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]

After looking under the tank to determine how much room there is for a filter, it turns out that everyone's concern of clearence is unfounded. Once the airbox is removed, the amount of room is well over 6 inches. A long straight filter or an offset pod filter will both work fine.

Being the case, I've ordered a two-stage pod filter from Uni, through my local bike shop.

The part number that you'll need, if you follow my directions directly, is UP-4200-ST.
Variants of the first number changes the LENGTH of the filter. For example, if you wanted a 6" filter, you simply ask for a UP-6200-ST

[Easy Rider]

Originally Posted by jonathan180iq

I'll be using this post to follow my attempts at jetting a GZ250 without a jet kit.

The main jet in the 2006 model is a 120.

What's contained in a "jet kit"?

Not at home now to look at the manual but my thought if and when I ever get around to doing this was to make it match the European specs. How do your selected mains compare to that?

[Easy Rider]

Originally Posted by jonathan180iq

Being the case, I've ordered a two-stage pod filter from Uni, through my local bike shop.

OK, I realize that this is kind of necessary for anybody who has removed the air box.............but do you really think this will end up being a significant improvement over the stock filter (with the "mouth" opened up a bit, maybe)?

[jonathan180iq]

Pod filters require the removal of the air box. In doing so, you are eliminating some of the low end torque and baffling of intake noises that are created in the air box. I'm not sure what the air box theory is called, but it has to do with vaccums being created. This will also require a home made vent for the crankcase gasses, as they are no longer able to piped back into the air box. If you are looking for a little more juice while cruising, or on the top end, this mod should be pretty beneficial.

In doing so, the stock jetting will need to be adjusted to meet the demand for more fuel. A factory jet kit, usually supplied by an aftermarket company, comes in what are called "stages". They have stages 1,2,&3 depending on the mods done to a bike. For something like an air filter and a slip-on exhaust muffler, you're looking at stage 1 or 2 jet kits. These things contain several different jet sizes, needle clips, tapered needles and idle screw settings, depending on some R&D done of a specific bike. These kits usually sell for around $80. However, for us, we don't even get the option, as there is no jet kit available for the GZ250. This is why I am attempting to show you guys that you ca achieve the same results by yourself. It will require tweaking on more than one occasion, but that happens even with a factory jet kit.

I'll be using a "T" adapter which will connect the crankcase vent hole to an escape tube on one side and a small automotive crankcase breather on the other. The breather will keep out contaminants while the draintube will be a catch for the oil blow-by.

I'm not sure how these mains relate to the European model. In my service manual, there was only one bike that used a larger main jet than the 120. I don't know what model it was, but it called for a 122.5 main.

Also, it seems that there is a secondary tube that injects air into the carb. It's the seemingly useless black tube that runs along the top of the throttle side of the carb. That hole will be plugged.

I'll be removing the air box this evening and photos will be updated sometime tomorrow.

[Easy Rider]

Originally Posted by jonathan180iq

If you are looking for a little more juice while cruising, or on the top end, this mod should be pretty beneficial.

I'm not sure how these mains relate to the European model. In my service manual, there was only one bike that used a larger main jet than the 120. I don't know what model it was, but it called for a 122.5 main.

Good luck! Sounds like it's a lot more than most of us are willing to tackle.
Have you considered trading up to an S40! ;-)

I think I will try enlarging the hole at the end of the filter, since that seems to be the only real point of restriction.

Are you using a "real" Zuki manual or Clymer, etc. ? My manual clearly shows the difference between the U.S. and two varieties of Euro carb configurations.

I do appreciate you sharing your experiences. I'm sure a lot of folks will learn from at least some part of it.

[jonathan180iq]

Thanks for the words of encouragement. I'm using a Suzuki service manual, the official one.

Well, it looks like we've already come across a snag. I went out this afternoon to go ahead and get the airbox out. This is a PAIN IN THE ASS!
It requires removing both side covers, removing the tank, removing the fuse box and wiring harness connector and pulling like hell to get the air box to slide back in the close clearances. Also, the tank mount brace sits lower than the tallest point of the airbox, so you have to really slam it in order to get it to slide under it.

Once it's loose from all connections to the carb, it slides back against the rear fender and then pulls directly out.

There are large connectors from the wiring harness that sit on top of the carb inlet duct. They make it a bit difficult to manuver around in that area. However, my original tought that there is PLENTY of room under the tank is correct. With the airbox out, there is also TONS of extra room under the seat, if anyone is so inclined to build a little storage box of some kind.

This whole ordeal took me a little more than 1 hour, cost me one bloody knuckle, 45 mosquito bites and a bit of swearing.

I'm beginning to think that this may be not worth the hassle.

We'll see,
Jonathan

[xt477]

things that I read on thegsresources.com stated that for pod filters go up 1 jet. for pod filters and opened up exhuast go up 2 jets. The gz seems so lean out the box that may have to add 1 jet size just to get a good starting point.

So I'm banking on the 3rd jet you have.

[jonathan180iq]

The problem with generic advice like that is that it is never the same. Jetting can vary greatly from bike to bike and even town to town in the same region of the country. It's important to note that my jetting settings may end up being completely different to someone else.

It may be possible that you are right. But simply hoping to get your jetting spot on with advice like "one jet for pods" doesn't really make any sense, if you want your jetting to be correct and functional.

[Badbob]

One thing the air box does is help to keep water out of the engine when it rains. If you can mange never to get caught in a downpour pod filters will work great.

Do you have to remove the air box to get the carburetor out? I've had mine out and I don't remember removing the air box. Perhaps the four carburetor Yamaha thats a nightmare to get apart made it seem so easy I didn't notice.