gs750 carbs are killin me

[chuck hahn]

And another VERY IMPORTANT fact to remember is that as the EPA crap came into being, they changed jetting on the 750s a few times. This chart shows that era of chaos and notes that the jetting is subject to CERTAIN ENGINE NUMBERS as well. Check down on the flat spot on top of the tranny on the right side to see what your engine number is and compare to the chart below.

NOTE >>> The chart shows the MIXTURE SCREWS ( side ones ) settings. Pilots ( bottom ones ) are set at 3/4 to 7/8 out.

[liftedresarch]

Thank you again for the responses. When you say "turn in gently" how gently are we talking? And im in the san fernando valley just north of Los Angeles

[chuck hahn]

Like hold the screw driver with your thumb and finger.....gently turn them in till they stop...DO NOT force or torque them down. Then count out 3/4 to 7/8 for the bottom ones and 2 out for the side ones. These are starting points and you do the fine tuning of the fuel to air mixture with the side ones. Leave the bottom ones alone once set.

[liftedresarch]

So i turned all the screws out to what was recommended, and still, nothing. Im not sure what else it could be.

[gustovh]

Check for spark.

V

[Phred]

Two things. First is that when I went to start my bike up after cleaning the carbs and putting it all together after the bike not running for 20 years, I had a problem. The bike would not start. It would turn over, there was fuel in the carbs, but it would not fire up. I drained the battery trying to get it to fire up. While recharging the battery I pulled out one of the plugs and could not smell any gas. Aha! Checked the float bowls and they were full, no leaks. I went over to my neighbours place and took one of his insulin syringes, filled it with gas and shot a squirt down the first and fourth cylinder after removing the plugs. Replaced the plugs and went to start it. It fired up right away and has been firing ever since. I think there was an air lock in there somewhere and all it took was a tiny squirt of fuel to turn it over enough to unblock the airlock. I did not want to put too much fuel down the cylinders as that would not be good for the bearings, and did not want to have to go through the process of changing the oil because there was gas in the oil. For me, it was something like Zen and the art of motorcycle maintenance. I had to wrap my head right into why it was not firing and then figure out how to solve the problem without putting too much fuel down the combustion chamber. It worked, not in a conventional way of thinking, but in an abstract way of thinking. The second thing is that I discovered that there is a product called the Morgan Carbtune. I spent most of a summer taking the carbs on and off to get them adjusted properly. One cylinder was running hot and I played with a lot of things to get it right. The Morgan carbtune is a glass sparkplug that enables you to look right into the combustion chamber as it is firing. You take the plug out and replace it with the glass one. There is a high tension lead that you connect to the spark plug cap and a little mirror that lets you see what is going on in the combustion chamber. As you rev up the engine what you are looking for is a bunsen blue flame, not orange. This enables you to adjust the flame precisely. Believe me, it's the cats meow, and saves a lot of guess work. From England at Morgan Carbtune. Takes about ten days for shipping, and they also have the synch meters as well. Best of luck with getting it running, I know how frustrating it can be to get it right.

[SVSooke]

A little correction Pred,this is a Carbtune http://www.carbtune.co.uk/carbdtls.html .The product you're referring to is the Colourtune http://www.carbtune.co.uk/colortune.html just to be clear

[Phred]

Oops sorry, my mistake, you are correct Thanks Phred

[Neon_95]

OK I'm new. Just to get that out there. I was trying to find the right forum to find out exactly which gs750 I have. It's an '82 but theres E's T's who knows what else I just wana know how I can find out which one I have
Thanks!

[Nessism]

OK I'm new. Just to get that out there. I was trying to find the right forum to find out exactly which gs750 I have. It's an '82 but theres E's T's who knows what else I just wana know how I can find out which one I have
Thanks!

Go to this website and download the various GS750 manuals and then match it up. http://members.dslextreme.com/users/bikecliff/

[KiwiAlfa156]

Hi there, I think I might be able to help here. I rebuilt a set of Mikuni VM32s carbs on my GS(X)1100S Katana, and have spent sometime sorting out the slow running/idle circuits. I posted the following on the Katana Australia Forum, but it's not visible to non-members of that forum, so I thought I'd paste it here. I couldn't find anything that gave straight forward instructions on how to tune VMs that have both an air screw (side screw) and fuel screw (bottom screw) beyond trail and error. So after about a year of tinkering I've arrived at the following system that has given me good low end carburation.

Incidentally, one thing I also discovered was that off idle richness that can't be dialed out using the screws is a sign that the needle jets are likely to be worn, or corroded. These flow fuel just off idle and if are bad will create blubbering. These are the corroded culprits (the carbs sat for 30 years...).





So the post on idle/slow/pilot circuit tuning is as follows:

"I think I have finally got the the low end carburation sorted. I couldn't find anything definitive on setting up the slow circuit on carbs that have both air screw and fuel screw adjustment. Interestingly, it seems that this two screw system was used on the VMs fitted to Suzukis, but not so much on Kawasaki fours of the same era. I had been using the general instructions for VMs from Basscliff, and other relevant posts on GS Resources, but this all pretty much boiled down to initial screw positions and then juggling between the screws until it runs right and the plugs look good. I did this and got a better result by leaning off the fuel screws, and leaning off the air screws too. But it was all a bit hit and miss and I got the feeling that although the idle and off idle was now better the idle was uneven, indicating that the settings weren't uniform across carbs. There was also a little roughness at 90-100 kph at cruise, but above that it was crisp.


Looking at the carbs and diagrams of the VM32 slow running circuit, showed that the fuel mixed by the air screw and idle jet feed two outlets, the pilot and the by-pass. The fuel screw only meters the pilot outlet -the furthest downstream orifice which is on the engine side of the slide. The bypass sits under the slide forward of the needle/needle jet. The layout is the same as the diagrams below (although the air for the slow running circuit in the diagram is air jet rather than an air screw).





A bit of further research revealed that the bypass is a multi-function orifice (Yes Glen, its true). Admitting extra air into idle mixture that is exiting the the pilot outlet when the slide is shut, and as the slide is opened it stops admitting in air and begins flowing fuel mix, effectively becoming a secondary pilot outlet. Apparently the bypass also flows more fuel as the throttle is opened, effectively enriching the mixture, which is why these things get away without accelerator pumps.





The accepted knowledge on tuning the slow running circuit on two screw VMs seemed to be that air screw adjusted mixture, and the fuel screw (preset at the factory) was to adjust for minor differences between the cylinders. Quite frankly I think that's bollocks. The diagrams suggest is that the VM fuel screw has most effect setting the mixture at idle only when the bypass is adding air. After all, at idle it's metering the only outlet flowing fuel. At all stages when the slow circuit is boss the mixture is set by the air screw. At idle this mixture is modified by the bypass and adjusted by the fuel screw.


So armed with this knowledge, out with the vintage Colortune plug for about the sixth or seventh time on the Kat. I'd ridden it about 15-20 Km and it was fully warmed. Set up the cooling fans, and set to it. For each cylinder I first set the idle knob to have the engine running at about 3,000 rpm and adjusted the air screw rich to an orange flame and leaned it out until it turned blue. Then I dropped the idle to 1,100 rpm and adjusted the fuel screw the same way. Checked the flame colour while slowly dialing it up with the throttle and it was uniform. Tried blipping and the flame colour went orange then back to blue, which is what its supposed to do. The initial reading on the colortune confirmed my suspicion that the idle mix was all over the place. Two cylinders were richer that the other and one was quite a bit richer (this also matched the plugs as I took them out). The engine was now way smoother at idle and responds to blipping from idle much more cleanly.


I took it out for a road test and my ass dyno tells me that its pulling harder from 2,000 - 4,000 rpm. It feels crisper. Cruising at 100 kph, it felt may be a tad lean (slightest hint of surging, although I might have been imagining it). Any how I dialed in an extra 1/8th of a turn rich on the air screws and she seemed to be all good. I'm closer than I've ever been and hopefully my theories are close to the truth."