Lepew Posted April 21, 2004 Report Share Posted April 21, 2004 What does bored stock carbs do? How does it effect the power on a banshee to increase the size of the throat of the carb? How does it effect the jetting if thats all you change? What are the drawbacks? I've been working with the jetting on some bored out carbs and I just want to better understand them, so I'm looking for your opinions and input, Thanks Quote Link to comment Share on other sites More sharing options...
sredish Posted April 21, 2004 Report Share Posted April 21, 2004 (edited) What does bored stock carbs do? How does it effect the power on a banshee to increase the size of the throat of the carb? How does it effect the jetting if thats all you change? What are the drawbacks? I've been working with the jetting on some bored out carbs and I just want to better understand them, so I'm looking for your opinions and input, Thanks Boring out carbs is basically making them bigger. If you take the stock 26mm and bore them to 28mm, then you basically have 28mm carbs. The object of opening or boring, or getting new bigger carbs, is to allow more air into the motor. More air equals more fuel equals more power. You add air, then you need to add fuel to keep the air/fuel the same and that extra air/fuel adds extra power. BINGO!! If you have a stroker kit or some heavy porting, the need for more air is imperative and that's where bigger carbs are a necessity. Small carbs can only flow so much air, and that can hold you back with extensive motor work. Although, I wouldn't do shit to the stock carbs except throw them in the fire pit and watch them burn! Edited April 21, 2004 by sredish Quote Link to comment Share on other sites More sharing options...
Banchetta Posted April 22, 2004 Report Share Posted April 22, 2004 I tried bored stockers on my shee w/ the mods below..I lost 2hp across the whole curve, but that was w/ no porting....the stock carbs are perfect for no porting...Bored or larger carbs benefit once you port it... Quote Link to comment Share on other sites More sharing options...
Lepew Posted April 22, 2004 Author Report Share Posted April 22, 2004 How does it effect the jetting if thats all you change? I'm sorry maybe I wasn't that clear on this question. I have a ported motor with increased compression and advanced timing, how would the jetting be affected between stock carbs and bored stock carbs on a motor such as mine? I know a little how bigger carbs will affect 4 strokes and understand unless you increase a motors "volumetric efficiency" a bigger carb isn't needed. This probably is the same for 2-strokes. You will probably get poor throttle response, bog and hesitations if the carb is to large for the motors needs. If I only had my own dyno. Quote Link to comment Share on other sites More sharing options...
sredish Posted April 22, 2004 Report Share Posted April 22, 2004 How does it effect the jetting if thats all you change? how would the jetting be affected between stock carbs and bored stock carbs on a motor such as mine? Well, you'll be sucking more air so you'll have to have more fuel going in, how much depends on your "volumetric efficiency". (I just wanted to get that in there ) If you have stock carbs and you get them bored, you'll have to rejet and will be jetting it richer than it was to match the extra air. Sorry, I didn't read your question right. Quote Link to comment Share on other sites More sharing options...
locogato11283 Posted April 22, 2004 Report Share Posted April 22, 2004 dont bore them, just have them honed like a cylinder....right?? no but seriously, on my cr500 banshee i had a stock 38mm carb i think it was. i bought a 3mm and bored it to 40mm and there was quite a difference. Quote Link to comment Share on other sites More sharing options...
Banchetta Posted April 25, 2004 Report Share Posted April 25, 2004 I had to go up 3 main jet sizes on mine...w/ stock porting.. Quote Link to comment Share on other sites More sharing options...
The Rebel Posted April 25, 2004 Report Share Posted April 25, 2004 How does it effect the jetting if thats all you change? how would the jetting be affected between stock carbs and bored stock carbs on a motor such as mine? Well, you'll be sucking more air so you'll have to have more fuel going in, how much depends on your "volumetric efficiency". (I just wanted to get that in there ) If you have stock carbs and you get them bored, you'll have to rejet and will be jetting it richer than it was to match the extra air. Sorry, I didn't read your question right. Not necessarily sredish. You won't allways need to jet richer just because you have a bigger throat dia. in a particular carb design. It will depend on the pressure drop across the emulsion tube in the main power circuit of the carb. I have bored carbs before and actually ran less main jet to get the A/F ratio on line. Sometimes that happens, but that is where the " Volumetric Efficiency" (you like that, don't you) of the engine set up comes in to play, plus the design of the carb. That's where I'm kinda like you on the stockers, I'd much rather run Keihins than Mikunis. Getting back to the jetting question, if you have ever read a tuning manual on jetting for max power you know to always start on the fat side on the main jet. Then work your way down till you get a good plug reading(cocoa brown works for me), and the engine is pulling strong in all the gears. After that you might have to tailor your needle setting or run different taper needles to get your 1/4 to Full throttle response dialed in. Looking more and more like it would be easier to run the single carb set-up, but that's another story. Bottom line: Start out bigger on the main jet than you would have in the stock carbs on the engine with the same mods, then work your way down. Every set-up is different. Quote Link to comment Share on other sites More sharing options...
Lepew Posted April 26, 2004 Author Report Share Posted April 26, 2004 Thanks for all your replies. I'm going to replace the carbs on this bike back to stock size bore. The carbs have been bored out to much (27.3mm). I concluded you can only bore a stock banshee carb to 27mm because A: the slide is 27mm at the smallest dia. and B: at 27mm you will barely get the motor to idle because the slide won't go down enough to close the throat. The carbs I have have a slight air gap on the side of the slide that is allowing air in with out pulling in fuel. I have to have a larger pilot to make up for this. I should try 28mm carbs and see if the motor needs them. Your real close "The Rebel" on your jetting. I was running 320 main ,needle #3 and #40 pilot and was a little rich on the pilot. I will probably be around a 320 main, needle #3 and #30 pilot with stock bore carbs. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.