Sharing my experience re LOW COMPRESSION on '81 XJ650 4k1 SECA

I only got the bike a few months ago and during my 're-commissioning' many things have needed to be done.

Some history.. bike was running very rich when I got it, valve clearance was on the loose side (yes, loose, not tight) and cam chain had enough slop to score the inside of the valve cover.

After getting carbs to somewhat 'Church of clean' (for the inside only, I did not split them) standards I got Valve clearance to within spec, adjusted the cam chain tensioner and re-set the idle mixture screws from 2 1/2 turns out to 1 3/4 turns out (as they are the coarse thread type) and vac synced.

One of the things I had to address was the low compression as initial compression tests showed: - 115 -95 - 100 - 120 on 1 - 2 - 3 - 4 - mind you, the bike seemed to run fine. After valve clearance was sorted the compression went up, but not enough. I squirted a bit of motor oil in the cylinders and retested - 3 and 4 went way up and 1 and 2 only had a slight increase. I decided to try the ATF/Acetone mix.. a week of rotating the back wheel in top gear, fresh oil and a warm up and retest yielded the compression numbers in red. All I can say is I am very impressed with how well this has worked.

Prior to using the mix I was SURE the next logical step would be a re-ring and re-seating the valves..

I am so glad that I tried the 50/50 mix as a 'maybe this will help' PHEW!

 

thats great news. a lot cheaper to try the a&a mix to unstick the rings and see if it works first than just pulling the top end off

 

So the mixture is 50% ATF and 50% Acetone and 1 tablespoon per cylinder?

 

I have not heard of this method for low compression, only for a seized motor. So now you have me thinking if it might work for my situation. I just completed restoration on a very low mileage '84 Kawi GPz900A1 (4600 miles). I got it running very well and feels like it has full power but cylinder #1 shows only 70psi where the rest are over 170psi. A leak down test indicates it's the rings. I'm also amazed you got such a huge jump with putting the motor oil in prior to retesting. I didn't get nearly those results (+10psi on #1). I was going to have the top end rebuilt but my mechanic suggested it just may need to be ridden. I am curious if those numbers will hold up. Did you put any real miles on the bike after the 50/50 mix or just a compression test afterwards? Will you be doing another compression test after you've put on some miles? Sorry for all the questions just really impressed by those results.
Thanks!

 

Cylinder 1 tested 60psi when I got the XJ650. Gave all cylinders a good shot of PB Blaster and cranked with the spark plugs reinstalled, let it sit overnight. Tested the next day and it was up to 120psi. Then poured in some ATF and cranked with the plugs installed then let sit. Then put in some 0w20 synthetic oil and cranked with the plugs. Tested again at 170psi, let sit a couple days for the oil to drain and got 130psi. I'll test again after I ride it for a few hundred miles, just got some Yamaha Ring Free fuel additive to try as well.

 

I think if your GPZ (collectible bike BTW) has been running, the rings have already settled in as good as they get. No soaking in anything will better the violence and heat of combustion.
If a cylinder is down after running you pretty much need to pull it apart to know what’s wrong.

 

Thanks, will be very interested in the results of next test. Could be useful knowledge on a future project.

 

I'm inclined to agree...just wishful thinking on my part I guess. Not gonna dig in any further, though, just runs too well and has amazing pull all the way to redline. I did have a similar experience with another bike of mine. I am the original owner of an 81 Suzuki GS550 that sat for several years. When I got it up and running it had around 100psi across all 4 cylinders. After several rides they were all up to 120+psi. But at least they were all much closer in spec than the Kawi.

 

Things can remain stuck in a running engine - how many hydraulic valve lifters have not been freed up by ATF over the years?! - so personally I would have a go before taking an engine apart.

 

I can agree with what you suggest, I suppose if the rings had carbon deposits stuck holding the ring a solution to dissolve it might work.
But if it was my bike sitting for amount of time I’d take a look for sure.
Old 4-cylinder bikes always have one cylinder taking in the atmosphere-
@Blackbird you did say cylinder #1 which could mean several years sitting on the side stand. I’m not suggesting your 900 was stored improperly. I’m just jealous

 

Tom Cruise top gun has immortalized your bike