Back fire is always timing, valve or spark. Magnetron conversion issues (I had one as well) are on older engines, but you will get no spark if the polarity is wrong.
Knowing you are fully cognizant of the operation and parameters of this engines, I hesitate to offer desk top troubleshooting. But you said you got shocked while using the spark tester, and that the fuel fumes from the tank ignited from spark.
Are you leaking spark? The last tec I did, I had several problems, but one of them was the fact that the magneto wire was sliced near the flywheel, and allowing a spark jump once in awhile.
I know you got this. It's disconcerting to have something "done" or rebuilt, then have to go back to the drawing board. Been there. Hang in there Pete.
Ooooh, this little baby ended up having way more issues. Sometimes it pays not to follow the rebuild instructions in the manual and do what your gut says to do.:facepalm: I tested the spark in the workshop with only an incandescent on, nice solid blue spark. No leaking. What caught on fire was the ether as this dumb @$$ forgot to drill out the vent holes on the cap after clear coating.....but read on.
Easiest quick check is use your other flywheel off the other engine, inspect the key while it is apart. You can do this swap in 20 minutes and you will have the shroud off replacing the rope so you are half way there.
Yep, did this along with a new key. Then I tried an aluminum flywheel that I knew had the correct polarity. Had plenty of spark with all three, no fire though.
youre close, double check the basics,
compression, how much psi?? 8000 series don't have much, maybe 60psi for a minimum
spark, you can get a zap but still have weak spark, will it jump a gap?
check the ignition timing.
try firing it up off a can of carb cleaner, that alone should rule out all but the carbie
cam timing correct?
Ah, Briggsie-San, you are getting closer to the issue. No go with a shot of ether in the intake port w/o carb attached. Don't have a true compression tester, but I have one with a rubber tip that I wedged in place....30 psi. Pulled the head to make sure valves were sealing, and checked lash all were good.
I remember the last Tecumseh I was working on, I didn't torque the flywheel down all the way. It was "tight"...but not the 55lbs of required torque that the flywheel key needed..and it was throwing my spark off until I torqued it down to the right spec.
You didn't mess with the cam timing did you??
Flywheels were at 55 lbs-inch. The Bellville washer is kind of tired, though. I ordered one from OldMiniBikes and supplier cancelled order. As to cam timing, that's where I was anxious. I eliminated everything on the outside first before I opened her up to find.....:shrug:.
Very true. I used to find a lot of flywheels that had kicked timing just a tiny bit. That was my comment on looking for a shift line on the key. I have never seen a Briggs flywheel that was on to tight, saw many of them that were too loose though.
.....metal shavings in the bottom of the sump. After many pulls, I noticed more oil than expected on the piston. Rotating the piston down showed the walls were getting scored by the rings. Per the rebuild instructions, I did not hone the bore beforehand. Ring gap was in spec as was the edge to piston groove clearance. Timing, btw, appeared to be correct. What wasn't happening was compression.
So my nice, clean and purdy 1964 Briggs ended up being toast. This was after 2.5 hours this morning.
The next several hours were spent building a "new" engine. I had a sleeved block and correct piston, so this engine got the usuals done plus a honing. Transferred parts over, made absolutely certain cam and crank were timed (a bearing crank, no less). Put on the white shroud, the gold tank and carb. Filled her with 19 oz of oil (20 overflowed), took her outside and after 10 "I really want to start" burps, she fired up.
Compression was the culprit in the end. Now all I have to do is clean the new block, paint it gold, and clear. I'm adding a new starter clutch and recoil spring for it when Amazon delivers.