98 yamaha 760xl won't rev past 2800rpm

[tmarch]

New to the forum with an issue thats driving me crazy. I have a 1998 yamaha 760xl. Last year it lost power and I thought it was a fuel problem so I pulled the carbs and rebuilt them which didn't help. Pulled it back out this year put in a new battery, changed fuel filter, bypassed fuel selector valve(read on a forum) new plugs, started fine, put it in the water and it won't rev over 2800ish. Idles fine, but when you start to throttle up it wants to die. Thought maybe I had water in the tank so I cleaned it out put new fuel in w/some seafoam same problem. Seams like I have good spark on both plugs but it will fire right up with the front cylinder plug out but won't with the back one out so I figured front cylinder carb must not be pumping fuel. Took the carbs back out went through the front one (which was full of fuel) restarted it and the voltage alarm went off which i thought was probably was from cranking it for about 20sec to get it reprimed. Idle'd it around for a few mins thinking the voltage alarm would shut off but it never did. Am I chasing the wrong problem? Should I be checking the charging system and would this make it run slow? Going back to the lake in a couple of days with my meter to see if the battery is charging and look for any loose wires or bad connections. What other tests should I do while I'm there? Thanks for any help

 

[Dr Honda]

Yamaha can suffer from the same issues that our Seadoo's, have. I'm thinking you have a regulator issue, and it's either over volt, or passing AC. When that happens, the CDI will do strange things.

So... try this... charge the battery, and then disconnect the voltage regulator. You won't be charging... but if the problem goes away... then replace the reg.

 

[Dennis]

You may have more than 1 problem. I would test compression. Should be 150 psi. The 760 was known for head gaskets blowing.
Also, if it is overheating then the computer will go onto "limp mode" and won't let the engine rev. :cheers:

 

[tmarch]

Had a few minutes with it the other day. Put a voltage meter on the battery it was reading 12.08v started it up and ran it for a few minutes at 2000rpm and it might have dropped slightly just under 12v but it never increased. Should this be charging over 13? Did'nt have a chance to disconnect the voltage reg or compression but will do so soon. By the way where is the voltage reg. located? Also it was there were no loud beeps when I put in the code to unlock it.
thanks

 

[Dr Honda]

yep. You should be seeing a little over 13v at around 5000 rpm.

Sounds like you may have a few electrical gremlins.

 

[tmarch]

Just checked compression. 25 in the front cylinder and 130 in the back.

 

[RutlandRacing]

thats not good, its pretty much running of the rear cylinder

 

[Dr Honda]

Yep........ time for a top end.

 

[Dennis]

They are known for having head gasket issues ( as I stated in post #3). Remove the head and take a look. Hopefully it just needs a head gasket and the pistons/cylinders are OK.