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]

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.

So I ended up taking the entire engine apart to rebuild, everything looks pretty good. Outer crank seal was perhaps bad; I also never used threadlocker to seal the engine when I was first putting both cases back together and the crank seals too.

HOWEVER!

I did notice while de-salting everything, that the exhaust manifold water bunge was clogged(the one that's underneath the manifold). Perhaps this was causing some sort of water back-flow? Or probably just unrelated to the loss of power at 7k rpm.

 

[mikidymac]

You don't use thread locker to seal the cases. ThreeBond 1184 is the stuff to use and you need to pressure test the engine after assembly.

 

[exoh_e]

You don't use thread locker to seal the cases. ThreeBond 1184 is the stuff to use and you need to pressure test the engine after assembly.

Roger, I will use 1184. Thank you MIKI! A man of wisdom. God I think I built two or three engines without it now. Take care! (Upon relooking at the manual it's 515 not 518.) Than saw your post from 2019 about this.

Will report back; for archiving users.

I see this mystery problem occuring alot.