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1979 gs750L panic! Can't single out the problem.

  • Thread starter Thread starter big_dave_1992
  • Start date Start date
B

big_dave_1992

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Hey! I'm new to the forums, my name is Dave.

I recently traded a beat-up but well running 2007 ninja 250 for my first vintage jap bike. It's an almost unmolested 79 gs750l. I knew it needed work but I had no idea what I was getting myself into, and now I'm beginning to feel silly for it. When I recieved the bike, it had a few mis-fires, burned a little oil on startup but stops after it gets warm. It idled really high and quite rough, and it would miss and give the occasional little flame fart out of the tail pipe. Welllllll, I spent $250 and sent my carbs into the shop for a rebuild and bench sync, get home and it still kinda varies at idle (I set the idle to factory and it varies 1-200 rpms while sitting in neutral.) And when I take off I'm not sure what the problem is but around cruise rpm the bike begins to shudder. I have not replaced boots or O-rings yet, will be doing that soon (although they arent in bad shape.). I'm running a factory air box with kerker 4-to-1 exhaust, VM carburetors, and the factory petcock (with an inline fuel filter), and still not the greatest of luck.

I'm at a loss, I really don't know where to go from here. I don't know if it's electrical, mechanical, or fuel. I have no idea how to check valve clearances or do a valve job. Has anyone experienced this before? I very much appreciate all of you taking the time to read this. I also have the shop receipt, the mechanic wrote down every screw setting and jet size, so if thats helpful let me know and I'll type it up.

I just wanna get this beautiful bike on the road!
 
Did you or they vacuum sync the carbs?

There is a how to on valve clearance checks here. Also, pm steve for the shim sheet if that is an 8 valve.
 
Replace those intake manifold O-rings (www.cycleorings.com) and check for any other leaks at those manifolds, adjust the valves and make certain that you replace the stock main
jets in those carbs (100) with either 107.5's or 110 Mikuni jets if they haven't been changed yet. Report back after those have been done.
 
Here are the specs on the carbs from the mechanic. They havent been vacuum synced yet.

Air/Fuel screws set at 2 turns out
idle air pilot screws set at 1 and 3/4 turns out
main jets - 112.5
 
Replace those manifold O-rings, adjust those valves and drop the main jets down a bit. (That sounds familiar for some reason :confused: )
 
Will do! Can somebody link me to a thread on valve adjustments that's fairly thorough? And when I step down should I go down to the 110 or try another size. Also, how do those screw adjustment sounds, are those correct?
 
Try the 110's first. In those smaller jet sizes one size makes a BIG difference. There is a fantastic valve adjusting tutorial on Basscliff's website. Don't forget those O-rings!
 
Try setting fuel screw to 7/8 turn out and air screw double that, then adjust for best idle using the air screw. Turning air screw out makes it more lean. Adjust valves before synching carbs or you'll have to synch carbs again after valve adjustment. I would replace boot O-rings or you'll won't be able to get air/fuel mixture right.
 
I'll try all of that :) and just for clarification, on VM carburetors, the fuel screws are the ones on top and the air screws are the ones on bottom?? Or is it the other way around? This is my first time having any experience with slide carbs.
 
And thanks for all the advice and time! I combed articles here but never really found an article describing my symptoms, so even if it's been said before, your advice is extremely helpful to me :D
 
You might want to consider taking off the inline fuel filter, if it is the wrong type filter it can restrict your fuel flow to the carbs. I tried running a bike inline filter once, didn't work, off it came, no more fuel flow problem.
 
big_dave_1992 said:
I'll try all of that :) and just for clarification, on VM carburetors, the fuel screws are the ones on top and the air screws are the ones on bottom?? Or is it the other way around?
Click to expand...
It's the other way around.

Set the fuel screws (the ones on the bottom) about 7/8 turn out from lightly-seated. Start with the air screws (the ones on the sides) about double that, or 1 3/4 turn out from lightly-seated. After the bike starts and warms up, turn the air screws slowly, listening for highest idle speed on each carb. Turn the master idle speed down a bit, if necessary.


78550GS said:
Also, pm steve for the shim sheet if that is an 8 valve.
Click to expand...
Info is down there, in my sig.


big_dave_1992 said:
Will do! Can somebody link me to a thread on valve adjustments that's fairly thorough?
Click to expand...
Here ya go: Valve Adjustments (8 valve).

.
 
Did you ask if the said shop replaced ALL the orings? Or did they simply poke out the jet holes and soak your wallet for a half azzed job.

Here the VM tutorial that you can use to check their work. And I would get ahold of them and ask for specific details as to what the tech did do..and if theres some hesitation or the answers seem vague then YOU ask about the orings..did they check the tips on the pilots to see if they were broke or bent..did they set the float hts and at what spec. Things like this will quickly tell you if they did it right.

http://members.dslextreme.com/users/bikecliff/images/vm_carb_rebuild.pdf
 
Steve, Thank you for the information. I'll PM you for that information before I work on the bike next. It's currently in my brothers garage and I'm at college

Chuck,
I took the carbs off and took them to spring valley cycle in Spring Valley, AR. It's a shop owned and operated by a man and his wife, and I trust them. he did a very thorough job of explaining everything he did to them in the shop for me, and was very detailed on the work receipt. He bought comprehensive rebuild kits and replaced every single piece of rubber and internal part in the carbs. He even replaced the long throttle rods that are inside the slides because they were bent, and removed the broken jet needle tips and replaced them all, as well as all gaskets. The only issue I had was he didn't wet test the carbs, and the drain screw on one leaked small drops of gas onto the engine, That's why I havent done a comprehensive road test, also my rear master cylinder went out so I can't brake effectively. He told me that the kits came with washers instead of O-rings, and I already have a set of rings around that I'm going to place on each drain screw. I've just been thinking that I'm having a huge adjustment/jetting problem, and that small leak from the carburetor furthest right.

I see that you live in Norman, that's about 4 hours from me. If I ever get this thing up and done and happen to be riding near you I'd love to stop by and show you the bike. It has an interesting story, it was owned by a preacher who babied it, always garage kept it and maintained it, but once he got too old to ride, it sat up for quite some time. It changed hands twice, both very brief, before I got it. It's been a bit of a case but I hope to finish it by March.
 
So he got comprehensive rebuild kits. Not to be bad mouthing him, but those aftermarket kits are generally full of chit needles with the wrong tappers, junk seat, and many other inferiorly made parts. Do a search or just wait to see who chimes in and seconds my accessment.

Did he tell you he dipped the bodies? And if so, for how long? Or did he run them in an ultra sonic cleaner? Like I said..not beating up on the guy..just saying there are a lot of guys out there that dont fully understand the vital importance of using quality parts AND doing a OCD styled cleaning of everything..inside and out.

And yeah..head this way anytime you want. When its warmer we can hook up and go riding. This winter drizzle is getting old fast..seeing that it was 70 last weekend and we rode a good 4 hrs!!!
 
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