Buckshot carbs (Dual 44's)

[Seb62T]

I recently tried 105Low 115Main with 2.3/80g spring.
I just have a random hesitation when leaving the idle. If à Pass the idle hole, the jet is awsome ! everywhere, even in he bottom end.
If i put the jet face to a wall and I open full throttle, it dies.
So my popoff is too low...
The popoff is measured at 15PSI...
I think I shouldn't get lower (carb shall leak at lowers popoff ??)

Maybe I could compensate this too low popoff by adding some prefilters on my very permissive RIVA aifilters ?? Or going to a stock airbox ?

when I get the carbs, I have 2.5/95 setup.

Can someone explain to me what is the difference between a buckshot and other carbs construction for the low RPM circuit ??
As I can see, buckshot have one bypass hole and 4 low speed holes...
OK, but some modified mikuni carbs have the same holes... So what is making these carbs to have no bottom end response like everybody tells ????

Other question, I know that my jet setup seems to be OK. In which propotions shall I decrease my low jet when I lower the popoff ??
For example if I go 2.5/80 shall i use 100/115 ??

regards,

Seb

PS : Superjett, I tried all your tests, to set the Low speed screw this is an easy way to find the goo setup.
-From idling, opening full throttle : it dies. opening slowly to full throttle, it revs very wellonce the hole is passed.
-from 1/4 throttle, opeing full throttle, No hesitation, crisp response, the same for the 1/2 Throttle test.

 

[waxhead]

when you leave idle you start to go onto the low speed jet and thats comeng from from the brass spigot
because of it lack of signal the fuel moves lower
thats why you need to add alot of fuel down low to make up for this lack of fuel response


i would be running a larger pilot and a smaller main
ie reverse jetting to let the fguel start to flow

put the fuel in down low and trim the top end fuel with the main jet

 

[Seb62T]

when you leave idle you start to go onto the low speed jet and thats comeng from from the brass spigot
because of it lack of signal the fuel moves lower
thats why you need to add alot of fuel down low to make up for this lack of fuel response


i would be running a larger pilot and a smaller main
ie reverse jetting to let the fguel start to flow

put the fuel in down low and trim the top end fuel with the main jet

Hi Waxhead, thanx for your explanation.
What you mean is most of the fuel flow is going thru the main jet instead of going first to the main jet.
In this case, that would mean that the high speed adjuster would have some influence in the low RPM mixture ?
I already tried large pilot jets like 110 or 120. With the 120 pilot jet... The engine ran very rich... It didn't idle clearly it sounded FAT !!!
I used the setup Pilot : 120, Main : 110; popoff 2.0/80, measured at 18PSI (instead of 21 in theory), this setup was tested with 1 month old fuel (rich in oil)

 

[waxhead]

what happened when you grabbed the throttle
did it pull cleanly

and welcome to bad signal carbs

 

[Seb62T]

what happened when you grabbed the throttle
did it pull cleanly

and welcome to bad signal carbs

Yes, after passing the idle gap it pulls cleany.
The problem is only located from the idle !! if i grab from higher Rpm, it pulls with no hesitation.
The hole is located precisely in the first 1/4 of the throttle.

On question, if the fuel is more routed to the spigot, I should be able to see the fuel leaking from it ??

Which setup would you try ?
I can try 120Low 100 high and I don't know what to do about the POpoff. 120 low will make me too rich in the low RPM (considering that I have a good response with a 105 pilot with low popoff), shall I compensate with a higher popoff like 20 (2.0/80) or 25 PSI (2.0/95) ???

 

[waxhead]

try more turns on the bottom
does this fix the issue

 

[Seb62T]

try more turns on the bottom
does this fix the issue

No, I never go over 1.5 turn or less than 0.5 turn on both screws (considering that over that range, there is a jetting problem, am I right ??).
Opening to 1.5 turns slightly reduce the phenomen, but it is not as clean as 1 turn when leaving the idle (it revs like with a bigger pilot jet : fat, then OK) and the engine dies if opening full throttle while idling.

 

[waxhead]

your right it normally means that you need more fuel in the low speed or lower pop off
but by adding the fuel from the mixture screw you get to see if it fixes the issue then you can make the change and go back down on the turns out
it is a pure testing situation

 

[SUPERTUNE]

One thing to look at is...make sure the butterfly throttle plates are NOT beat in at the edges from the shaft endplay not tight enough and causing wear to the throttle valve edges and drawing air there does/would cause this type of tuning problem. Buckshots are fussy to get right and stay right when the weather changes. Use stock Mikuni 46mm's with my set-up and will solve this problem.

 

[Seb62T]

No, the plates and the carbs are in great shape.

One thing I noticed, one day, I lost my idle screw.
The the throttles were full closed => the engine can't idle... But, without idling, the response was good (the hesitation dissapeared).
Maybe to increase response I should close more the throttle at idle and screw my low speed screws.

Once again, my Riva filters very permissives, maybe the signal would be better with taucetis and prefilters ???

If I can't set up these carbs until the end of the year, i'll go on dual stock 46's.
I found some XL760 mikuni carbs. It is write 46mm... But I think they are 44's on this 90HP 760 ??? Someone could tell me if dual 46's can be found on XL760 (1999) ??

Ther are no lucky buckshot's owner there ?? FU..... Carbs !!!!

thanks

Seb

 

[Seb62T]

One thing to look at is...make sure the butterfly throttle plates are NOT beat in at the edges from the shaft endplay not tight enough and causing wear to the throttle valve edges and drawing air there does/would cause this type of tuning problem. Buckshots are fussy to get right and stay right when the weather changes. Use stock Mikuni 46mm's with my set-up and will solve this problem.

Happy new year to everybody !! I hope that 2007 sill be the year of the hard pulling ski !!!

I followed your advices and bought a pair of XL 760 44mm stock mikunis.
So this is the stock setup : 115/130/1.5/65g.
I also change my 833m ported cylinder for a 62T 84mm 760 stock cylinder from yamaha.
do you know where does theses 760 62T cylinder come ?? I heard about 62T 701... and 64X 760, but not about 62T 760.

So now, what to do with these carbs ?

Here is the full setup :
62T cases
62T 760 84mm cylinder
Thundercats dome head 38cc/84mm (180PSI) / ADA girdle kit
Coffman lightning exhaust (the big purple one)
R&D ported manifold with delta VFORCE 2 Reeds
Dual 44's with a modified 701 airbox (not yet done)
3° Static advance with tunable stock CDI unit (limiter)

Would you please help me to start with a safe base setup.

Seb