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Compression numbers Question

  • Thread starter Thread starter ryonker
  • Start date Start date
R

ryonker

Guest
Ok, so I've been working on my 79 GS 750 and I just took its compression numbers; 112, 122, 122, 112 (cyls 1,2,3,4). I know the numbers aren't great. What do they mean?

Thanks for your help!

Rick
 
Probably good news, they are all close enough together, if a bit low. Was the engine hot when you did it? If not it won't read as high.

Has it been sitting for years or has it been ridden a lot lately?

What is your elevation, where I live (5,500 ft ) everything reads lower.

Strong battery and a good fast cranking RPM?

There is a lot of variation in gauges too.
 
Compression

Compression

I live in MN. No, the engine was cold, but I have been riding it. The battery does crank really well; especially since my coil relay mod (-:
 
One other thing that can cause low numbers ... was the throttle held wide open while cranking?

You think the coil mod helps cranking speed?
icon_confused.gif

Try the headlight cut-out mod, too.
pepper1.gif


My wife's bike has Accel coils and has never had a problem with spark, but I installed the headlight cut-out mod, and it really helps.
beer3.gif


.
 
Compression numbers

Compression numbers

I don't recall holding the throttle open. But I had the carbs totally off when I checked teh compression. Does this make a difference?

What is the headlight relay mod?

Rick
 
putting the headlight on its own relay for power instead of it going through the harness.....
 
ryonker said:
I don't recall holding the throttle open. But I had the carbs totally off when I checked teh compression. Does this make a difference?


Rick
Click to expand...
Yep, that's good enough.
How does it run?
Smoke?
Use any oil?

Try a wet compression test, and do it with the engine at operating temperature...

And have you adjusted the valve clearances? Almost all low compression on these engines is caused by the valves being too tight, they normally get tighter with wear, and a tiny bit too tight will affect a compression test and make it hard to start when the engine is cold.
 
Compression Check

Compression Check

Ok;

So I will re-check the compression after riding. And I will definitely check the valves. How much can valve clearance affect compression? 10 plus psi?

So to do a compression check do I;

1. Ride the bike and warm it uo.

2. Pull out all four plugs.

3. Hook up compression tool.

4. Turn over bike---switching tool to each cylinder

I am a total beginner so any step by step is great. I didn't see any articles on this yet--I am devouriing everything I can read on this bike.

Thanks again so much for your help guys!!!

Rick
 
the engine runs pretty well. Points need replacing. No real smoke and it doesn't seem to use/burn any oil.
 
ryonker said:
Ok;

So I will re-check the compression after riding. And I will definitely check the valves. How much can valve clearance affect compression? 10 plus psi?
Click to expand...

It can bring the compression to zero, if the valves are not able to close.
Usually some exhaust valves will be burnt at this point, when they can't seal, they get hot, flames leak past, it's all bad.

ryonker said:
So to do a compression check do I;

1. Ride the bike and warm it uo.

2. Pull out all four plugs.

3. Hook up compression tool.

4. Turn over bike---switching tool to each cylinder

I am a total beginner so any step by step is great. I didn't see any articles on this yet--I am devouriing everything I can read on this bike.

Thanks again so much for your help guys!!!

Rick
Click to expand...

Don't forget to open the throttle all the way when testing each cylinder.
 
Last edited:
Ok, now for a really dumb question. Does my 79 GS750,L have 8 valves or 16? I have looked through my clymers and can't seem to find it.
 
You have 8.
The 16 valve engines started in 1980, they have rectangular cam end covers, where yours are round.
 
80GS750 said:
putting the headlight on its own relay for power instead of it going through the harness.....
Click to expand...
Actually, I think the Headlight Relay is actually a cut-out relay, so that when you hit the starter button, the headlight momentarily shuts off, so the coils recieve full power. One i plan soon as well as the coil relay mod...
 
One of the good 'ol boys here reckons that when they were racing using the old 8 valve engines,they used to run their engines down to 70 psi before they rebuilt them.Don't know how well they ran,but he assured me they still go down to this sort of level.My 79 'S runs at about the mid 120's and fairly even.Cheers,Simon.
 
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