Cold Compression 1982 Suzuki GS300L

[Kentman7]

Does anybody have any idea what a good cold compression reading would be for a 1982 Suzuki GS300L?

Bike won’t start for a hot compression reading.

Is there a substantial difference between hot and cold compression readings for these bikes?

[Rijko]

welcome.
So what compression reading do you have, and how are you measuring ?
Is this a new to you bike ?

Tell us more on the bike, we love pictures, too.
Do you have a workshop manual ?

[Kentman7]

Hello!

I recently bought my 1982 Suzuki GS 300 L with a known starter clutch issue. That was fixed easily.

The bike was running a month before I bought it with no issues, which was back this year in the spring time.

The bike has only 1400 original miles on it when I bought it. So it sat for the majority of its life. But, it was stored well.

The previous owner before me put on 800 miles on it before he sold it to me, he also cleaned and rebuilt the carb.

After I fixed the starter clutch issue, I thought for sure it would fire right up, but it didn’t, not even a cough. The bike has not turned over for about 6 months now

I checked all electrical, spark plug,fuel , carb , petcock, and air box issues and everything was fine.I even sprayed starting fluid in the carb to get it to start, and once again, not even a cough.

I can also feel the compression on the spark plug holes and the air escaping the tailpipe.

The last check was compression, cold compression. One cylinder was 70 PSI and the other was 50 PSI. I then added some marvel mystery oil in the spark plug holes and let it sit overnight. When I came back in the morning, all the oil was gone, I figured it drained. I then tested the compression again. 85 PSI for one and still 50 PSI for the other. One cylinder compression changed 15 PSI.

Could this be a compression ring issue? Or maybe a valve clearance issue?

Also, could it potentially fire on this low of compression from a cold start?

Thank you, Kent

[Rijko]

You can get a digital manual here : https://gsarchive.bwringer.com/mtsac..._manual-82.pdf

Compression should be way higher.
You are measuring with throttle wide open ?

For a bike that stood a long time, compression is usually low due to rings sticking.
Usually that improves during the first couple hundred miles.
But since the previous owner drove it for 800 miles recently, that does not seem the issue.

[rphillips]

Nothing that drastic should have happened from spring till now, rings shouldn't stick nor valves should get tight. I'd suggest check the the valve clearances.

[Kentman7]

Could valve clearance issues cause that low of compression?

also, throttle was wide open during compression test.

the digital manual says the engine needs to be warm before a compression test, and that the compression should be 150 PSI.

do you know where to purchase shims that would fit this 1982 Suzuki GS300L ?

[rphillips]

Test should be more accurate with warm eng., not 100# different. Check valve clearances to see if they are too tight. That shouldn't be it, but easy to check and a that low, completely possible. If I remember right those bikes don't use shims, they have adjusters on rocker arms.

[Nessism]

Why are you asking if valve clearance could cause low compression? Go to the garage and check the valves, NOW. That is not a question that should ever be asked.

[Kentman7]

Mr. Nessism, with all due respect, when I asked if a valve clearance could cause compression issues, I meant compression issues to spike down as low as 50 PSI.

I wouldn’t think that valve clearance would even be an issue at 1400 miles.

But, I am a very amateur mechanic at best, and I appreciate all your help and advice. Thank you.

[Rijko]

I wouldn’t think that valve clearance would even be an issue at 1400 miles.

I agree, but, i would not expect rings to be the issue either.

So there we are, and i think the only way to determine what the issue is, is to check things.
Valve lash check on a new-to-you machine should be one of the first things to do anyway, and is easier than dismantling the engine to check bores and rings.

[Nessism]

Most likely, the rings are gummed up due to inactivity. The valves and/or the seats could have crud on them also. This is a common problem with bikes that have sat around for a LONG time without running.

It goes without saying that the valves should be checked. This is very easy...except, the valve cover gasket is likely to be petrified, and scrapping it off is a royal pain in the butt. Always buy OEM Suzuki gaskets, if at all possible.

And since you are at this point, I'd give each cylinder a heavy shot of Marvel Mystery Oil. This will help loosen up the rings, if they happen to be stuck. After you get the bike running, ride it good and HARD. This will help burn off crud in the cylinder, and give the rings some load, and help them reseat.

[rphillips]

Running with no problems this spring, now compression that low???? Valves too tight "or" too loose would explain low compression. And what he said about the Marvel Mystery Oil, do it now and you'll have a head start on addressing that if the rings are the problem.

[bwringer]

You have enough compression to start, so there's little point to chasing that rabbit much further down its hole right now.

As stated above, it's quite common for rings to get sticky when an engine sits for a long time, and it's also quite common for this to clear up pretty quickly once you get it running. If you're worried, get a bore scope cheap on Amazon and take a peek to make sure there are no giant rust pits in the cylinders.


The other thing that is VERY common is for a compression check to give artificially low results because it's performed with the carbs in the way. Even with the butterflies open, you can't get a good compression check with CV carb slides blocking much of the airflow. A blockage in the exhaust is rare, but possible, and will also give false compression readings.

Heck, I've seen an engine that was accidentally assembled without piston rings start and run... just not very well.

In short, don't condemn an engine or invest in head gaskets and such until everything else has been handled.

Now, back to what Ed said; although it's a low-mileage engine, I also have a suspicion that you could find enlightenment by removing the valve cover and ignition cover and checking valve clearances, operation, and timing. A sticky valve, for example, could cause all sorts of weird stuff and failure to start, even with ether.

Skipped timing also comes to mind; you have spark, you have compression of some sort, but... are they happening on the exact proper schedule?

To give just one example that has happened more than once... maybe some curious soul removed or unscrewed something on the cam chain tensioner, there was a faint "kapwing", and they slowly backed away without breathing another word...

So I'd definitely check for stuff like that, and basically take a good hard look at the proper operation and timing of the valvetrain and its relation to the ignition timing. Assume nothing.

There are, of course, things to look for I haven't mentioned, but I think a peek under the valve cover is pretty clearly the next move.


You had also mentioned a starter clutch failure... IIRC, one of the things known to happen with these on occasion, at least on the larger engines, is a substantial kickback that causes the cam chain to skip time.

[Kentman7]

Thank you all for the kind advice!

I have the valve cover off, and my first initial observation was the timing chain. It was way too tight. I couldn’t even move it up or down. I know you want a little play in it.

Now that I think of it, The bike had the cam tensioner replaced from the original to a manual one. When it was installed it looks like it was put on too tight. I wonder if that could affect the valve clearance?

Anyways, next step is valve clearance check. I’m also going to check the timing.

[Rob S.]

I've seen an engine that was accidentally assembled without piston rings start and run... just not very well.

Holy compression, Batman! Reminds me of the time I had some sort of work done on my '68 Camaro, and the brake pedal was way too close to the floor.

They sent me home with one of the brake drums missing!