LOUD "POP" when Restarting within 2 minutes. Why?

[Turk]

Happens about every time I restart my 1991 Toro with a 5HP B&S engine, usually within 2 minutes.

It has happened for just the past year. It runs on ONLY Non-Oxy gas since 1991.

What causes this?

 

[The motor guy]

probably just premature ignition. Since the engine is already extremely hot from heat soaking for 2 minutes, it practically lights off of the heat of the engine. So there is no where for it to go since the exhaust valve may be closed at that time so it will backfire out the intake.

 

[NateDN10]

Probably some unburnt fuel lingering in the exhaust. When the engine starts up again, the fuel goes boom! Could be because the engine is running a bit richer than it was before?

 

[Colt45ws]

I agree with the pre-detonation hypothesis.

 

[SatinSilver]

How many miles on the Toronado?

 

[bubbatime]

Originally Posted By: NateDN10
Probably some unburnt fuel lingering in the exhaust. When the engine starts up again, the fuel goes boom! Could be because the engine is running a bit richer than it was before?


+1, this is a sign of unburnt fuel in exhaust, probably time for a carb rebuild

 

[147_Grain]

Unburned fuel in the exhaust or a sticking valve in the open position when the spark ignites the fuel mixture in the combustion chamber.

 

[user52165]

Does this have a spark arrester and if so have you ever cleaned it?

 

[GreeCguy]

Originally Posted By: motor_oil_madman
probably just premature ignition. Since the engine is already extremely hot from heat soaking for 2 minutes, it practically lights off of the heat of the engine. So there is no where for it to go since the exhaust valve may be closed at that time so it will backfire out the intake.


+ 1 - and maybe some carbon buildup in the combustion chamber which increases heat. When was the last time you cleaned the motor itself of grass and dirt?

 

[Turk]

Thanks, guys. Kinda figured it was unburned fuel, but wanted others input. I was questioning if it has something to do with 91 octane + Seafoam used as a stabilizer in stored gas...

I spray down the engine every Fall. Oil never gets black. Carb never rebuilt & it runs great.

Air Filter gets dirty easy and I take it out and bang the dirt a few times a year.

Think I'll replace the air filter, then the spark plug, (as they're needed anyway...) then clean the carb.

EDIT: Pulled the plug and there's a lot of black carbon inside...

 

[Turk]

Update:

Replaced the air filter with a new one. Mowed the lawn. Did 3 hot starts and it did not POP once.

Small sample size, but it's a start.

 

[wag123]

Originally Posted By: Turk
Update:

Replaced the air filter with a new one. Mowed the lawn. Did 3 hot starts and it did not POP once.

Small sample size, but it's a start.


You found your problem. The air filter was restricted and the engine was running rich. That combined with the fact that the compression release works by holding the exhaust valve open slightly when you are starting the engine and that is how you got the backfire.