1977 GS750 Valve Issue ?

[XGS750X]

I was riding the bike at about 130 km on the highway the other day and I lost all engine power. The throttle was literally doing nothing when turned. The bike still had electrical power and the motor would rev up when down shifting. On the side of the road the motor was hot (I had been riding for a couple hours at various speeds) and when I tried to start the bike it would kind of start, flutter and die instantly.. so I towed it home and let it sit for a few days (I was busy with work).

Last night I got around to turning it on and boom it started up and warmed up like normal. I pulled the plugs to see if there were any learnings to be had there. Plugs 2-4 we're super super black and cylinder 1 was white / brown on the tip and black brown on the base of the plug. I replaced all the plugs and now the bike is smoking like crazy!

Otherwise, there is a bit of a tick when the bike is running and a rattle at about 2,500 - 3,000 rpm which sounds like its coming from the valves. I think the tick might be a loose chain but can a loose / warn out chain cause these types of problems? My old man thinks it might need valve / head work. Thoughts?

The bike is on a stock air box with 4-1 pipes.

Any suggestions / experiences with similar problems would be appreciated!

-Jon

 

[chuck hahn]

Add a location to yur signature..if folks know where you are you may get an experienced member to come and take a look

 

[tkent02]

Start here:

http://members.dslextreme.com/users/bikecliff/

A well maintained motorcycle is a beautiful thing.

 

[chuck hahn]

Maybe a valve or piston ring went away?? Start with a compression test. Wide open throttle and roll over till the needle on the gauge has reached its maximum read. This will tell a great deal right off the bat.

 

[tom203]

Still got a vacuum petcock?? Check its operation in case it failed and added fuel into crankcase, thru #3 carb. Check air filter element for gas dampness from excess blowby.

 

[8ball]

Check valve clearance. You might have a couple tight ones.

 

[XGS750X]

+ location

I'm going to get a compression tester this weekend. I still have a vacuum petcock that was checked not long ago - I'll check again. If the piston compression is good, I'm going to check valve clearances and chain tension next.

Thanks all!

 

[XGS750X]

Just did a compression test. These are the results:

Cyl #1: 115
Cyl #2: 115
Cyl #3: 110
Cyl #4: 108

I then sprayed a bit of oil in each cylinder and redid the compression test. I've been told that if the PSI increases with oil in the cylinders it means that air is getting between the piston rings and the cylinder walls as the oil is helping create a better seal. Thus meaning that the piston rings need to be done.

Readings with oil:

Cyl #1: 130
Cyl #2: 140
Cyl #3: 140
Cyl #4: 125


So, with all that said. Can anyone else vouch for this type of problem solving with the oil in the cylinders? I'll likely try to do the work myself but it will take a lot of time and so i'll take the insurance off the bike if I'm getting into a top end rebuild.

 

[8ball]

Did you check valve clearance? And did you do the compression test hot? Valve clearance first....then compression test. A tight valve lash clearance can skew your compression test

 

[XGS750X]

Haven't checked the clearances yet and the motor was cold. Going to do both tomorrow night. Thanks for the tip. I got the order all wrong!

 

[XGS750X]

Checked the clearances and couldn't get a .003 (smallest feeler gauge size I have) in between any of the shims / cam lobes! I guess it's time to buy some shims? Is it worth buying an entire kit? I really don't have any idea what sizes I'll need.

 

[Guest]

Don't buy a whole kit. You need to buy some smaller feeler gauges to fit in there. Then you can either do the math, or E-MAIL Steve for his 'shim spreadsheet' to find out what shims you need in each bucket to achieve the specified clearances. Search 'shim club'.

 

[chuck hahn]

Take the readings on each shim and record them. Then do the math and see what each will need. Next. PM Ghostgs1 ( Ray at the SHIM CLUB ) and tell him what you need. When they arrive you swap them out and send Ray the old ones..nothing to buy and just a day or two to get the replacements.

http://www.thegsresources.com/_foru...spreadsheet&highlight=simplified+spread+sheet

http://www.thegsresources.com/_forum/showthread.php?122394-GS-Valve-Shim-Club

 

[XGS750X]

Finally got some time and the tool to check out the shims. Here's what was in there:

Cyl #1 EX: 2.70
Cyl #1 IN: 2.70



Cyl #2 EX: 2.75
Cyl #2 IN: 2.75



Cyl #3 EX: 2.75

Cyl #3 IN: 2.75



Cyl #4 EX: 2.70

Cyl #4 IN: 2.80

I took a couple of the 2.70's and put them where the 2.75's were to see if this made a difference and i'm still unable to get a .04 feel gauge in those valves? Is it normal to have all the valves like .10 + out? I feel like that's a huge difference but again this is my first time working on valves. I'm just trying to get a sense of what shims I will need to order? Right now i'm thinking 3 x 2.60 and 4 x 2.65.

 

[8ball]

I assume you mean .04mm feeler gauge. It is tough to estimate what you need when they are so tight you can't get a gauge in. Try this. Start the measurement process again, but replace each shim with a quarter. It will be thinner than the shim. Measure the clearance and add that clearance to the measured thickness of the quarter. The shim you need will be that total minus .08 mm (the high end of the spec). Replace the shim and move to the next valve.

 

[XGS750X]

Nice tip! Will do tomorrow.

 

[XGS750X]

Ok. Measured with a Toonie. You Americans need these as they are almost the exact size as the shims.

The Toonie is 1.80 mm thick and I could just barely fit a .80 mm feeler gauge in each valve. So I'm thinking I need 2.55 shims. Here's the math - 1.80 mm + .80 mm = 2.60 mm - .05 mm for clearance = 2.55 mm. I think I got this correct?

 

[chuck hahn]

Read the manual again...or type the right measurements..but its .08 MM as the MAX gap and .03MM as the minimum. A .80 MM gauge is wrong to use.

 

[XGS750X]

I used my larger set of feeler gauges as the toonie was only 1.80 mm thick and created a pretty significant gap. I went up in feeler gauge sizes until the gauge just barely fit which was .80mm. Adding those two thickness together would create a 2.60mm "shim" which would have no clearance. I then subtracted .05 mm which in theory would then be my clearance (right in the middle of spec). So as mentioned I think 2.55 mm shims should be a safe starting point.

Maybe my previous post made no sense but to me these numbers make sense or am I from Mars - which is totally possible!

 

[8ball]

.05 clearance would be OK to shoot for, but I tend to shoot for the MAX clearance. Primarily, because the clearance wears tight with time, and even a slightly over clearance (.09-.1mm) clearance won't do any damage. It will give you more time between lash adjustments, and according to the guys who used to build the Vance and Hines FZR motors actually provided a very small, but measurable gain on the dyno. This was information I got from a tech at V&H many years ago when I was club racing FZR400s.

You might want to get a few 260's in case the gap is too large