You are way off on how to rejet a carb MAC. You have to do it properly or you will not gain anything and doing it the way describe it will kill any performance that you will gain by running any aftermarket parts. Be patient as this PROPER description will be lengthy.

First off there are 3 circuits in your carb that relate to THROTTLE POSTION and not RPM. The first circuit is idle to 1/4 throttle that is controlled by the A/F mixture. The next circuit is 1/4 to 3/4 throttle which is controlled by the needle and the last circuit incase you haven't figured it out is 3/4 to wide open throttle (referred to WOT from here on in) is controlled by the main jet. All these circuits overlap and deliver fuel at the same time when at WOT only. When at idle the primary circuit is the only thing that delivers fuel, at 1/2 throttle the primary circuit and needle valve meters how much fuel is coming through the main jet. At WOT the metering rod is fully removed from the main jet and the full amount of fuel is being delivered. The problem with MAC's reply is if you are lean or rich after changing the main jet and you adjust the A/F mixture screw you will gain very little if any. This is because the pilot jet is so small that the amount of fuel it meters is miniscule compared to the main jet. The proper way to rejet is as follows:

1. Do a plug chop at all throttle positions to begin with and after doing each adjustment a plug chop should be done. Do not go in and change everything at once as you will not have a clue what you screwed up if the thing runs like a POS. If you find that the plug looks good then you have saved you a crap load of work. Use a new plug for each position and for starters do idle, 1/2, and WOT with the engine under load. A plug chop is running the engine and shutting the engine off at that rpm, removing the plug and seeing if you are rich or lean.

2. You find that you are rich or lean after the plug chop so you must change the main jet to smaller or larger depending on the plug read. Do WOT throttle runs until you find the proper jetting.

3. Now that you have the main jet figured you can go to the ½ throttle position. If you are rich hear you move the washer on the needle up so that it lets less fuel through the main jet and opposite if you are lean.

4. Now it is time to do idle. This is fairly easy as you have the engine idling and adjust the idle screw in or out to attain the best idle. The screw either meters fuel or air depending on what carb you have. As on the Grizzly it meters fuel if I remember correctly and you screw it in to lean it out or screw it out to richen it up.

5. After all this you have to go back and do plug chops at the same throttle positions to make sure that you are not rich or lean. If you change any of the carb settings you have to go through the same sequence to see where you are at as each circuit will have a slight influence on each of the other 2. When you have it all done you are good to go. It is also better to be a tad on the rich side as if you are slightly lean any vacuum problems can lead to a really lean condition and melted piston.


This is quite lengthy but to do it properly and gain the most HP out of the engine just throwing in a jet will not help at all unless you are extremely lucky and nail it with just a jet change. I am going to make this a sticky as there are very few people that know how to rejet. The difference between the machines that always seem to run a little faster than most are the ones that have had the carbed tune by someone who knows and understands the carb and what changes influence the machine. If you do go buy a lottery ticket. I am going to try and find a diagram that show the carb circuits to help clear up my description.

I copied this from another forum. Good info!

 

good post , very detailed and more work then i want to mess with.

 

Exactly my point! Most owners just create a monster when they don't know what to do or how to do it.

 

Well .......

I oughta know better than chime in on this - but here we go .

The post you copied is a first step in understanding carb tuning - but not the end all and be all to carb tuning . There is a lot more to it than what is written in any manual , tuning guide , or any other piece of paper . (or my post for that matter)

The basic layout of carburetor circuitry is pretty well correct - or at least what the engineers of the carburetors wish they would work .

When I tune a machine I keep in mind INTENDED USE - the same engine to be used as a woods race or a dune runner will not be jetted the same , there is always a compromise when working with a carburetor - it's not fuel injection . -->(which I make no claims of being an expert )

Read the above information and understand this is how the carburetor is supposed to work - this will give you a good understanding of the basics .

Plug chops are something I may do after I know I have the job done right , I don't find it an accuratte way of learning much - and I think it gives most people the wrong impression that the engine is tuned right . I can't tell you how many times I have talked to customers that have done plug chop after plug chop and then burn things up when they put the quad in service . --- It can be hell I tell ya --- :-)

Thanks for your time boys - just remember - just cause you read it don't mean its always so - and - maybe a grain of salt on my comments as well . :-)

Your old buddy ,
Rick Ritter

 

RickRitter, Come on Down, you win the bonanza.

You're right, what I posted is what should happen in a perfect world. Most tuners differ in what and when a throttle chop is made. I like your comment about how riders use their equipment. I hope riders that advise others to just "rejet" read these posts and consider the events that they may set in motion.

I was also going to post on how to read a spark plug, but that could be a disaster.
Save that for another time.

 

Spark plugs are easy to read --- the letters are on the upper part of the porcelain -- sometimes you have to take the cap off to see them though . hahahaha

Rick :-)

 

Groan!!!!

 

Good post, I was looking for carb adjustments for my 30 Sp 700, since I put the 800 headers on it.
Are there air/fuel screws on the Mikuni BST 34mm carb?

 

Ill add my 2 cents, with my experience in jetting i always had to jet up or down as we said in the old days, not up and down as it relates to Mains and Pilots but as it relates to speed and torque, one jetted on the rich side will feel better on the lower end of the power as leaner will feel better up top, ive always had good results with changing jets and always taking it out for a test ride when Im jetted correclty it means im happy all the way around. But iahave altered so needle tip contours and really made a difference, and have changed all jets and made no difference, and have worked for days and made no progress, and then took the new supertrap muffler off and put the stock exhaust back and it runs better all the way around, So sometimes yeah ive done plug chops and used charts , but there aint replacement for seat of pants tuning and I always remeber Air fuel fire exhaust have to work in unison. when i increase one I may have to increase something else until it runs bad, then decrease to the last Increase, and it should be spot on, right now my machines all run mid range power increases, machine power doesnt mean stink if you cant ride it. but Mongrol and rick ritter have shed some Great light on the Jetting Dilemna.

 

yes i have the mikuni bst 34 carb on my 500 scrambler with the 500 sportsman engine in it and there is a adjustment right in front of teh boal were it goes to the engine intake. in teh centerthing that sticks out you will need a narrow screwdriver and might have to spin the carb or take it alltheway out to adjust idk i just found mine a few days ago it makes a big difference. mine wass 4 turns out and i moved it in to 2 and 5/8 were the book sais for the 500 sportsman