Boooom ! Back fire !!!! 760 timing

jonny waters

jonny waters

Location
Miami, FL
I know there’s probably a couple threads on this already but I can’t really get a definitive answer. Ok so here’s the deal. Just blew up my previous 760 cuz it over heated from a clogged water line. Just finished rebuilding it from the crank up. Went to start it finally and it fired for about 2 to 3 seconds and then all of a sudden BOOM ! Backfire. ( made me spill my beer ! All good tho I had more). Tried to start it again and wouldn’t fire. Poured a little fuel down the carbs and boom again. Yes! Everything is put together the correct way. Pistons are facing the correct way, carbs are pulling fuel and retiring beautifully. Only real question here is the timing. Mods to my sj are as such
760 ported with 185psi
Dual 44s with reed spacer
Factory p pipe
Msd with 61x full electrical
. So my real question is. What is the correct position the stator should be in for the correct timing ?


***** if you have any other ideas on why it would backfire, please feel free to go at it.
 
S

Storbeck

Location
Minnesota
There may be some small amount of performance to be gained by slightly more or less advance than the "stock" setting with the combination you have, but if the timing is stock then that's not the reason it's backfiring. Something else is wrong.
 
jonny waters

jonny waters

Location
Miami, FL
Storbeck said:
There may be some small amount of performance to be gained by slightly more or less advance than the "stock" setting with the combination you have, but if the timing is stock then that's not the reason it's backfiring. Something else is wrong.
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Yea there we go. So even if the timing is off (advanced, stock, or retarded) it will still start? Well that leaves me with everything else to go back though.
 
TheBuzzard

TheBuzzard

Location
the jetski grave yard
a backfire is unburned fuel and air mixture in the exhaust pipe/waterbox that gets ignited. it doesn't mean there is anything wrong with the ignition or timing or anything. it means you had fuel and air vapors floating around in the exhaust system that ignited. if it constantly backfires instead of runs right, yea you got an electrical issue.
 
jonny waters

jonny waters

Location
Miami, FL
TheBuzzard said:
a backfire is unburned fuel and air mixture in the exhaust pipe/waterbox that gets ignited. it doesn't mean there is anything wrong with the ignition or timing or anything. it means you had fuel and air vapors floating around in the exhaust system that ignited. if it constantly backfires instead of runs right, yea you got an electrical issue.[/QUOTE

Yea that makes a lot of sense. Just guess I gotta go through it again to see why it doesn’t wanna start and why it wants to do that big back fire.
Click to expand...
 
bored&stroked

bored&stroked

Urban redneck
Location
AZ
Swap out the MSD. I just finished this after blowing my waterbox to pieces cranking my new engine. Runs great with the MSD out.
 
Harbor

Harbor

Location
Stockton
You might verify your carbs aren't dumping fuel into crankcase then to exhaust somehow, ei new seat partially open and tank pressure.

You can check your cranking timing with a cut msg cover, find and mark cyl 1 TDC on flywheel and cover or your case and check while cranking with a timing light. Shouldn't be more than 12 ish degrees or 3.5 teeth worth from your tdc mark. (counting top of each tooth, top of first tooth is 0, top of second tooth is 5ish, top of third tooth is 10ish, top of 4th is about 15 or so....

Last one I did that backfired was more than 60 degrees out, very obvious with a light...

Cheap eBay china stator just bought was the cause on that engine.

Woodruff key sheer mentioned already,

Edit: helped another guy, backfired on first start up and sheered the woodruff, no start and backfiring, nothing wrong just the first backfire sheered the key...

Edit: if your facing the engine while checking your timing with a light, the TDC mark on the flywheel should move to the left side of your case/cover TDC mark for timing that's advancing.... (intake side) ........if it's retarding the flywheel TDC mark will be on the right ( exhaust side) of the case/cover TDC mark....
 

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jonny waters

jonny waters

Location
Miami, FL
Harbor said:
You might verify your carbs aren't dumping fuel into crankcase then to exhaust somehow, ei new seat partially open and tank pressure.

You can check your cranking timing with a cut msg cover, find and mark cyl 1 TDC on flywheel and cover or your case and check while cranking with a timing light. Shouldn't be more than 12 ish degrees or 3.5 teeth worth from your tdc mark. (counting top of each tooth, top of first tooth is 0, top of second tooth is 5ish, top of third tooth is 10ish, top of 4th is about 15 or so....

Last one I did that backfired was more than 60 degrees out, very obvious with a light...

Cheap eBay china stator just bought was the cause on that engine.

Woodruff key sheer mentioned already,

Edit: helped another guy, backfired on first start up and sheered the woodruff, no start and backfiring, nothing wrong just the first backfire sheered the key...
Click to expand...


Right on brotha. Thank you so much. That was a very helpful. I’m going to get into it after I get back from Bimini and I shall let you know how it turned out
 
bored&stroked

bored&stroked

Urban redneck
Location
AZ
jonny waters said:
Msd has never given me an issue before. And it was running beautiful right before the rebuild. The msd wouldn’t go bad from just sitting for a 3 weeks
Click to expand...
I'd say your right, except that's EXACTLY what happened to me. Worked fine for years. Did top end rebuild. Tried to start rebuild. MSD bad.
 
jonny waters

jonny waters

Location
Miami, FL
Ok guys update on the ski. Finally got back from my trips and had time to figure it out. Took of the flywheel and everything looked great. Didn’t shear the woodruff key. Looked to still be in perfect stock timing. So i ended up taking the carbs off and saw that I must’ve mixed up the diaphragms that I had on the bench. I had one with a red little nipple and one with a silver one. They were obviously not the same size. So put both the silvers ones back on and she fired right up.
 
jonny waters

jonny waters

Location
Miami, FL
Harbor said:
You might verify your carbs aren't dumping fuel into crankcase then to exhaust somehow, ei new seat partially open and tank pressure.

You can check your cranking timing with a cut msg cover, find and mark cyl 1 TDC on flywheel and cover or your case and check while cranking with a timing light. Shouldn't be more than 12 ish degrees or 3.5 teeth worth from your tdc mark. (counting top of each tooth, top of first tooth is 0, top of second tooth is 5ish, top of third tooth is 10ish, top of 4th is about 15 or so....

Last one I did that backfired was more than 60 degrees out, very obvious with a light...

Cheap eBay china stator just bought was the cause on that engine.

Woodruff key sheer mentioned already,

Edit: helped another guy, backfired on first start up and sheered the woodruff, no start and backfiring, nothing wrong just the first backfire sheered the key...

Edit: if your facing the engine while checking your timing with a light, the TDC mark on the flywheel should move to the left side of your case/cover TDC mark for timing that's advancing.... (intake side) ........if it's retarding the flywheel TDC mark will be on the right ( exhaust side) of the case/cover TDC mark....
Click to expand...



Thank you for all your help. All your suggestions ended up helping solve my issue. I just explained right above this post what I think was the issue
 
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