Determining Carb Size and Tuning Carb

[YFS240]

ok so i have carb sizing problems and tuning issues i need to solve.. I have a pretty stout motor that im trying to feed.. .80 over, 3mm stroker crank (no spacer plate), Port and Polish (was ported to trinity stage 5 banshee specs), reed spacer, right hand side pipe with toomey silencer..

I am currently running a Pro Design 34mm intake with a snowmobile 34mm carb.(all i have at the moment) which the carb cant keep up..

I dont know if its the porting that is causing no low end or the carb.. The powerband is like a on of switch.. it has a powerband similar to a banshee.. I have a Video im posting of how it sounds going down the road.. but towards the end you can hear spuddering which is how it runs majority of the time if your not pinned all the time..

What i need to know is what size carb is best for these mods on this engine and where to start with jetting.

 

[sicivicdude]

what kind of "snowmobile" carb is it? Mikuni? series number? pics of the carb? Current jet sizes and other adjustments?

Do you know the actual porting specs? Port timing? head displacement? squish clearance?

When was and what were the results of the last compression test and leakdown test?

Most of the time, without significantly adding displacement, adding power to an engine requires "recalibrating" the torque curve. Generally moving it higher which means more horsepower but less bottom end. This results in a "peaky" engine or light switch powerband (ride a 125cc dirtbike if you want to know what that is like LOL). Pipe options, head work, carburetor choice, and ignition setup all come into play in tuning around an engine's output goals.

In other words, what did you expect?! You can maybe tame some of that peaky-ness out but you won't be able to get rid of all of it...

 

[YFS240]

what kind of "snowmobile" carb is it? Mikuni? series number? pics of the carb? Current jet sizes and other adjustments?

Do you know the actual porting specs? Port timing? head displacement? squish clearance?

When was and what were the results of the last compression test and leakdown test?

Most of the time, without significantly adding displacement, adding power to an engine requires "recalibrating" the torque curve. Generally moving it higher which means more horsepower but less bottom end. This results in a "peaky" engine or light switch powerband (ride a 125cc dirtbike if you want to know what that is like LOL). Pipe options, head work, carburetor choice, and ignition setup all come into play in tuning around an engine's output goals.

In other words, what did you expect?! You can maybe tame some of that peaky-ness out but you won't be able to get rid of all of it...

well i basically dont know much right now off the top of my head.. I dont know the model of the carb.. this engine was just put together.. upon building the engine i was finding these parts for a steal of a deal and just put them together.. I had a machinist put it together so all the clearances and porting were done by him..

All im trying to do is to get it run smoother and make the powerband a little broader..

If V-Force 3 reeds will help i can always put them in because i do have them but not installed..

 

[sicivicdude]

Some of the information needs to be verified before you ride it much more. Too lean of a mainjet or needle in the wrong position will cause a violent powerband and can seriously shorten the lifespan of your engine.

 

[Awk08]

yep, i'd play with the needle clip positions and see if richer or leaner on the needle helps, then possibly go to a different needle if you can't get it right
i had similar problems with my 34mm keihin, till i called carb parts warehouse, told him what i had, and all my mods, he suggested a move from the "klh" needle in it to a "dgh" needle
huge difference and tuneable, gave me all my bottom end back

heres the link for cpw, lots of good info on there too.........

Carburetor Parts Warehouse

 

[Blaaster]

Thanks Awk, the CPW site is a doosie, I am about to fit a MIKUNI TMXX 35mm and my questions were answered.

 

[YFS240]

talked to the builder today.. Im mistaken.. its a stock crack with hotrods rod and stuffer blocks in it.. And i found the numbers on the carb and it says F1Z.. I lowered the needle and it seems to help alot with the powerband but while racing around the yard the carb sucks all the gas out and cant keep up.. if i powerslide 3 times left and right no matter what way by the end of the 3rd time and barely getting to the 4th powerslide it bogs and pops and idles high as if it ran out of gas.. i ride it with it on reserve becasue i feel as it can keep up with filling the carb back up.. i was going to try and drill the petcock to the next larger size to see if that helps..

 

[sicivicdude]

Pull the fuel line off the carburetor body and hold it out to side and then turn the petcock on "on" and see how much fuel flows out. If it's running a stream, you have good fuel flow to the carb. If it's not running freely, pull the petcock out of the tank and clean the screens on the stand tubes.

If you have good fuel flow to the carb inlet, pull the carburetor off and clean it thoroughly. While it's unlikely.... trash could block the inlet of the float needle and cause the carburetor to starve for gasoline. Definitely something to check out because if you're pre-mixing and starving for fuel you're starving for oil!

Take some pics of the carb and post them to photobucket,tinypics, or flickr. Those numbers might mean something to Keihin but they're jibberish to everyone else. Visual identification is relatively easy with good photos.