1977 GS750 Low compression

[Andersonspartsplus]

Hope everyone reading this is doing good!
I own and love a 1977 GS750 15,000 miles, pod filter, rejet, 4 into 1 with Kerker muffler.
We got a 50 degree day today here in MI. I got out the GS750 and went for a solo ride. I noticed after a while of riding that it did not seem to pull quite like it did in the fall. At first I thought it may be a lean issue with it only being 50 degrees. Sitting on the stool in my shop I got the idea to check the compression. NOT GOOD! 90 90 90 60 I checked the clearance in the spring '11, seemed good and rode it all year in late Nov aprox. 5000 miles.

My questions are:

(1) Could valve shims cause this much of a compression loss? I will admit to not being as "on top of it" as I should with checking the clearance.

(2) Vendors for sourcing piston rings?

(3) Recommendations for gasket brands?

(4) Has anyone seen this much of a compression loss related to valve shims?

I have rebuilt several Hondas, but never a GS. Any Do's and Don'ts? Besides do not drop any small bits in the crank case.

Thank you
Jim

[SqDancerLynn1]

First question DID you hold the throttle wide open when checking the compression. That will effect your reading. Was the battery fully charged ? If the rings are just gumed up ??? You may be able to pour some cleaner in the spark plug hole.. Otherwise it sounds like the rings need replacement.

[Andersonspartsplus]

Throttle wide open YES

I did hold the throttle wide open! I never get that lucky, but I did soak it with Sea Foam after getting the sad readings. It has sat for 3 months or so.

Battery fully charged: YES

Throttle wide open: YES

Thanks for the reply

[hampshirehog]

It's only done 15k - shouldn't be the rings. I would check those valve clearances before doing anything else.

[Nessism]

Was the engine warmed up? That will effect the readings too.

1) YES. Tight valves will burn and damage the parts

2) Original Suzuki rings are available from many dealer ship sources. Boulevard Suzuki and Part Shark are two good ones to check

3) OE Suzuki gaskets are superior to any aftermarket brand. You can sometimes find good deals on OE gaskets on ebay.

4) Same as 1)

Part of your power loss could be due to jetting. Did you perform plug chops to judge the mixture strength.

[razor02097]

Yes mis adjusted valves can give a low compression reading. Put a tea spoon of motor oil in the cylinder and get another reading. If the compression improves a lot then the rings are fried. If not then it is a valve issue or maybe a blown cylinder head gasket.

[Andersonspartsplus]

I am an idiot!


Sorry for the wasted space on the forum!

After the first reply I pumped all the sea foam out, ran it a bit to dry out the cylinders. Let it cool down. Checked compression again, same same. Then I realized my throttle lock was not holding the throttle wide open.

Held the throttle wide open by hand rechecked the compression.

150 150 150 150.

A note to anyone reading this, make sure the throttle is wide open as the previous member noted!

It's not the bikes fault the owner can't find his ass with both hands!

[Andersonspartsplus]

Valves will be done!

Jetting. I did preform plug chops and it seemed to be good. However if I need just a bit richer or leaner can I use the fuel and or air screws or is jetting the only way?

The main seems to be spot on, but the pilot seems a bit lean when the temp is low around here. On a warm summer night it will run a hole in the wind! When the temps drop it seems to lean ( colder temps more air in a small space) a bit in the low to midrange.


After this lesson I will soon be checking valve clearances.

Thanks again for all the input!

[hampshirehog]

You haven't wasted anyone's time Jim - you've got a 750 that'll be fine now.

Oh and by the way....

Any Do's and Don'ts? Besides do not drop any small bits in the crank case.

Don't drop any big bits in there either....