Top end rebuild - question on compression and rings

[Guest]

Currently have the engine on my '78 GS750E torn down to the lower case. This is a new bike to me and the major engine work I've ever done. Prior to pulling the head, I tested compression (engine was out of frame and obviously cold). Numbers were low (105, 95, 85, 100). Ran a wet test after and numbers elevated (125, 135, 140, 130). I would have thought this was a slam dunk for worn rings/scored cylinders but all looked pretty darn clean. Trying to determine why the numbers were low. Some relevant info:

55k on the clock
Wouldn't start when I bought it - prior to that it was said to run great, but hadn't run in 3 months or so
Replaced plugs - fired right up - as it was snowing out, I never did run it. Ran for maybe 60 seconds
Head gasket actually looks to be in nice shape
3 valves were identified as tight (see my other post)
Good amount of oil/carbon build up at exhaust ports
Piston heads looked fairly clean
Valves seem pretty dirty w/ good amount of carbon build up, but I don't have a frame of reference to know how bad

Basically trying to make sure this thing is worth putting back together w/ just new rings and gaskets or if I need to do more. Perhaps the numbers were that low simply due to it being cold? Thoughts?

[Joe H]

You need to do a leak down test, it will show you the leaks.

[JTGS850GL]

You need to do a leak down test, it will show you the leaks.

I would say it's kind of hard to do a leak down test now that the engine is apart.

You could have been seeing low numbers due to stuck/gummed up rings. Probably would have loosened up after being run for a while. Since it's apart you might as well do a simple top end rebuild. New rings, hone cylinders and a fresh gasket set. After that you'll need to get those carbs torn down and rebuilt. More then likely they were the cause of the no start issue but you'll have to check for spark as well. Don't forget to get all those valves adjusted before you try to start it again.

[Nessism]

Agree on the basic freshen. Use a bottle brush hone on the cylinder, not a straight hone. 240-320 grit. New rings, new valve stem seals, OEM Suzuki gaskets. Don't skip here or you are likely to get leaks.

[Steve]

You pulled the head because the (COLD) compression was "only" between 125 and 140?

Personally, I would have been elated at the wonderfully-high numbers and continued with the stuff that really needed to be done, like valve adjustment and carb cleaning.

Low compression numbers, especially on a cold engine that had not been run for a while, might come up simply by running for about 1000 miles or so.

Now that you have it apart, you are committed to trying to do the rest of it right. As mentioned, a light hone on the cylinders, new rings, valve adjustment, carb cleaning. Do this with OEM Suzuki gaskets, especially the base and head gaskets. When you get to the carbs, do NOT get "carb rebuild kits", get a can of Berryman's (or GUNK brand) Carb Cleaner Dip and a set of o-rings from member Robert Barr at cycleorings.com.

Next time, maybe ask some opinions before taking things apart, which will cost you a LOT more money.

.

[sclark900]

Thanks for the feedback. I actually pulled it apart for the experience, not necessarily because it needed it. Never done it before and always wanted to. And since I'm repainting the frame and pulled the engine, it seemed reasonable just to get it done while out considering the mileage. It'd need it sooner or later!

Top end refresh will certainly be done. Just trying to determine if the cause of the lower compression may have been the oil leaks and residue found, particularly at the exhaust ports. The rings moved freely and the piston head (tops) took very little work to shine up. Cylinder walls also looked great, so I was a just a bit stumped on why I may have been getting lower numbers and why the exhaust ports were so gunked up. Sounds like I'm a-o.k. Thanks all!

Already have the cycleorings kit and Berryman's cleaner. Ordered a new Honda R/R from duanedage (sp?) as well - purely a preventative move. Read all the stator papers cover to cover as well. I did check the valves and actually have another question posted on that. Maybe you guys could shed some light on that? Here's the link:

http://www.thegsresources.com/_forum/showthread.php?229059-Valve-clearance-confusion-78-GS750E

Anyhow, thanks for the replies.

[Sandy]

As stated your compression numbers are great for a cold motor and putting a bit of oil in the hole will normally cause them to come up a bit. You've probably got 36 year valve seals in there so best do those while you're at it.

[sclark900]

That's encouraging. I'll add valve seals to the list as well then. Thanks!

As stated your compression numbers are great for a cold motor and putting a bit of oil in the hole will normally cause them to come up a bit. You've probably got 36 year valve seals in there so best do those while you're at it.

[GS1150Pilot]

You can't properly do a top end overhaul without doing a valve job, so those seals were on your list already.

[sclark900]

Ha! Thanks. Glad to see you folks are keeping me straight. Need all the help I can get!

You can't properly do a top end overhaul without doing a valve job, so those seals were on your list already.

[Joe H]

[QUOTE=JTGS850GL;2145521]I would say it's kind of hard to do a leak down test now that the engine is apart.

This is true.... The worst part is I read it several times before I posted!...
Now you have good advice to follow ... Good luck on the rebuild!

[sclark900]

To be sure... this is the valve seal we're talking about, correct?:

http://www.z1enterprises.com/ItemDetails.aspx?itemDescription=Valve+Stem+Seal+Suzuki&item=KL14-6386-1

If this isn't the seal, could someone point out what's needed on a microfiche diagram? I've been looking but can't quite tell what's needed.

Edit: Found the microfiche. Is part #14 what I'm looking for?:

http://www.procaliberbend.com/OEMpartfinder.htm#/Suzuki_Motor_of_America,_Inc/GS750EC_(1978)/CAM_SHAFT_-_VALVE/02780008/5602780010

Sorry - rookie engine rebuilder here.

As stated your compression numbers are great for a cold motor and putting a bit of oil in the hole will normally cause them to come up a bit. You've probably got 36 year valve seals in there so best do those while you're at it.

[Sandy]

To be sure... this is the valve seal we're talking about, correct?:

http://www.z1enterprises.com/ItemDetails.aspx?itemDescription=Valve+Stem+Seal+Suzuki&item=KL14-6386-1

If this isn't the seal, could someone point out what's needed on a microfiche diagram? I've been looking but can't quite tell what's needed.

Edit: Found the microfiche. Is part #14 what I'm looking for?:

http://www.procaliberbend.com/OEMpartfinder.htm#/Suzuki_Motor_of_America,_Inc/GS750EC_(1978)/CAM_SHAFT_-_VALVE/02780008/5602780010

Sorry - rookie engine rebuilder here.

By the description it shows the more expensive ones are Viton (the rubber material) seals while the others don't mention what they're made of. Viton is supposed to be a much more durable material than the standard rubber/vinyl?? that the cheap ones are probably made of. They both would probably work fine but I'd put my money on the Viton ones.

[1_v8_merc]

Get a set of Motorcycle/ATV Ring Compressors.
It will make your job a Million times easier, make sure you OIL them up before use! As long as the Kit fits 65mm Pistons, you're good.
The kits can be had for Under $30 on Ebay,(I recommend Two compressors, One for You and One for a Buddy)

when it's time to compress the rings, you must compress Two pistons at a Time. Start with the Inner Two.
Simply Crank the motor Clockwise until the Inner pistons Max out at the Top, slowly slide the Cylinder Head down, and hold it there. This where you Stop, and check to see if you remembered the Lower BASE HEAD GASKET! lol
Okay...Now crank the motor Clockwise again, so the Inner pistons go down a few inches or so...now slide the cylinder head down more, and simply place a Small clamp on any Cylinder Head Stud, so the Head rests on it while you work. Now keep cranking the motor again, until the Inner pistons come back Up and touch the Head....now you and your buddy compress Both Rings...now slowly Tap the head down with a Rubber Mallet, until the pistons are completely In the cylinder. If you feel any resistance, or the head suddenly stops moving down, STOP, double check that a Piston ring is not stuck on the Lip of the cylinder.(usually the Top piston ring slips out of the compressor, and takes a few tries to get the Feel!)
Once the Inner's are done, slowly crank the motor, while your buddy holds the Head and ensures the Pistons dont Pop back out. Now do the Outters and DONE!
The reason you do the Inners first, is because when the motors in the bike, there's not much room to do the Inners, so you do them first. And the Outters are on the Outside....so who cares! lol.
I know it sounds like a Lot, but once you start playing with the Assembly and Clearances and see how it works, you'll understand How to do it.


PS... Can't remember if You need a block of wood or something to keep the crank assembly from moving while you Tap the head onto the rings.

[sclark900]

Thanks for the comprehensive response 1_v8_merc. I'll certainly keep this on file for when the time comes. The engine is currently out of the frame and the top end disassembled. Cylinders/cylinder head are at the machine shop for a tank cleaning, hone, and valve lapping. In the meantime, I'm ordering up the gaskets. It's a chore finding OEM gaskets that are reasonably priced!