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#1 |
Registered User
Join Date: Jan 2008
Location: Hawthorne,CA
Posts: 114
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1969 GMC backfires
I have a 1969 gmc c25/pick up. I put two cherrybomb glasspacks about 2 weeks ago and the truck backfires when i decelerate from high or medium rpm. It never did before. wats wrong. All it is is the glasspacks and no muffler/exahusts.
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#2 |
1970 Daily Driver
Join Date: Jun 2007
Location: Glenns Ferry IDAHO
Posts: 1,037
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Re: 1969 GMC backfires
Nothing is wrong. Should sound pretty good. My 2 year old son claps and giggles when my truck back fires on deceleration.
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It's not truly yours if you didn't build it yourself or if you don't do your own maintenance. |
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#3 |
Cool Truck
Join Date: Dec 2008
Location: Bowie,MD
Posts: 1,869
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Re: 1969 GMC backfires
my truck backfires when I shut it off
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1968 Chevrolet C20 Longhorn "Cool Truck" 1937 Chevrolet Pickup - Project Looking for a patina'd 68 50th Anniversary longbed |
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#4 |
classic=of recognized value
Join Date: Sep 2008
Location: Wichita, KS
Posts: 317
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Re: 1969 GMC backfires
Mine does that too, pretty cool, huh! My neighbor calls it my Jake Brakes.
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#5 |
Registered User
Join Date: Oct 2005
Location: Overland Park, Ks.
Posts: 5,217
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Re: 1969 GMC backfires
Your idle jets are lean. Try turning them out about 1/2 turn. I my young days we used to turn the idle jets way in to get it to do this. It sounded cool. I don't have to be cool anymore.
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#6 |
Registered User
![]() Join Date: Jan 2007
Location: Columbus, Ohio
Posts: 4,339
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Re: 1969 GMC backfires
Sounds like an exhaust leak near the manifold - that can cause a backfire when letting off the gas quickly after accelerating...
Just had my new exhaust installed - it would backfire as loud as can be (scare the crap out of me) when I took my foot off the gas on the highway. |
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#7 |
classic=of recognized value
Join Date: Sep 2008
Location: Wichita, KS
Posts: 317
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Re: 1969 GMC backfires
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#8 |
Registered User
Join Date: Apr 2007
Location: King City, Ontario, Canada
Posts: 974
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Re: 1969 GMC backfires
yip, mine does it when driving with open headers, it winds down real nice like 6000 rpm to 2500 or so, then crackles down to an idle. i think it sounds cool
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- Jason If you can turn, Youre not going fast enough. Favorite caption: first car to 200 mph in the 1/4 mile, no parachute, drum brakes all the way around.. ..back when men were men. |
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#9 |
Size Matters
Join Date: Aug 2007
Location: Lyman, SC
Posts: 9,189
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Re: 1969 GMC backfires
When you close the throttle and the engine is still trying to suck throught the carb, it gets a richer mixture. This rich mixture is not needed since you are not under power so a lot of it is not burned on combustion in the cylinders and finds its way into the exhaust where it finds lots of oxygen in a free flowing exhaust. Heat from exhaust + rich mixture + Oxygen =Result....Ka Booom!
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"Reading the owners manual is only for quitters." Jason Harper, Bloomberg News. ![]() ![]() |
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#10 |
Account Suspended
Join Date: Apr 2009
Location: Oklahoma City
Posts: 4,709
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Re: 1969 GMC backfires
Yeah it's unburned fuel igniting inside your new glasspacks. What's your mpg like?
![]() If you have a manual trans and a carb you can turn your engine off on the highway wait a few seconds and then turn the switch back on and listen to that explosion! ![]() |
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#11 |
Registered User
Join Date: Jan 2008
Location: Hawthorne,CA
Posts: 114
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Re: 1969 GMC backfires
well how can i stop the backfiring i need help to stop it.plz.
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#12 |
Account Suspended
Join Date: Apr 2009
Location: Oklahoma City
Posts: 4,709
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Re: 1969 GMC backfires
There are alot of things to check. Your spark plugs and wires, distributor cap and rotor. Your carburetor. (what type of carb are you running?)
Do a compression test on each cylinder. If you have one that's low, it's probably not igniting the fuel like it should. |
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#13 |
Registered User
Join Date: Jan 2008
Location: Hawthorne,CA
Posts: 114
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Re: 1969 GMC backfires
its a 4 barrel edelbrock carb
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#14 |
Account Suspended
Join Date: Apr 2009
Location: Oklahoma City
Posts: 4,709
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Re: 1969 GMC backfires
What model? How old is it? Missing any linkages? Does the choke work like it should? Do you know how to adjust it? Is it leaking gas anywhere?
Last edited by highperf4x4; 06-02-2009 at 01:24 AM. |
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#15 |
Account Suspended
Join Date: Apr 2009
Location: Oklahoma City
Posts: 4,709
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Re: 1969 GMC backfires
First off, a glasspack is a muffler. And when you say no exhausts do you mean no pipes behind the glasspacks? What type of exhaust manifold/headers are you running? How far are the glasspacks from them. What size are your glasspacks/headers/pipes? Provide as much info as you can. What type of heads/intake. How old is the motor? Do you know how to do a compression test?
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#16 |
Registered User
Join Date: Jan 2008
Location: Hawthorne,CA
Posts: 114
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Re: 1969 GMC backfires
i mean there is no pipes out of the glasspack its just the glasspacks and notin else..
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#17 |
Account Suspended
Join Date: Apr 2009
Location: Oklahoma City
Posts: 4,709
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Re: 1969 GMC backfires
I'm sorry razr I don't have enough info to help you much. I will say this, if you put a pipe after those glasspacks they will probably stop the vacum pop.
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#18 |
Senior Member
![]() Join Date: Dec 2006
Location: Studio City, Calif.
Posts: 2,873
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Re: 1969 GMC backfires
Just fix the leak.Tighten (to recommended torque)or replace doughnut gaskets as needed.
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'69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. |
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#19 |
Registered User
![]() Join Date: Jul 2007
Location: Toppenish, WA
Posts: 15,518
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Re: 1969 GMC backfires
Before you play with the pipes, check the timing and vaccum advance. They will backfire when slowing down or pop through the pipes if the timing is late.
But I will agree that it may be going a bit lean when the throttle plate closes. My truck was doing pretty much the same thing and according to my vacuum gage the timing was late but when I swapped carbs I went up several sizes on the jets so I don't actually know which change did the most good but it doesn't pop when I slow down now. |
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#20 | |
Senior Member
![]() Join Date: Dec 2006
Location: Studio City, Calif.
Posts: 2,873
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Re: 1969 GMC backfires
Quote:
So putting on glass-packs will change the timing and carb settings?? interesting But I would still just find and fix the leak in the exhaust.
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'69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. |
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#21 |
Account Suspended
Join Date: Apr 2009
Location: Oklahoma City
Posts: 4,709
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Re: 1969 GMC backfires
How do you know he has an exhaust leak?
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#22 | |
Size Matters
Join Date: Aug 2007
Location: Lyman, SC
Posts: 9,189
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Re: 1969 GMC backfires
Quote:
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"Reading the owners manual is only for quitters." Jason Harper, Bloomberg News. ![]() ![]() Last edited by Longhorn321; 06-02-2009 at 10:57 PM. |
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#23 | |
Senior Member
![]() Join Date: Dec 2006
Location: Studio City, Calif.
Posts: 2,873
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Re: 1969 GMC backfires
Quote:
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'69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. |
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#24 |
Registered User
Join Date: Jan 2008
Location: Hawthorne,CA
Posts: 114
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Re: 1969 GMC backfires
how do i adjust the carb and how do i fix the exhuast leak ...wat do i have to do
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#25 |
Senior Member
![]() Join Date: Dec 2006
Location: Studio City, Calif.
Posts: 2,873
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Re: 1969 GMC backfires
If it wasn't backfiring with the other mufflers,forget the carb for now. Make sure the exhaust manifold is torqued to specs.(inside bolts 30,outside 20) -assuming it's a small block with stock manifolds.Then check the dougnut gasket clamps just past the manifold. I can't remember the torque for these,get a manual or just make sure they are very snug.If you see signs of exhaust blowing out of the joint and the nuts are tight,you may have a bad gasket.
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'69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. |
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