xj650j carbs on a xj650g motor - any problems?

I have been having poor mileage since I got the bike running well. I completely rebuilt the carbs and did the valve shims to spec. Bike fires right up and runs good, choke needed when cold but when it's warm lights right up. I feel like most of the time, power is good.

But I'm getting between 20-25 MPG. I've checked the brakes, bearings, etc. Can't figure out what's up. I do know that I have an engine from a different bike in there, or at least part of it - the serial number on the frame is for a 1982 xj650, but the serial on the engine is for a 1980 xj650. The carbs that are on there, including all the adjustments made to bring them into spec, are for a 1982 650.

Could any of that be related to the poor fuel economy? I'm going to do one of two things very soon:

a) pull the motor and replace it with a correct year motor of lower mileage (assuming I can find one) or rebuild this motor completely.

-or-

b) sell this bike and pick up something newer, preferably watercooled with fuel injection that will get me reasonable fuel economy.

Could there be something wrong with this motor? Compression was ok when I checked it a year ago.

 

What is OK? Do you recall the numbers? Or can you test again?

That MPG is low. There are no differences that I know of in 650 carbs through the years. YICS showed in in 82 I believe on the 650's but that's in the head and the carbs are the same. You can verify the jet sizes with chacal's catalog to be sure you have the correct ones. May also need to measure their opening. Do you possibly have fuel leaking into your crankcase or overflow?

A rebuild would be difficult to get all the parts for I believe.

 

Aren't we discussing this in another thread? http://xjbikes.com/Forums/viewtopic/t=20773.html For like a year?

The fact that you've got a 1980 motor in an '82 bike means nothing; everything motor-wise is 1980 spec and there really was no difference.

I went back and re-read your whole other thread and I have a new question.

Should I post it here or there?

 

my 81 said YICS on the motor.

for $200 you can have an auxiliary gas tank with all connections. i have 3 extra gallons i can pack if need be. giving me a total of 6.4 gallons.

 

Well, yes, there is the other thread, but this question is specific to... the title question. I wanted to put it in a different thread since, although it is related, is independent of the other issues. It is a general question. I agree, the motors are very similar, and the carbs from everything I've read are virtually identical. But I wanted to ask the community at large since my knowledge is limited.

Thanks for the responses!