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Help - backfire on deceleration - tried EVERTHING
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andk5591
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PostPosted: Fri Sep 17, 2021 4:59 pm    Post subject: Help - backfire on deceleration - tried EVERTHING Reply with quote

Long read, lots of detail:

Customer car - glass buggy. I BELIEVE it is a stock 1600 DP. Has dual canon exhaust. 34 PICT3 Solex, and now Pertronix SVDA. Blip the throttle or drive and shifting or decelerating get serious back firing. Not just one, but multiple. Both sides of exhaust.

Supposedly did not do it when they last drove it (year or so ago). I put in fresh gas to play it safe. BUT had big can vac only distributor where as soon as you opened the throttle at all, you got full advance. I do NOT know how much timing it had since it was wrong and I was gonna fix it. Also had somewhat eratic idle.

So, did tune up, set the valves and swapped the distributor with Pertronix SVDA. Set timing max mechanical at 30. Get good vacuum signal, so will advance into the 40s with vac connected.

My initial suspicion was a vacuum leak. We already replaced the intake boots and gaskets when we had the engine out. Ran fine on test stand. But that was mostly timing, carb adjustments. Steady idle now.

I have gone over EVERY joint and all over the intake itself with small shots of starter fluid. No change. My one tech suggested hitting the intake with water as well. Tried that. No change.

Went through the carb tuning several more times. (mix adjusted so the idle speed just drops.) Tried adding or removing some timing. Seems a little better when a little fatter, but not enough,

Swapped the carb with an Empi 34P3. (55 idle, 127.5 main) No change. Tuning is almost identical. Played with mix and timing again. Same deal, fatter up the mix and its a little better, but still unacceptable.

Checked the plugs - 3 and 4 look perfect color. 1 and 2 are little whiter than I like, but not burned or anything.

Empi carb had a 55 idle. Put in a 60. No real change.

I am pulling my hair out. What am I missing?
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PostPosted: Fri Sep 17, 2021 5:08 pm    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Backfire on decel can be exhaust leak.
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PostPosted: Fri Sep 17, 2021 5:23 pm    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Idle mixture screws too lean or idle speed set too low. Bumping up initial timing. Try backing out the idle mixture screws maybe a 1/2 turn or so. Set idle to at least 675. I prefer 800 usually as its better for the engines splash oiling system and cam. It also keeps the fan rpms up which cools better in traffic.

It could be fixed with a better exhaust too.

Check if exhaust valves leak. Could be burnt valve?
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PostPosted: Sat Sep 18, 2021 12:11 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Stuff a rag in the exhaust while the motor is running and it will show if the exhaust is leaking.
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PostPosted: Sat Sep 18, 2021 5:28 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

It seems to me that there has to be raw fuel in the exhaust to make it backfire... is richer the best way to cure that?
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andk5591
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PostPosted: Sat Sep 18, 2021 7:46 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Forgot to mention - fuel pressure is 3 lbs. May try the exhaust plugging to find a leak - BUT backfire is both sides and we had not touched the exhaust at all.

Already played with idle mix. fatter, leaner on both carbs. fatter seems to help a little. Idle speed has been played with - 900 to 1100.

My tech ad already set the valves to .006, but I double checked to play it safe....He was damn near spot on.

Also played with timing some....started out 30 max mechanical. Gave it a little more and a little less (did not verify amount, but guessing 3 or 4 degrees) no real change.

I know how to set up 34s for the initial throttle plate setting. Same deal, both carbs act the same. I do NOT know what the jets were on the Solex that was originally on it. Obviously I can check on Monday, but assuming stock jetting.

This is a personal mission at this point. The engine should run fine and it does except for the backfire. I can also almost guarantee if I put a different header on it, the problem would go away. Have run into this before, BUT lots of folks run these dual canons and seem to be fine.
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PostPosted: Sat Sep 18, 2021 8:15 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Dumb question!
Have you looked at the inside of the distributor cap for carbon tracks?
I my mind, if they are present. You could have a misfiring plug when an intake valve is open.

I know, DUMB QUESTION. Shocked
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PostPosted: Sat Sep 18, 2021 9:27 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Sounds like op changed out everything but the exhaust, why not try that too? Process of elimination...
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PostPosted: Sat Sep 18, 2021 9:30 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Hmmm....Pertronix....electronic points...
Make sure you have a full and fat 12V at the coil. Much less and it'll backfire quite aggressively.
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PostPosted: Sat Sep 18, 2021 10:18 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

oprn wrote:
It seems to me that there has to be raw fuel in the exhaust to make it backfire... is richer the best way to cure that?

Basically if it's too lean it will backfire on deceleration when letting off the throttle while moving higher RPMs in gear. It will backfire NOT becuase there is TOO much fuel in the mixture, but for the opposite reason. It's actually so lean that it can't even ignite in the chamber, so unburnt fuel quickly accumulates in the muffler over a few cycles and then combusts in the exhaust.

The reason this happens is due to the intake vaccum spike that occurs when the throttle plates are closed abruptly while the engine is rotating fast but also trying to slow itself down with compression. This hinders the ability for the carb to deliver the proper amount of fuel but air is still able to get sucked in, too much. The mixture is so off at that moment, it simply does NOT combust in the chambers... just dumps right into the exhaust... then "pop, pop, pop."

Does it have the stock intake and heat riser system?

For the above reasons everything needs to be finely tuned to work right, especially with a single carb when you deviate from the stock setup. That's why I think you either need bigger idle jets, carb adjustment, or a better suited exhaust system. Or something else with how it's all tied together (ignition system.) You could just tell him to live with it, but I'd want to fix it too. I think it probably always did it (at least since whatever external mods were done,) but who knows... "customer always right, etc..."
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Last edited by evanfrucht on Sat Sep 18, 2021 10:29 am; edited 1 time in total
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oprn
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PostPosted: Sat Sep 18, 2021 10:27 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Ok, that makes sense. My Street Buggy with Kadrons and cannons did it no matter what I did but the cannons were so loud anyway that it really didn't matter. I was annoying people both with the throttle on and with it off. My sand rail with the stock carb and cannons didn't do it at all come to think of it...

Anyway I have 2 sets of cannons up for grabs if anyone wants them. I don't.
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PostPosted: Sat Sep 18, 2021 11:07 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

To answer you.
Brand new distributor. Cap looks good.
Coil.....I believe its a Bosch Blue, but now that you mention that, not 100% sure. I am pretty sure my tech verified that its at least 3 ohm.
Carb idle mix/jet - went fatter and leaner....Fatter was a little better. Stepped up to a 60 idle jet. No real change.
Stock intake with heat risers.

Now here is the kicker - the owner has his own shop...BUT has had the car for several years. Bought it from a relative. Would rather I deal with it raher than him. Really nice guy and assures me that the backfire was not there before except if going do a real long hill and that the dual canons have been on it for years. I really want to swap the exhaust...I have a NOS S&S that would work well on this and I am pretty sure it will fix it and the car will run better.

Bottom line for me is that I hate throwing parts at it, but I can't spend a lot more time on it either. There is some good info, but nothing blatant that I missed. (not my first rodeo...LOL) So gonna swap it and thne see what happens.

And yeah on used dual canons, I have 2 or three sets that I took off of cars sitting in my one trailer....
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PostPosted: Sat Sep 18, 2021 12:02 pm    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Reading back it seems you haven't looked much into the exhaust. Its could easily just be a leak at one of the flanges or joints. Also if the muffler is glass packed that stuff burns out eventually and will cause the muffler to become less restrictive and louder (not sure if that particular exhaust you have is... never looked into it or cut one open myself.)

Dirty carbs could be causing a lean condition as well. Even if it's just a little debris at the bottom if the float bowl or stuck somewhere.

Just some extra fairly obvious ideas.
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PostPosted: Sat Sep 18, 2021 12:45 pm    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

My duals don't backfire on decel. I know a lot of people hate the duals but the exhaust design is not the problem.
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PostPosted: Sat Sep 18, 2021 3:33 pm    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Q-Dog wrote:
My duals don't backfire on decel. I know a lot of people hate the duals but the exhaust design is not the problem.


maybe not, but can't say he tried EVERYTHING if he didn't try the exhaust...
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PostPosted: Sun Sep 19, 2021 12:39 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Check the 12 volt feed wiring to the coil . Maybe as the engine twists decelerating its cutting the power to the coil for a moment, letting out unburnt mixture then firing it.
.
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PostPosted: Sun Sep 19, 2021 12:07 pm    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Have not done anything with the header. Thats on tomorrows task. These dual canons have NO baffling or anything. Gonna try the rag, but same question - would expect that on one side, not both. BUT 1-2 plugs are whiter than 3-4...so maybe?


Carb dirty - nope original was cleaned before installing, second carb was brand new.

Coil wire coming loose. Nope. New wiring harness - plenty of slack in that wire.
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PostPosted: Sun Sep 19, 2021 2:11 pm    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

It probably used to backfire less when there was no carbon buildup inside the exhaust system.
It is normal that exhaust systems develop a layer of carbon inside.
just as it is normal for this thing to backfire.
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andk5591
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PostPosted: Mon Sep 20, 2021 7:18 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

modok wrote:
It probably used to backfire less when there was no carbon buildup inside the exhaust system.
It is normal that exhaust systems develop a layer of carbon inside.
just as it is normal for this thing to backfire.


Once again - we havent touched the exhaust and the really goofy thing was that the original disytibutor on this thing was a large can vac only, so one you got off idle. you got full advance... I have a couple things to try. One is looking for exhaust leaks (so much fun on a VW), disconnect the vac advance, different header and put his original distributor back in....
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"John's car" 64 VW woodie - The first ever
Maxine 61 Cal-look bug - Cindy's daily driver.
Max - 73 standard Beetle hearse project - For sale
66 bug project - Real patina & Suby conversion
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PostPosted: Mon Sep 20, 2021 8:50 am    Post subject: Re: Help - backfire on deceleration - tried EVERTHING Reply with quote

Easy way to check for an exhaust leak is to flip the hose on your shop vac (clean it out first) so it blows and put it in the exhaust pipe with a towel and hold it there and spray the joints and gasket flange areas with soapy water. It will bubble if there is an air leak and takes just a few minutes.
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