99- carb theory 101 (running poorly)

Well see I got this here 99 on which I have installed a JD red needle, airbox free, blue and grey cut, otherwise stock. It seems to be running a bit funky and I'm looking for a little feedback. I'm at sea level and its hot and humid here, I'm running a 168 main, i reduced the pilot jet one below stock (sorry forget the number) which helped the starting characteristics allot. It´s low end power is good... pulls hard for stock. The top end is also good, but the transition from say... about .5 throttle into .75 throttle is a big stumble. When it clears it´s throat and goes into .75 to full it´s strong again, rips good, but its got a BAD stumble in that transitional area. Any ideas????

Second issue, this has been a problem since I bought it and still have found the cause, should be fairly easy for a carb guru type to nail the issue. It has a problem where if you shake the bike hard smaller hard bumps, the bike stutters and if severe enough will die. It´s worse when you are going up an incline, incline and bumps equals a stalled bike. I have adjusted the float a bit to make for less gas in the bowl, and I think that helped but it´s getting pretty problematic cuz I'm riding harder. So, I think I´m smelling gas when it quits... so were likely talking about a flooding condition rather than a starving condition right? Does that equate with the mechanical arrangement of the carburetor in relationship to the "incline" problem? Even though my float valve needle is new (the seat is old), seems I'm looking at either needle seat leakage or a still improper float level? Could the seal around the OUTSIDE of the valve seat (where joins the carb body) be bad? Sorry this is so wordy, but I'm looking to understand this problem as well as I can before I order parts. Basically I´m wanting to at least confirm that this is a problem of EXCESS gas rather than a momentary starvation problem.

thanks ya´ll


My first thought was float level too low...???

When it comes to solving ANY jetting issue, the carb must be disassembled for a cleaning of biblical proportions.

On our carbs, the air jets have been known to plug. This can be caused by excessive oil in the air filter, or not letting the air filter oil "set" prior to firing off your engine. The air jets are found under the air horn on the airbox end of the carb.

On my 99, I removed the entire air cut valve assemby and sealed off the unused ports. I changed the air jets (just the MAJ I think) to the YZF.

Also, you can improve your accelerator pump performance by installing a set screw under the diaphragm to limit stroke time of the pump.

According to Factory Yamaha, the optimum stroke is ~ 0.5 seconds. My 99 was 3.5 seconds! :busted:

I replied to one TT'er that posted in the jetting forum about installing our FCR onto his DRZ. Do a search there, under my user name.

Any other Q's, feel free to PM me! :excuseme:

Kevin :bonk:

Check your plug also. The JD jetting specs may not correct for you bike because it is older and more worn and not as fresh. If your plug is blackish, then go down in the main jet, if it is white then go up in the main jet. Also you want your plug to be a slightly dark tan, but not black, so this way you dont run lean.

Though I spend very little of my riding time at anywhere near full throttle, my plug is a nice tan color, middle of the range. My problem seems to be in the transitioning from half to three quarter, my inclination is to go down from 168 to 160, drop a clip leaner, and go one less in pilot.

Yes, I know... one thing at a time... I can never seem to do that :bonk:



Pilot jet is not going to affect .5 to .75 throttle. Thats the upper half of the needles range. And the main takes over at .75.

Although i am probably wrong like i seem to always be.

Right, but due to the fact that I get no response really from fuel screw changes (from .5-3 turns), and I get no popping on decel, I´d say it´s rich, though Im suffering no ill effects.

Am I right huh huh? Probably not... really though... I get didly when I change the fuel screw position... nada, SOMETHING must be off, no?


dont drop the main, just go as lean as you can go with the needle, and if you are already there, just drop the main once, then richen the needle. 3/4 range is needle.

Well, I will do just that then. thanks, ill let ya know.


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