Looking for some advice on jetting and float height

[Guest]

So I've been trying to fine tune my bike for a while now. Here's my setup: 81 gs750 cv carbs with k&n filters, 4 into 1, dyna s ignition and coils, running 122.5 Mikuni mains, stock pilot, fuel screw 2.5 turns out, dynojet needles 2nd position from the top. I'm having problems at cruise typically around 3000-3500 rpm the bike will sputter a little and act almost like it wants to misfire but if I roll on more throttle it clears up and pulls hard and if I cruise at any rpm higher than that no issue. Bike idles great and isn't hard to start and has good acceleration besides this one problem spot. When I noticed it I started playing with the needles and went leaner and it got better but now I'm one position away from topping out the needle. Would this indicate too rich a mix in the float bowl? I'm already at the higher end of the spec on height at 23.4 mm. Should I go higher than this to try it out. Any advice is greatly appreciated.

[Steve]

Your "higher end of the spec" is actually making everything leaner. Remember, you are setting the distance from the carb to the float, and you are doing it upside down, so you are actually keeping the fuel level farther down in the bowl. Try raising the level by setting the floats around 22. If that does not fix the mid-range surging, move the clips down one notch on the needles to raise the needles and richen the mid-range a bit.

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[howardshane2]

Steve, thanks for the reply. Maybe I can pick your brain a little more. What I'm getting I wouldn't call a lean surge more of like a drowning out/sputtering so that makes me think rich but I could be wrong and if I roll on the throttle just a bit when it does it goes away, so this makes me think that if I introduce more air into the mix by opening the throttle I leaned out the slight richness. When I started trying to troubleshoot this problem I leaned out the needle and got better results, it's still there but not as prominent. Do you think me setting the floats to the leaner end of the spec could cause this? I'm just trying to learn and get a good understanding of what's happening in the carbs.

[Steve]

Suggestion: let your spark plugs tell you what's happening. After all, they are the only ones that can see it first-hand.

Find someplace where you can get them to act up. Hold the throttle at that setting just as long as you can, about a minute, if you can. As quickly as you can, pull the clutch, hit the "kill" switch, coast to a stop in a safe place. Pull the spark plugs, take a picture of them, if you can. You are looking for a light tan coating on the ceramic. Lighter than that (approaching white), you are running lean. Darker than that, you are running rich.

Note the actual throttle position, too. That will determine which carb circuit was in use.

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[steve murdoch]

That is a great description on doing a plug chop.

[howardshane2]

Thanks for the advice. I wasn't 100% on how to do the plug chop I just always thought you pulled the plugs whenever and they would tell you what the bike was doing. I'm gonna give it a shot today.