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Old 06-16-2007, 12:50 PM   #1
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Question backfiring on down grades?

Hi All,

I've made some performance mods to our 88 345: a ;new bigger exhaust system including headers and a bigger ram-air style air intake. I also rebuilt the carb. We just drove over the Sisikyou summit in Northern California / Southern Oregon --she pulled well. (Would love to see the 454 behind a gear vendors overdrive some time).

On the downgrades, we are getting some backfiring. Seems to have gotten a bit louder since I added the ram-air intake. Anyone experience a similar issue or have any advice on how to adjust the mixture to compensate? If I remember correctly, I may have to drill some holes in my quadrajet to get access to the mixture screws.

Thanks,

Nick
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Old 06-16-2007, 12:56 PM   #2
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Quote:
Originally Posted by nickglase
Hi All,

I've made some performance mods to our 88 345: a ;new bigger exhaust system including headers and a bigger ram-air style air intake. I also rebuilt the carb. We just drove over the Sisikyou summit in Northern California / Southern Oregon --she pulled well. (Would love to see the 454 behind a gear vendors overdrive some time).

On the downgrades, we are getting some backfiring. Seems to have gotten a bit louder since I added the ram-air intake. Anyone experience a similar issue or have any advice on how to adjust the mixture to compensate? If I remember correctly, I may have to drill some holes in my quadrajet to get access to the mixture screws.

Thanks,

Nick
Nick,

Sounds like your engine is running lean. That is a cause of backfire on deceleration.

Bill
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Old 06-16-2007, 01:14 PM   #3
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An air leak on the exhaust side can cause backfire.
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Old 06-16-2007, 02:38 PM   #4
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It is, of course, always going to be difficult to get the settings right when using a non-standard combination of after-market devices. I had exactly this problem of back-firing down hills on a race-car I built (Westfield 7). The best investment I made was to take it to a specialist with a rolling road. The guy took two hours to transform the performance by changing the various carburetor jet sizes on the twin Webers, as well as checking the rest of the the ignition and exhaust systems. I know we like to do our own work, but this was one of the rare occasions when I called on a specialist, and I was most impressed.
Nick.
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Old 06-16-2007, 08:57 PM   #5
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Quadrajet Carb

If your quadrajet is a mechanical carb, you can remove the steel plugs covering the air/fuel mixture screws on your throttle plate (if not previously removed) to adjust the mixture. Additionally, assuming your running too lean, you can remove the air horn and look into the base of the float bowl to change your primary jet size.
I would not suggest drilling out the jets (2), except as a last resort! Your jetting is trail and error (suggest "small" increment changes). The inside of your tail pipe will let you know if your getting too rich.

An electro-mechanical quadrajet can be identified by a two wire plug connecting to the mixture control solenoid on the air horn. Should you have this set up, special tools are required to adjust the dwell between 28 and 32 degrees. There is a lean stop and a rich stop with a paddle that requires 3/32" travel (up and down). Hopefully you do not have this setup.

Before involving myself with the carb, I would see what the inside of the tailpipe looks like and then pour some ATF through the primary side of the carb while holding the RPM's up (1400-1600). Any area you see smoke (other than the exhaust pipe) indicates an exhaust leak(s). The ATF does no harm to your engine. I have used it (or water) to decarbon engines. Using the ATF will leave you smoking out your tailpipe for a couple of days before returning to normal. I assume your engine is running good. An engine that is "too" lean will run higher temps.

Good luck,
Larry
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Old 06-16-2007, 10:37 PM   #6
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Quote:
Originally Posted by lsinclair
Before involving myself with the carb, I would see what the inside of the tailpipe looks like and then pour some ATF through the primary side of the carb while holding the RPM's up (1400-1600). Any area you see smoke (other than the exhaust pipe) indicates an exhaust leak(s). The ATF does no harm to your engine. I have used it (or water) to decarbon engines. Using the ATF will leave you smoking out your tailpipe for a couple of days before returning to normal. I assume your engine is running good. An engine that is "too" lean will run higher temps.

Good luck,
Larry
I have also used this method of finding exhaust leaks, as I am hard of hearing, and can see a leak before I could hear it. An exhaust leak can also contribute to a backfire on deceleration. Just be sparing with the ATF, I have smogged out my neighbors doing this, they were not happy.
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Old 06-17-2007, 08:45 PM   #7
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Backfiring on downgrades

Quote:
Originally Posted by overlander63
I have also used this method of finding exhaust leaks, as I am hard of hearing, and can see a leak before I could hear it. An exhaust leak can also contribute to a backfire on deceleration. Just be sparing with the ATF, I have smogged out my neighbors doing this, they were not happy.
Terry;
You have to admit the smoke temporarly takes care of the mosquitos in your neck of the woods!
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Old 06-18-2007, 08:38 PM   #8
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Thanks all, I'm going to check for exhaust leaks, then probably start with the idle adjustment. My guess is that when I'm dragging the engine (no gas), she's just using the idle jets? Overall, I was a bit disappointed by our mileage ~6 mpgs, though we did tow about 19,500 lbs (with toad) over multiple 4000 ft passes.
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