If I don't get any bites on my DS soon I may start tweaking with it and was wondering that since I have a boat load of DJ jets on hand if I'd be worthwhile to go down the street and buy the DJ needle (they're about 3 miles from my house)? Right now I'm running the stock needle on the last notch with a K&N and open end cap, comments?

and while I'm thinking about it..... how did the airbox lid on/off debate settle out (or has it) in relation to the stock carb. Obviously If I pick up a DJ needle that'll leave me with plenty of adjustment to go open air box *IF* it's been found to make gains.

Thanks guys.

Rich

 

you will still have a problem with the slide opening fully without the airbox lid unless you do some carb mods.

 

Well I'm wondering about of that as the wife's Z400 has basically the same carb and the instructions that came with her white bros pipe said to remove the lid and rejet (came with a fatter main jet). Now, we went ahead and put a DJ kit in it after we put the pipe on and it seems to run great?

Just trying to understand why this forum is about the only place where it's mentioned to not remove airbox lids.

Thanks in advance.

 

because its been proven to loose peak hp numbers...and not open the slide fully.

 

How do you know the slide is open or not other than running with the intake tract removed (so you can physically see the slide)? Also, help me out here (seriously)....The slide is fed air via the tube that tees into the intake tract, so if you remove the lid more air should be moving through the airbox shouldn't it, and if there's a bigger volume of air moving through the box (and into the tract) you'd think the slide would open just a good with the lid open or close. I'm just trying to understand why it does or doesn't work.

 

thats what i thought also....so i made a custom airbox lid and everything but after riding it for drags and top speed there was no way the lid off pulled as hard or fast on the top end as it did with the lid on.... oldmanracing said that even with the slide holes drilled and the vacume tube taken outta the air intake tube it still ran best with the lid on....on the bottom and mid the lid off makes the quad pull hard but it just dies up top...the only reason i can think of is that the slide doesnt open all the way.....or all that air rushing around in the airbox isnt doing as much as you think.

 

CV carbs have one major inherit problem.
Air flow threw the carb must be straight, uni-directional and constant velocity.

Air entering the carb cannot be swirling or tumbling.
Three problems are generated.

First issue is fact for all carbs - HIGH TURBULENCE CAUSES LOW VELOCITY.
CV carbs are designed to control velocity going into motor. If the air going into the carb
is not controlled, the effectiveness of the carb to control velocity from the carb to the motor is greatly reduced.

Second issue - If air is not controlled, the air passing threw the venturi will have fluctuating negative pressure, causing bad slide control and fuel delivery. Also,
when the turbulent air hits the butterfly the velocity will be lowered more and the end result is reduced total power.

Third issue - Airbox resonance does not occur unless air pressure threw airbox and carb have high inertia. If airbox does not maintain even pressure then during reversion the slide will be pressurized and come down. This high speed flutter not only causes reduced airflow but fuel delivery is not uniform. Lid must be on and air controlled. Resonance is caused by spiking the natural frequency of the airbox system. Like ringing the bell.

Now you can understand why I have had more success with running with the lid-on.
I am trying to work around the inherit design of the CV carb. Because the DS is a single cylinder motor, the need to controll pressure and reversion is critical during the time that the intake valve is closed.