180 main jet (help!)

40 replies to this topic
  • CHris_PArks

Posted April 29, 2002 - 01:10 PM


well, for you guys that run lean jets, what is all that other stuff you ajust. i live a sea level and run my wr in the dunes pinned alot. i run a 45pilot with a 178 mian and the P needle 4 down. i also have it yz timed all the wr crap off it with a full WB system open with no disks. could it be that there is a huge difference in everyones jetting do to where they ride and in what conditiones. would you guys running 160 mains take them out to the dunes with a paddle at sea level and runn them wide open all day?

  • endurodog

Posted April 29, 2002 - 01:24 PM


There is a difference in the WR timing to the YZ and the jetting you need to use.

  • Taffy

Posted April 29, 2002 - 10:23 PM


chris parks
check my sinature. i live 25 feet BELOW sea level.

there is no discernable difference between YZ and WR jetting. YZ may be one MJ leaner (we're talking just the timing alone here right?).

it's not too difficult to make a bike run ok slightly rich. but if you go lean for even a split moment the bike is a useless piece of junk. the art is getting right down to the correct jetting and not crossing the line.


  • endurodog

Posted April 30, 2002 - 01:42 AM


I was using a 160 MJ with my WR timing and had good results. Switched timing to YZ and upped my jet to a 178 and it runs great. That would be a noticable difference to me. I also notice that most of the jetting specs listed in the tech area of this page the guys with YZ jetting all went richer a minimum of one step and there comments say it works for them.

  • Rich_in_Orlando

Posted April 30, 2002 - 04:24 AM


I retimed my bike to YZ timing and went leaner (180 to 158) on the carb.

[ April 30, 2002: Message edited by: Rich in Orlando ]

  • Mark_Cantrell

Posted April 30, 2002 - 04:51 AM



Not trying to start anything, just a basic question. You started with WR timing and a 180. You ended up with YZ timing and a 158. How much of this is coincidence? If you went back to WR timing, would you be back at a 180 main? On my 250F, I went back from YZ to WR and didn't notice much change in jetting.


I know it's out there but are 'Taff timed' cams close or in the direction of YZ cam timing? I'm curious why the cam timing makes such a large difference (155 to 180), if it really does.


  • LarryCO

Posted April 30, 2002 - 05:26 AM


Just my .02 here...

Granted I'm in CO (the Av's sign above the visitor's locker room says 5280 feet above sea level...go Av's!), but I have WR timing and am running a 150 pilot jet. I've tried to mimic Taffy's jetting by compensating for altitude, which I've done, and the bike runs fantastic! FYI...my buddy out here has YZ timing (although I havent convinced him yet to lean his bike out like mine) and he's also running a 150 pilot jet.

Run what you feel most comfortable with...but there's quite a few folks out here leaning out their bikes considerably and are much happier for it. :)

Good luck!

  • Rich_in_Orlando

Posted April 30, 2002 - 09:13 PM


I posted that because Endurodog stated that "I also notice that most of the...guys with YZ jetting all went richer a minimum of one step and there (SIC) comments say it works for them." My experience is the opposite of what he observed and I just wanted to share that.

When I still had a 180 MJ, I tried YZ timing and it ran like crap, popping and sputtering like it was way too rich. I went back to WR timing for quite a while until I started messing with the carb. At least with WR timing, the bike would run with the 180 MJ. With YZ timing, it would not. When I decided to finally change to YZ timing for good, I steadily reduced the MJ. Before I did the complete "Taffy jetting", i.e. needle, PJ, PAJ, etc., I had changed the MJ to a 170 and it ran a lot better than all the larger jet increments between 170 and 180. But it still had shortcomings. Then "Taffy jetting" cleaned up the whole thing.

But, you know what? If you're happy with the way your bike runs, then I say leave it the way it is. I wasn't, so I changed the jetting. To each his own. :)

[ April 30, 2002: Message edited by: Rich in Orlando ]

  • MichaelAngelo

Posted April 30, 2002 - 12:37 PM


Sorry Boyz,
Didn't realize that I was opening a can of worms with this post. I was just curious about going bigger. It sounds rather simplistic, and it is, but I figured more gas to the piston the faster it would go. I have the wr timing, the bike breathes well with the T4 complete exhaust system and lid removed, and I was just wanting an edge over my buddy. I was just concerned about the possibility of squirting too much gas in there and causing valve damage. Seems nobody responded to that concern.
Thanks for all the interest though. :)

  • Taffy

Posted April 30, 2002 - 12:46 PM



did you change the timing to YZ and do the jetting THEN go ride the bike?


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  • Rich_in_Orlando

Posted April 30, 2002 - 02:46 PM


Originally posted by MichaelAngelo:
...It sounds rather simplistic, and it is, but I figured more gas to the piston the faster it would go...

I'm having
deja vu

  • Mark_Cantrell

Posted April 30, 2002 - 05:39 PM



Dude, I can't agree with your basic assumption. The bikes run best when lean. Most people that I respect here (and a couple are replying to this post) agree on this. I am thoroughly convinced from my tests and experience. More gas (after this healthy lean I'm describing) just makes it too rich, reduces the snap and robs the power. Way too much causes carbon deposits on the inside of the engine, plugs the spark arrestor, and fouls the plug.

The down side of lean is that it makes it run hot. The leaner the hotter up to the point it starts missing (you would hear rough running or popping at that point). The heat of too lean, combined with running it hard, can cause catastrophic engine problems related to melting holes in the piston and burning valves.

You are jetted correctly when lean enough to get the max power and snap but not so lean you damage your bike. The factory, not surprisingly, errs on the safe side, too rich. Too rich on the pilot, too rich on the needle, too rich on the main, and too rich on the AP. They know the savvy riders will correct this and the not so savvy ones will not hurt their bike.


  • MichaelAngelo

Posted May 01, 2002 - 06:16 AM


Thanks Mark...that about covers it. I appreciate your thoughtful reply to my question. I will try and see what going lean does.

  • Mark_Cantrell

Posted May 01, 2002 - 07:09 AM



You heard my point and that's cool, but let me say it another way anyway.

All gas engines (except early rotary airplane engines which had no throttle) directly control the power by controlling the amount of A/F mix into the cylinders. This is done on cars with a butterfly valve or throttle plate in the throttle body of a FI car or at the bottom of the carb otherwise. This directly controls the amount of air into the motor. Pushing the 'gas' pedal of a car doesn't directly control the gas, it controls the air. If the throttle is closed, almost no air gets in, instead there is a vacuum of air pulled against the closed throttle plate.

The job of the carb (or of the fuel module and injector) is to measure the amount of air, and for whatever amount of air that is, supply the right amount of fuel for it, i.e., give that air the right mixture of fuel.

In a fuel injector this is really simple. They measure the volume of air through a throttle body restriction (Bernoulli's equation and differential pressures), then measure the air density (with a barometric sensor), the temperature (with a thermocouple) and the humidity (with a humidity sensor). With this, and the standard percentage of oxygen in air, they plug in the richness they want and calculate the amount of fuel to inject. Just a simple set of equations done on digital computers hundreds of times a second. The sensors are simple. You could easily write an Excel spreadsheet that given the pressure drop across the throttle restriction, the temp, air pressure, and humidity, calculate the fuel required. The equivalent of this spreadsheet is repeated executed in your cars fuel injection module.

In a carb, which is very much a complicated analog computer, the vacuum of the air flow sucks in the appropriate amount of fuel. The adjustments for temperature, barometric pressure, altitude, and humidity are done by you, not the carb. You have to change jetting for conditions. But for the condition it is jetted for, it provides the correct amount of fuel for the air passing through the carb. The combination of the air volume (or velocity), pressure, temp, and humidity is called the MAF or mass air flow sensor and is sometimes a separate module from the fuel injector module.

As for more gas equaling more power, that would be correct if stated as 'more air with the appropriate ratio of fuel equals more power' or even as 'more fuel with the corresponding quantity of air equals more power'. The problem with the latter is that we don't control the flow of fuel with the throttle, we control the amount of air and the carb provides the correct amount of fuel automatically.

This really rambled. If you skipped to here, you might just let it go.


p.s., even worse, sorry in advance. The fuel injector in a modern car is driven by a target air fuel mix only when starting and under rapidly changing conditions (accelerating and decelerating). At steady state (driving down the road), it changes from this predictive calculation (predicting the amount of fuel to inject into the measured air mass) to using a feedback PID controller based on the amount of oxygen in the exhaust, provided by the O2 sensors in the exhaust manifolds. Basically, if things are stable, screw what we calculate it should be, instead adjust based on the how complete combustion actually is. In MX or SX or even trail riding, we are on/off the throttle so much a feedback controller would be useless. I am all in favor of a predictive FI for bikes. There would be no more jetting changes for conditions.

  • Mark_Cantrell

Posted May 01, 2002 - 07:12 AM


I just reread what I posted, what a bore. Anyway while writing that reply I went back and added the MAF line to the wrong paragraph. The MAF has nothing to do with a carb and everything to do with the fuel injector. Mentally move that line up one paragraph and it won't be so stupid.


  • MichaelAngelo

Posted May 01, 2002 - 07:55 AM


Mark, way cool dissertation on the workings of "air/fuel mixture devices". I think I get it.
Simplistic again...but...if one were to go up on the main jet (175-180) wouldn't a corresponding change in in pilot jet (say 38-@45-50) do exactly what you are saying in your reply? Still curious,

  • Bill

Posted May 01, 2002 - 11:21 AM



You do not have to change PJ when changing the MJ. Depending on what PJ or MJ you may want to change to the appropriate PAJ or MAJ.

SJ= when choke is on
PJ= 0-1/4 throttle
needle= 1/4-5/8 throttle
MJ= 5/8-WFO


  • Guy

Posted May 01, 2002 - 11:33 AM


:) Cue the music from............................
"The Twilight Zone"

  • MichaelAngelo

Posted May 01, 2002 - 11:36 AM


That's very funny Guy. Thanks for the info Bill.


  • S_Phillips

Posted May 01, 2002 - 04:44 PM


Mark -

I agree fuel injection is much easier. Just throw
in some bigger injectors. Make it breath better by opening up the ole exhaust say to about 3"(if your a ricer) Bigger turbo,,, etc. Mod the ECU
to over ride the stock maps. Then it's off to the races :)

Sorry guys just a little car talk :D

There is so much test/change and test/change again with carbs. Yes a lot of work but if you have time it's just as fun.........


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