Hesitation on roll on after rolling off throttle

[Blueoval78]

Hi everyone, I have just put my '89 GS500 back together after taking the carbs off to clean them.  I cleaned every passage with copper wire and carb cleaner.  I have replaced every o ring and float valves.  I also adjusted the floats with a vernier.  The bike starts and idles perfectly.  When riding, it accelerates normally and seems to have normal performance.  The issue is when I'm cruising or accelerating, if I roll off the throttle and immediately roll back on the throttle, the bike seems to hesitate for a bit and then accelerate properly.  Any ideas before taking everything apart again?

[mr72]

Vacuum leak.

[Blueoval78]

Could a hole in a diaphragm cause this?

[mr72]

yes absolutely a hole in the diaphragm would cause that.

or a vacuum leak, like o rings between the intake boots and the engine, or other vacuum leak in the carb, missing "little o rings" under the top caps.

But if there's a hole in the diaphragm then this will absolutely do this.

[Blueoval78]

Thanks for the response.  One thing I forgot to mention is when I put the bike back together and was syncing the carbs, the slides seemed to "flutter" when I would crack the throttle.  Is that normal?

[The Buddha]

Gawd guys, what is this - carb trouble shooting roulette

Hesitation when opening throttle is usually float level.

Just for future reference - A vacuum leak will usually show up at idle and keep it form idling - either rev higher or suddenly stall - or annoyingly both.
A diaphragm pin hole will not cause a vacuum leak or even mimic a vacuum leak.

A diaphragm pin hole will keep the bike from revving past a certain throttle position usually ~1/2 but can be even less depending on how big a leak.
The bike will rev progressively lower and lower with increasing load. In your driveway in neutral - you'd be able to redline it, 1st gear 7k, 2nd 6k, 3rd 5k and so on.

My first thought is float level.
My second thought is - slide rising too fast. But before you start trying to fix the second one - post more details, cos I don't like mangling the original slides to solve some non problem.

Cool.
Buddha.

[Blueoval78]

I don't think it's float level because I adjusted them as per the manual with float valve just seating (no weight or pressure) and put it right in the middle of the spec using a Vernier caliper to measure properly.  Also, If I hold the throttle long enough, it gets past the hesitation and accelerates normally.  I'm thinking if floats were too low it would starve at WOT, right?  It seems like it's a little hesitation immediately after releasing throttle and re-applying immediately.  I'd describe it as quick off and quick on.  One more thing I tried is applying choke and it made the bike run worst so I don't think I'm running lean.  Re-iterating my one question, when I put the carbs back on the bike and started it, the slides were fluttering up and down really fast when I applied throttle; is that normal behaviour?  Also, the top of the diaphragms seemed a little wet.  The bike does not pop or backfire.  I can't think of any other details...

[Joolstacho]

Just like my '79 Bonnie, and that ain't got no fancy diaphragms! My bet is slide cutaway.

[Blueoval78]

One more thing I forgot to mention is that as I was pulling the carbs apart, the previous owner had the mixture screws set to 4.5 turns out (even though all jets are stock).  When I reassembled the bike, I put them at 3 per the manual.  I am understanding that these screws only control idle and shouldn't affect this situation, correct?

[mr72]

Just for future reference - A vacuum leak will usually show up at idle and keep it form idling - either rev higher or suddenly stall - or annoyingly both.
A diaphragm pin hole will not cause a vacuum leak or even mimic a vacuum leak.

A hole in the diaphragm IS a vacuum leak.

The effect of the vacuum leak depends entirely on where the leak is. Engine vacuum is highest during WOT and acceleration so the effects of a vacuum leak will be most noticeable under those circumstances. This is true in my own experience, in addition to the symptoms you suggest.

If the vacuum leak is on the engine side of the slide then it will prevent the slide from rising quickly enough and cause hesitation on opening throttle just as the OP describes. A pinhole will do the same thing, cause the slide to not rise in response to vacuum and cause hesitation opening throttle.

A vacuum leak on the airbox side of the slides will cause other symptoms like those you describe. I think a very common vacuum leak on these old bikes is the intake boots, and that will cause hesitation on opening the throttle. Anything else you do to make it lean when going to the main jet will also cause the same hesitation, like too small jetting, sticking slide/needle, clogged carb, low floats, etc.

[The Buddha]

The slide does not have an airbox or an engine side. Its a vacuum opposed by the spring. A leak there will keep the slide from rising in that carb cos it will never develop that vacuum.

OP: Your float - my guess is that its a shade high. Also are you describing the rpm rev and catch up stage as hesitation ? Truly its not.

Cool.
Buddha.

[X-ray]

Do the easiest test first; use the clear tube method the check your real time float heights.

[mr72]

The slide does not have an airbox or an engine side.

Of course it does. That's how vacuum exists. It's a pressure differential. On the cylinder side is low pressure when the throttle is opened, on the airbox side is higher pressure. Difference is vacuum. The pressure doesn't get equalized until the slide is lifted by action of the diaphragm in response to vacuum, at which time air actually flows in volume into the intake. Then vacuum is reduced because there is less difference in air pressure between the cylinder intake and the carb inlet.

Its a vacuum opposed by the spring. A leak there will keep the slide from rising in that carb cos it will never develop that vacuum.

Yep. Exactly. And keeping the slide from rising results in a hesitation on acceleration during the time of high vacuum, which is precisely a transition from closed throttle to open throttle. That's the time when the slide is down and vacuum is reduced. This is the principle operation of a CV carburetor.

OP: Your float - my guess is that its a shade high. Also are you describing the rpm rev and catch up stage as hesitation ? Truly its not.

Very well may be, worth a check for sure.

Hope you get it sorted.

[Blueoval78]

Just to answer below, it's definitely not the RPM catch up stage; it really feels different than when I was riding before cleaning the carbs.  Would the mixture screws have any effect on my condition?  Reason I ask is that is the only delta other than float height from before and after cleaning.  As I said, before cleaning, they were at 4.5 turns but I've set them to 3.

[Jim Moore]

Well set them back to 4.5 and see what it does. You can reach in with a flat head attachment to a multi-screwdriver. A lean idle will cause what you're describing.

As a note, 4.5 turns is pretty far out. It might be time to go with the next bigger pilot jet and reset the idle screws to 2.5 or so.

[The Buddha]

The slide does not have an airbox or an engine side.

Of course it does. That's how vacuum exists. It's a pressure differential. On the cylinder side is low pressure when the throttle is opened, on the airbox side is higher pressure. Difference is vacuum. The pressure doesn't get equalized until the slide is lifted by action of the diaphragm in response to vacuum, at which time air actually flows in volume into the intake. Then vacuum is reduced because there is less difference in air pressure between the cylinder intake and the carb inlet.

Its a vacuum opposed by the spring. A leak there will keep the slide from rising in that carb cos it will never develop that vacuum.

Yep. Exactly. And keeping the slide from rising results in a hesitation on acceleration during the time of high vacuum, which is precisely a transition from closed throttle to open throttle. That's the time when the slide is down and vacuum is reduced. This is the principle operation of a CV carburetor.

Sorry man, the airbox side and the engine side are the same. Lets say that the slide is the gate between the 2.
The other side is semi blind - fed by 2 holes and leakage through the needle hole. Other than those 2 holes its blind. Unless there is a leak in it.

Cool.
Buddha.

[Blueoval78]

Ok, so I turned mixture screws up to 4.5 and went for a nice long ride.  No improvement.  I'm not sure if this is more of a clue but at first I was thinking it was more of a roll off and roll back on situation but after tonight's ride, I'm thinking it's more of a when I'm at constant cruise at 5000 rpm's, if I roll on the throttle slowly it hesitates but if I twist it to 3/4 or full throttle it accelerates linearly.  Any ideas?

[The Buddha]

Ok, so I turned mixture screws up to 4.5 and went for a nice long ride.  No improvement.  I'm not sure if this is more of a clue but at first I was thinking it was more of a roll off and roll back on situation but after tonight's ride, I'm thinking it's more of a when I'm at constant cruise at 5000 rpm's, if I roll on the throttle slowly it hesitates but if I twist it to 3/4 or full throttle it accelerates linearly.  Any ideas?

Yikes. If it was the opposite it will be slide lifting too fast. And sorry I don't think the slide lifting too slow is a valid condition, the slide will definitely lift faster than the engine can rev.
You may have a float problem. I can only imagine your float is a shade high, your slow opening of throttle doesn't use up fuel to lower the level enough. If you whack it open you use up fuel faster and temporarily get it low enough to work right.
Steady throttle operation isn't affected by high float unless its so high its spilling over into the manifold.
Cool.
Buddha.

[mr72]

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.

[sledge]

I am with Mr72 on this one.

I dont comment on carb issues. There are are better and more reliable sources of info relating to BST (slingshot) carbs on the net than this one and thats where I go......dunno_black:

BTW.....why are they called slingshots? Because in section the diaphragm looks like a.......slingshot