Jetting: Part 1

[jonathan180iq]

I'll be using this post to follow my attempts at jetting a GZ250 without a jet kit. This will not require any type of drilling or permanent alterations, aside from drilling out the idle mixture screw cover, which does nothing to affect the fucntion of the carb. You can consider this as a HOW-TO as well as a MODS post.
Hopefully, upon following my posts, you'll be able to avoid any kinks that I come across.
By the end, I suspect that we will have a total jetting HOW-TO with a total parts cost of under $20, minus the pod filter.

7-18-07:
The main jet in the 2006 model is a 120. The main jet size varies depending on the model number on the side of the carb (this info can be found in the service manual) and the year of prodcution.

Today, I went over the my local Suzuki shop and asked for the 3 next sizes up.

122.5, 125, & 127.5

At $3.00 a pop, this is not going to burn anyone's wallet.

Since the main jet is the first thing that needs to be adjusted for proper jetting in the other ranges, this is the first place to start.
When jetting, it is important to find the main jet that gives the BEST pull under 3/4-full throttle. I did not say good pull. I said best, given the options. Discovering which jet gives the best performance requires getting several jet sizes and testing them on the open road. This will require installing a new jet, testing it, coming home, pull the carbs and repeat. If you are switching over to pod filters, this will be LIGHTYEARS easier, as the removal of the airbox will no longer be necessary.

I purchased the three increasing jet sizes to have a pretty wide field. In a perfect world, I should have also bought the 3 lower jets and maybe even a 130.
However, I highly doubt that I will need a smaller main jet, as my current factory jetting is pretty spot-on and I feel that a 130 would be too rich.

Once I do some work this weekend, I'll post my results and then we will move on to the needle adjustment.

[jonathan180iq]

Drilling holes in the exhaust is kind of a waste. Just wait and save up for a real slip-on muffler at least.

If you're only going with the K&n filter, you won't need a 140 main jet.
Stock is 122.5 or something close. Start with a 125. Maybe a 128 or 130 should cover the intake.

EDIT: (Second thought, just slap a 130-135 on there. Worst case, you'll be a tad rich but that just helps the bike run a little cooler.)

Be sure to order the K&N filter for the GS500. That's the one you want.

Everyone, even stock, should do the needle shim. So go ahead and plan on that too. It's not hard at all.

http://www.gz250bike.com/forum/showthread.php?t=16

[Etsola]

Hey thanks for the reply ! I had thought I would've had an email notification, but I didn't.

I will probably go for an aftermarket exhaust in a bit, but in the meantime I just wanted it to at least sound better

And yeah, in Canada, stock main jet is #120. So, since we get colder weather, I'll definitely get 130-135-140's to test.

Looking at part #'s, what type of carb is it ? It says MIKUNI BSR32SS, but for jets it only says something like SR ... only 2 letters and there seem to be 2 types of very similar jets. Any idea ?

I already have a small pod filter that I tried to use on my Bandit GSF400 ... but tuning 4 jetted carbs is not that easy at all So I'll reuse it with the GZ ...

I'll probably create a thread with my bobber build.

Also, thanks for the link for the needle

Cheers

[mainlinecoffee]

For what it's worth I run a 140 just to be cautious,with a pod and slip on you can't be too safe. Plus more fuel never hurt,itl get burnt just the same

[raul10141964]

carb modding

1) main jet control the maximum amount off fuel available to the engine (part#16) factory 120 upgrade to 140 (I found my at my local motorcycle dealer , look for small mikuni main jet or take the old one with you)

2)needle shine control the fuel at mid range move the washer (part# 9)from the top of the E-ring (part# 10) to under the e-ring to improve the mid range

3) Pilot jet (part#26) control the fuel from idle to about 1/8 of Throttle factory set under 2 turns from the close position new set 2.5 to 3 turns, you may need to remove the plug (part#26A) to gain access, improve idle and reduce the fluctuation be twin col and hot idle

4) Upgrade air filter (I modify the factory filter see second picture)



http://www.gz250bike.com/forum/attac...9&d=1408145033

http://www.gz250bike.com/forum/attac...0&d=1408147180

[pfaffe1]

Hi, will it gain power to jet the carb and change the airfilter to GS 500 ( without tube inside ) with the original exhaust ?

Can i use the bigger jets from the S40 with 8mm top N100604 ? I have got some in 125, 130, 135

Thanks

[pfaffe1]

130 Jet instead of 118, PilotScrew 1 3/4, and this airfilter from GS 500 WITH large Tube inside. Not annoying loud and Drives perfekt with much power in all ranges, Floatlevel 13 mm

[kilo0195]

I ported, repiped the exhaust to 1.5, installed an emgo shorty glasspack, k&n filter, and rejetted to 147.5. I shimmed the needle by moving the silver washer to the bottom and made some more washer shims by drilling and cutting some pieces of a coke can. I also did the 16t sprocket and it will get up and move with the mods.

[spldart]

Unless your gonna bend the heck outta the inlet tube you need to remove the two 10mm bolts either side the airbox and shift it rearward about a centimeter to clear the carbs removal.

[spldart]

Oh! And I anxiously await your jet test results mr. 180.
I have found whenever I through some K&Ns on a bike I had to go up 2 or 3 jets.
But I haven't done any mod like that on this bike... yet...