Cant get new SBT 720 to start

[kmendo88]

Little backstory - Picked up identical '97 GSI's at the start of summer 2020. One of them ran great for two weekends and then lost all top end on the third. Took it to a ski mechanic, pressure tested it and said it needed a rebuild. Attempted to disassemble the motor and it was a saltwater corroded brick that needed some serious equipment to salvage.

Fast forward to now and we got a fresh SBT rebuilt 720, got the ski put back together with new fuel lines and can't get it to start. Plugs looked good but replaced them anyway, both appear to be visibly sparking and getting wet with fuel. Between air, fuel and spark seemingly not an issue, I'm wondering if there could be an issue with the timing or rotary valve? I lined up the stator timing mark with the mark on the case as per the manual and followed SBT's YouTube video for the rotary valve and felt like I got them both pretty dead-on.

My concerns with the rotary valve / cover is that they looked pretty scored coming out of the old motor. However, SBT lists this particular RV Cover as out of stock on their website so I didn't have an option to get it fixed. I ordered a new Rotary Valve but installed the old one since I was never able to fix the scoring on the cover and figured the ski didn't have problems starting prior to the rebuild. I've read that mating a new RV to a scored cover could actually worsen the problem.

Any thoughts you guys might have would be much appreciated.

 

[NickCawks]

Did you rebuild the carbs?

 

[Ckrawiec]

Have you tried removing the plugs, squirting some pre-mix directly into the spark holes, putting the plugs and wires back in (hand tight) and firing her up? If she kicks / runs even briefly, then I'd go into the carbs as your most likely suspect. Fuel delivery is the most likely problem.

You have verified that you have spark - YES?

Have you checked compression in both cylinders?

If you have spark and fuel and compression.....even if the timing is slightly off, she should fire on pre-mix directly in the holes.

 

[kmendo88]

Did you rebuild the carbs?

This one is just a single carb and it was rebuilt last season before the last trip out when it lost compression.

Didn't compression test it. I'll try adding some premix directly in and report back.

 

[Ckrawiec]

This one is just a single carb and it was rebuilt last season before the last trip out when it lost compression.

Didn't compression test it. I'll try adding some premix directly in and report back.

Ah....did the re-build involve genuine MIKUNI parts? If it a'int factory parts and specs....game over.....the carb(s) are the most finicky beasts.....they HAVE to be spot on and getting AM carb kit is a fool's errand.....many have tried and failed...myself included until I knew better. It's not worth the fight....go Mikuni or go home! Good luck with her!

 

[kmendo88]

Ah....did the re-build involve genuine MIKUNI parts? If it a'int factory parts and specs....game over.....the carb(s) are the most finicky beasts.....they HAVE to be spot on and getting AM carb kit is a fool's errand.....many have tried and failed...myself included until I knew better. It's not worth the fight....go Mikuni or go home! Good luck with her!

Honestly Im not sure. If I have to rebuild it again, I will.

Anyway, tried the direct premix into the plug holes and got nothing, just kept turning over from the starter. I wasn’t sure how much so I did about 8 drops with a dropper in each cylinder with fresh premixed 87.


Edit: So I watched a video of a guy putting together the rotary valve on a 720 and in that video he said to set it to 147 degrees. The video I watched from SBT set it to 130 degrees. Which one is correct, or does it depend on your engine? I never knew it varied and now I think I may have found a potential issue.

 

[mikidymac]

Factory service manual has the procedure for your year and engine, follow that not SBT or YouTube.

Yes, rebuild the carb with genuine Mikuni parts if you want that new engine to last.