Fixing low compression

1985 Maxim-X, showing low compression.

I'm guessing availability of oversize pistons, rings, etc. is nearly nil for these guys.

Looking for advice from gurus on how best to address compression issues with these motors? Should I just swap? Fixable if it's valves but not if it's bottom end? Oversize rings readily available or not? Sorry for being so nooby but, well, I'm a noob at these motors. The thing runs great except it's down on power and compression is low. I'm riding it now and it's fun but I know it's not anywhere near the performance it should have.

-iX.

 

First you should get your valves in spec then maybe a few drops of oil into each cylinder

 

Valves are done and in spec. I'll do the oil drop test tonight.

 

Keep in mind that the engine is shared with the FZ750. While there are cam differences, the lower end is identical (cylinder to case angle being the exception). This should help you to source oversized pistons and rings if they are needed.

 

Ah, good info, thanks.

Compression numbers:

Cylinder: Dry / Wet
1: 90 / 100
2: 55 / 70
3: 40 / 50
4: 70 / 90

I can't understand how it even runs if it' supposed to be 140?! It actually runs great, smooth, sounds good, just way down on power from what it should be. However, if nobody told you it's supposed to turn 12 second quarters and you had never ridden a fast bike, you'd think it was just perfect.

I'm not sure how to proceed from here. Any thoughts folks?

 

Your numbers are not coming up that much. Double check valve clearances the re-test

 

The numbers don't concern me so much as the spread across cylinders.
How long has the engine been in service ( or back in service since sitting)?

 

Briefly explain how you are doing the compression test.

Gary H.

 

Wide open throttle while you crank?

 

The engine wouldn't run with compression numbers that low..........

 

I admit, I never looked but when you open the throttle for the compression test, do the slides go all the way up?

 

Exactly.

Gary H.

 

Might it be wise to try compression test again with a different meter? Could be the tool is reading too low. Doesn't explain the difference between cylinders, though.

 

He could do a leak down test to see how that comes out.

 

Procedure
1. Remove all plugs.
2. Fuel tap to 45 degrees to block fuel flow
3. Disconnect ignition circuits
4. Screw gauge adapter in to test cylinder
5. Hold throttle wide open
6. Crank until gauge stops moving higher, write down number
7. Use pressure release button
8. Unscrew adapter from cylinder
9. Squirt oil into cylinder
10. Screw adapter in to test cylinder
11. Throttle wide open
12. Crank until gauge stops climbing, write down number
13. Release pressure
14. Remove adapter and repeat on next cylinder

This gauge has always been trustworthy and has a high-quality indicator. We fabricated an adapter (by gutting a spark plug) so that we could screw it in, we were not using a pressure-fit setup.

This engine has about 60k miles on it and as far as I am aware was never out of service for more than a winter.

It sounds good and runs well. It just doesn't have the power it should.

-iX.

 

By the way we did the valves literally last week. I am 100% sure that they are in spec, we checked them several times before closing up the engine. But most of the exhaust valves were tight prior to the service and I was afraid the valves might be burnt as a result.

 

You are probably right in the valves getting cooked. At 60 K it may need a good fleshing up or find another mill with less miles.

 

I think this has been discussed before, but when checking compression not only is there a set procedure, but the setup counts also. So, you need to be careful and not increase the already small displacement of the these engines. Fabricating a tool to screw in could do this, as well as not having a Schrader valve at the bottom of the hose that screws into the plug threads. I believe both of these situations could cause low readings.

 

Imo borrow a compression gauge with the proper adapter. Hope this helps.

Gary H.

 

You know... In my experience... engines of Japanese variety as opposed to the older Norton Triumphs of the 60's and 70's, very rarely need a re-bore unless there has been horrible maintenance and poor luck. I have taken apart engines with 100 thousand klicks on them, with no appreciable wear. What I have found is ring wear more than cylinder wear. So a re-ring is probably all you would need. If you use a straight cylinder hone rather than a ball hone, you could remove a thou or two on a cylinder, and even that would not really require new oversize pistons. I have often simply bent out the "ears" of the slipper skirt pistons to take up the slack. New rings would be in order. Also often I have found low compression to be a result of rings seizing in the piston grooves. Make sure you carefully remove then and to not break the rings. You might have to soak them in penetrating oil to remove from the pistons. Then carefully clean the piston grooves, and then replace with new or if you are on a tight budget, you can reuse the old rings. With the new surface via honing, the old rings as well as the new rings will seat again and return your compression to normal.

 

@mikeforbes:If the rings are sticking how can the cylinders make enough compression for his bike to "sound good and run well"? I agree with Chacal... if the numbers are truly this low his bike shouldn't be running. I agree with what your saying if the numbers are this low, his bike doesn't start, the plugs are oily wet and it's been determined the valves stem seals aren't leaking.

Gary H.

 

I have heard a lot of people who are unfamiliar with what a good-running XJ sounds like comment about how good their poor-running XJ sounds. The bike will run on two (possibly three) of the cylinders at startup, and could begin running on the rest as it warms. While I aso suspect that the compression test is not giving accurate results, I would not discount the thought that he just has stuck rings. Drop in a tablespoon or two of Seafoam or a 50/50 mix of ATF and Acetone. Let it work for a week, and then run it. That usually frees up stuck rings.

 

Get a can of PB Blaster penetrating oil, squirt lots and lots of it into each cylinder (after you pull out the plugs.) Let that sit for a bit. Put the bike on the centre stand and put the transmission in top gear. Now two or three times a day, rotate the rear wheel.... After a week or so you might be good to go... Otherwise as I find, a cylinder head and barrel removal is in order. I often am able to cheat.... If the head gasket comes off really nicely I use a permatex copper coat spray on it and reuse... The bottom barrel gaskets are usually made of tissue paper, so you would definitely need one.... Put new valve seals once the head is off. unless you don't have a spring compressor. You can take the head into a small engine shop and have them install new seals. Before you go to that shop, test the valves by making the combustion chamber as clean and dry as you can... then pour gas, solvent, gunwash... whatever you have about the place... and fill the ports to look for any seepage into the combustion chamber. If it remains dry for a 10 min period, check the other valves. After this test is passed, take it in to have the seals replaced. any small shop should be able to do that for you. Its about a 20 -30 min job. If it is a nice friendly shop they will look at your valves and guides to make sure they are not worn (on overhead cam engines that the camshaft pushes onto a bucket.... The guides hardly wear. My old Triumph's and Norton's and pan head Harley's could wear out valves and guides in 15k... Take the cylinders into that same friendly shop and have them run a ball hone into each cylinder... and they also can measure the barrels to see if there is any major damage from wear or from rust. These ideas are not really brand centric. I do the same process on all brands of bikes. Not just the Yamaha. If you shop around for rings, you can find some rings usually from the manufacturer that supplies the oem (in this case Yamaha) Yamaha does not make rings, pistons, bearings... They are all purchased for Yamaha from outside vendors. Often these same vendors will sell to a distributor who can supply the frugal bike mechanic. Anyhow if you need rings measure the old ones carefully. Measure the thickness of the rings... to make sure you don't get sent the wrong rings for the Yamaha that has two sizes of rings for the top and scraper... (ask me how I know this lol)

 

I have a 1978 Yamaha XS750 triple. One cylinder was at 50 pounds the other two at 140 when dry. It ran reasonably well for the PO. It did occasionally bacfire...... Wet the compression came back up to 130. So Broken rings you say? No... It was seized rings in the piston groove. I carefully removed the rings after loosening them up, honed the cylinders put the same rings back, and voila.... all cylinders were 130-135. This is why I suggest that it might be seized rings in the piston groove. If your budget allows I would recommend new rings and gaskets. In my case I cannot justify spending 4-500 dollars on parts for a bike that is likely to get 2000 as its sale price. Unless I got the bike for free. Even then, if I justified my labour at 5 dollars an hour, I would be better off working part time at a McDonalds. But it is a hobby, and I am simply a bit bike crazy, as you all are... lol

 

Mike,
This ⬆ is very informative and easy to understand. I just didn't think a bike would run with such low compression across the board. I know one will with one or maybe even two on the low side. I've nursed my bike home on three cylinders before. Learn something new every day. Thanks for sharing.

Gary H.

 

But how much would you spend on a bike that you never plan to sell? For some of us, the bike is the profit, not the resale!
PS I really like your posts. Lots of useful information. I usually read 'em a couple of times to digest it all!

 

I have a lot of bikes. They are either used by me, or friends, or they are for sale.
Norton N15 1968
Triumph Bonneville 1970
Honda CM400A 1980
Yamaha XS750 triple 1978
Yamaha XS400 1980
Yamaha XS650 1978
Yamaha XJ650j 1982
Yamaha XJ650 1981
Kawasaki KZ550 1980
Kawasaki KZ750 1982
Honda CB750 1978
Honda XL350 1978
Suzuki GS850 1980
Suzuki GS750 1980
Honda CX500 1980
Honda GL500 1980
Honda CX650 1980
Honda CM400 1981

I think thats all of em... Cept for the parts bikes....
I dont run the old Norton and Triumph anymore. I am too old to kick em.

 

A very nice stable indeed.

Gary H.

 

Given that we saw only a small rise when oiling the cylinders, is it likely rings? We were kind of thinking valves. Getting 20 valve seats ground and 20 valves ground or replaced is not likely going to be especially cheap...

I have sourced a motor that is on my extended commute route, that is showing 150-155 across all cylinders for $300US, that was running well last year until the coils died, I think that's the way I will go.

That said, if it turns out to be an easy fix on this motor after the swap, it may be worth fixing the mill, because everything else in there is in great shape... clutch, transmission, etc so if it's just rings -- and I can get them -- it would make a great item to sell to somebody or use to revive a dead bike...

How hard is it to get rings for these things?


The "squirt PB blaster, rotate wheel once a day" thing sounds simple enough, I will try that the next week I am home long enough to do that. I think I can probably borrow a different gauge to use too.

But the bike really is down on power from where it should be. It starts well, idles well, revs smoothly to the redline, pulls away smartly, but never even hints at lifting a wheel or squirming from the back end. I would say at the low end it's just about equal to the power my old XJ650 had, and at the top end, somewhat less.

 

When you add oil to the cylinders and the compression does not increase the pistions and rings are good so you are looking at valves and a valve job.
That is the woes of having to adjust the valves on these bikes all the time. If it does not get done then its hard on the valves and seats. If the exhaust valves are BBQ'd
i would not even mess with them and just put in new ones. Intakes rarely need replacement bacuse they generaly don't run as hot as the exhaust side does.

If you can get a different mill for $300 I would say go fo it because you will be down along time and its going to be expensive between a machine shop for th head and gaskets
and all the other fun stuff a top end job needs. Hang on to your old mill incase you need any parts or decide to freshen it up and put it back in.

 

I will be picking up the motor in a couple weeks after he finishes stripping the bike. He's going to leave it in the frame which I think will make transport easier and probably make it easier to handle on the bench also.

From this donor bike I have also taken a complete factory exhaust system, front fender, and front braking system. Really good guy, former motorcycle mechanic. Always takes me an extra half hour when I go by to look at something because we get chatting about bikes and I get interested in all the other stuff in his driveway.

 

I've been running my 81 650 at 80,60,90,50. Like a bad boy. But my father, who's wisdom j never neglect unless it's wrong, said that it sounded smooth, and runs, so it's okay, and the only problem was that power and efficiency would be down. Knowing now what I'm reading, I'm very excited for next week. New Pistons and rings came in from ebay, and I just am waiting on all new gaskets. I know it's not valves because of the oil drop compression test came out max compression across the bank, and the bores when I lifted the head off a few nights ago felt smooth. She's got almost 27k, but everything else is splendid.
only thing I've experienced is a sort of sputtering and surging at low revs, which is probably simply in need of a sync, and a sort of topping out, almost as if at redline, at about 5000 rpm. Will report when the gaskets come in and I've put everything together again. And syncd, of course.

 

So I think I did ok with that motor I bought... it came with a few extras attached.




Snuggled in the garage ready for surgery.

 

So now that the carcass looks like this:



I have a box of parts to get rid of.

I was looking for a little advice on how to go about it...

1. I have the steering head, without forks. Bearings are shot, but I can source them industrially pretty inexpensively. Should I a) sell the part as is without bearings b) sell it with new bearings to be installed or c) install the new bearings an the head into the frame and sell them together?
2. I have most of the rear end minus one shock. Should I a) sell the swingarm, driveshaft, final drive, brakes and wheel as a unit b) sell the swingarm, final drive, wheel, and brake kit as 4 separate items or c) bust it up into all dozen or two individual bits and sell them off one by one? The tire is shot by the way.
3. Would the purchaser of a frame (if such exists) want the inner fender with it? Or should that be separated? How about the kickstand and center stand?
4. I'm thinking of keeping all the various cables, switches and sensors, just in case I ever need them if mine break or die. False economy? Should i just sell them all off?

 

Expand ^ to see my advice. Imo be honest about description/condition and sell all parts as is.

Gary H.

 

Thanks Gary, much appreciate your advice.

-iX.

 

Welcome. I hope all goes well with the restoration.

Gary H.