Void in powerband

[Jeepin it real]

thanks to the incredible knowledge this site has to offer my new(to me) 1981 GS550L is running after it had sat for 10 years. but theres one small problem. it will idle perfectly all day and rev up to about 1500 very smoothly, but when it reaches about 2000 it kinda stumbles and loses power but once it reaches 3-3500 it SCREAMS all the way up to redline. what could be causing this "void"?

[BassCliff]

Hi,

Air leak? Raise the jet needle? Do a plug chop where it stumbles and read the plug to see if it's rich or lean.


Thank you for your indulgence,

BassCliff

[AJ]

Agree with Cliff above. Check for an air leak. After that....

Assuming your carbs are stock setup and you've just cleaned and adjusted them.

a) Any chance you have even a pinhole in one of your diaphragms?

b) Ignoring the starter ("choke") circuit (which is sort of its own mini-carburetor system independent of the main carb), you have the pilot jet ("slow circuit") and the main jet ("fast circuit"). By the sound of it, your fast circuit is fine. The problem is the transition between the two circuits. I've copied another post below for more info.

See the attached picture. There are three tiny holes at the top of your carb throat associated with the slow circuit. The big hole in the top is the starter circuit, and I assume that is clear.

1) The small hole nearest the engine is the one metered by the "air screw" or mixture screw that you had to uncork and adjust. If it's idling well once warm and you're off the starter circuit, then I assume that is operating properly. Let's call this hole #1/idle.

2) There are two small holes in the top further back near the throttle butterfly valve. One is on the engine side of the closed butterfly (hole #2 low), the other is behind the butterfly (hole #3/mid) and is activated as you open the butterfly. I suspect one of these holes is partially blocked.

If hole #2/low is blocked, you could still adjust the mixture screw to compensate and get it to idle ok. If hole #3 is blocked, you'd get a stumble as the butterfly is opened and revs build.

Recommend that you remove your mixture screws and spray carb cleaner in there. Hold the butterfly open and verify that cleaner comes out all three holes. If necessary, you can probably poke a wire into the holes to clear them, although it will be hard to get that back one #3.

Final thought, if your carb synchs are wildly off, then that might also cause problems with like this. Did you bench synch the carbs?

I found this page the other day. Not for a Suzuki, not even CV carbs, heck, he's talking 2-stroke mixture, but still some very good info.

http://justkdx.dirtrider.net/printcarbtuning.html

I especially like the part explaining the different circuits in the carb operation, which I have copied below. He also has nice explanations of mixture, the needle jet, etc.

The Basics
When people talk about jetting a carburetor, tuning a carburetor or "breaking out the brass" they're talking about manipulating the carburetor's 4main circuits to optimize gasoline delivery and therefore engine performance. They might adjust the air screw, adjust the jet needle’s clip position or exchanging the pilot(slow) jet, main jet, throttle valve (slide) or jet needle for one of an appropriate size. A perfectly tuned 2-stroke engine/carburetor delivers a 12.5 to 1 air to fuel ratio

No jet acts independently of the others but rather they work together to deliver gasoline to the engine. They do however target specific throttle openings and have the most effect is that area. See below:

• The air screw is most effective between idle through 1/8 throttle.
• The pilot (slow) jet is most effective between 1/8 through ¼ Throttle
• The slide valve is most effective between 1/8 through ½ Throttle
• The jet needle is most effective between 1/4 through ¾ Throttle.
• The main jet is most effective between 3/4 through wide-open throttle.