Pop Backfire through exhaust on decel and between gears

[tristanblue]

I have 70 bus with rebuilt engine we installed. The engine is a stock 1600 Single Port from 1968 bus. Has a stock 30Pict1 carb and 009 distributor. Valves just checked and fine.

While out driving the bus it runs great on cold start up (auto choke closed) with zero issue. But once she gets warmed up she will give a loud POP between 1st and 2nd gear and will make some rumbling popping noises when decel in gear and throttle closed.

-009 is set at about 28-20 total advance.
-I tried increasing the main jet size and no change to the popping
-I heard could be an excessively lean condition. I am not seeing any leaks but what techiniques should I employee to check for lean condition?
-I do have the stock single pipe bus muffler on the engine but I do NOT have the tail-pipe/resonator installed....could THIS be causing the issue.

[Maxpower]

I would look for exhaust leaks

[evanfrucht]

Your idle is set to low. Been there.

It might sound funny but trust me, turn in the idle speed adjustment screw a little and it should fix it.
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1967 Bug ( the daily rod )
1964 Fury Wagon ( the pavement shredder )

[Ohio Tom]

Classic signs of the idle mixture screw being set too rich...

[c77owen]

[tristanblue]

-Exhaust leaks, brand new exhaust and J pipes with German gaskets, not a single leak...BUT I did not have the bus resonator pipe and tail piece on hand. So it just has the single exhaust port open with NO tail pipe. Could THIS be a cause?

-Idle too low, idle is on the nose. I did try increasing very high and still pops.

-Idle set too rich comment, I have her tweaked in right where she should be. Could it be too large an idle or main? Not sure if the too rich could be it as she runs great when engine is cold and choke is closed. Would seem like it is a lean condition or vacuum leak possibly but I am could be wrong.

[c77owen]

Lean popping typically occurs during acceleration or steady light load. It could be that your jetting (specifically idle) is too rich and loading the exhaust with fuel which in turn fires or "pops" when you let off and it is hot

[evanfrucht]

If you are certain it's not the tune then possibly something is going on with the exhaust.

It could have something to do with the resonator tip. In my experience they do change the exhaust tone significantly and maybe provide some sort of back pressure that helped the stock engine maintain proper fuel mix ratio on decel. Not really sure but just an idea.
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1967 Bug ( the daily rod )
1964 Fury Wagon ( the pavement shredder )

[bastardbus]

I have "heard" the resonator and extremely small single tip on the stock bus muffler had something to do with back pressure but can not recall specifics.

The tone with out the resonator is extremely throaty almost like a header sound. I took a new beetle tailpipe I had on hand and put it over the hole while engine was running and AMAZING the sound difference. The problem is that tail pipe is much larger and will not fit in the the hole so I can't get her on the road and test it out. Guess I will have to wait til the pipe I ordered get's here. That being the least amount of work I will try first and proceed from there.
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Samba Member #95

[VW_Jimbo]

It is running too rich AND has an exhaust leak. That’s what makes the big “Bangs”!

To find it is a smoke fest, but it will show you where your exhaust leak is. Mix some brake fluid into some water. Get the engine to normal operating temperature, THEN rev it up high a dribble that mixture down the intake throat. Keep the idle up and go looking for the smoke coming out of the exhaust.

Do this test outside in fresh open air. Tell everyone that you are going to smoke the neighborhood out, so DO NOT call the fire department. Because THEY WILL CALL!

Good luck! Hope you find it!
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Jimbo

There is never enough time to do it right the first time, but all the time necessary the second time!

[bastardbus]

There is no exhaust leaks, 100 miles on rebuild with all new german exhaust EXCEPT I am missing the resonator-tail pipe which could be the issue. But it is not a leak so to speak. The tail pipe just arrived today so will be testing that to see if it is the cause.
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Samba Member #95

[evanfrucht]

Try the exhaust tip first that is most likely it

Could try a different idle jet size? and/or verify the current size

Where is your idle mixture set to now? How much have you tried adjusting it?

Could try adjusting the mixture screw leaner and then back off the idle speed screw. If that doesn't work try the opposite, as in, adjust the idle richer and then turn up idle speed/open butterfly to compensate. Could be an ignition advance issue too. Also if it has a vaccum advance make sure that is working.
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1967 Bug ( the daily rod )
1964 Fury Wagon ( the pavement shredder )

[tristanblue]

The resonator/tail pipe came today and just installed and took it for a drive....WOW! That was it! I am completely stunned at what a difference just that little resonator makes in how this thing sounds and runs

Not a single pop, fart, backfire or odd noise and it absolutely purrrrrs like a kitten. Unreal a stoopid little resonator would make THAT much difference.

Hope this info might help someone else down the road!

T