Some Jetting Basics

[mxbanshee]

O.K., I've been reeding some of these threads and have some information for guys aksking all these jetting questions.

 

-No one can tell you what to use to tune your quad.

-It's yours and it is very different from the next guys.

-There are way too many variable for someone to help you with all of them.

-Do it on your own.

 

Some Basics

-If you do ANYTHING to your quad that affects air or fuel flow, you need to re-jet.

-Jetting is all about AIR to FUEL ratios.

-Lean = to much air compared to fuel.

-Rich = to much fuel compared to air.

-Look at your plugs often, they have a lot to tell you.

-You have to jet (or adjust) for each throttle position.

-There is something mechanics refer to as carburetor circuits. (Pay attention now)

-A circuit is the order in which you tune your carburetor for different throttle positions.

 

Do as follows:

-Start at the bottom

-Then jump to the top

-Then work back down

 

Bottom: Start with the idle mixture screw or more technically the pilot air screw. This ONLY controls throttle position form closed up to 1/8 open. This is the only externally adjustable jet setting, it is on the left side of each carb. Now this is an air screw and NOT a fuel screw like on a 4stroke. So, you screw it in and you restrict the air(richer), screw it out to allow more air(leaner). Now just cause lots of guys like to KNOW were to adjust things to I'll tell ya for this one. It is APPROXIMATELY two turn out from bottomed out. Now I told you this cause it does not have a huge influence on performance. Set is as stated then reset your idle using your idle speed screws. Remember, idle mixture will affect idle speed. And oh ya, this will help a backfiring engine. If when you let off or chop the throttle your getting a backfire, richen (or screw in) your idle mixture. Perfect, we've got this thing idling, (with no worries of overheating an engine) now what? Oh ya, jump to the top!

 

The Top: This is your main jet. See, idle speed and idle mixture are required to start your quad, warm it up to running temp, to be able to test your main jet. This is important because in finding the optimum settings we do not want to do damage to our engines by running them to lean (read hot here). So, back to the main jet. The main jet controls 3/4 to full throttle ONLY. O.K. This might sound wierd to you, but go through all those nice shiny brass main jets you have and find the largest one you have. That's right, the one with the biggest number on it. Throw it in and start it up. What you have to do here is start real big, with lots of fuel, so your rich with no danger of overheating. Go for a ride, warm it up slow, then pin it. What gear you ask? If your racing MX use fourth, if your into drags use sixth. But rember, if your jetted for MX don't expect huge top speeds (nor would you need them). To get real technical if I'm going to a track that hase some long straights were I do reach fifth, I pull her apart and jet for fifth. Anyways...Back to the test ride. Wind er out in any gear ya like. It should skip! Yep, it might run O.K. or even real good untill you hit 3/4 to full throttle then the engine skips and sputters. What did you expect, your drowning the poor thing with fuel. Go back to your garage, (or wherever) and try one size smaller. Repeat joy ride! Keep going down one jet size until you don't have a stumble and normal throttle operation. REMEMBER, this is only 3/4 to wide open. If it sputs and farts getting there who cares. Just make her scream on top. Now, I'm going to forsee a problem so I'll adress it now. You might ask me. What if the biggest jet I have is not big enough and I blow her up? Well here's how to tell, if your going faster at 3/4 throttle than you are at full, your to lean (or not a big enough number). Make sure that from 3/4 to full throttlr your gaining engine rpm's, if not back off and crawl home. Go buy some more jet's, there cheap! Good, now we have the bottom and the top. Now let's worry about yanking our arms off!

 

Moving on down: Now it time to play with the jet needle. This controls 1/4 to 3/4 throttle. Picture here a needle going into a long brass tube with holes in it to allow air to pass through. The mixture of fuel to air is metered by the height, taper, and diameter of the jet needle as the air/fuel mixture passes around the needle and ultimately into the engine for cumbustion. Remember, what I'm going to suggest is for 1/4 to 3/4 throttle. It actually takes a bit of practice and concentration to be aware of throttle position, so do this preferably on a nice straight long trail or back road. O.K. Set your needle right in the middle position. Why not eh? It gives you play both ways. Now go for a ride. If you have a slight hesitation, just not quite there kinda thing, without it actually stumbling (AT THE 1/4 to 3/4) your too lean, so raise the needle. Remember, to raise the needle you need to lower the clip. If you have a hard stumble, like when going to big on the main jet, then your to rich. Lean out the mixture by raising the clip position on the needle. Easy so far right? It gets a bit more tricky. If your lean at 1/4 throttle and righ at 3/4 throttle you need to change the needle. Needles have different tapers. The diameter of the needle controls how much fuel escapes around the needle while still inside the needle jet (remember the holes). Again, this is just trial and error.

 

There are two more adjustments that you can play with once you have the above done. Truthfully, I have never played with these and my Bansh rips. But just so you know, you can adjust the slide cut away. This involes buying different slides with a different sized cut aways in order to allow more or less air to pass under the slide at 1/8 to 1/4 throttle. Another is the low speed pilot jet. This controlls fuel flow at 1/8 to 1/4 throttle. Both of these jets are ussually not affected by most pipe, air filter, and reed modifications. You know, you didn't give Yamaha all that money for nothing. They did do some things right that we don't need to mess with. They did afterall engineer those carbs for that engine.

 

There you have it. That's how you make your engine run at it's peak. I know reading this seems like "Man, I gotta do a lot of wrench'n". Well ya! Instead of spending so much time in here asking what size main jet you need, go out into the garage and to some wrenching. It takes time, some trial and error, and some dirty hands. Take it apart, follow this guide, and have some fun!

Edited April 8, 2005 by mxbanshee

[89Steeda]

Thanks

[fixitrod]

Pretty good right up.

 

There is also BenBB's faq that's been around for a long time here http://www.dfn.com/benkaren/ and some other good tuning info to include a spreadsheet for figuring gearing, electrical, plug chops, etc...

 

MX banshee, with stock cylinders some of the people on here can at least get someone within a couple of jet sizes with basic mods such as pipes and filter mods. I don't give specific numbers because mine wasn't stock long enough.

 

Where are you located anyway.

[mxbanshee]

Hey fixit,

I'm up in Canada.

[PUSH THE THROTTLE]

Nice write up MX

[sheefreak]

Well done!

[mxbanshee]

Thanks guys, I was just getting so frustrated reading all those questions! Figured I give some guys a hand, or at least my 2 cents. Afterall, jetting and suspension setup are the two most complicated (and time consumming) things to dial in.

Ah ha, great idea. I'll start a suspension thread... rebound, compression, camber, castor, traction, body roll... all fun stuff!

 

GO HERE:

SUSPENTION SET UP!

Edited April 8, 2005 by mxbanshee

[sheefreak]

I could use some help on the suspension, please do!

[richybanshee]

Good info.