carbs stumbles on takeoff

[Guest]

I have 1979 GS750 I bought new. Engine number GS750-77112. Engine has 850 cylinders with 750 head, 4 in 1 pipe and Dyna ignition. The carbs have been in cleaner overnight and are clean. They have 115 main jets, 20 pilots, needle number 5DL36 at one groove down from center.. I have to use choke cold and starts instantly warm. It idles good and has good torque there so will pull away with no throttle until I try to turn it up. It stumbles just off idle with throttle roll on then catches and jumps forward. Past that point throttle response is good. I have had the 4into1 pipe a few years and some of the baffles have rusted until it sounds a little louder then when new but is not plugged. I don't think this is enough to effect tuning much. Any help with this problem would be appreciated...I need help with this one.

[bomber737]

Have you tried to use a little choke to see if that helps with the stumble? That will tell you if the slow speed jet might be too small

[tom203]

Do you have airbox or pods? Is this a recent problem? I assume that you've tried riching up the pilot fuel screws with no luck.

[arnie]

carbs stumble

I tried the choke thing and it didn't help. It has K&N washable filter in stock air box. It was ok last year with the 750 cylinders. I put the 850 cylinders on in the winter last year and have been messing with it since and cant get it right. I don't think changing cylinders should make that much difference.

[arnie]

carb stumbles

the pilot fuel screws are adjusted so I can use air screws to make final adjustment

[azr]

Are the carbs a stock set of VM26's? what is your fuel needle turned out to and where do you have the air mixture screws at? have you done any plug chops yet? sure sounds like you're close to having right.

[Steve]

I don't think changing cylinders should make that much difference.

You should be using settings for an 850, rather than a 750.

I don't have much experience jetting for a situation like yours, but in general, with the pipe and a stock airbox, you should probably have mains that are 3-5 sizes larger than stock, the fuel screws out about 1 to 1 1/4 turns and the air screws about double that.

.

[arnie]

carbs stumble

The fuel idle jet needles are all a little different. I set them until the air screws would slow idle down when turned in then backed them out some. I think around 1 to 1 1/4 on the fuel screws. Not sure where the air screws are but not all the way out. The plugs are black with soot from trying different things and idling and riding around couple blocks which I think opposite from what they should be from the way it acts. I have had 15 pilots in there too. I'm thinking 22.5 pilots now. I rode it about 30 miles in the country and they came back chocolate brown on insulator and still black on the bottom of the outer base. Of course this is not the way to read plugs. I never chopped yet which I know I should do but its hard to chop for this condition because of the narrow band where the problem is. Also its hard to find a good place to do it here. I'll be 80 yrs old in march and don't like to be along the side of the road with a dead bike ,..I have changed from ngk 8 to a hotter 7 plugs. I could buy new plugs and try to chop I guess but not sure how to do it for this problem. I have a 10X jewelers loupe

[arnie]

carb stumble

I put some 127.5 mains in because I had them and they were too rich. Do you think I should try something between 115 and 127.5 then?

[arnie]

carb stumbles

I tried 127.5 main jets because I had them and they were too rich. should I try something between 127.5 and 115 then?

[azr]

I agree, sometimes it's hard to do plug chops. It would help if we knew exactly what carbs your are running on the bike. I'm guessing they are stock VM26's as you have the fuel and air mixtures. Sometimes though you can find yourself with a set of 28mm VM's. Good to know what you have. I would try 117.5 mains and 25 pilots and see what that does. Like Chef said your air screws should be about double out, but you can tune those with the fastest idle method. WOW 79 years of age and you're racing around on a hoped up GS? I would REALLY like to see a picture of you my good sir, one of you and your bike of course. Glad to hear I've still got a chance at a few more years of riding at 44. Even more impressive is you are 79 and wrenching on your own bike....hats off to you..

[Big T]

Change your plugs back to B8s

Hotter plugs are just screwing up what you're trying to do

Set fuel screw about 1 turn
Air screws about 2.5
Main about 107.5
Pilots 17.5

And go from there. Your overall jetting is too rich, you need to get closer to fine tune it

and yes, the lack of baffling will change things. Pull out the baffle and repack it

[arnie]

carbs stumble

The carbs are the stock vm26,s. I think you have give me good advise and I am going to try the jets you recommend. It's hard to beat a gs Suzuki. I have had mine in most of the states west of the Mississippi and always dependable. My old bike is a sleeper and when things are right it is REALLY fast. I am going to ride it until I can't hold it up then put training wheels on it.
I would like to send pictures but don't know how. My bike looks pretty good for its age...better then me I built a reverse trike on a 1983 gs1100e and didn't like the way it worked. It wanted to throw me off in corners so I cut it up. Now I am building a tilt wheel reverse trike on the same 1100e. I have pictures of that in progress too.

[azr]

Yes, I was reading through your previous threads. You've been on the site for a few years now just haven't heard much from you. Post up some pictures of your bikes and 'projects', would be most interesting.

[Coady]

Mate I've got a similiar setup, except for a foam uni-filter rather than K&N. Sounds like you're way too rich all over to me.

As a reference, I'm running 15 pilots, 107.5 main and same needle as you in the middle position (1 notch richer than stock). Fuel screw at 1.25 turns and air at 2 works best for me. I guess you'd go a little richer than this across the board, but the settings you're talking about seem too much.

Btw I found jetting wasn't much different going from from 750 to 850 and I drove myself crazy assuming it would be till I went back to what I started with.