Back fire caused by carbon fouled spark plug?

[Vintage Engine Repairs]

Anyone else experienced it? I never thought it could do that, but this coil has timing advance (and retard) and at low rpm it’s not far off sparking at TDC so I guess weak spark didn’t ignite the charge well and it propagated slowly enough that it was still burning as the piston travelled back down allowing it to shoot back through the transfers?

Anyway, I checked timing via degree wheel and timing light, and was showing 10 degrees BTDC via a drill spinning it, which seems late, but this coil has ignition advance built in so i suppose not considering the drill doesn’t spin that fast. I checked it against another identical coil that had identical timing. Checked the spark arrestor, everything. Till I just realised I had nothing to lose and changed the plug… this is the first time ever that I have had a plug be the reason an engine wouldn’t run let alone back fire and it wasn’t even that badly carbon fouled. Plug was a Bosch wrsf6

 

[maisl]

Yes - had it (and not running due spark plug) a number of times on motorcycles. Sometimes the original will work fine once started with a different one.

Nippon Denso in UK late 80's were terrible for it in 2T & oily 4T; even brand new would regularly fail.

 

[Vintage Engine Repairs]

Yes - had it (and not running due spark plug) a number of times on motorcycles. Sometimes the original will work fine once started with a different one.

Nippon Denso in UK late 80's were terrible for it in 2T & oily 4T; even brand new would regularly fail.

Thanks for sharing, much appreciated. I’m going to hang that plug off the curtain rail in the workshop at eye level to remind myself of the time wasted!

 

[huskihl]

Less carbon with Dominator

 

[Wonkydonkey]

A friends generator (Chinese 6hp engine) started running all funny. popping/farting/dying/over reving etc.

I said could be timing probs or the plug, check the plug it’s easy., anyhow he rev'd the nuts out of it and it worked ok for a bit after the popping and banging stopped.

The next time he wanted to use it he couldn’t start it. Lol. I came along and pulled the plug out, it was black and fouled as well as the gap being huge . I guess that was the carbon pre igniting the fuel before.

I cleaned it and set the gap and it worked as it should, but said maybe a good time to give it some love and change the oil

I explained about engine type and it’s not got an oil filter and oil getting contaminated and losing its lubrication qualities and thus more engine wear…and maybe that’s why the s plug fouled up


I guess he don’t love it that much cos he hasn’t bothered with the oil change

 

[5155]

198? Wheel horse Kohler. 3000 hrs.
Started back firing on hot shut down.
Removed head, carbon coated with chunks.
Soaked and cleaned, fixt. Changed oil to,,,,
clean oil.
This was after an AM carb for 15 mins, Then the oem carb got soniced and a white "pill" removed.