'81 550T very low compression on all 4 cylinders

[heater63]

Hey everyone. So I bought an'81 550T in non-running condition about two months ago and have spent many nights and weekends trying to get it running again . I had two other posts going in the fuel carb section and the ignition section detailing the first major repair work. Basically after doing a complete carb rebuild, I was able to barely get the bike to run for short periods of time. I needed to keep the engine at like 2500 rpm to keep it from dying, and I could only rev the engine to a max of 6000rpm for the short periods I could keep it running. I noticed that I wasn't getting a consistent spark on cylinders 2 and 3, and replaced the coils and performed the coil relay mod. That seemed to help, but it ultimately ended up being a loose ground connection on the ignitor. Even after improvements to the ignition, I still noticed that the exhaust headers on Cyls 2 and 3 weren't very hot after getting the engine to run. I verified that the plugs on 2 and 3 were getting fuel, because they were wet when I pulled them. So today I bought and compression test kit to see what the compression was on all the cylinders. I followed the procedure in Clymer manual with throttle wide open. To my surprise I got the following: Cyl 1: 68 psi, Cly 2: 55psi, Cyl 3: 60 psi, Cly 4: 62psi. I then followed the procedure in the Clymer manual and poured about a tablespoon of oil in to each cylinder and then perform the test again. The results were then Cyl 1: 110 psi, Cly 2: 130psi, Cyl 3: 120 psi, Cly 4: 140psi. This confirms the piston rings are shot correct? Is this a valid assumption?

How screwed am I? What are your thoughts? Is it a difficult repair (assuming thats all thats wrong). I knew it was going to take a lot of time and some money to get the bike running again, but I guess I haven't decided where I draw the line.

I'd appreciate your thoughts and comments. Thanks.

[Nessism]

Those compression numbers are really low. You sure your gauge is any good? I didn't think an engine will even start with compression that low.

Did you adjust the valves yet?

When an engine sits for a long time the rings can stick in the piston grooves so you don't get good compression. Some Marvel Mystery oil on the pistons, allow them to soak for a few days, may help loosen them up. Some MMO in the fuel helps too.

In many cases the compression will increase if the bike has sat for a while inactive. I'd work on getting it to run as well as possible and then start thrashing it to work the ring as much as possible. I doubt your reving problem is due to the compression but I'm not sure. Low compression keeps the engine from starting, but once it does, it should have the ability to rev up.

Good luck


Edit: just saw your old thread and apparently you haven't adjusted the valves nor changed the intake boot O-rings yet like people suggested. I'm dumbfounded that you think these critical and rudimentary maintenance items are optional. I'm done trying to help.

[heater63]

Hey Nessism, thanks for the quick response. I just checked conencted the compression gauge to my air compressor and compared it to the value on the air compressor gauge. The values match. So I think I can rule out a bad gauge.
I have not checked or adjusted the valves yet though. I'll take a look at them tomorrow.
Any other suggestions/tests to confirm bad piston rings?

[Missile99]

Definitely check the valves and add some Marvel, PB or something and let it sit a few days like Nessism advises. Check the compression again. Make sure the compression tester seats against the head like a plug would. Don't worry too much about worn rings and such yet. It's probably not the case. I let mine sit for a week with Marvel or PB before even trying to free it.

[7981GS]

As the others have said, spray about a teaspoon worth of a good penetrating oil or MMO into each cylinder.
Turn the crank with the 19 mm bolt about 45 degrees every 8 hours by hand and let it sit.
After a few cycles of this and new clean plugs, take it for a (smokey) ride for about 20 miles.
Clean the plugs and ride it another 20 miles and see if that doesn't help free the rings up a bit.
Recheck your compression and see if it has improved.

Eric

[surviverguy]

Like others have said- don't give up so fast...the numbers can go up... the rings might seat themselves again as the rust on the cylinder walls is craped off/ and or the stuck rings in the pston grooves become unstuck. You could also add some two cycle mix to the gas to help the process which would add alittle oil on each intake stroke. Yeah it'll smoke a bit and it might foul out some plugs...but could save you lots of time and money! Good luck and keep us posted.

[tkent02]

Usually after a few thousand miles the compression will be very good. Adjust the valves first.

[heater63]

So guys, I put the work on the bike on hold for a few weeks and just got back to it this past weekend. I checked all of my valves and had to adjust most of them. They many were pretty tight (0.03mm or less). I replaced the shims so that they were about 0.08-0.09mm. The bad news is that I am still exhibiting the same symptoms (won't idle below 2K rpms, and engine has a ceiling of 6k rpms). I also still measure very low compression (60-70 psi) on each cylinder.
One interesting thing I did notice tonight was that if I have the throttle fully open, and then pull the choke out, I can get the bike to like 9-10K rpms. This makes me think maybe I have a carb problem. I did recently rebuild the carbs. Does this sounds like it could this be a main jet issue?

[Nessism]

Is the bike stock or modified?

You might want to check the Newbie Mistake thread linked in my signature to see if anything rings a bell.

[matt1100]

Sealed the airbox? Replaced the intake o-rings, boots?