vfr jetting after air filter / bigger exhaust

Discussion in 'Mechanics Garage' started by FstaRocka, Jun 9, 2013.

  1. FstaRocka

    FstaRocka New Member

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    Hi all - been doing some reading about vfr jetting. I have a rc24 with K&N air filter. Bike runs sweet - but i put on a rather larger S/S exhaust on - and the power droped noticeably! while still fun to drive id like to regain some power back! The bike race shop here said i have to go up a jet size or 2. I have some micro drills and i plan on opening them up carefully with my dremel. Are the jet sizes in 1/2mm increments or something off like 1.18mm, etc? I have a micro set in quite a few variations.

    I did fill out a form with all my specs / upgrades and it recommended 130 mains with my current setup - does that sound about right?

    thanks!
     
  2. kennybobby

    kennybobby New Member

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    Hey FR,

    The number on the jet size is the orifice diameter in hundredths of a mm (0.01 mm). They are relatively inexpensive and would be hard to measure to verify if you were to drill them out.

    The stock main jet is 118, and there are several [120, 123, 125, 128 ] to try on your way to 130.

    Just have to say that i find it hard to believe that you would really need a 130--that is going up 5 jet sizes from stock.

    Try making changes one at a time then test to see if it has an effect.

    Here is a suggestion that costs nothing and can be done without pulling the carbs--open up your idle mixture screws 1 turn out and see how it does. If it is too much then close them back 1/2 turn, if not enough then open up another turn and try again. Then go either 1/4 or 1/2 turn closed depending upon the results of the second test. If it needs more fuel than this, put the mixture screws back to original position and try running a test with the choke lever opened 1/4, 1/2, 3/4 etc. That will dump a lot of extra fuel into the intake. Just a quick and easy way to test effects of going to bigger mains before pulling on your pud too much.
     
  3. FstaRocka

    FstaRocka New Member

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    makes sense - ive tried riding with the choke a bit closed, cant really notice much of a difference.. ill try the air screw.. i wanna take the carbs off and check float / needle seat settings anyway and replace those teeny orings (i might have lost one before), maybe its that? .. after leaving the bike for a year with petcock open i found 3 litres of fuel in crank case!!!


    i might just try and put on a smaller exhaust and see - maybe i lost velocity who knows. i did fiddle with the screws aa bit - i set them all the same, maybe its that. I havent syncd them yet, and maybe im looking in the wrong place, but so far everyone has suggested going up a jet size. btw - do the vfr's also jet the rear carbs a bit higher to the front since they tended to run a bit warmer? I see stage 1 and 2 kits with 114 and 118 jets and washers. Can anyone explain what the washers do, and how to use them (if i need to that is).. just curious since they ship with jet kits.


    * anyone know if the vf750 mitsubishi fuel pump is repairable? mine has an elec issue, ive dismantled it a few times.. the elec whizbang thing under the top cover just opens, but doesnt diconnect and repeat, ill either put a flasher unit on it lol, or something else. diaphragm works 100%.. just the elec jobby at the top - dont have $$ now for a replacement, and grav feed is killing me!
     
  4. kennybobby

    kennybobby New Member

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    The 'choke' is really an enrichment circuit that bypasses the throttle valve--it doesn't choke the flow like a lawnmower or automobile carburetor, but lets more flow around the valve as well as add fuel to the extra air. That's why the rpm goes up when the lever is pulled back--i refer to that as opening and maybe you call it closing the choke, but as long as we are on the same page about what it is doing...

    The washers in a jet kit are installed in the diaphragm chamber under the main-jet needle to raise the needle higher than the stock position. The idea is that for a given vacuum level the needle will rise up to a point with a smaller diameter exposed in the main-needle jet due to the needle taper. Basically get onto the taper sooner which puts more fuel into the mixture.

    These are all very subtle changes that can be made to fine tune and tweak if you have a good way to measure power output, e.g. dynamometer.

    Since you had a stuck inlet needle and a crankcase full of fuel you probably need to get the carbs in correct working order before making a bunch of changes.

    One main jet size would be from 118 to 120. Some of the bikes did go with larger mains on the rear cylinders. The same effect can be regulated using the mixture screws for the 95% of the riding that most folks do--unless you ride at full Wide Open Throttle the main jets hardly come into action for most folks...The pilot jets are used most of the time and control most of your fuel usage--i'll bet they are clogged unless you pulled the carbs and verified they were clear. One jet size on the pilot adds quite a bit of fuel. Clogged pilots and you run lean and won't feel like it's running with good power.

    Do a bench synch using drill rods or feeler gauge will put you so close to perfect you won't need to fiddle with the synch gauges. Unless someone has messed with them they don't normally go out of synch.
     
  5. FstaRocka

    FstaRocka New Member

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    hi - thanks kenny. I stripped the carbs last year and meticulously cleaned them blowing / checking as many orifi (correct?) as i could possibly find, and the unturned each mixture screw about 1 1/2 or 1 3/4 out from seated. I also verified float heights.. From standing a yr all the gas ran into crank case, so something is a miss there. Does turning counterclockwise make the mixture richer or leaner? I hae quadrajets on the brain which afaik r the opposite, in = leaner :)

    Im gonna try make those holes accesible while on the bike.. I actually mixed and matched from 2 identical sets of carbs, and noticed the screws dont face outwards on all of them. surely this is the way they should be right? or do they face inwards? I really want to be able to tune them on the bike..

    Il leave washers and jet drilling on the table for now - glad i didnt just jump in and make minkey assumptions. one step at a time, path of least resistance / consequence lol
     
  6. kennybobby

    kennybobby New Member

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    the mixture screws open up the circuit as you back them out ccw, which makes a bigger opening and lets more fuel out the port, hence richer mixture.

    The mixture screws should face out on all the carbs, e.g. 1 and 2 on the left and 3-4 on the right. If they don't then it will be very difficult to adjust. Can't really make any meaningful adjustments off the bike, it needs to be running as you tweak the screw settings...

    Use a number 12 guitar string to run thru the pilot jets to ensure they are open properly plus make sure the cross-drilled holes are fully open.
     
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