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Old 03-29-2020, 12:56 PM
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Default Loud backfire

Loud backfire 2 days ago . Now loss of power with difficulty idling. Friend thinks power valve. So before I start throwing parts at the engine, what's a logical course of action?
Engine in sig. 70 GTO

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Old 03-29-2020, 01:03 PM
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What carburetion are you running?

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Old 03-29-2020, 01:06 PM
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For sure Roger change PV & new gasket, how old is it?
Check timing as step 2
Jeff

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Old 03-29-2020, 01:11 PM
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Thanks Jeff, will do. Carb is an Holley 850 dp.

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Old 03-29-2020, 03:22 PM
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Put a Q-Jet back on and see if the problem goes away.


...could be a dead sparkplug ( we got backfire from a shortd-out new.

Wiped exhaust lobe cause backfire up the carb.

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Old 03-29-2020, 03:33 PM
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Sounds like a Pertronix issue. If that is what you have, nix the Pertronix.

But judging by your signature, you are smarter than that.

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Old 03-30-2020, 01:09 AM
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My distributor managed to loosen itself years ago and cause some issues. It’s easy to rule that one out. Also - cross firing plug wires?

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Old 03-30-2020, 06:09 AM
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Look down into all 4 bbls of the Carb with the motor idling , if you see any fuel dribbling out of a booster then it's a Carb issue and likley a blown power valve.

Back firing out the Exh and not the Carb is a sign of being too rich at near idle rpm and ruff idle goes hand in hand with that.
Excess fuel collects in the muffler(s) until the Exh temp gets hot enough to light it all off with a Big Bang!

It's also possible to have the firing order screwed up yet not have the motor backfiring Thur the Carb, so if you have had the wires off to do some work then double check the firing order.

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Old 03-30-2020, 05:18 PM
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A little more info will go a long way in diagnosing cause.
Did it backfire thru the intake or exhaust?
What were operating conditions when it occurred? (IE:cold/hot, idle/ light load/ WOT, etc)
Just out of storage?
Is carb dripping fuel?
Listen to exhaust ton,e idle and 2K RPM.
Do you hear a misfire from one cylinder/pipe, does it clean up or get worse with increased RPM?
Crank without ignition and listen for skip/loading in the cranking sound, or pop off valve covers and check for any valve train problem!


Logically deduce if backfire was the cause.... or an effect....of the problem.


Last edited by STEELCITYFIREBIRD; 03-30-2020 at 05:30 PM.
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Old 03-30-2020, 09:09 PM
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Thanks everyone for your suggestions. New Pv and fresh set of plugs and all is well. However I will be jetting down from the 78 to 76. Plugs looks like I have been running rich.

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Old 03-31-2020, 03:12 AM
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The rich plugs were probably the result of the blown power valve and previous jetting could have been okay. So we know the result of the backfire but not the cause.

Also, Holley finally put a check valve in the line to the power valve to protect it, and the check valve kit is available and can be installed in older carbs. Then the next backfire won't take out the valve.

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Old 03-31-2020, 12:22 PM
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"Logically deduce if backfire was the cause.... or an effect....of the problem. "

Time will tell!

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Old 04-01-2020, 10:00 AM
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Default For future reference

Quote:
Originally Posted by steve25 View Post
Look down into all 4 bbls of the Carb with the motor idling , if you see any fuel dribbling out of a booster then it's a Carb issue and likley a blown power valve.
Blown power valves leak gas through the passage/hole in the bottom of the base plate. Runs rich without nozzle drip.

Clay

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Old 04-01-2020, 10:38 AM
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Quote:
Originally Posted by lust4speed View Post
The rich plugs were probably the result of the blown power valve and previous jetting could have been okay. So we know the result of the backfire but not the cause.

Also, Holley finally put a check valve in the line to the power valve to protect it, and the check valve kit is available and can be installed in older carbs. Then the next backfire won't take out the valve.
That is why my original post had to do with the ignition system. Large backfire out the exhaust is generally caused by a cut-out in the ignition system. Temporary delay in ignition, build up of unburned fuel in the exhaust, re-ignite..... BOOM!

Apparently no one here owned these cars back in the day and turned the ignition switch off then on while tooling down the road.

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Old 04-01-2020, 11:04 AM
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[QUOTE
Apparently no one here owned these cars back in the day and turned the ignition switch off then on while tooling down the road. [/QUOTE]

Coming down this long grade, and had my daughter with me. Slap into 2nd gear, and turn off the key, and back on...BOOM! Daughter thought it was hilarious so I did it a few more times.
Words something to the effect of "this is the last time", and when turning the key back on, sent the muffler to the pavement, and guess who had to replace it, and of course my young daughter just had to tell mom !??

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Old 04-01-2020, 11:28 AM
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Quote:
Originally Posted by gtohunter View Post

Coming down this long grade, and had my daughter with me. Slap into 2nd gear, and turn off the key, and back on...BOOM! Daughter thought it was hilarious so I did it a few more times.
Words something to the effect of "this is the last time", and when turning the key back on, sent the muffler to the pavement, and guess who had to replace it, and of course my young daughter just had to tell mom !??
LOL! Had a lot of friends who's mufflers looked like footballs back then from doing that. Ahhh.... The good 'ol days! You weren't cool if you couldn't brag about how many tires and mufflers you went through.

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Old 04-01-2020, 11:45 AM
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I was taught check the float levels, good, shut idle mixture screws, still runs, power valve likely.
....
When I was a teen, 16 to be exact, coasting down big long hill to save fuel in my 69, forgot to take car out of gear, turned back on key POW, 1 week old dual exhaust system reduced to two oval ends with nothing in between! Saw the shrapnel sparking in the rear view mirror. My friends called my Harley with drag pipes the fire god, I liked to light them off in the Fort Pitt Tunnels.... when I was an azzhole punk kid. We all did stupid chit, and occasionally may still do.



Still curious if it was an intake or exhaust back fire...or both?

I could see a carbon'ed up cylinder, glowing carbon ember, light off the incoming mixture.


Last edited by STEELCITYFIREBIRD; 04-01-2020 at 11:51 AM.
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Old 04-01-2020, 12:01 PM
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Yep, ignition cutoff and back on while coasting will blow out the mufflers.

George

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