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Discussion Starter #1
Hi guys,
Have another question about my 1980 750 LTD 4 cylinder barn find...

All four cylinders have about 120-125 psi compression. This is kind of low for this bike, right? What kind of compression do I want?

So, since then I have adjusted valves, cleaned it up, worked on the carbs, new plugs and oil change. I have not checked the compression since I adjusted the valves, but I doubt it would change because none of the valves were WAY out of adjustment.

My question is since all 4 cylinders are almost the same compression, when it comes time to rebuild would I be able to get away with just new rings and a hone? Or do you think I'd be looking at valve job, boring and new pistons?

There isn't that many miles on it... like 15,000. It doesn't use any oil that I have noticed. It runs well and has lots of power so I was actually a little surprised to see the low compression.
 

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Sorry to ask the same old question but was the engine up to temp, and was the throttle wide-open?
 

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What you have is acceptable, especially as all four were close but a really healthy one would be putting 180-190psi out. Could be the rings are slightly
stuck as it's stood.

If you do feel the need to do the work then maybe just a glaze-bust would work if the existing rings are ok, should be ok wi that mileage.
 

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Did you do a wet test? (squirt some oil in the cylinders, if the compression rises dramatacally, it's rings, if not, probably valves) if you were to replace the rings, a valve job is easy and would be recommended. If it runs good, I think you should do nothing! The compression may improve as Oneshade said, long sitting engines can have stuck rings. Oh and you may want to test your compression tester! Who knows?
 

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GHOSTRIDER
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Hi guys,
Have another question about my 1980 750 LTD 4 cylinder barn find...

All four cylinders have about 120-125 psi compression. This is kind of low for this bike, right? What kind of compression do I want?

So, since then I have adjusted valves, cleaned it up, worked on the carbs, new plugs and oil change. I have not checked the compression since I adjusted the valves, but I doubt it would change because none of the valves were WAY out of adjustment.

My question is since all 4 cylinders are almost the same compression, when it comes time to rebuild would I be able to get away with just new rings and a hone? Or do you think I'd be looking at valve job, boring and new pistons?

There isn't that many miles on it... like 15,000. It doesn't use any oil that I have noticed. It runs well and has lots of power so I was actually a little surprised to see the low compression.
As long as the bores are within service limits and not too oblong, a standard hone to brake the glaze,and new stock rings will be fine!:wink:So you dont feel alone, my readings are exactly as yours, 125-128psi on all four and I intend to install new rings this winter. Ive got 28k on mine, 4000mi since last year and the readings are down 5-8psi from a year ago.
 

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As long as the bores are within service limits and not too oblong, a standard hone to brake the glaze,and new stock rings will be fine!:wink:So you dont feel alone, my readings are exactly as yours, 125-128psi on all four and I intend to install new rings this winter. Ive got 28k on mine, 4000mi since last year and the readings are down 5-8psi from a year ago.
I live in the NW corner and it still is winter here, so get to it!!!Haha, just kidding, z-man. If the compression is somewhat low and consistant throughout, when do you decide to rebuild? I think as long as it's running fine and no smoke, it's OK. I wouldn't want to do a rebuild before its time! (Oh I miss Orson Welles!) I'm probably wrong, though...
 

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Discussion Starter #8
Thanks for the advice guys.

The bike was running before I found it, but the guy was kind of clueless about bikes, so I don't think he did more than just start it.

I have a new compression tester, so I would sure hope its ok....

The engine was in fact not warmed up - how much of a gain would I really see though?

The throttle was wide open.

I will try putting some oil in the cylinder to see if its the rings or valves.

I was going to leave well enough alone for now... I was just doing some long term planning so I can break the news to the wife...slowly but surely.

BTW, whats all involved in a valve job? Valve seals, springs and cutting new seats, which in turns means most likely all new shims for each valve, right?

Also, how do you check the cylinder for out of round (or eggshaped)? Is a digital caliper accurate enough? What kind of tolerences am I looking for?
 

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Or do a google search for mbsween and download the factory service manual.

If you've ridden it for awhile, try another compression check, you may be pleasantly surprised.

About 10 lbs difference between hot and cold testing on the 650 and 750 fours (the only ones I have compared)
 

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Discussion Starter #11
Thanks Guy, I really appreciate the advice.

I think I may have a combined 40 miles so far in the last year on this bike (and I have no idea how long it sat before then). I got it running, but never licensed/insured it so it only took quick test runs around the block. I also didn't want to get too far from home and break down... I'm rather sick of pushing them home :)

As an FYI, I contaced Z1 about oversized pistons for this bike...they are discontinued, but they found some in Holland... and they aren't cheap.
 

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Forget oversized KZ750 pistons, just get a Wiseco 810 kit, it's probably cheaper anyway (~$500 complete).

But you don't need it, just ride the bike around for a week or two then double check the compression. Ought to be up to at least 150 by then.
 
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