'79 CA FI, Startup back fire

[TonyFio]

'79 Super, CA FI
Symptom:
Typically, this only happens when the engine is cold. Occasionally when starting, we get a backfire that causes the large rubber elbow hose to the Aux Air Valve to go flying off (it hurts when hit in the wrong spot).
Once warmed up, it will start and shutoff many many times with no problem (no accel pedal given).

Question:
What might cause this?

Background:
This symptom has been there as long as we've had the car.
When we first purchased it didn't run at all.
Changes performed have made incremental improvements to performance but I don't think it runs as well as it should.

Work done (no particular order):
Valve adjustment (several times)
Replaced plugs, points, etc
Set/checked timing multiple times (28-30 btdc @ 3500 rpm)
Replaced fuel injectors
Replaced distributor with rebuilt
Fixed kink in fuel lines at gas tank
Replaced alternator (was binding causing mechanical resistance)
Checked vacuum, replaced lines
Checked fuel pressure
Checked various resistances at various test points (temp sensor, etc)
Verified that the cold start valve operates and does not leak

Also, while we haven't touched anything with the air/fuel mixture, the previous owner may have. The PO also may have replaced the catalytic converter.

Any ideas would be greatly appreciated!

[busdaddy]

That 28-30 timing is with the hoses off the distributor, correct?

I'd try removing the cold start valve when parking overnight and bagging it to check for a slow leak. If that checks out OK put it back in and try unplugging it. And I'd also try an alligator clip test lead jumper between the #15 (+) on the coil and the D+ post on the alternator to rule out ignition switch issues, remember to disconnect that after or it won't shut off.
Try one at a time to narrow things down.
_________________
Rust NEVER sleeps and stock never goes out of style.

Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!

Слава Україні!

[jinx758]

Typically a pop out the carburetor means a lean condition/possible vacuum leak & a pop on deceleration is an exhaust leak.

I am COMPLETELY clueless on fuel injection in a VW bug.
U-tube for a cheap, easy smoke machine to check for vacuum leaks.

Problee no help ... stay safe

jinx
_________________
" It's not valuable unless you learn something from an experience. " Henry Ford

It's not unlike the same difference ...

My Craigslist rescued 100 footer :
1971 Standard Bug
1776cc dual port
034 distributor
38mm EGAS Carburetor

[volksworld]

basically you're describing what these engines did when they were new...the basic problem with early generation fuel injection was that it was set up very lean for emissions purposes, and as the engine got some miles on it and wanted a little more fuel, the system couldn't supply it...once oxygen sensors found their way into the systems they could self adjust and they became what we're used to today...what you basically have to do is make sure there are no vacuum leaks and ignition is good...the head temp sensor is what tells the computer to richen the mixture (longer injector pulses) when the engine is cold...you may have one thats resistance is "in range" but is still too low ....other "fixes" would be to open the air flow meter and readjust the flap a tiny bit to richen the mixture across the entire rpm range ...idk where you live or what you go through for inspection but this could fix your issue but you'd fail an emissions test...and don't firmly clamp that hose that keeps blowing off, if you do the backfire will bend or break the flap in the air flow meter...original ones had a blow off valve in the flap, but these also broke so they discontinued them

[busdaddy]

Yes, no clamps!

A smoke test is always time well spent, there's always a leak or ten: https://www.thesamba.com/vw/forum/viewtopic.php?t=620083
_________________
Rust NEVER sleeps and stock never goes out of style.

Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!

Слава Україні!

[TonyFio]

[TonyFio]

[TonyFio]

[TonyFio]

[busdaddy]

It could be either, I've suggested some simple tests to help narrow it down before getting into invasive stuff like mixture, etc....
_________________
Rust NEVER sleeps and stock never goes out of style.

Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!

Слава Україні!

[TonyFio]

[TonyFio]

It took awhile to get back to this issue. As always, life gets in the way...
Did the smoke test, no issues.
Did the cold start valve test. With it disconnected electrically and under fuel pressure, there is no leaking. Reconnected and it activated (fuel squirted into the plastic bag).
Our ambient temperature is about 85 degrees right now. Should it activate at this temperature?

[busdaddy]

[TonyFio]

[busdaddy]