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GS750 CARBS - jetting, where to start???

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Anonymous

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Sorry to post again about this! Chopped 1979 GS 750 only runs on full choke with unidentified cone filters.... I've had some helpful replies - thanks guys.
I've never done this before -is it really the case that I just have to keep buying and installing different jets until the bike runs?
It seems I have two options: stick with my unidentified filters or buy K & N filters so I know what I'm dealing with. K & N recommend a 15% increase in jet size, but I think I'll need to go up (or is it down?) a little as my exhaust is a bit 'straight thru'. Would 125 jets be a good place to start? How does the exhaust affect the jet size required?
If I stick with the unidentified filters that came with the bike, I guess I just have to risk paying out for a few sets of jets before I get it right, is that how it works?
I haven't got a clue, but I'm learning! Thanks for all the help.
2R.
 
The amount of backpressure created by the exhaust is one of the many components that dictates the amount of air that is going to be drawn into the combustion chamber. "Jetting" is the art of matching the correct amount of gas that is mixed with the air to make the "fuel". This is why using a random or modified exhaust is so difficult to deal with in order to get a proper burn.

You should however be able to figure out a rough idea of where you should be with jets. Form there yes, you may need to simply try a whole bunch of main jets, needle positions, pilot jets, air jets, float heights, etc.

Your bike only running on full choke indicates that your current jetting is too lean. This, of course, assumes that all the other carb components are in perfect working order and everything is clean and free.

I wouldn't bother buying new filters, as you are already working with a somewhat mystery exhaust. You mentioned it was "straight thru", did you modify a stock or aftermarket exhaust?
 
If you stay with the cheaper pod filters, try 117.5 or 120 Mikuni mains.
If the jet needles are in their factory settings, be sure to raise the jet needles by lowering the e-clip. 1 position richer may be enough for the cheaper pods. Then turn out the pilot fuel screws (underneath) a 1/2 additional turn from where they are now. Adjust the side air screws for maximum rpm. When you move the e-clips, the carbs MUST be re-synched. It's a good idea to replace the manifold o-rings when removing the carbs.
 
Thanks for the replies, guys, and my apologies for being a complete novice and knowing nothing!
In answer to Keith: what is a e-clip? I cant see anything that it might be. I need to learn more about these carbs - are there any good books or anything on the web?
In answer to Jethro: the exhaust came with the bike. it looks like someone has just cut off the original pipe, just rear of where the four come into one, and welded a piece of pipe on! a conical piece of metal has been mounted in the end of the pipe, pointed end facing into the pipe, I dont know if this is meant to act as a baffle. Apart from that, the pipe is totally empty!
Thanks again, all help really appreciated.
 
Not sure if the 750 had clips. Older bikes had 3-5 grooves in the needles. small e-clip was used to determine positioning in relation to the slide.

Back in late 70s, Joane Claybrook/Jimmy Carter was trying to outlaw motorbikes (had already killed muscle cars) and 1 stunt was to start trying to kill performance by introducing pollution specs on street bikes*.

The GS I believe had a single position so you could not modify the factory setting. The trick as reported in CycleWorld Magazine was to lift the needle out of the slide. On top of it was a nylon spacer. You put the spacer below and reinsterted it into the slide. It effectively raised the need ~1-2 settings.
On a stock bike, this allowed it to run half way decent until the temps got into the 90s when it was spot on.

If one has a modified air box, filter and/or exhaust, this trick is not going to correct enough to accomplish what you are trying to do. You are going to run too lean.


john

* try as they may to stop them, the Japanese beat the US Govt by coming up with bigger and bigger mills that would loaf along at idle @ 55 mph and get 40-45 mpg. Off idle, the other circuits would kick in and pin you to the seat, but the govt test was run only at idle and 55 mph in a lab. I doubt our litre bikes ever would have been built if not for these clowns in Washington. You might even remember the fuel injection system on the Kawa kz1000 LTD that was the forerunner to all performance systems today.
Some of you may even remember the secret front wheel drive bike they built, but nobody could figure out how to drive it without falling off.
Man, am I ever showing my age...
 
The '79 GS750 does have adjustable jet needles. All VM carbs have adjustable jet needles.
Yes, you do need a manual, preferably a factory Suzuki manual. The carbs have a lot of pieces and you need to work slowly and take notes when learning.
 
Thanks for all the replies - I'll get a manual!
Just out of interest - what jetting do you think would suit carbs without any filters and a straight-thru exhaust? I ask because it seems to make no difference whether the filters are on or off. And someone did recommend 145 jets! I'm just wondering if that would be over-the-top or a starting point. Failing that, I'll take Keith's advice and try 120s.
(I promise to stop talking and buy some jets now!)
Thanks everyone.
2R
 
Do your bike a favor and be sure you run air filters, regardless of brand.
The cheaper pods WILL lean the factory jetting. Removing them or running velocity stacks will lean it even more, regardless of if you can tell a difference. A lean mixture is dangerous for the motor.
I still suggest my first reply as a starting point. I won't make suggestions for running without filters. 145 mains will not work on your bike.
Before re-jetting, the carbs must be clean, all rubber parts/o-rings good, MANIFOLD O-RINGS should be replaced, check the manifolds too for cracks/hardness, float levels checked, carbs VACUUM SYNCHED. Check ignition timing/point gap/condition and be sure all connections are good at plug caps, coils, and the plugs are good and gapped correctly.
If you KNOW all this stuff is good, then you know any problems are most likely jet related.
PS: besides the initial vacuum synch, if you later move the needles, you must re-synch the VM carbs. Be sure to put all plastic spacers you find on the jet needle back in the same order, when moving the e-clip.
 
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