Just wondering how much of a corelation between intake velocity and jet size. Say your using a larger main jet than most people say a 190 (keihen size) does this mean you have a low intake velocity and should be running a smaller carb? assuming of course you dont have any mechanical problems such as air leaks and such. thanks!
intake velocity
- Thread starter Surfrjag
- Start date
well idk if this helps but it would come into play... a bigger carb alows air for top end and a smaller carb gives low end... to big and you kill your low and mid.. to small you have no mid/top
Jet size is only one part of it. The taper of the needle, length of the needle and size of the emulsion tube opening all play into the "carburetor process"
You can have a BIG jet, fat needle , set very low and a tiny emulsion tube and then have a HUGE crankcase compression ratio (drawing lots of air in), your still not going to get a lot of fuel in the motor.
There is no real formula to go by, since there are so many variables.
Main Jet Size
Emulsion Tube Size
Emulsion tube length
Emulsion tube profile (straight or tapered)
Needle Profile (taper, length and shape)
Needle Height (installed clip height)
Carb Bore (straight, tapered or stepped)
Slide style (Round vs. Oval vs. Flat)
Slide Cut Outs
Crank Case Compression Ratio
Engine Intake Size
Engine Intake Length
Engine Intake profile (taper, stepped or straight)
Sq. in. of reed area
Carb intake Size
Carb intake Length
Carb intake profile (taper, stepped or straight)
Then you add into the equation such things as "power up" devices that increase velocity across the emulsion tube at low throttle positions and that throws a whole new monkey wrench into play.
You can have a BIG jet, fat needle , set very low and a tiny emulsion tube and then have a HUGE crankcase compression ratio (drawing lots of air in), your still not going to get a lot of fuel in the motor.
There is no real formula to go by, since there are so many variables.
Main Jet Size
Emulsion Tube Size
Emulsion tube length
Emulsion tube profile (straight or tapered)
Needle Profile (taper, length and shape)
Needle Height (installed clip height)
Carb Bore (straight, tapered or stepped)
Slide style (Round vs. Oval vs. Flat)
Slide Cut Outs
Crank Case Compression Ratio
Engine Intake Size
Engine Intake Length
Engine Intake profile (taper, stepped or straight)
Sq. in. of reed area
Carb intake Size
Carb intake Length
Carb intake profile (taper, stepped or straight)
Then you add into the equation such things as "power up" devices that increase velocity across the emulsion tube at low throttle positions and that throws a whole new monkey wrench into play.
Assuming that everything is equal to the other bike, but your jetting and your plugs look identical, then a larger main jet could mean a lower crank case compression ratio (i.e. lower intake velocity) that requires the larger main jet to produce the same amount of combustion energy.
Things that can cause this are reed spacers. Spacing the reeds further from the motor increases the crank case volume, thus reducing the crank case compression ratio. Port timing changed, transfer ports enlarged, etc.
if your running a spacer, move it to the other side of the reeds, spacing the intake boot away from the motor and moving the reeds closer. See what effect that has. personally I dont like intake spacers. they have never been proven to me to make any advantage.
You are either moving the reeds away from the motor and reducing the charge circuit
or
You are moving the carb from the reeds, increasing the length of the intake track and causing greater restriction in air flow past the slide/emulsion tube.
Neither of which make much sense on these lil motors. Now on BIG Motors (500+cc) with HUGE charge circuits (16:1 and above) and gigantic carbs (50mm +) with an over sized tapered intake tube from the carb to the reeds that moves the carbs away from the motor has been proven to increase top end HP. The power band is very narrow and the motor is ran WFO.
You are either moving the reeds away from the motor and reducing the charge circuit
or
You are moving the carb from the reeds, increasing the length of the intake track and causing greater restriction in air flow past the slide/emulsion tube.
Neither of which make much sense on these lil motors. Now on BIG Motors (500+cc) with HUGE charge circuits (16:1 and above) and gigantic carbs (50mm +) with an over sized tapered intake tube from the carb to the reeds that moves the carbs away from the motor has been proven to increase top end HP. The power band is very narrow and the motor is ran WFO.
You're exactly right AWK. All of these big carbs are robbing their owners of low end to mid throttle response. If you're only running in drag races, then that's OK. But put really simply, if you go bigger on the carb throat, you'll decrease velocity at the jets, not just the main, all of them.
Do a little experiment;
Purse your lips like you're going to whistle, but suck in through them instead. Then widen your lips a little as you inhale. When the hole is small, you can feel the vacuum pulling on your lips exactly as the air pulls fuel from your jets, as you increase the diameter of the opening, you'll lose that pull on your lips. This is the venturi principle that all carbs are based on.
When your opening is larger you'll have a lower velocity and you'll need a bigger jet (pilot/mid/main - all of them). At some point, you'll only have a good vacuum at WOT and very little vacuum at the low end. But all along, you'll be making a trade-off, giving up bottom end in favor of high end air/fuel flow through the engine.
Richening the pilot and needle clip position will provide more fuel but it is dribbling fuel and not mixing well. So at some point, you will be running LEAN in the low to mid ranges no matter what you do b/c the fuel is not atomizing well. Not only that but, you'll be forced to use only the top end of your throttle range. Very little throttle range = very little control.
Now if you've done some engine work and your air flow is a lot higher, then going up a little (28mm-30mm) is probably a good thing. But you will always be making a trade-off to some degree.
Hope this helps a few people understand their jetting a little better I:I
(see the "But" to Surfrjag regarding plug chops two posts down)
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Hey Surfrjag,
To answer your question directly; probably yes, you should be running a smaller carb. I'm not familiar with aftermarket carbs since I'm only running the stock Mikuni's on my Banshee and Blaster, but even if your plug chop is looking perfect, you're probably ignoring low and mid unless you're doing the plug chop at each range. And from what I read on here, very very few people do all three ranges.
On the other hand, if your doing the plug chop in all three ranges (use the throttle stop screw and start at off-idle) and you're getting them all just right, then you're fine. I:I
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Yes you're right, physicists will call it Bernoulli's but engineers will often call it Venturi Effect (the shape of the carb throat) that the Bernoulli principle works on. The name isn't so important as the understanding most will get by pulling air through their lips as described.
http://en.wikipedia.org/wiki/Venturi_effect
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interesting thoughts. do a google search for Boswell Energy Systems or Boswell Carbs. Interesting to say the least.
I have one brand new 38mm boswell left. Dont have anything to run it on at the moment, but Ill find some interesting project sooner or later.
Too bad the guy was such a crook and went to jail for it, he is truly a mad scientist when it comes to carbs
I have one brand new 38mm boswell left. Dont have anything to run it on at the moment, but Ill find some interesting project sooner or later.
Too bad the guy was such a crook and went to jail for it, he is truly a mad scientist when it comes to carbs
i cant find anything on his theory or how his carbs worked or setup but i did find that they were pretty good. He was convicted on june 28,2001 for securities fraud. He robbed 400 investors of over 2.3 million dollars. stay away from that guy!!
keep searching, there are pics of the carbs around. Ill grab some pics of mine this next week and show what all he did just to the outside.
Pretty cool stuff, what I can figure from coments on how people have to go about jetting his modded carbs is he apparently found a way to make the circuits (low/mid/hi) more interactive to get better fuel atomization right where we've been saying it's needed - in the low to mid range on high diameter carbs.
Btw, that vortex throat on 99LRDblaster's ^^^^ avatar pic is pretty interesting too. Looks like it would help some. What brand/model carb is that?
I do believe it is a fudged image.
I landed here looking for http://www.blasterforum.com/engine-mods-swaps-129/improving-velocity-48827/index5.html
Higher velocity intake resulted in smaller jet needed in our experiment.
The "swirls" in the intake, exhaust, and head are not fudged images at all. They're quite real and quite useless. Wall effect makes them complete bunk...
They were a product of Hollywood Dirt Productions many years ago. Fancy looking but produced no more power than a "regular" portjob. Of course because it *looked* more powerful they charged a premium.
