General Yamaha 701/760 questions

[LaceyLeeLz]

Hello all, I recently got back into PWCs from not having any contact with them since childhood. I bought a 94 VXR Pro last year and absolutely loved it as it was, up until this year I picked up a WB2 with the 760 and between the two it's like a night and day difference. (Because one of them is running flawlessly) Having somewhat of an automotive background, I can tell there's something wrong with the carbs on the 701 - it only responds to half throttle or less and sputters almost like the pump is cavitating (but it sputters out of water as well) I'm looking for insight on whether (to the experienced wrenches in here) it sounds like a pop off, main jet, or oil fouling (or maybe something else)
Also, on a completely different stride - what's the difference between the 62T and 62X blocks/heads AND what makes a 760 different than a 701 besides for displacement and carb diameter?

 

[LaceyLeeLz]

EDIT: THE 701 STARTS RIGHT UP, RUNS GREAT, AND THROTTLES UP SMOOTHLY UP UNTIL IT GOES PAST HALF; WHERE IT STARTS TO SPUTTER.

 

[john zigler]

Hard for any of us here to say for sure. By your description, it sounds as maybe your spark is breaking up, like you are on the rev limiter early. This is a pretty common issue with the old Yamaha's. It can be caused by a bad over heat temp sensor, bad CDI, coil, stator coil assy, etc.

If it is running out of fuel, you need to go through the fuel system. Check/clean the tank, fuel select, replace the fuel filter, rebuild the carb and fuel pump etc.

I would start with a spark and compression test.

 

[Smokey72013]

Could be just like others said if could be fuel delivery it could be spark breaking up it could be a bad cdi or stator. Tighten the sparkplug gap a few thousand. Like what they run .028 of a inch? If so knock that down yo .020 and see if it runs better. If your cdi or stator is weak sometimes you can see a better running condition by resting vs changing parts throwing money at it

 

[Myself]

I've seen this several times on the Yamaha twins. Check AC voltage of the 'charge' coil while running. Slowly increase throttle and watch voltage as you rev it and it starts breaking up. If the AC voltage drops then the 'charge' coil is bad.

 

[LaceyLeeLz]

Hard for any of us here to say for sure. By your description, it sounds as maybe your spark is breaking up, like you are on the rev limiter early. This is a pretty common issue with the old Yamaha's. It can be caused by a bad over heat temp sensor, bad CDI, coil, stator coil assy, etc.

If it is running out of fuel, you need to go through the fuel system. Check/clean the tank, fuel select, replace the fuel filter, rebuild the carb and fuel pump etc.

I would start with a spark and compression test.

This sounds exactly like what I'm experiencing. The only thing that makes me think it's fuel related is: when I go from resting to full throttle without easing into it, it acts like it's bouncing off an imaginary rev limiter. However, I know the battery is bad, and the stator doesn't produce enough power to keep it running unless it's hooked on the jump pack for atleast 15 sec. Could the stator attempting to charge a bad battery affect the ignition system?

 

[LaceyLeeLz]

Could be just like others said if could be fuel delivery it could be spark breaking up it could be a bad cdi or stator. Tighten the sparkplug gap a few thousand. Like what they run .028 of a inch? If so knock that down yo .020 and see if it runs better. If your cdi or stator is weak sometimes you can see a better running condition by resting vs changing parts throwing money at it

It does seem to run better after letting it rest. Could a bad battery cause symptoms of a bad CDI / Stator? The battery is a known part that is on the fritz.

 

[Myself]

Once it's running, the stator powers everything.......BUT, trying to troubleshoot with a known bad battery is a fools errand.

 

[LaceyLeeLz]

Once it's running, the stator powers everything.......BUT, trying to troubleshoot with a known bad battery is a fools errand.

UPDATE:
Replacing the Battery, rebuilding the carb, and re-gapping the plugs to .024 (they were WAY out at like .05) worked and the 701 runs flawlessly throughout the rev range! Can't wait to get it out on the water and see what it's like to ACTUALLY ride it. Thanks all for the help and insight!