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Carb backfire and Fear
Posted by: Bernie ()
Date: September 20, 2018 03:01PM

Ok guys, I have a 65 Monza with a 110.

lately, I have a problem. Car has been a pretty strong runner in the past, but now has been backfiring through carb at deacceleration and I have a slight bit of hesitation.

timing is set at 14 degrees BTDC, Seth Emerson Dist. NGK BP5HS plugs gapped at .30.

plug wires fairly new and in good shape.

Did a Compression test. Results:

1) 110lb first time. 120 second time with teaspoon of oil. 120lb third time dry.

2) 110lb first time. Checked again dry 120 second time.

3) 120

4) 120

5) 120

6) 120

Carbs at one time were synced by a highly respected member of local corvair club.

At the same time, though I keep carbs clean they have seen better days.

Also, checked for vacuum leaks using starter fluid. No problems.

As for exhaust, I need some work. Ears busted on both manifolds. Waiting for new manifolds to come back from ceramic coater guy. Until then, exhaust seems to constantly need to be tightened up and often sagging.

It sure seems like a lean-out condition, and possibly a carb problem,

What's your thoughts?

Thanks,

Bernie out of Washington State.

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Re: Carb backfire and Fear
Posted by: Bernie ()
Date: September 20, 2018 05:05PM

Nobody?

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Re: Carb backfire and Fear
Posted by: Lane66m ()
Date: September 20, 2018 05:20PM

Probably needs new accelerator pump cups. If they are a few years old, ethanol fuel can make them a gooey mess that don't seal well. Get rebuild kits from John Sweet (jasvair). His kits have Viton cups that stand up todays fuels. Take a few hours to clean and rebuild the carbs. Easy to rebuild, especially if you have Bob Helt's Rochestor H carb book. Have a knowledgeable person help you setup carbs to include balancing them.

Al Lane
Ellabell GA

1966 Monza Coupe, 110 hp, 4 Spd
1968 Camaro SS Coupe 350 CI 295+ HP PG
1964 Greenbrier Deluxe, 6 dr, 80 hp car engine, PG
2015 Chevrolet Malibu 2LT
2018 Chevrolet Silverado LTZ Z71 Centennial Edition
1947 Farmall A tractor 15 hp


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Re: Carb backfire and Fear
Posted by: MattNall ()
Date: September 20, 2018 05:44PM

Quote
Bernie sez:
backfiring through carb at deacceleration and I have a slight bit of hesitation.

Never Fear!! ggg

You have two problems most likely...

1 Hesitation, as Al stated Replace the Accel Cups....no need to Kit / rebuild the carbs.. instructions for the CUP are in our FAQ's

2 Backfire on Decel? Most likely your lousy exhaust! Leaks cause backfires during Decel!





MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

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Re: Carb backfire and Fear
Posted by: Lane66m ()
Date: September 20, 2018 05:52PM

Probably will need carb air horn gasket and viton cup for each carb. John sells just them too.

Al Lane
Ellabell GA

1966 Monza Coupe, 110 hp, 4 Spd
1968 Camaro SS Coupe 350 CI 295+ HP PG
1964 Greenbrier Deluxe, 6 dr, 80 hp car engine, PG
2015 Chevrolet Malibu 2LT
2018 Chevrolet Silverado LTZ Z71 Centennial Edition
1947 Farmall A tractor 15 hp


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Re: Carb backfire and Fear
Posted by: American Mel ()
Date: September 20, 2018 07:44PM

Matt is most likely correct.
Carb back-fires, lean condition on accel.
De-cel is most likely tail-pipe back-fires, not through the carbs.
On de-cel, you are running with the butterflies closed.
This causes an extremely rich condition.
This unburned fuel is then being pumped into your extremely HOT exhaust pipes where it comes into contact with fresh oxygen the is coming in through those leaky manifolds.
BooM ! ! !
Spontaneous Combustion !

.
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WA. state, 1 mile south of the Canadian border,
I am not at the end of the world, but you can see it from here.
'66 Monza Coupe - 4spd, 140 Daily driver beater
'67 Monza Vert. - PG, 140 Daily driver beater
'67 A/C Moredoor Monza - 140 4-spd. driver

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