Suzuki gs750 1978 low compression

[SanderG]

Hello Guy's, Here is a new member Off the forum ( proud to own a Suzuki gs )

did compression test the other day ( valves checked and all good, one was 0.09mm but guess that's no problem ).

compression was low.( For information : did compressiontest hot, throttle full open, valve clearence good.)


Dry. Wet
Cilinder 1: 114 121
Cilinder 2: 106 112
Cilinder 3: 104 107
Cilinder 4: 107 113

what do you Guy's think? As I see it there is not that much difference between wet and dry.
what can be the problem?

Would love all information you Guy's have!

kind regards,
Sander

 

[tom203]

Has it been sitting for a while? If it runs, run it a bit and recheck - valves might not be sitting properly.

 

[Guest]

Run the bike for a bit and retest. Or have you already been riding the bike for sometime? and I don't mean 100 miles.

 

[chuck hahn]

Yup run the snot out of it and beat the engine with some speed jamming through the gears. Get those rings reseated and shaken loose again.

 

[hillsy]

Maybe your compression gauge is faulty? There's not a huge difference between the dry and wet readings which would indicate the rings are sealing fairly well.

 

[SanderG]

Have not been riding it myself for alot ( 4months away from my driving licence ) , i did run it stationary for like an half an hour or so before compresson test. But i guess that's not enough? what do you guys mean with get them seating again? Because wet dry doesnt make a big difference as you see in the readings.

Also the bike has 88.000km on the counter.
So i will have to wait 4months to drive it. Suggestions to what i can do this moment?
Thaaanks!

 

[chuck hahn]

The rings seat under load as thats when the most resistance is applied to the cylinders. Ride the snot out of it and go through a lot of RPM changes and cycles. One thing that will free any Carbon sticking in the ring grooves is a 50 /50 mix of any tranny fluid and Acetone. A few ounces down each plug hole and let it set. Place a rag over the plug holes and crank it a few times to eject the fluid out when youre ready to out the plugs back in and ride it. DONT crank it with the plugs in until youve ejected the residual fluids from the cylinders first.

 

[hillsy]

Have not been riding it myself for alot ( 4months away from my driving licence ) , i did run it stationary for like an half an hour or so before compresson test. But i guess that's not enough? what do you guys mean with get them seating again? Because wet dry doesnt make a big difference as you see in the readings.

Also the bike has 88.000km on the counter.
So i will have to wait 4months to drive it. Suggestions to what i can do this moment?
Thaaanks!

Was it blowing a bunch of smoke, making bad noises or gutless when you rode it last? If not, then just ride it.

I'm still thinking your compression gauge might be faulty - sometimes it doesn't take much to make them inaccurate (drop on the garage floor, etc). Maybe get another gauge to compare.

 

[Guest]

I think he's saying that he can't ride it yet without his license. Some people frown on that, but my suggestion if that is the case, would be to have someone else run the snot out of it.

 

[chuck hahn]

I would just ride it...they cant write a ticket until you stop !!!!

 

[Guest]

You ran it stationary for 30 minutes? Hope you had a fan on the engine to move air through it.

 

[SanderG]

mrbill5491, not for 30minutes no, brother took it driving for 15min and then I kept it stationary idling for like 15 more. what Would be the concequence if you run it for long time stationary? The engine does not make lot of rpm so I would think the heat production would be minimum.
Or not?

 

[SanderG]

Hello Guy's,

I have like 400 miles on the clock since I bought the bike and did a retest of my compression. Here are the old ( 400 miles ago ) and new results in PSI ( tested it in Bar but I think you guy's don't work in that unit. )

Old / New

Cilin 1 : 115 / 116
Cilin 2 : 106 / 109
Cilin 3 : 104 / 109
Cilin 4 : 107 / 107

So as you can see, they are not much better then the old numbers. Since the bottom compression described by Suzuki is 100 PSI my question is :

Is this bike worth putting lot of effort and money in it?

Because this winter I was about to break down the carburetors and change all the rubbers/clean everything. But when I see that this all will cost me about 250 euro alone I would like to know what you guy's think of it? ( PS: not afraid to put some money in to it, but wasting money on something thats gone is a bit stupid )
Visually is the bike in pretty good condition. With a nice Red color.
Is there anything you guy's would do to it that would increase the compression and doesn't involve giving the bike a full top end rebuild?
Due to low compression the bike obviously is hard/not to start when it's colder, that's what bothers me the most.
Let me know!

ps: the bike has 88.000km/ +- 55000 miles.

 

[SanderG]

https://photos.app.goo.gl/j4T8aZ0oxiCdtdLJ3

 

[swanny]

Let me start out by saying - I don't know what the compression should read, however, if it really is low and the reading is virtually the same dry vs. wet, then it's the valves. That's engine basics 101 assuming the test was performed correctly with an accurate device.

 

[Nessism]

If the bike runs decently enough I'd just continue to ride it. If it falters, then decide. For guys that have a love for older bikes the GS is worth putting some money into. If you just want to ride then I'd get something newer that needs less work.

 

[SanderG]

@ nessism : I love older bikes, that's the reason I got one in the first place. I am only 24 but I can appreciate an oldtimer!

I also love the build of the gs750 and love to work on it.

This winter I will take on the intake ( airboxsealing, cleaning carburetor + replacing carb + intake boot o -rings + gaskets ) allready ordered everything.

Then this summer I will ride it hard

And next winter or after that I will consider ( but probably do ) a top end rebuild. Problem with top end rebuild is that I mostly don't know how far to take it. Just do gaskets, lap valves and check/replace anything out of spec.
Or Also let a shop : flatten head + cilinder, hone cilinder, cut in new valve seats, change everything that is close to out of spec,...

What are you guy's opinion on that ?

Thanks for listening to my "story"

 

[Nessism]

Sounds like you got it. Measure first and then make a decision on what to do. 88,000km is not all that much for a GS. Most likely new rings and some valve cleanup will fix up the compression.

 

[Charlie G]

Nice looking motorcycle! Welcome.

 

[steve murdoch]

Have always gone with as long as the compression numbers are within 10% of each other it is not a huge concern.
I agree with Ed, ride it.