717 backfiring after rebuild

[MAL]

Well, I blew it up last year. I wanted to get it on the water for the rest of the summer so I "temporarily" built it with questionable crankshaft. But when I tried to run it there was a constant back fire. I assumed that I had put the rotary valve in wrong and I gave up on it for the rest of last year. I built a different engine with a good crankshaft for this year. When I fired it up for the first time, it backfired just like the other engine. I am in the process of checking the rotary valve, but I don't think It's installed wrong. What else can cause constant backfiring? ignition timing? CDI? I am looking to make a list of things to check.

 

[azsxr]

Pistons in backwards? Arrow towards the exhaust? Been there and done that it sucked lol

 

[MAL]

Pistons in backwards? Arrow towards the exhaust? Been there and done that it sucked lol

I will check that tomorrow, but it would have to be a pretty big brain fart to do that twice when I know which way the arrow is supposed to go.

 

[MAL]

I couldn't wait till tomorrow. The arrow is pointed at the exhaust on both cylinders. I was starting to think that was the problem.

 

[MAL]

My rotary valve cover has consistent scratches across the surface nothing real deep but concerning. I did use this rotary valve cover on both engines. I think the rotary valve was correct, but I flipped it too many times to remember which way it was to begin with. It is correct now.

 

[MAL]

Rotary valve is timed correctly. I noticed that the rotary valve cover is covered in light circular scratches. some almost catch a fingernail. The valve clearance is just out of speck. at .015 So I need a rotary cover, but is that enough to cause backfiring?

 

[azsxr]

Rotary valve is timed correctly. I noticed that the rotary valve cover is covered in light circular scratches. some almost catch a fingernail. The valve clearance is just out of speck. at .015 So I need a rotary cover, but is that enough to cause backfiring?

I wouldn’t think so maybe cause hard starting Id check the flywheel and make sure the key didn’t sheer

 

[MAL]

I put it together and tried disconnecting the red wire from the regulator rectifier. It did not work...still back firing through the carbs I will check the for a sheared keyway in the flywheel tomorrow. Is there a way to check the MPEM?

 

[azsxr]

Swap it with a known good one?
I think Westside can test them?

 

[MAL]

Well...I guess I was over thinking this...it was just a bad spark plug. Thanks for the help.

 

[azsxr]

It sucks it took your time but nothing like a cheep fix.