Stumped beyond all reason, 720 cutting out at 5-7k rpm

[exoh_e]

Hey everyone,


A while back, I posted a video of my '95 XP. I got an engine from a GTI—it only had about 10 hours on it, but it was seized because a small nut got stuck in the crankcase. I removed the nut, put it back together, and installed it in my '95 XP.

The problem: It revs nicely up to about 5500 RPM, then just dies and bogs back and forth.

What I've tried so far:

• Swapped MPEMs—no change.
• Tried a different set of carbs—same issue. (Two different set's of Duals.)

Recently, in my Twin 720 Challenger, one engine had a small screw come loose and strip out the rotary gear (I think). So, I swapped the engine from my '95 XP into the Challenger. I also swapped over the single carb from the old Challenger engine.

Same exact problem: Revs up to 5500-6500 RPM, then cuts out and bogs back and forth. (I also swapped the regulator from the broken Challenger engine before this.)

I figured maybe it's the stator? So, I took the stator and magneto from the broken Challenger motor and swapped them in. Still the same issue!

It's weird—they both climb nicely to 4-5k RPM, then the good engine keeps going steady, but this one starts acting erratic it'll hit 7k rpm and cut off(now).

At this point, I've ruled out:

• Carbs (tried three different ones)
• MPEM
• Stator
• Rectifier
• Regulator

Has to be the crank, right? But oddly, no oil is leaking anywhere.

Mind you before this, before the other engine broke the boat was running "well" enough.


Any ideas or advice? Thanks! I've never felt so defeated from a damn engine problem. What a time sink

 

[mikidymac]

Check compression just to be sure.
Also check rotary valve timing and valve cutout angle.
It sounds like you have ruled out anything electrical or fuel related so all we have is mechanical.

 

[exoh_e]

Check compression just to be sure.
Also check rotary valve timing and valve cutout angle.
It sounds like you have ruled out anything electrical or fuel related so all we have is mechanical.

Unfortunately I painted over the old SKU marks to see the model engine. I also have limited knowledge to the old engine(picked it up on facebook marketplace for a deal). But yes double confirmed timing with two different valves. Thanks for the quick reply.

 

[exoh_e]

Put some paint remover on it.