Poorly running XS650J.... where do i go from here?

I am helping a friend of mine get a 1982 XJ650 Maxim running again. The last time it was on the road was three years ago and supposedly ran fine, according to the previous owner, as it was tuned by a known decent Yamaha shop. There are even the plugs to prove that they used an exhaust analyzer to set it up!

The bike has 12,500 miles showing and is in excellent condition. Purchased a new battery, fully cleaned and rebuilt the carbs, set fuel level, changed the oil, and installed new fuel lines with a sintered filter.

When firing it up it started with full choke. Idled roughly for a few minutes, had to restart a few times, but settled in at half choke to 2k. The bike would not run on zero choke, so i brought the idle screw up a bit and it sat at 1200 nicely.

When attempting to give it a little throttle it would die. Restart. Half choke, a bit of throttle and it would stumble a bit and then rev up to 4k and then very very slowly float back down to 2k. After a bit, it would run at 1200, no choke. You could give it gas, and it would hesitate a bit and then zoom to 4k (it would go higher if pushed) and then float very very slowly down to 2k. Nothing i could do would bring it to a lower idle.

I am very familiar with two cylinders like the XS650, but the XJ bikes are a bit new to me. So far to me, it seemed like it could be valve or piston related on top of carb syc.

I was able to drive around the block with it, the idle was oh so slow to come down and really kept me going at a steady speed. It pulled well under load and really didn't stumble though the rpm range. I ran the bike up to a steady 4k, cut the engine, and coasted back to the garage.

I ran a compression test. All plugs out and grounded. Engine hot. Compression was pretty consistent. Cylinders 1 and 2 ran 90psi, while 2 and 4 ran 100psi. I also repeated the test with a bit of oil in each cylinder, it did not change the compression readings.

The plug on cylinder 3 did look pretty black, while the others looked nice.


So from my estimations it seems as the piston rings are still solid. Unless this is really low compression for this bike??? I couldn't find any references for what they should be....

Am i facing "simple" carb sync? Are the valves out of adjustment? Are the piston rings shot? If you could give me a little direction it would be much appreciated.

skott!

 

Two things immediately popped into mind while reading your post that I would check: vacuum leak and carb synch... well, actually there is a third: check your throttle cable for proper action and lube as needed.

Hope that helps!

 

Compression readings are low. New is 156 PSI.

 

Skott, with 90-100psi showing on 12K, there is enough evidence to suggest this engine did not receive proper preventitive maintenance. You should be seeing 125+ across the board. I would offer two possiblilities:
1. P.O. ran the motor with low oil levels and the rings and bore aren't very happy.
2. Sitting for three years can cause rings to stick easily.
My bet is on the sticking rings if you find the rest of the bike is in very good condition (and at 12K it ought to be if it wasn't left out of doors). Check your alternator brushes, if they are near 11mm, I would say it justifies the sticking rings. You changed the oil, run it a while and see if the numbers don't improve. You could also run a bit of SeaFoam through the crankcase to clean things up a bit before you change the oil (Don't ride it with SeaFoam in the oil). Best of luck.

 

Gamaru: There is definitely not a vaccuum leak, i did check for that right away. The cables are also freshly lubed and not binding anywhere. For the sync, looks like i will have to build that YICS tool first. I have a manometer though

Robert By run it a while, how much do you mean? up and down the block twice, or a hundred miles? I don't want to foul too much up. With proper amounts of oil, can i mess anything up? I will check the alternator brushes tonite though.

Does anyone have a good link with diagrams on the carb sync procedure for this bike. The whole YICS thing is blowing my mind right now.....

 

A couple of hundred miles should do nicely. I consider 300 miles a good flush point. That way the bike has had several heat/cool cycles to work the sticky parts loose. I have successfully used Marvel Mystery Oil in my efforts to free up sticky rings in the past with no discernable downsides.
Good diagrams are available in the XJCD and the service manuals that are readily available. A book and/or the XJCD should be the first "tools" you procure for the effort of maintaining your bike. They will pay for themselves the first time you use them.

 

Tell me more about this mystery oil? How does one use it????

 

Here's a product write-up...

 

I don't see anyone mentioning valve seals/seats. If there was no difference, the valves aren't effected by the oil added to the cylinders like the rings are.
Tight valves, worn guide or seats. The seals probably make more difference on the fouling alone that compression numbers.

Of course, if you only added 1/2teaspoon, there wont be any noticeable change. I think 1 to 2 tablespoon should be plenty.

Steve

 

Before you start Engine Cleaning ... you need to consider?

Did you run the Compression Check with Wide Open Throttles???

Do you know if the Carb Diaphragms would PASS a "Clunk Test?"

Search: Clunk Test
Author: RickCoMatic

I'd refinish those Bores and them Polish them ... You'll get your accurate Throttle Response Back and the Bike will run nice if you forgot to do W-O-T's for the Compression Check!

 

RickCoMatic: I did not check compression with WOT!!!! DOH!!! I will warm up and redo soon and post the results. It was 2am before i got to do the compression test and was already a little tired. I only added about a teaspoon of oil. The manual called for "a few drops"

I know for sure the carbs pass the Clunk Test. I read your entire post about cleaning these carbs before i dug into them. They did not prior to cleaning though.

I will retest a bit later and post the results.

 

Replace the sparkplug, check the wire loom for bad connections. It can't hurt to go through the forums here and recheck everything you find, where symptoms are similar.

For your other question on carb sync check here:

Carb sync and colortune info