no compresion on one cylinder

Mark257 · Posted: January 4, 2009, 3:51 pm Post subject: no compresion on one cylinder

i just got this 1981 xs650 it was sitting outside for a few years (like 15) took it all a part got electrical going and got spark cleaned carbs but i have a problem there is almost no compression on one cylinder i took carbs off and when i put my hand on the boot one cylinder is sucking hard and other almost nothing , but when i take the exhaust heathers off and check for suction its same on both sides (really hard) is that normal , and what should i check next the bike has only 9k miles on it
thank you for your help

jayel · Posted: January 4, 2009, 4:38 pm Post subject: Re: no compresion on one cylinder

almost any time a motor is stopped one or more valves will be open, if that bike set for fifteen years with a valve open it won't be seating properly now.....

Mark257 · Posted: January 4, 2009, 4:41 pm Post subject: Re: no compresion on one cylinder

i thought i will have to rebuild top end , do i have to remove the whole motor out of the frame to do that?

cpotts454 · Posted: January 4, 2009, 4:52 pm Post subject: Re: no compresion on one cylinder

You will unless your frame isn't stock. This motor in the stock frame is a pretty tight fit.

Retiredgentleman · Posted: January 4, 2009, 5:24 pm Post subject: Re: no compresion on one cylinder

Mark;
Before removing engine, check for proper valve clearance, in case a valve is being held open. Intake=.004" and exhaust=.006".
This link has good info and pictures.
www.bikerworld.co.uk/t...index.html

Mark257 · Posted: January 4, 2009, 5:40 pm Post subject: Re: no compresion on one cylinder

thank you all for your help will start removing the motor this week

BigGeorge · Full Member Joined: Apr 13, 2008 Posts: 267

Mark before you yank the engine out, buy or borrow a compression gauge the kind that screws into the plug hole to determine your compression numbers on each cylinder. If one cylinder is low try squirting about 1oz of oil in the cylinder to see if the compression comes up at all. If compression comes up its rings if not its valves..........BG

Mark257

i did it with oil in the cylinder and nothing changed there is hardly any compression ,

BigGeorge · Full Member Joined: Apr 13, 2008 Posts: 267

Mark if you have the carbs off and the exhaust off at the headers you can look in the intake and exhaust ports and see if your valves are opening and closing. You may have a valve thats stuck just off the seat......... BG

thorshammer · Full Member Joined: Dec 12, 2008 Posts: 58

Another check is use a rubber tipped air chuck with cylinder on TDC with valves closed apply air if valves are bad you will hear it out the exhaust or intakes, but if it seems to come out some other place such as breather you know its piston or maybe some po forgot rings compleatly.

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

I'd figure out which valve is sticking. Then I'd roll the crank until the piston on that side is at bottom dead center. Squirt a generous shot of Marvel Mystery Oil around the top of the valve-stem seal. Then, me being a crazy bastard and all, I'd get me a pin-punch or a brass drift, and a ball-peen hammer. Put the punch/drift tip on top of the valve stem and whack the drift/punch with the hammer. Knock the valve farther down (open) and let it bounce back on the spring again. Do that a couple few times and you may find that the valve comes back to the seat like it's supposed to. Use plenty of MMO while you work the valve down and up.

I saved myself doing a valve job on a 7-horse Wisconsin engine with a similar trick. The old Wisconsin was a flathead engine, so I had to work from the head of the valve instead of the stem: I knocked the valve down onto its seat with a brass mallet and then used a pry bar in the adjuster port to lever the valve up and down against the spring while, now and then, hitting the stem with another shot of MMO. I left the old gasket in the engine and sealed it up with silicone. I torqued the head down while the silicone was wet and, after it set up overnight, ran that engine two more years before I sold it to another dude. He bolted it on his garden tiller and, as far as I know, it's still running.

Mark257 · Posted: January 5, 2009, 3:15 pm Post subject: Re: no compresion on one cylinder

ok iam back again i did the air test like thorshammer suggested and its leaking air like crazy valve its not sitting correctly at all plus there is all kinds of dirt and smudge inside tried cleaning it out but its hard to get to , before i will start taking it apart i will try to bang it into place like jimmythetrucker said thanks for your help all of you and i will keep you posted

yamaman · Posted: January 5, 2009, 3:22 pm Post subject: Re: no compresion on one cylinder

If you want to rebuild it, you should quit stuffing around and pull the motor. Yep you could smack it around a bit with a hammer and get it going for now, all the time causing more damage. Or you could pull the head, sort it properly.
All the crud your talking about will probably end up scoring a bore, then you'll need a rebore & new pistons. If you fix it now, you might get away with a head job & a new set of rings.
Unless of course all you want to do is plough!

jpowell · Posted: January 5, 2009, 4:50 pm Post subject: Re: no compresion on one cylinder

I gotta agree. I kept farting around trying to keep my abused bike I got in aug running long enough to get me into winter. It died on me on the highway. Finaly pulled it to find a burnt valve and the timing chain gaured completely missing. Fix it now before you do more damage. I can pull the motor fix it and put it back in in one good day expect for the waiting for parts.

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