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Old 12-09-2012, 02:37 PM   #1
jasonroman
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whats the cause for backfire thru exhaust while shifting??

picked up an 83 c10.
st8 six. 4 spd.
notice that when shifting gears...not all the time, but a good majority of the time, i here a backfire type noise. only happens when switching gears.
had a new cat put on. my old one was hollow and muffler was beat up.
so i had a new cat put on and a muffler i had on my 85 welded up.
money is tight now. lost home and 2 vehicles in storm.
anyway so i had new cat put on and old/newer muffler put up. no piping from muffler
guy gives me back truck and tells me that its still backfiring. could be distributer or carb?? he says

so i tuned the truck up. and still have it.
timing is a little advanced right now. but not much. is that whats causing my pops?
or do i have to figure out how to mess with the carb?
or could it be something else??
appreciate any imput.
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Old 12-09-2012, 04:07 PM   #2
68gmsee
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Re: whats the cause for backfire thru exhaust while shifting??

What did you do to tune it up. That is a real vague term and everyone does it differently.


Some things to check but not necessarily in this order:

Before digging into the carb, check for any exhaust leaks real carefully. Make sure you don't have any cracks on the hoses and that all ports not used are plugged.

Check all the spark plugs for signs of oil burning and replace them if in doubt.

Check and/or replace spark plug wires and distributor cap and rotor.

Check vacuum advance to make sure it's working and not leaking internally. If you don't have a vacuum pump use mouth suction to insure it's not leaking.

Adjust your timing slightly retarded or advanced to see if it makes any difference.

Adjust the idle air adjusting screws for the highest vacuum and/or the smoothest idling. I prefer to use a vacuum gauge.
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Old 12-10-2012, 02:12 PM   #3
jasonroman
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Re: whats the cause for backfire thru exhaust while shifting??

Quote:
Originally Posted by 68gmsee View Post
What did you do to tune it up. That is a real vague term and everyone does it differently.


Some things to check but not necessarily in this order:

Before digging into the carb, check for any exhaust leaks real carefully. Make sure you don't have any cracks on the hoses and that all ports not used are plugged.

Check all the spark plugs for signs of oil burning and replace them if in doubt.

Check and/or replace spark plug wires and distributor cap and rotor.

Check vacuum advance to make sure it's working and not leaking internally. If you don't have a vacuum pump use mouth suction to insure it's not leaking.

Adjust your timing slightly retarded or advanced to see if it makes any difference.

Adjust the idle air adjusting screws for the highest vacuum and/or the smoothest idling. I prefer to use a vacuum gauge.
tune up consisted of, plugs, wires, distributor cap and rotor
pretty sure vacuum advance working properly. only because when my buddy checked the timing. we noticed the change in sound when the advance was disconnected and vise versa.
guess i will loosen distributor and check if i get different result.
dont have a vacuum guage or am i sure where and how to test with guage.
but i will wait to mess with the carb for last.

would the backfiring also be linked to another issue i noticed.
when i am driving, let say 4th gear on the park way. if teh road has an incline. the truck slows down. no power to move at the same speed prior to incline. eiter down shift to 3rd or end up going from 65 to 35 till road flattens out.

also notice that if i get on the gas while shifting. the truck boggs down rather then accelerating?? just falls on its face for a second then will pick back up.
is that a carb issue? timing??
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Old 12-10-2012, 02:45 PM   #4
Lee H
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Re: whats the cause for backfire thru exhaust while shifting??

Back fire on deceleration is usually a sign of an exhaust leak.
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Old 12-10-2012, 02:49 PM   #5
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Re: whats the cause for backfire thru exhaust while shifting??

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Back fire on deceleration is usually a sign of an exhaust leak.
well i believe only issue right now with exhaust is that the muffler came off my v8 truck and is now on the str8 6. after the muffler, there are no tailpipes. so its a bit louder. thought bad cat was cause of the backfire. but now theres a new cat and still have the backfire.

any ideas on why truck boggs down when i get on the gas?
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Old 12-10-2012, 03:05 PM   #6
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Re: whats the cause for backfire thru exhaust while shifting??

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Originally Posted by jasonroman View Post
any ideas on why truck boggs down when i get on the gas?
Very vague question. It could be a dozen or more things.

Low compression, bad fuel filter, timing out of wack, accelerator pump shot, carb improperly adjusted, bad fuel pump, worn valves/seats/guides, bad gas, floats not set right, clutch slipping, restriced exhaust, etc...

Backfire could be a lean issue, timing out of wack, or just a really bad exhaust leak...
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Old 12-10-2012, 09:39 PM   #7
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Re: whats the cause for backfire thru exhaust while shifting??

If the Smop Pump Diverter valve has straight vacuum ran to it. It will cause a popping noise it the exhaust when decelerating.
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Old 12-11-2012, 12:05 AM   #8
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Re: whats the cause for backfire thru exhaust while shifting??

Carb adjustments or bad accelerator pump can be the cause of bogging during acceleration. May not be getting enough gas so check the filters or replace them and also check float level. A weak fuel flow from the fuel pump could be suspect. And to add to the long list of potential problems a worn cam will cause similar symptoms.

Write the possibilities down and start by eliminating the easiest and cheapest ones first.
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Old 12-11-2012, 10:35 AM   #9
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Re: whats the cause for backfire thru exhaust while shifting??

damn. didnt expect there to be 100 possibilities...
im pretty sure fuel pump was put in prior to me buying truck. looks new.
then again. the truck sat for almost 1yr since p.o. passed.
guess i got alot to learn.
aside from the backfire. which i thought the cat would fix. and if not, maybe the fact that i have a muffler from a 350 truck on the str8 6 with no tailpipes might have been the culprit.
but the fact that when im driving and get on the gas there is a hesitation, leads me to believe i might have a couple things wrong.
needed a daily driver. this one turns on and takes me to and from work. so its serving purpose. would just like to get to the bottom of the little flaws.
truck has number of issues...guess if i were more of a mechanic id be able to pin point it better.
i know the timing is set a bit advanced.
and a guy at work said he smelled the truck. said to be running rich?
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Old 12-11-2012, 06:42 PM   #10
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Re: whats the cause for backfire thru exhaust while shifting??

Ok lets start at the beginning. Catalytic converters DO NOT fail on there own nor are they going to cause a running problem. The running problem is what caused the other cat to fail. Generally a backfire in the exhaust is a sign of a rich condition and your hearing the unburnt fuel lighting off in the exhaust pipe when the fresh air hits it. If its an old worn leaking carb that would be first suspect. Timing needs to be verified. If the truck is in fact running rich it will cause your new cat to melt down as well.
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Old 12-12-2012, 10:39 AM   #11
68gmsee
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Re: whats the cause for backfire thru exhaust while shifting??

I agree that there's more than one problem going on. You have the backfiring and then the bogging down when trying to accelerate and loss of power going at a slight incline.

The backfiring, like cableguy mentioned, is unburned fuel in the exhaust. Carb problems, timing, ingnition components like plugs, plug wires, points, distributor, ect. can cause this as well exhaust leaks and worn valves or cam.

The bogging when accelerating could be the accelerator pump. Easy enough to check by manually activating the throttle while looking down the throat of carb to insure it squirts fuel into the intake.

Lack of power when on an incline could be worn cam or a combination of all the other factors mentioned before. That's why I mentioned to make a list and scratch off each one as you check them.

Think of it as a quest. These old trucks have seen a lot of years and need help.
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Old 12-13-2012, 11:34 AM   #12
jasonroman
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Re: whats the cause for backfire thru exhaust while shifting??

thanks for replies guys.
today after work. i messed with distributor a bit to get the idle down. then carb started poping. so i messed with a screw on carb to adjust idle. and then again with distributor.
didnt have my light with me to dile in timing. i know it was a bit advanced. not sure now.
took it for a ride. seemed to pop alot less. and might have gained a little accelaration threw gears. not perfect but noticed a bit of a change.
ill get light from friend and see where im at now.
hope its not valves or cam right now. not sure how to fix or the dollar tag that goes along with it.

think my rear main seals leaking too.

noticed while messing with carb and distributer that when i manuallly reved truck up.
id get a smoke cloud from what looked like the front of the carb??

also noticed that theres a good amound of oil leaking from around the distibutor.

guess i got alot of issues with the old 83
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