'79 GS850 wont' start after top end rebuild

[Guest]

I've been working on this bike for a few months now. I've cleaned the carbs, replaced the rubber boots, corrected the timing and stuff like this. I've had it running but the middle two cylinders had low compression. About 40psi. So I bought some new shims to correct the gapping between the valve shims and the cam lobes. I decided to take the top end apart to clean it and just make sure everything looked alright on the inside while I was waiting for shims in the mail. Anyway, I just put it all back together and corrected the gapping between the valve shim and cam lobe and now the bike doesn't start and all the cylinders have low compression. Between 40 and 60psi.

What would cause all the cylinders to have low compression now? I know the gapping is correct. None of the valves were bent and were seating properly. I can't believe that something would be wrong with the rings in each cylinder all of the sudden. I didn't replace the gasket between the head and the cylinder because it looked like it was in good shape.

Does it usually take a long time to get the bike to start up the first time after something like this?

Also, I accidentally put a little coolant in the oil. (The jugs are the same size and shape) I tried to drain it out the best I could. How bad is this?

I need help.

[azr]

You pulled the top end apart and didn't put in new gaskets? good for you for taking on such a project but there is likely a multitude of things that could be wrong now. You almost for sure will have a leaking head or base gasket. The piston rings are likely not properly positioned. You might have the cam timing off and that is why the bike won't start. Coolant won't hurt thing too much. Once you get the engine started let it warm up and change the oil again.

[Tanerrx]

I did replace the gasket on the bottom where the cylinder connects to the lower end and I replaced the top gasket where the cams are housed. I didn't replace the gasket between the cylinder and head because it was a different material that seemed pretty sturdy and in good shape. A replacement is also like $70.

I tried not to mess with the rings when I took the cylinder off but they are probably not positioned correctly like you said, azr.

I'll double check the timing and cam chain to make sure they are correct too.

Anyone else have any ideas or suggestions?

[DanTheMan]

That $70 head gasket is extremely important. Once you've removed the head it has to be replaced. It holds the compression in the cylinders, and when it's been compressed once it is no longer the proper thickness to be compressed again to hold the pressure in the cylinders. A new one is required.

[tkent02]

I have re used head gaskets a few times, they had compression enough to run fine. One I needed to get across the country, the other just wanted to make sure the bottom end worked before doing a top end rebuild, one was just to sell a cheap running bike to a friend who needed it to get to work but couldn't afford it. They all ran OK but they always leaked or at least seeped a little. Not so bad really.

There was a thread on here a while ago, some of the engine gurus were saying to use Permatex sealer on the head gasket and use it again.

[Tanerrx]

I went ahead and bought a gasket set for the bike already. I wanted to replace some stuff on the exhaust and the valve seals could stand to be replaced too I'm sure.

I hope this is all it takes.

[Big T]

I hope you bought a OEM gasket set, that like $200

[overboostin]

I bet your timing is off since the compression is low across the board

[Dogma]

I bet your timing is off since the compression is low across the board

That's what I'm thinking. Like when I used the wrong TDC mark on the crank.

[Tanerrx]

So I replaced the head gasket. Not with an OEM one. I'm just trying to get this thing running so I can sell it to a buddy. I'm not worried about the gasket lasting another 40 years.

I'm pretty sure I'm doing the timing right. I line up the "T" mark on 1-4. Put the exhaust cam in with the "1 arrow" pointing forward even with the top of the cam. Then I count 20 link rivets or whatever they are called and have the 20th one line up with the "3 arrow" on the intake cam. Then I put the chain tensioner and top chain guide back on. What else is there?

[terrylee]

Your bike has points and condenser, might check and make sure they aren't pitted or not set right. If you haven't replaced you can clean them up sometimes with nail file and reset gaps. Then you can check that ignition timing too (see manual). terrylee

[Tanerrx]

Your bike has points and condenser, might check and make sure they aren't pitted or not set right. If you haven't replaced you can clean them up sometimes with nail file and reset gaps. Then you can check that ignition timing too (see manual). terrylee

Why would electrical stuff affect the compression? Am I missing something here?

[Grimly]

When you pulled the barrels off to replace the bottom gasket, did you closely check the piston rings condition? They might be dead tired or stuck.
What compression figures do you get with oil squirted down the bores?

[Tanerrx]

When you pulled the barrels off to replace the bottom gasket, did you closely check the piston rings condition? They might be dead tired or stuck.
What compression figures do you get with oil squirted down the bores?

I get maybe 5 more psi if I put oil in down the spark plug hole. I think my compression gauge might not be working properly though because I corrected the timing today and the bike would start. It would die after a couple seconds, so I don't know there is really still low compression. Seems like a fuel things now. Anyone have a suggestion on where to go from here?

[Trogdor]

Low Compression

I just had a similar issue with my 78 gs 550. I took the top end apart and checked the cylinders - about 20k on the bike now - cylinders were in fine shape, so I ruled out warped cylinders and the bike got good compression prior to the rebuild (~130 psi in each cylinder). But I had oil leaking out along the block/head interface (along the headgasket). I replaced the gasket and rebuilt - stopping along the way to lap all 8 valves. after the first time I relapped I couldn't get compression. partially because of tappets that were too thick and partially because a couple valves had bent very slightly. I found this using a small flashlight.

If air gets through, so should light. Try this:

remove the carbs and boot and exhaust manifold.

Rotate each cylinder through a closed intake and exhaust valve. while the valve is closed, shine a light through the sparkplug hole and look through the port of either the intake or exhaust. If you see light, you've either have a bent valve or oversized tappet.... or your timing could be way off, thats another story though.

If its an oversized tappet, you shouldn't be able to get your feeler gauge in the gap. thats how I determined that it was bent valves.

Try disconnecting the fuel lines and Plug wires and using the battery to crank the motor. The intake side should be sucking (with carbs removed) and the exhaust side should be blowing. anything else, and youve narrowed your problem down to the culprit cylinder and problem...