Between 2500 and 3500 rev's running bad

[GT500Andy]

Need help with my GT500. Between 2500 and 3500 rev's running bad
When I drive slowly gentle on the gas by 2500 rev’s it runs bad, hesitating gurgling holding back. Also when I hold it at 3000 just the same. if I open the throttle it clears and is smooth acceleration to 7000 no problem. And if I accelerate hard full throttle through the gears no problem.
Bike starts good idle good, when I rev it up no noticeable misfire
What I tried:
Ignition timing correct with the stroboscope
Spark plugs nice light brown colour
Carbs completely dismantled and cleaned no sign of debris blown out the jets


Andy

[Zunspec4]

Hello Andy,

Can you gauge how far you have the throttle open when it goes bad. This would give you an indication of what part of the carb range you are working in. Air screw first 5%, pilot jet the next 10%, Needle jet and needle starting to become effective etc. Once past 50% you are working with the main jet and final needle taper. It could be something simple like adjustment of the air screw or a blocked pilot jet as I imagine that is the part of carb opening you are probably in.

On the other hand just use bigger throttle openings and power through it

Cheers Geoff

[Vintageman]

Carbs completely dismantled and cleaned no sign of debris blown out the jets

Did the Jets look like new where it counts! These need to be very exact. For example did you look inside the Needle jet first 1/4 inch where it meters with jet needle to ensure it was extremely smooth? Need good light and magnifying glass to tell (or at least great youthful eye sight and bright light) The exit is round and tapered and jet needle looks perfect (no nicks or wear marks). Are the jets the original and mikuni brand. Are all the air passages open. Is the two little holes in the carb body (pilot holes) open... you sprayed cab cleaner into pilot jet and watched it come out both those little holes? Carbs synced

Can you gauge how far you have the throttle open when it goes bad.

+1

It doesn't take much throttle to leave pilot circuit and be on the needle jet circuit

Also week battery or bad plug caps at these low rpms for generator may not be providing any energy.

[GT500Andy]

Hi,
I think about 1/4 throttle just gently opening the gas 3000 rev’s is a good cruising speed in a built up area. I didn’t see any wear on the needles or jets. Yes I’m of age reading glasses etc. but I have a magnifying glass with light on my work bench
What for me is strange I’ve never had this problem before, the carbs are about 40 years old there isn’t much adjustment any more air screw adjusting doesn’t make any significant difference. I just set the cables with a little bit of play and adjust the slides that they come up together using two wire rode resting under the slides and opening the throttle too see that they fall together, air screw 1 ¼ out.
Jets etc. are standard

Andy

[GTandcbr]

Andy its important to note that carb problems(if indeed this is a carb problem) can be identified not by the speed of the engine but by throttle position. To identify which component is causing the problem i use this method. Wind masking tape around the throttle tube close to the switchgear. Take a large marker pen and mark the place where the throttle is at rest. Then open the throttle fully and make a mark there. Using a tape measure mark halfway and quarter and three quarters. Ride the bike and take note of where the problem begins. If problems occur up to 1\4 throttle pilot system is at fault. 1/4 to 3\4 the needle jet and jet needle most likely and 3\4 to full throttle main jet most likely

[GT500Andy]

Ok I will try in the weekend thanks for the good advice
Andy

[Vintageman]

I didn’t see any wear on the needles or jets.

Have to looked inside the Needle Jet (not the Jet Needle: glad you checked jet needle but that one is easy to see wear). The needle jet is harder and ou have too look into the barrel where it meters ~ 1/4 inch from top down. It is hard to see in there and need bright light and good eyes. you need to rotate to see all parts of the inner barrel, Like Looking down the barrel of a tiny gun Tell me this is exactly what you did and you know exactly what I mean. It must be smooth if not shinny in there.

Sometimes cleaning loosens the decayed material and it falls off finally. The bike is old the Needle Jet inside are junk unless proven otherwise

Skip the headache and buy new Needle Jets if you are going to take it a part again ~$30 -$40 pair. Unless you know exactly what I mean and they are shinny and smooth in there (unlikely). The problem you describe might be something else (timing spot on?), but if Needle Jets as I think it will cause what you say.

Also check the air passage that feeds needle jet via carb body. And, Look at the little mating air hole in brass needle jet itself. if a spec of something gets caught there it wedges and blocks air (even a little) and prevent automatizing fuel fully. Once you get above 1/2 throttle it less important and main jet kicks in.

You are right not much adjustment. Pilot air screw only affect things a little. All brass items must be pristine. Change all brass items with OEM mikuni parts