Hesitation on roll on after rolling off throttle

[mr72]

I am with Mr72 on this one.

May wonders never cease.     

[Blueoval78]

Thanks everyone for the advice but after reading Crackin's recent post, I am starting to wonder if it could be my floats?  I could swear I had measured them bang on with a Vernier but after doing the clear hose method check today, the fuel seems high.  I don't have fuel in the airbox or crankcase but the level is higher than the bowl seam.  Could this cause the hesitation when accelerating lightly from a cruise at 5,000 rpm's?

[The Buddha]

Once again rpm isn't the right way to talk about jetting. Throttle position is.
Anyway fuel at the bowl gasket level and you have to set the 2 equal, you have 1mm leeway, but please do not adjust it to correct something else. Set both equal and @ the bowl gasket level. Non negotiable.

Now, 5k is 1/8th throttle @ no load, doubt you have hesitation you can feel then, and if so, try air screws before float level.
5K is 1/2 throttle when climbing a hill in 5th or 6th. if your float is more than a 1mm off the gasket face, or left and right aren't exactly the same, then set it to that level and equal and make the run again. May not fix this problem, but rest assured, you've nailed down a very important variable.

Cool.
Buddha.

[The Buddha]

And sorry I don't think the slide lifting too slow is a valid condition,

Sure it is. With a hole or tear in the diaphragm or a vacuum leak on the engine side of the diaphragm, the slide will lift too slowly or not at all. You know these carbs are used on a lot of bikes besides just GS500s, and those bikes' aficionados have their own forums where they discuss this, often with a completely different mixture of experts. I have learned a lot more about the operation of these Mikuni BST carbs on these other forums than I have learned here. This slide-not-lifting behavior due to damaged diaphragm or vacuum leak is commonly reported and resolved elsewhere.

Of course you seem to be unwilling to try and understand what I posted before about this, instead arguing inexplicably about how this is not possible. I don't get it, but I'm tired of arguing about it.

You're quoting 1/2 a sentence to make your point more valid ?
I say 'don't eat that shaZam!" and you happily edit out the don't and quote me.

Ok if you must, there is an engine side for the diaphragm, and a black plastic top side. Happy ?
Airbox side = engine side. The other side = plastic top side.

No one is unwilling to understand anything, these carbs were on GSXR's, katana's in the Suzuki's and something very similar on a few yanmaha's etc.

Cool.
Buddha.

[mr72]

Haha. From a vacuum perspective, or in terms of air pressure, "black top side" is the same thing as "engine side".

The slide restricts air flow into the engine, like a valve. The side facing the piston which I call "engine side" is necessarily at lower pressure than the side facing the atmosphere or what I call "air box side". This lower pressure is commonly known as "vacuum" and it is that pressure differential that pushes the slide upward via the diaphragm. You can think of vacuum pulling it up or pressure on the air box side pushing, both are true. A leak either in the diaphragm or anywhere on the lower pressure side will reduce this pressure difference (vacuum) and result in the slides failing to lift as it should. If the leak is big enough the slide won't lift at all.

[The Buddha]

Haha. From a vacuum perspective, or in terms of air pressure, "black top side" is the same thing as "engine side".

The slide restricts air flow into the engine, like a valve. The side facing the piston which I call "engine side" is necessarily at lower pressure than the side facing the atmosphere or what I call "air box side". This lower pressure is commonly known as "vacuum" and it is that pressure differential that pushes the slide upward via the diaphragm. You can think of vacuum pulling it up or pressure on the air box side pushing, both are true. A leak either in the diaphragm or anywhere on the lower pressure side will reduce this pressure difference (vacuum) and result in the slides failing to lift as it should. If the leak is big enough the slide won't lift at all.

Awesome, I'll just refer to my engine as the plastic top then.
Cool.
Buddha.

[mr72]

Awesome, I'll just refer to my engine as the plastic top then.

And you wonder why I said you are deliberately trying not to understand what I am writing.

If you can't understand it, then I give up. I can't explain it any better than that. Maybe the OP understands and it helps in the event this is related to his problem.

[Blueoval78]

Sorry, I should've been more specific, I meant 5K rpm cruise in 6th.  If I twist the throttle slowly to accelerate slightly from that point, it hesitates.  If I twist it more, it gets past that hesitation and accelerates linearly.  Regarding the air screws, I thought the screws being on the engine side of the carbs were fuel screws and only affected pilot circuit?

[mr72]

Regarding the air screws, I thought the screws being on the engine side of the carbs were fuel screws and only affected pilot circuit?

I am not sure what "air screws" he was talking abut but I am guessing (as I think you are) that he meant the mixture "screws" accessible on the bottom of the carbs.

In this case, you are correct on both counts, those screws (needles, actually) control fuel and not air, and they primarily effect the pilot circuit. But depending on throttle position (and load, to some extent) the bike may run on pilot at surprisingly high rpms. It may be running [mostly] on pilot at 5K rpm at very light [or no] load. When you open the throttle under these conditions you must switch from pilot to the main jet.

If you're stumbling or hesitating when doing this then it could be the slide not coming up quickly enough as I said before, but barring that it may also benefit from a larger pilot jet if you haven't already upsized so there is more fuel available at that throttle position allowing it to transition to main more linearly.

[1018cc]

I am not sure what "air screws" he was talking abut but I am guessing (as I think you are) that he meant the mixture "screws" accessible on the bottom of the carbs.

On some carbs, the air is regulated into the engine, not the fuel. In the end it achieves the same result of adjusting the idle mixture between rich and lean - just by a different method.

[Jim Moore]

IT may not be a torn diaphragm. It may just be seated incorrectly. It's easy to do. When you seat the diaphragm, line it up, install the top, then jiggle it a little as you put it into place. You will feel it as everything lines up.

[The Buddha]

Sorry, I should've been more specific, I meant 5K rpm cruise in 6th.  If I twist the throttle slowly to accelerate slightly from that point, it hesitates.  If I twist it more, it gets past that hesitation and accelerates linearly.  Regarding the air screws, I thought the screws being on the engine side of the carbs were fuel screws and only affected pilot circuit?

Throttle position please.
5k in 6th on level road depending on your weight etc etc could be anything from under 1/2 throttle to ~2/3rd - smack in the needle range.
Hesitation likely could be rich or lean, and issues when opening throttle is usually rich.
So you have a washer under the needle ? take that out.

1 change at a time only and study it well, fine tuning is all about your bike and your carbs and your running conditions. What applies in Colorado wont apply in Florida. That's what fine tuning is.

Cool.
Buddha.

[The Buddha]

Awesome, I'll just refer to my engine as the plastic top then.

And you wonder why I said you are deliberately trying not to understand what I am writing.

If you can't understand it, then I give up. I can't explain it any better than that. Maybe the OP understands and it helps in the event this is related to his problem.

I'm sorry my plastic top is not running right between 5000 and 7500 rpm. Let me make sure it runs better and also @ exhaust temps of 6-700 it wants to melt. So let me address those and get right back to you.

Cool.
Buddha.

[Blueoval78]

Ok everyone, thanks again for all the replies.  I FINALLY got around to working on my bike this weekend.  (Home renovations eating up all my precious riding time!)  I started by replacing the diaphragms since they were easy to do without taking the carbs off and I had purchased new ones from Suzuki that were not returnable.  Went for a nice long cruise after doing so and the bike runs perfect now.  I am not sure if the originals were torn or maybe not seated properly but there is a substantial difference in feeling between the old and new ones.  The old diaphragms feel "rubbery and stiff" while the new ones feel "silky and smooth".