1975 Bus - FI; BIG backfire upon start-up

[BellePlaine]

Last Friday afternoon I went to start the bus, which had otherwise been running fine, and I hear a big bang from the engine. One the first stroke, BANG! I go back to see wtf and white smoke was everywhere in the engine compartment. Nothing was on fire, thank God.

The S-boot dislogged from the throttle as well as most of the hoses attached to the boot. What caused this? I did notice that the wiper tracks has started wearing thin the right side of the wiper board. Could that have caused this?

[Amskeptic]

Last Friday afternoon I went to start the bus, which had otherwise been running fine, and I hear a big bang from the engine. One the first stroke, BANG! I go back to see wtf and white smoke was everywhere in the engine compartment. Nothing was on fire, thank God.

The S-boot dislogged from the throttle as well as most of the hoses attached to the boot. What caused this? I did notice that the wiper tracks has started wearing thin the right side of the wiper board. Could that have caused this?

Sounds like an intake valve was not fully closing. Engine been sitting around? Slow cold driving only?
Sounds like a sparkplug wire crossfire. All plug wires good and separated from each other?
Minor incident .....
Colin

[BellePlaine]

Last Friday afternoon I went to start the bus, which had otherwise been running fine, and I hear a big bang from the engine. One the first stroke, BANG! I go back to see wtf and white smoke was everywhere in the engine compartment. Nothing was on fire, thank God.

The S-boot dislogged from the throttle as well as most of the hoses attached to the boot. What caused this? I did notice that the wiper tracks has started wearing thin the right side of the wiper board. Could that have caused this?

Sounds like an intake valve was not fully closing. Engine been sitting around? Slow cold driving only?
Sounds like a sparkplug wire crossfire. All plug wires good and separated from each other?
Minor incident .....
Colin

I'm glad to hear that it's considered a minor incident. The bus seems to be running like it's old self now. To think about some of your questions: the engine had ran fine in the morning, on the 17 mile drive into work and it was a frosty morning. An hour before the backfire, I did drive it for two minutes to move it to another parking lot on campus. The plug wires are typical Bosch deals and only 3 years old. Since June, I'm running one NGK plug (B6ES, I think) and three Bosch W8CC (non-resistance).

[Amskeptic]

Last Friday afternoon I went to start the bus, which had otherwise been running fine, and I hear a big bang from the engine. One the first stroke, BANG! I go back to see wtf and white smoke was everywhere in the engine compartment. Nothing was on fire, thank God.

The S-boot dislogged from the throttle as well as most of the hoses attached to the boot. What caused this? I did notice that the wiper tracks has started wearing thin the right side of the wiper board. Could that have caused this?

Sounds like an intake valve was not fully closing. Engine been sitting around? Slow cold driving only?
Sounds like a sparkplug wire crossfire. All plug wires good and separated from each other?
Minor incident .....
Colin

I'm glad to hear that it's considered a minor incident. The bus seems to be running like it's old self now. To think about some of your questions: the engine had ran fine in the morning, on the 17 mile drive into work and it was a frosty morning. An hour before the backfire, I did drive it for two minutes to move it to another parking lot on campus. The plug wires are typical Bosch deals and only 3 years old. Since June, I'm running one NGK plug (B6ES, I think) and three Bosch W8CC (non-resistance).

Frosty morning? Add, clean and dry inside of distributor cap. "Only" three year-old spark plug wires tells us nothing. Anything can happen at any time, so do not preemptively forego an inspection. I have seen new new new wires tear when installed in the plastic hold down clips.
Colin

[vdubyah73]

I've had that happen on both air and water cooled engines. check timing for to much initial timing, check for sticking vacuum and/or centrifugal advance. did the back fire happen just as you released the key because you thought it was about it fire? I've done that too. just as the piston comes up to the squeeze and go bang position, you release the key the piston squeezes, the spark ignites at 7-10* before top dead center, the engine doesn't have the momentum to pas top dead center because you let go of the key a moment to soon. the engine fires off backwards fills the engine compartment with unburnt fuel air mixture and is hard to start after you settle down to try again. most probably minor, how does the engine turn over? does it drag down on compression, almost stop turning sometimes?

[Amskeptic]

I've had that happen just as you released the key as the piston comes up to the squeeze and go bang position, the spark ignites at 7-10* before top dead center, the engine doesn't have the momentum to pas top dead center because you let go of the key a moment to soon. the engine fires off backwards

Isn't that excellent? Your letting go of the key can act exactly like breaker points opening, firing off a spark from the coil. It should be noted that a sloppy ignition switch can do that. A tight switch will not give you that little momentary loss of 12 volts to the coil.

I like more retarded timing than many suggest, because I hate the thought of pistons trying to drive the engine backwards ... ever.
Colin