High End Power/Carburetor Issue? 82 Yamaha 750

Yahoo Answers won't help me on this issue..

I've got a 31 year old, 1982 Yamaha seca xj750rj that idles and runs great up until 4500rpm, where the techometer will jump to 6 grand and the engine doesn't have the power that it should. I think there's a little problem with the gauge because the engine doesn't jump with it, it revs up fine. at the same time when I get to 4500 the exhaust gets louder and sounds sorta hollow, like long "whomp".
It also jumps from 7 to 8 grand. not the engine, just the techometer. I did a compression test on cylinders, hot and cold. They all read about 60psi which I find odd. New plugs, oil, and oil filter. Planning on getting new plug wires.
I can't afford to put money into this bike but I can fix a lot of the stuff on my own. Some people said it's a carburetor problem, which is fine I can do carburetors, but I'd like some more opinions first because those carbs are a real pain to get out.

Also, not really related, I've got it's little brother, a 1982 Yamaha Seca xs400rj. This is a 2 cylinder that i've had now for a couple years and has the same problem but on a minor scale. when you get to about 5000 it has about the same power as lower rpms but when I get to 6.5 or 7 it'll haul a$$. I wouldn't doubt that if I drag raced these two bikes (not saying I will) they would be pretty equal, which obviously isn't good since one has almost twice the engine than the other one.

Does anyone have any ideas?
Carburetor, compression, or something else?
Do think it's the same problem on the 400?

Any help or suggestions are appreciated,
Thanks!

Here's my 400: 1982YamahaXS

I'd say a combination of things, compression sounds very low and if it's been sitting carbs are probably gunked up.
I had an "81" Seca nice bikes very little problems and very fast when running correctly. I my still have the service manual around, I'll look for it later today.

That's probably true, the guy I bought it from said it was sitting a couple of months, but he also said there were 21,000 miles on it. After purchasing I found that the odometer doesn't work!
I do have the owners manual, I'm not sure if that's different from the service manual but let me know if you find it!

Owners manual and service manual are 2 different animals...you'll need a service manual...here's a link... Yamaha Motorcycle Service Repair Workshop Manuals

Add a little oil in the cylinders and do another compression check,if pressure goes up it shows bad rings,could also be bad head gasket or valve seats...

If I'm reading right and the engine revs but the bike doesn't speed up that could be the clutch slipping...good luck...

Great thanks!

So if the clutch is slipping would that be a bad clutch or maybe it just needs adjusting? I found the adjustments but haven't messed with them yet, its a different setup compared to my 400, which I've messed with a lot.

Thanks again! I'll look into this stuff

Could be both....try adjusting it,if that doesn't work then you have to go deeper...

Well I played around with the clutch, that didn't change anything.

Out of gear it does the same jump as in gear. The tech shows a jump from 4.5 grand to 6, but the engine is steady.
In gear, it'll do it now matter what gear I'm in.
As the rpms go up, the bike does increase in speed, however it has significantly lower torque than rpms 4500 and lower.
I wanna say it's just a scrwy tech because it jumps but the engine doesn't. However at the point the tech jumps, I lose power/torque, leading me to believe the problems are related.

Hopefully tomorrow I can do another compression test, this time adding oil. If the pressure doesn't go up I believe that means a cylinder ring leak since I have overhead valves and just replaced the valve cover gasket.

ONE THING I FORGOT TO MENTION: At idle I have a clicking on the right side of the motor, cylinder 3 or 4. my neighbor who played around with these bikes a lot when he was younger thinks it's piston ring/seal. But I should find out after doing the compression test.

No, I'm 3 hours away from there, 20 mi south of Kansas City.

Tach jumping could be a few things too, like a fubared tack,bad ground, bad wires causing arcing and misfire or a cracked coil same results as bad wires....

I found the service manual for the FJ750,
if you still need it send me a private message (PM)

There ya go!!!If you're gonna keep the bike ,you need them....