Any Carb experts here ?

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Nov 21, 2003 | 08:42 PM

Beach Bum

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Joined: Feb 1999

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From: Little Elm TX

Any Carb experts here ?

One of my vehicles is a nice rust free 4 wd 1981 Chevy Blazer.... the previous owner put in one of those low dollar GM 350 motors. The motor goes into detonation at about 1/2 to 2/3rds throttle, it helped when I put in the 93 octane gas, but still does it, just not as severe. Exhaust has burnt type lean smell to it..... however, motor starts up and runs smooth, just can't give it much gas.... also runs cool. This motor shouldn't have to run 93 octane at 8.5-1 cr. Also timing is at 6 deg initial.

I suspect the carb has a problem or two....

Anybody have an idea of what I should look for on the carb ? Anybody experience this same problem before ? I just want to know what to look for 1st rather beating around it for an hour.

.... I'm going to pull it off tomorrow, haven't even seen what it is yet, but probably a Rochester quadajet or something to that effect.

Thanks for any help !!

cheers,
Beach Bum

Nov 21, 2003 | 09:04 PM

CFI-EFI

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From: The Top of Utah

Re: Any Carb experts here ? (Beach Bum)

Yes. It should be a Q Jet. If the carb is old, the throttle shafts can wear the base plate. I would look at the choke pull off diaphram and hose for a leak. Also, make sure the distributor is hooked up to ported and not manifold vacuum. Good luck, and...

RACE ON!!!

Nov 21, 2003 | 10:14 PM

SWCDuke

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Re: Any Carb experts here ? (Beach Bum)

The problem is probably your spark advance map, which I assume is vacuum and centrifugal advance on a conventional HEI distributor. Use a dial back light or put some marks on the balancer to characterize both the vacuum and centrifugal advance, and let us know what it is.

Mid throttle acceleration detonation was somewhat typical of the late seventies early eigthies era as OEs tried to meet emissions and achieve high fuel economy.

Duke

Nov 22, 2003 | 12:07 AM

Vetterodder

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From: Fountain Hills AZ

Re: Any Carb experts here ? (SWCDuke)

Quote:

The problem is probably your spark advance map, which I assume is vacuum and centrifugal advance on a conventional HEI distributor.
Duke

:iagree:

While a quick advance curve is usually a benefit in a light performance car, it can give exactly the results you're getting in your not so light Blazer. While there are carburation issues that could be the problem, I'd look at the timing curve first. If the curve isn't the problem, my next suspect would be something retaining heat in the combustion chamber such as carbon buildup.

Nov 22, 2003 | 03:06 AM

The Green Rocket

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From: Garden Grove CA

Re: Any Carb experts here ? (Beach Bum)

Todd, Some of those Goodwrench engines have the pistons so far down the hole that they cannot help but have detonation problems. Get it to a shop with an exhaust analyzer to help with carb calibration, many of the Q-jets from that era used really screwed up calibrations to pass epa smog testing.

Thomas

Nov 22, 2003 | 11:22 AM

clem zahrobsky

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From: delmont pa

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Re: Any Carb experts here ? (Beach Bum)

if the EGR is not connected up you will have this problem as the engine needs the colder exhaust gas to lower the combustion chamber temp to prevent pinging. i went thru this back in the 80s trying to remove the EGR and it alway cause this problem. :chevy