GS450 Low Compression after rebuild

[JRHemmen]

That compression problem by itself isn't enough to make it not run. Something else is wrong.

Hmm. If that's the case, then I can only assume the timing was bad. Also, my spark plug boots pull right off the wires, so I'm sure my spark isn't the most dependable. I tested them by sitting on the head and they both work, but I don't know how strong the spark is.

[jdvorchak]

Boots pulling off is a problem. You can pull the boots off and trim about 1/4 inch of spark plug wire off then you should be able to screw the caps onto the wires and have them hold. There is also a resistor inside the plug caps that should be about 5k ohms measured with an Ohmmeter. From plug cap to plug cap you should find about 22k ohm resistance. The fact that you have at least some spark tells me they can't be all that bad.

I also respectfully disagree but I think it won't run with those compression numbers. If you are sure the cam timing is correct they would have to be leaking a lot to get only 40 lbs compression. Or the valve clearance is too tight and not letting one or more valve close tight.

[JRHemmen]

Do these valves look sufficiently lapped? I only did two because I didn't want to do the others if I had to do them all over again.

[tkent02]

The second picture, the exhaust valve still has a few pits in the sealing edge, as does the exhaust valve seat. A jeweler's loupe or even a pair or strong reading glasses will help you see the pits. Test them with some solvent or oil or something to see if they seal..

Doing this on the XL500 right now, I thought they were good enough but two of them are still leaking.

The good news is they don't have to be perfect for the bike to run, but as long as you're in there might as well get them to seal as well as you can.

[JRHemmen]

Thanks, I'll head back out there and work some more. How important is it to grind down the faces and the backs of the valves? I watched someone do it with a brass wheel to help the tool stick better. I didn't do it because I don't have the grinder at home, but I'll do it at work if it makes any significant difference.

[tkent02]

Why would you grind those?

[gearhead13]

I use a wire brush on a drill to clean the fronts and backs of valves, cleaning the front(top) helps the suction cup hold better. You need to lap the valves until you see a nice smooth sealing surface around the entire seat and valve circumference. A little flashlight and magnifying glass works well to inspect them.

[JRHemmen]

Why would you grind those?

What gearhead said. I ended up not using the suction tool and I used a drill with a hose attached to the drill and valve. I know this is frowned upon, but I went at a slow speed and switched directions often and I think they look pretty good. Tomorrow I'll rent the spring compressor and try to get it all back together.

[tkent02]

Frowned upon, but it works just fine.

[mottacycle]

The bike was unable to start in its current condition. I'm sure the valves are leaking, but I think the timing should have been fine. Now I'm taking it apart to lap the valves and make sure the timing is right. Hopefully after this I'll be able to actually start it.

fine grit lap compound and clean clean clean meaning wipe off check, then clean with carb cleaner then reapply more, use a light in the ports (in and ex) to check for fit.. the way i do it.... also by hand is the best no drill attachments they willl give u more problems with high and low spots just my exp.. just posted didnt realize u already went the drill bit way opps..... good luck

[jdvorchak]

If the tool sticks ok than not needed but preferred. You can't have them too clean. Also if you have carbon build up on the pistons it won't hurt to wire brush them as well. Looks like you're getting close. Would like to have seen a before pic to see if they were bad enough to leak badly. But the seats don't look burned. How is the head gasket? I still can't believe you got 40 and 70 pounds compression. For those kinds of numbers you'd have to have a blown head gasket or a burnt or misadjusted valve and those valves don't look like they are flame etched.

[jdvorchak]

Are the cylinder walls scored? Post some pics of everything you're seeing with the heads off.

[JRHemmen]

Would like to have seen a before pic to see if they were bad enough to leak badly. But the seats don't look burned.

Heres the exhaust valve from the cylinder putting out 40 PSI. My guess is that my timing was off, because the valves didn't look that bad and I have a brand new head gasket.

John, I'll post a picture of the cylinder walls in a bit, I'm headed out to get the spring compressor now.

[JRHemmen]

On a different note, (maybe this has something to do with my poor compression), It looks like something has condensed or leaked all over the head, and its starting to take my new paint off with it when I wipe it away. Its all the way around the cam cover gasket, so I'm wondering if its the oil that I rubbed on that gasket before putting it on. However, it's also in random spots on the head, specifically in the low spots on the sides where the spark plugs screw in. I'll try to post a picture of that later.

[jdvorchak]

Agree that valve and seat don't look bad at all for 40 lbs. I am really starting to doubt that it is really 40 lbs. I have seen more than once a guy with low numbers borrow another tester and like magic that 40 or 50 becomes 120.

I have two compression checkers. One an old MAC tools and a new Harbor Freight. I bought the HF unit because it was like $19 and the proper adapter for my old MAC, to fit motorcycle spark plug holes, was about $12. Both of those read within 10 lbs of each other. But I also have one of those old units with the rubber cone on the plug end and you are supposed to hold it into the spark plug hole. I found one of my sons checking compression on a Chevy with it and complained to me he was only getting about 80 lbs compression. I then used the MAC and like magic he had 130 lbs. And the leak down test showed he had ok, not good, sealing. His complaint was burning oil and it turns out the valve stem seals were shot. The 130 number came from the engine having 230 K miles and was getting tired.