third.

10.75:1, factory

8-10 BTC.

Shell, premium

Size of carb? Don't really know. Stock quadrajunk for this year.

No spacer.

I'm thinking of sticking the old carb back on for testing purposes.

All feedback appreciated!

Reply to
none

The detonation is from the gas. You can't run just today's premium if the engine is built to 69 specs. My 70 needs at LEAST 95 "pump octane" (R+M)/2 GW

n> Bryan Lee said

Reply to
Geoff Welsh

Geoff Welsh said

How come I had no detonation prior to switching carbs?

Reply to
none

My only thought is that it was running very rich before. THAT would coincide/agree with another's comment that it is very/too lean now. Also of note, mine rumbles like mad in the exhaust. Pontiacs are supposed to do that. GW

Reply to
Geoff Welsh

Geoff Welsh said

Hence high combustion temps and detonation. Seems plausable.

Engine run on: Assuming nothing else has changed, except the carb, I'm starting to suspect this carb is more likely to fuel under closed-throttle, engine-shutdown conditions that it's predecisor. Grasping I know....

Wierd. It seems to come and go. Something is marginal somewhere. I'm going to spend quite awhile chasing this down. First task, put back the old carb and see how things go.

thanks for the help.

Reply to
none

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