Help - backfire on deceleration - tried EVERTHING

[modok]

"Once again"?

It still seems like you don't understand.

That engines backfire on decel, UNLESS they have some means of preventing it.

[evanfrucht]

Try to get more info from the owner...

When EXACTLY did it just start backfiring one day to the next? Is that even what happened. Maybe he didn't explain himself well and you misunderstood.

Sounds like there were a bunch of top end parts changed or messed with since the last time you saw the car?

You said new carburator? A lot of new chinese carbs are pure garbage, same with Brazil... get a rebuilt German Solex. New carbs can have machining debris stuck inside them among other issues, so perhaps the new carb isn't as trustworthy as assumed? Then get the proper well matched GERMAN distributor. THE PERTONIX COULD BE A PART OF THE ISSUE. For your setup I imagine an original SVDA would work well.

[74 Thing]

Swap the distributor back too as a last resort.

[andk5591]

Well I am a freaking idiot....Header gaskets looked good (at least on 2 and 4). But turns out that 1 and 2 were garbage paper ones. Felt around back and you all know how easy it is to differentiate between cooling air and exhaust leaks.....Well, chunks were missing on these 2 gaskets. Exhaust leak....backfire on deceleration...Go figure.

Replaced all the gaskets, cleaned up the flanges and all is good. The original carb is back in it and its jetted little fat, but leaving it as is. Car runs well except for a little bog when you jump on it...When I had the carb out last time, I noticed that the accel pump shot was maxed (probably to get around the advance issue with the old distributor,) but I carb access on this particular body is a major pain in the ass, so just leaving it as is...

Thanks all and sorry for wasting your time...
_________________
D-Dubya Manx clone - 63 Short pan,1914.
Rosie 65 bug - My mostly stock daily driver.
Woodie 69 VW woodie (Hot VWs 7/12).
"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
There's more, but not keeping them...

[andk5591]

I have to answer this even though I found/fixed the problem. Hoping to give this poster some useful information. I am not trying to be a dick even though missing the cause of the problem makes me foolish. Wont address the comments/questions about the actual problem since I messed up identifying the issue.

This is the first time I worked on this car. Primary reason it came in was for a new harness. BUT other issues were evident (there was a vac leak at the intake boots) as well as the carb/distributor mismatch.

Installed an Empi 34P3 as a test - His original carb is back on the car. I have installed quite a few (dozen or more) of these as well as their 30 PICT1. No issues, BUT many folks do NOT clean out ANY new carb or know how to properly set up a 34 PICT3 type carb. I do. Will not argue that a PROPERLY (Volkzbitz) rebuilt Solex is superior. I have used dozens of Tim's carbs.

Distributor - a big can vac only does NOT work with a 34 series carb. Yeah, on your 67 with a 28 or 30 series, they are great. But in this case, you cannot tune around that when the advance goes full as soon as you are off of idle. And I install a LOT of Pertronix. Same deal, I have some distributors rebuilt be Glenn and TASB. They are better and will last forever. The Pertronix works just fine though.

[evanfrucht]

Glad you got it sorted out. Backfires can be hard to track down so I'd say you got off easy! I've had terrible luck with Pertonix distributors but I know they can work.
_________________
1967 Bug ( the daily rod )
1964 Fury Wagon ( the pavement shredder )

[74 Thing]

Yes, glad you found it!

I sometimes use a piece of tubing as a stethascope to search for an intake leak, but you are right it will not work with the fan turning so you have to pull the fan belt and just run it for a short time and check both manifolds or other sealing areas.

[andk5591]

Ya know - I have a stethoscope and forget that I have it... Thanks for reminder
_________________
D-Dubya Manx clone - 63 Short pan,1914.
Rosie 65 bug - My mostly stock daily driver.
Woodie 69 VW woodie (Hot VWs 7/12).
"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
There's more, but not keeping them...

[Q-Dog]

I hope you don't charge the customer for all that wasted effort? PM me for where to send my consulting fee.
_________________
Brian

'69 Dune Buggy
'69 Beetle Convertible
'70 Beetle

[oprn]

My experiance with a Pertronics is that it worked... until it didn't. Which was about 5000 miles.
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[txoval]

I agree with Evan, backfire on decel is a lean condition, which is easy to fix with EFI and programming...

[veltror]

[evanfrucht]

[Ohio Tom]

Hmm. It has always been my experience that backfire on decel is the idle mixture screws are set too RICH.

So fuel is pulled thru due to the higher vacuum and goes bang when it hits Oxygen.

I used to do this on purpose in Highschool.
I would set my bugspray mix screws 1/4 turn fat. Big ol' flames shooting out on decel.

[evanfrucht]

[andk5591]

The only thing I had not addressed on this car (but the owner, who is a mechanic will take care of) was too much accel pump shot. I spent too much time on this as it was. I am 99.9% sure they did this to compensate for lack of advance in the original (wrong) distributor they had on it. I noted that it was maxed out when I cleaned the original Solex carb and fixed the binding throttle plate, but did not adjust since I was not sure what would happen. And the carb is a huge pain in the ass to remove because of the body style. There is a bit of a bog and smoke when when nail it, which is indicative of this. Otherwise, the car runs better than ever.

OMG - you should have seen the wiring that was in this - The owner got this car from a relative who was NOT a mechanic. It was one of this "well this quit working, so lets just run another wire." Had a second ignition switch installed since they couldnt figure out how to wire the early 70s ignition switch (3 wires - how hard could it be). Foot dimmer was on the tunnel, 4 ways worked only if the left turn signal was on.... Just a general rats nest. The only thing we did not fix was the horn. Had an aftermarket steering wheel and was missing the horn button components, se we just left the push button on the dash.
_________________
D-Dubya Manx clone - 63 Short pan,1914.
Rosie 65 bug - My mostly stock daily driver.
Woodie 69 VW woodie (Hot VWs 7/12).
"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
There's more, but not keeping them...

[oprn]

I have a school bus that Dad and I converted to a camper back in the 70's that always back fires on deceleration. In fact if you shut the ignition off for a second or two and turn it back on, it would go off like a cannon. I did that once too often and split the seam on the muffler wide open from front to back!
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[andk5591]