My Zilla came from Sea Level. Now it lives at 5200 feet.
Its stock, other than the pipe and the vented air box lid. (filtered vent)
I just pulled the main and it is a 400. Should I step back down to the stock 350?
In what increments do the main jets on Mikuini carbs rise? (350, 370, .... ?)
Thanks people!

 

There are way too many variables to be able to appropriately answer your question.

Here's my setup: I ride between 4,500' to 7,500' I have to re-jet for both extremes. Temp, and air density also effect performance/jetting.

I run Amsoil Series 2000 mixed at 50:1 (Which made my jetting richer than other oils, thus requiring me to reduce main and pilot jets)

I usually very between a 440 and 420 on the main jet (440 at the lower elevations)
I'm running a 32 to 35 on the pilot jet.

Mine has an FMF pipe, and a K&N filter directly mounted to the carb. Pretty well stock otherwise.

I definately wouldn't go any leaner (smaller) on the main jet, just because that's what the 'stock' jet size was, unless the machine is blubbering at WOT (Wide open throttle), then you could start by one step at a time. (Mikuni Jets make steps of ten (440 to 430 to 420 and so on...))

Just be careful, and read the plug and piston wash. You'll get it dialed in for your situation. (there are several useful sites on the net to describe what to look for, and how to jet)

 

I dont know what jetting you need exacly but I do know that 1987 has a different cutaway on the carb slide and I think a different needle than the 88+. What I am saying is 87 specs are different. It seems most 87 Ive seen run jets a little larger than 88+.

 

I am not telling you to ditch the stock carb, but the Kehin carbs are way less picky on jetting than the stocker, I fumbled with mine on no less than 3 500s over a period of 7 years before i switched to the 39mm kehin and even on a relatively stock 500 pipe, filter, ect, it was way easier to tune, I may be taking the easy way out and im sure someone knows how to jet the stocker like its going out of style, but for me the 39 has been a godsend

 

I tend to disagree ****. I've had just the opposite problem. Mikuni's have always been pretty easy to get tuned/jetted properly. (for me that is) The stock Kehin on my 250R was a tempermental sensitive piece of crap. even a small Temp change= jetting change, small altitude change = jetting change.. after two smoked pistons, I gave up and found a 38mm flat slide mikuni... problem solved.

Yes, I am probably way to **** about properly being jetted, and like it as close to perfect as I can get. I've heard many other people swear by the Kehin's... Just not for me.

If Mr350X wants to spend the $$ on a new carb... put a 44mm round slide on the mighty 500 and watch her scream. (Suzuki used a small carb in place of a rev limiter)

I really didn't mean to start a carb. bashing session here... just put in my $.02

It's more a personal preference kinda thing!

 

I had the same problem with me PWK39 I smoked a piston, Pain to jet right, dident fit the carb boots right. It is gone now.

 

<< If Mr350X wants to spend the $$ on a new carb... put a 44mm round slide on the mighty 500 and watch her scream. (Suzuki used a small carb in place of a rev limiter) >>




What does the 44 do for the low end power?, I am looking to change things up a bit and this sounds like it might be a lot of bang for the buck.

 

Truckracer,

The 44 will improve the top end dramatically, but there are always trade-offs.
You're bottom end will suffer slightly, but you'll notice the throttle response might be more sluggish than a real reduction in bottom end power.

The 500 has so much bottom end anyway... I don't miss it, but it certainly depends on your riding style, and what type of riding you do..

Dunes for me, so I need the top to run out hard..

If you MX, TT or woods ride, the 44 might be a bit too big... I'd consider a 39 or 40.. The stocker is too small.

You will find that the intake boot has to stretch a BUNCH to get it in there, but it will fit. Also, if you run the thumb throttle, the cable needs to be modified.