Maxim XJ750 Low Compression across the board

Not sure if I am posting properly, or in the right place. I'm new to XJ Bikes.

Very minimal knowledge when it comes to motorcycling, but consider myself pretty handy. Bought my bike two months ago only to have it crap out on me. Was told I had pretty low compression in all 4 cylinders, one of which was quite bad. Cost for a mechanic to diagnose and fix the problem was more then the bike is worth. Made the decision to take on the project myself, and maybe learn a little along the way.

My question is, I can't seem to find any good tutorials on removing and replacing piston rings. I don't even know where to start, I don't own a manual, can't even seem to track one down. Dealer says they never made one! I've managed to remove, disassemble and clean the carbs so far, next plan was the piston rings. HELP!

 

yes there is both a haynes (Yamaha XJ 650 & 750 fours) and factory (650 service manual plus a 750 supplement) books for these bikes.

did you get wet compression test numbers to differentiate between valve and ring problems?

 

No I got a very vague diagnosis of low compression. No numbers or anything. I think he was very busy and didn't really want to spend much time on my bike. Like I said, I really
Have no idea what I'm doing. Really just winging it. I basically
Have given up on the bike and plan to buy something next year. All
I really want from this one is to learn a thing or two. I just need a place to start. I will try and find these manuals. I knew the dealer was full
Of it telling me they don't exist.

 

Your best bet is to start from scratch find a friend with a compression gauge or get one and get the real numbers (if the mechanic didn't actually give you numbers who knows if he even did anything). Rings are not a cheap fix so it is best to positively Id the problem before you get too far into it. Don't give up on the bike till you find out what is actually wrong with it. You are also going to want to check your valve shim clearances. and if you haven't yet pull your rear wheel and check you back brake pads for delamination just to be safe. All easy to do and there are good guides here to show you how.

 

Thanks for the advice. I will take it all into account. My goal
By the end of this is to have a better understanding of how the whole thing works. What exactly are valve shim clearances? I'm really learning as I go here.

 

valve shims adjust the gap between your valves and cams and makes it so they open and close correctly. If they are too tight your valves stay open too long if they are too loose they don't open long enough both of which could end up ruining your valves. This could even be the source of your low compression numbers.

 

How long was the bike sitting before you got it? It is not uncommon to have low compression on a motor that has been sitting awhile. Rust can get on the valves and seats keeping them from closing completely. That will definetly cause low compression numbers.
I just finished a 750 Seca today that has not run in many years. The rings were stuck keeping the motor from turning. After some work with a BFW and oil it was free. Compression on 3 cylinders was below 40 and I could see oil spitting out the intake and exhaust ports on those 3 cylinders as we turned it. After a couple of miles running up and down the street and a few hours of cooldown the compression numbers are 140 to 145 on all 4.

 

I believe the bike sat for a year. That's what I had been told. I ran the bike with little trouble for 2 months. A few long distance runs as well. Suddenly just didn't want to start one morning, and the following days after it would start but impossible to keep the engine running without full Choke and throttle up. That's when I brought it in. My only answer from
The mechanic was low compression, and not worth the money to have him deal with it. I picked the bike up cheap, my intention was for it only to be my beginner bike. I don't want to sink a ton of money into it, hence why I figured I could cut my losses as is and try to tinker with it myself.

 

Yuo'll get more satification and enjoyment if you work on it yourself. Have you verified the compression test numbers or are you just taking his word? He may be giving you a line of BS because he does not want to work on it. Time for a second opinion, post you location in your siginiture. There might be a member nearby that can help with finding the problem.

 

what you need is a "leak down" compression tester, google that.
basically it's going to tell you if the rings are leaking or the valves.
maybe if you lap the valves and set them you'll be in business

 

leak down testers are nice, but he'd be fine with wet and dry numbers from a regular compression tester as well.

 

Any Dealer that tells you that a service manual was not available, and diagnosis a low compression issue without providing the test numbers never did anything with the bike except let it sit there long enough to take your money.

While I agree that a good compresion test needs to be done. Your description of the problem leads me to believe that you have a very common dirty carb problem with the bike.

You can get both the factory, and haynes manuals by clicking the XJ4ever banner at the top of every page, Then look for the parts catalog. While you are there take the time to check out the rest of the info there.

Finally. Always bring your questions here. You will deffinately get more reliable answers than going to that so called dealer.

Ghost

 

I'd like to throw out there's a possibility the cam chain jumped time and is responsible for the low comp...

 

What makes you think the cam chain jumped? I absolutely agree with MiGhost - sounds like he just didn't want to mess with it and made up a bs line to get you out of his shop. Don't tear into the motor yet. Probably just a carb and valve shim issue. You are going to need to remove the carbs in order to clean them properly. Search the threads on this forum for carb cleaning and follow them to a "t". Then do the same with valve shim adjustments. THEN check your compression numbers. What you are looking for is SIMILAR numbers across all four cylinders.

Tell us how many miles are on your bike - that helps in focusing on likely culprits to a bad running xj.

 

I agree. DO NOT start ripping the motor apart based on some vague statement from somebody who didn't want to work on the bike.

DO A COMPRESSION TEST yourself. If you get screwey numbers, do a wet test. Based on those findings, get the valve clearances in spec and re-test.

Don't simply assume the BS the guy gave you was accurate and jump off into an expensive major rebuild based on no hard facts.

Manuals are all over eBay or you can get them through XJ4Ever. A decent compression gauge kit runs maybe $25~$30 or so from Sears, AutoZone, pretty much anywhere. AZ may even have one to "loan."

It's going to cost enough in the way of the parts needed to get the bike running again if the motor is still solid. If you're worried about "not sinking a lot of money" into it, then don't even consider a full top-end rebuild.

Diagnose; don't assume. And never believe anything the PO or a shop tells you about an XJ; they're probably just making stuff up.

 

Nothing....just offering it up as a possible cause if it does actually have low comp. everyone always jumps straight to rings and valves...forgets jumped cam timing will do that too...if uniformly low across all.

 

Thanks for all the advice everyone. I will get myself a manual, and a compression gauge and start from scratch.

 

You can find the Service Manual on CD here:
http://www.ebay.com/itm/Yamaha-XJ750-XJ ... 2a1fe9000b

or here:
http://www.xjcd.org/

I am in the process of doing the same thing. My bike has very low compression in 2 cylinders. I just pulled the cylinder off this afternoon. Disassembly so far has been pretty easy.

 

haynes: https://docs.google.com/open?id=0B6ZEap ... TMwaF9uZms

factory books: https://docs.google.com/open?id=0B6ZEap ... TdRSnlnbm8

 

More possible causes are if the carbs are screwed up maybe the rings are dry and not sealing. Squirt some oil in the cylinders after the first compression test to see how the numbers come up.

MN