Transition from off idle to 1/8 Throttle? Looking for a mechanical explination!

[81gs7501166]

I'm starting a new Thread!
Just on transition from idle to 1/8 Throttle!
I want to know what's mechanically going on when this Happens?
And how many options on tuning this Area?
30 mile ride, mostly slow, where my little bit of problem still exist,right off idle transition below 3k,just a little gurgle sometimes.
I'm really Pleased with all the rest of the carb .top good, roll on good, so I gotta keep chasing it?
whats the mechanical thing that's going on?
wrong taper on needle?
Wrong tip on needle?
worn needle jet?
needle coming out of hole too soon?
Wrong slide spring tension?
My last move was clip in 4th from bottom. It now is smooth as Silk!
BUT, the roll ons arent nearly as strong,there best in 2nd from bottom, and their OK in 3rd from bottom.
1166, 34s,4 into 1,pods.
Their has to be a way of getting the good roll on, and smooth transition?

 

[Guest]

I have the exact same problem on my VM26's, slight sputter around 3500-4000rpm. I can't help but think its the carbs coming out of sync, when I take the carbs out, bench sync them balls on and reinstall the problem pretty much disappears for 200-300km. After that time it gradually starts doing it again.

 

[81gs7501166]

I don't believe it's sync. It's when the needle just starts to Move out of the hole.
I've changed, jets,main,and pilot. I set float at least 5 different settings,I've moved clip to every position.
I have not changed needles [dynojet now].
I have not changed the slide spring.[I do wonder though what a stiffer spring would do?]
I read somewhere a guy wedged 2 slide springs together to slow the opening of the Slide?
I kinda think it's the tip of the Needle? The dj needle is much fatter and short taper than the stock onengine?
I don't really understand needle design.
I'd like to try a set of FP needles,but a hundred bucks to experiment is out of my budget?
OK carb engineers,and wizards,let's hear some theory.

 

[posplayr]

Most likely sync. That is why you vacuume sync at 2500 rpm. My head was flow balanced which smoothed vibration . Ever think that sync might be what is aggravating your hands?

you were also told that those emgo carb blockers are crap.

 

[londonboards]

That whole area of throttle operation is using the idle circuit.

First thing to do is strip the carbs and make sure you have surgically clean idle circuits and idle jets (air and fuel).

You should be able to eliminate any problems by adjusting your idle air and fuel screws. Look for a thread that tells you how to do this.

There is good info here:

http://members.dslextreme.com/users/bikecliff/images/vm_carb_rebuild.pdf

 

[posplayr]

That whole area of throttle operation is using the idle circuit.

First thing to do is strip the carbs and make sure you have surgically idle circuits, idle jets (air and fuel).

You should be able to eliminate any problems by adjusting your idle air and fuel screws. Look for a thread that tells you how to do this.

There is good info here:

http://members.dslextreme.com/users/bikecliff/images/vm_carb_rebuild.pdf

he says he gets rid of it by changing the needle which suggests the carbs are clean.

he is running needle 4th from the bottom to remove stumble. I would suggest going up 1/4 step on the main and rset the needle to position #3 and balance the carbs at 2000-2500.

This is will all be good practice when he has to start over with real pods.

 

[Guest]

I have the exact same problem on my VM26's, slight sputter around 3500-4000rpm. I can't help but think its the carbs coming out of sync, when I take the carbs out, bench sync them balls on and reinstall the problem pretty much disappears for 200-300km. After that time it gradually starts doing it again.

A bench sync is a starting point before you sync the carburetors. They actually need to be synced after that to compensate for differences in each cylinder. On the VM carburetors a low speed stutter is usually the pilot circuit before you transition to the main jets. Try turning the screws in front of the float bowls counter clockwise about 1/8 turn at a time until the stutter goes away. Where are the screws set now from lightly seated?

 

[81gs7501166]

If the pods were the problem,it would be lack of flow at high rpm, not at 3-4k.
It screams to 9.5k and is still pulling when I Shift?
It could be the synk [ I haven't done that since I built the bike 15 years ago] it is in my plans to do.
I like the idea of upping the manis [waiting on 120s now] and raising the fuel, and put the clip back in 3rd from bottom.
I'll try that next.
Carbs are clean,every hole ran wire through, carb clean straw blow,then compressed. Air.
I still want to understand the mechanics of the transition?
I've ran into this problem several times before on customers bikes.
I usually set them with the lean needle for smooth transition, they never know what they are missing,and don't complain.
Thanks All!
And my pods, Christmas is coming, let's take up a collection and buy me Some? LOL

 

[81gs7501166]

My air screws are now 2.5 out. I've tried to set them for fast idle.
I can't tell any difference in the sputter no matter where the screws Are?
Since I've installed the baffle, it's way harder to hear or feel the stumble than with open Pipe?
I can't tell if it's rich or if it's lean at Stumble?
It got somewhat better with the larger Pilots?
But it also got somewhat better with a leaner needle Clip?
Some buddies coming by today for a Ride!
I put my clip back to 3rd from bottom yesterday, roll ons were kinda suckey in 4th?
It's the best compromise for Now!
Again I'll ask, What's happening when I just start to Crack the Throttle?
The butterfly is mechanically connected to the throttle cable,so it starts to Open!
Then what Happens? Vacuum starts to pull slide???,needle starts out of hole?,fuel starts coming up the main???
What about the slide springs, too soft, too strong? What about the air hole in the slide, too small,too Large?
Anybody got a set of FP needles they would let me Try? I'll buy them or send them back if no Improvement?
Thanks all!

 

[londonboards]

Please post a picture of your carbs.

I'm worried now that you are talking about CV carbs. You mention butterfly, vacuum, slide springs and air hole in the slide. These are all things associated with CV carbs not the VM carbs I have been thinking that you have.

Very different procedures involved.

This is the correct procedure for CV carbs:

http://www.factorypro.com/tech/carbtun.html

http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,lower_rpm_engines.html

 

[Spyderman]

My air screws are now 2.5 out. I've tried to set them for fast idle.

turn the air screws in a 1/4 turn to two turns out and then ride it and see the stumbling is most likely from the idle being too lean and compensating with the needle is not fixing it that is only richening the mix from 1/4 to 3/4 throttle position
you need to richen from idle to 1/4 and that is done with the air screws on the side of the card

 

[81gs7501166]

I'll adjust and c?
Thanks
Ps I thought richer was out?

 

[bobgroger]

My air screws are now 2.5 out. I've tried to set them for fast idle.

turn the air screws in a 1/4 turn to two turns out and then ride it and see the stumbling is most likely from the idle being too lean and compensating with the needle is not fixing it that is only richening the mix from 1/4 to 3/4 throttle position
you need to richen from idle to 1/4 and that is done with the air screws on the side of the card

If these in fact are CV carbs....
Adjust only when warm. Use choke until it gets there
I ended up 2 sizes larger on the pilots to get rid of the stumble. These are pretty lean out of the box. If you have pods you need Dynojet tapered needles if you are ever going to get close. Richer IS out with CV, opposite for VM

 

[81gs7501166]

I've went up 1 on the pilot,it did seem to help,my 120 mains finally came yesterday,put them in and raised fuel 1mm.
Haven't rode yet,rain all day yesterday, hope to get to ride today,Temps have dropped, highs in the 70s are gone,50s and 60s now. If the screws still don't work I'll order 1up Pilot?
I also unhooked one of the springs to reduce throttle effort.