77 FI intake backfire on start

[OG Velvet]

So I’ve had this issue for a year or so. But it appears to be getting worse.

About every 10th start I get a single backfire through the intake. It used to only blow off the hose from the S boot to the oil breather. Now it blows off the S boot, the oil breather hose and the hose to the aux air valve. There is a puff of smoke in the engine compartment. Bus runs and drives great. Like I said, it’s about once every 10th start.

77 westy
Stock 2.0 FI
Very recent valve adjust, dwell and timing (in effort to fix the problem)

What’s the problem?
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'67 13 Window
'77 Westy
'69 Sunroof Beetle
'65 Honda Dream

[honestcharlie56]

If it were me I would start by verifying an injector isn't leaking and then I perform a cylinder leakdown test. Please post the results.

[timvw7476]

spark plug / ignition wire lead failing.
any one of those eight.
^edit: if it's not the above leaky injector scenario^

[OG Velvet]

My oil smells like gas too. I will check my injectors.
_________________
'67 13 Window
'77 Westy
'69 Sunroof Beetle
'65 Honda Dream

[Wildthings]

Ethanol laced fuel can cause a buildup of gum on the valve stems which can cause them to hang up in their guides which can lead to backfiring or worse the piston hitting the valve. When I let an engine sit for a long time with ethanol fuel in it, I dump some upper cylinder wall lube into the fuel when I go to run it again.

[rottenkid]

A lot of people mention to check your ground strap and ground from the battery. My 78 bus would act like it would not want to keep going after starting. It would sputter and act like it was backfiring. I cleaned the grounds and applied die electric compound. That was some time ago, but I have never had a problem since.

[Amskeptic]

[OG Velvet]

[OG Velvet]

[Amskeptic]

[Abscate]

If your cold start valve is leaking excess gas into the boot that could be contributory. Pull it and check while cranking.
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.ssS!

[Randy in Maine]

YOu can get all of the injectors cleaned and calibrated for about $100 you know.

[SGKent]

Do a compression test cold and then one warm. See if the valves are all closing when it is cold.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[OG Velvet]

Thank you for the suggestions. I will get to work on them.

Unfortunately, my bus is now acting totally weird again. Now it wants to randomly shut off. Super bummer because I am going on a fishing trip tomorrow. Looks like I'll be sleeping in a tent instead...

These buses man... if I didn't love them so much, I'd hate them...
_________________
'67 13 Window
'77 Westy
'69 Sunroof Beetle
'65 Honda Dream

[SGKent]

try putting a jumper between the 12V+ and #15 on the coil and see if it keeps running. If it does you have a bad ignition switch. The switch uses a separate set of contacts when it starts. What may be happening is that occasionally it is running from the start contacts then it intermittently dies on the run contacts which creates the backfire.

DO NOT LEAVE THE JUMPER ON LONGER THEN NEEDED TO TEST THIS. If you put it on and walk away you will burn the points and cook the coil. I mean like put the jumper on and go start it. See if it continues to run. If it does remove the jumper and see if it dies.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[OG Velvet]

Problem is that this seems to happen after a few minutes. I just went out and started it up and ran it (not driven) for about five minutes and it ran fine. I will test more in a couple hours (I'm at work). I think it may be only happening once the engine is warm?
_________________
'67 13 Window
'77 Westy
'69 Sunroof Beetle
'65 Honda Dream

[OG Velvet]

[Wildthings]

Check to see what your fuel pressure does after shut down, there is a spec in the book. I am thinking that your cold start valve is leaking fuel into the plenum, excess fuel which gets picked up as soon as the engine starts thus leading to the backfire.

[THall]

[Wildthings]