Loss of compression

[friedshrooms]

I have a 1982 GS 450 L and I tore the top end apart a month ago, put on new OEM base and head gaskets and timed it with my Suzuki GS 550 manual because the cams looked to have the same markings and it's all I had for a manual. But the bike had 150 lbs of compression in each cylinder before the rebuild, now it has about 105. The 150 lb test was taken cold and the post rebuild 105 was taken warmed up. I am wondering if a bad cam tensioner or cam cover gasket would cause it to lose compression? Maybe I was a link off when I put the cams on? Was curious as to what this boards infinite wisdom thought.. any input appreciated.. Also I think I might have had piston to valve contact while turning it when I had the timing off with a wrench slowly, but if that happened it wasn't forceful, and I've been riding it without issue for almost 90 miles..

 

[Big T]

Cam timing would be the 1st place to look

 

[tkent02]

With a 450 manual from Cliff's site.

 

[Guest]

Did you break in the engine?

 

[friedshrooms]

Did you break in the engine?

Do I have to break it in if I just put new gaskets? I did nothing to the piston and rings.. We did take it very easy on the bike I'd say below 3,000 rpms for 20-30 miles. I even remembered to offset the rings before I put it back, putting the gaps in a Y formation.

 

[tkent02]

We did take it very easy on the bike I'd say below 3,000 rpms for 20-30 miles.

Hopefully that didn't hurt anything, as long as you didn't use new rings it's probably OK.
The cam timing is correct?
Did you do the leakdown check?
Are you sure the valve clearances are correct?

 

[friedshrooms]

I don't know if there was a procedure I was supposed to follow for breaking in the head gasket either. The cam timing was correct I'm 95% sure, seeing as it took me 3 or 4 tries since the chain kept coming off. Oh.. I remember what I did. I moved the valve buckets around.. sigh. Well if I take it apart again which do you think are the intake and exhaust? Is one usually larger? Or will i have to try the hopefully two combinations otherwise I'm screwed. Though I could calculate all the clearances before I tear it apart and find the corresponding valve shim that was the closest.. I'm sorry lol. I did do a leak down though 120 left, 100 right.

Hopefully that didn't hurt anything, as long as you didn't use new rings it's probably OK.
The cam timing is correct?
Did you do the leakdown check?
Are you sure the valve clearances are correct?

 

[trippivot]

where did you place the ring end gaps? does it smoke?

if the gaps were seperated right and you timed everything right you should have no trouble.

no smoke no compression bent non sealing valve

to test this you do a dry compression test

and turn around and put a squirt or 2 of oil in the plug hole. then do a wet re-test.

if the reading goes up on the wet test rings are not sealing. if the reading does not go up on the wet test it is valve sealing integrity. simple.

 

[Danjal]

Warm it up and do another compression test. Now put some oil in the cylinders. A couple shots out of a oil can will do. This should tell you if its a ring issue or a valve.

 

[hzoltaan]

Hello, I'm new here and I was looking for some ideas on internet because the very same happened with me too. :/ I had my cyl head redone by a specialized cyl head workshop... so, I put it together and I have measured compression 600kPa, (85psi). Originally it was around 140...

The bike starts, but runs very weak... My problem is that my manual deals only with the breaker-type ignition and the timing marks are different on that. What i did: I set the rhd piston on TDC and set the cam shafts as the manual said. I did this before (changed cyl head, changed shims, whatever) and this is the first time I have such problem. Might I have retarded or advanced the timing? (does anyone ha s a correct timig diagram for those marks on the electric ignition... what to line up with what?)

Thank you the help guys.

 

[Buffalo Bill]

Not much diff?

Not much diff?

My problem is that my manual deals only with the breaker-type ignition and the timing marks are different on that. What i did: I set the rhd piston on TDC and set the cam shafts as the manual said. I did this before (changed cyl head, changed shims, whatever) and this is the first time I have such problem. Might I have retarded or advanced the timing? (does anyone ha s a correct timig diagram for those marks on the electric ignition... what to line up with what?)

Thank you the help guys.

I have the older Clymer manual too, for breaker ignition. However the timing marks on the crank are the same as the breaker type: T I , capital T next to a vertical slash. The cams have the same marks too.
Sounds like you'll just have to open it up and check it all again. Could be the rings, maybe?
My bike (GS550T) is still disassembled, hope to do it right myself.

 

[hzoltaan]

nah, I measured the compression few months ago, it was okay, and since the bike was standing more than ridden thanks to some local self proclaimed mechanics. Now I have a new cyl head and I hope the problem is not there. the head gasket is new too, si I guess it's just me, I set the wrong timing.

by the way, earlier I managed to breake a few cam retainer bolts into the head, so as a tip: change them from time-to time. I have new ones of them too and still I got nervous just thinking of loosening them up again... :|

 

[hzoltaan]

thanks for everyone, I just screwed the valve timing. doublechecked it and fixed it, now I have 950-1000 Kpa compression.