1982 XJ750 Suspected Head Gasket Replacement

Hello all. I picked up a 1982 XJ750 Maxim last week. It was running when I first got it and after a few days has stopped. I’ve replaced the spark plugs and cleaned the carbs. I’m suspecting the head gasket needs replacement as there is oil seeping from the left side of the motor from the gasket. Additionally, I am hearing air noises come from the engine when I try turning the engine over. I can manage to get it started after about 10 minutes of trying. When it runs I hear what sounds like air leaks from the engine, similar to the sound the boost on a car makes. I’m new to bikes and wrenching on them but I’ve had a few friends take a look and they think the head gasket needs replacing too. I’ve been doing a ton of reading on this site and elsewhere that makes me think this is the case too. Wanted to see what the experts here think before ordering the part. Thanks in advance!

 

Is the oil coming from the cam cover gasket and running down to the head/cylinder joint?

Do you have a Haynes manual for your bike?

Do a compression test first to check the readings against the specifications in the manual. Then you will know for sure it the gasket is faulty.

 

What Franz said in the last post.
Also check your valves. When things wear on these engines the valve clearances tighten, the valves get pulled back into their seats. If this has gone on long enough you may have a, or some, valves that are not closing completely causing the air leak sound you are hearing. This will also make it very hard to start. If not fixed this will cause the edge of the valve to burn and never will seal again.
Checking and fixing valve clearance issues is way easier and less expensive than diving right into a head gasket..... and even if you do replace the head gasket you STILL have to check the valve clearances when you are done.
Do a good cleaning of the leaked oil and then dust the area with something like baby powder to see where the leak is coming from. More likely to be the valve cover than the head gasket. Head gasket failures are not common on these motors.
Good Luck!
John

 

Yes if the valves are not closing property compression test results will be lower. A problem caused by lack of routine maintenance by previous owners rather than a failed head gasket. Check all the easiest things first.

 

Thanks Franz and John for the info. Definitely a lot of deferred maintenance on this bike that I’m doing my best to catch up on. My Haynes Manual came in yesterday. So going off of your inputs, my plan will be to do a compression test and also clean the engine and use baby powder to spot the origin of the oil leak. Is a compression tester good enough for this diagnostic? Or will I need to do a leak test for reliable results? I’m having battery issues too so I’ll need to start while the bike is wired up to a jumper.

There seems to be oil coming from both the valve cover gasket and the head gasket but I will further investigate as I have read it’s tough to tell the origin of the leak.

 

When the valve clearances tighten up the valves either don’t open fully, or are always closed. The clearance never gets wider as the engine gains miles.

 

K-Moe, you are absolutely correct. I stated the result of bad valve mainteance just bass-acwards!

 

You guys both know your stuff but you're sniffing the same brain fart.
John stated it correctly. When there is no valve clearance the valve may not be closing all the way and may remain partially open.

 

Actually now that I'm thinking more about this, without valve adjustments the valve can only sink into the head as far the cam shaft will allow.
In other words, once the lash is taken up by the valve sinking into the head, it can't sink any further because the camshaft is in the way of any further travel.
Unless you account for cold motor start ups, now we have to debate what grows faster from thermal expansion, the head or the valve stem? Probably negligible.
But Yamaha say to check valve measurements with a cold motor, is valve lash greater or smaller when warm? Anyone know?

So I believe the valve lash can be reduced to zero but the valve will still be closed.
I suppose the exhaust valve can still burn out as it may be open longer, but is will still close.

 

from the web:

Aluminum expands much more than steel, when heated.

So I believe the valve lash will tighten when cold as the head shrinks in relation to the valve stem.
The valve seat would pull away from the valve as the head shrinks if there was no clearance.
Then when the cold motor is started it has no compression because the valves don't seal.
Am I right?

Back to the original post, sounds like your valve cover is the source of the oil leak.

 

To do a proper compression check , remove all the spark plugs , make sure battery is fully charged, while cranking engine over hold throttle wide open . Spin over till you get highest reading , if compression is less than 100psi dry , you can put a spoon full of oil down cylinder , try again see if compression is any higher.

 

I was getting readings less than 50psi. Put oil in the cylinders and then had about 120psi-150psi in each cylinder.

 

The bike has a REALLY hard time starting. I can get it started after about 10 minutes of trying and playing around with the choke setting. It will idle for a minute or so and randomly rev up to about 3500rpm without the choke or throttle touched. Not sure if I messed with something on the carbs when I cleaned them yesterday but that’s a new issue as of today.

 

Have you synched the engine (carbs) yet? If not, an out of synch engine will do all sorts of crazy things that appear to be due to some other fundamental cause, and you will chase your tail trying to fix things that ain't broke.

But those compression numbers don't sound to good.......

 

When you got the bike how was it accelerating? Or did you only have it running while on it's stand? Was it out of use for some time before you bought it? I am wondering if your piston rings are stuck and need freeing up with a 50/50 mix of acetone and automatic transmission fluid. You can put a little down each spark plug hole and let it soak for a few days then do another compression test.

 

A cold started engine with enrichment will normally increase RPMs as it warms up. Adjust lever so RPMs come down and after running a bit more the RPMs will go back up. Adjust again. This is normal. Hard starting is not.

 

I haven’t synced the carbs yet. I’m looking into different options of doing so. I’ve seen a couple DIY solutions for doing so but am trying to locate the tool as I wouldn’t trust my own makeshift method. When putting a little bit of oil in each cylinder, the compression test numbers got much higher but still 2 and 3 seem low, just under 100psi.

 

I’d like to first figure out the compression issue before messing with the syncing though as I’ve read and seen that the manual first mentions getting all the valve clearances spec’d before tuning the syncs on the carburetor.

 

that just means you would have to resync after you did shims.
but still best to wait

 

I have both the four vacuum gauge set up as well as the Morgan pro motion synch gauge , best purchase and much more accurate.