Jetting: Part 1
 

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.

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


http://upload9.postimage.org/345899/...0Stage2003.jpg

Re: Jetting: Part 1
 

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?

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)?

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.

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.

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

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.

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.

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.

Honestyl, Bob, I didn't try to get the carb out without pulling the airbox. I have never, on any other bike, been able to pull the carbs without removing the airbox, so I didn't even try.

I'll have the filters in by Wednesday, so you guys can expect more after that.

-Jonathan

Hi, I'm from Spain and I'm starting a similar project. The GZ250 is called "Marauder 250" here, and we've got a forum in Spain too: www.maraudercustom.com, successor of www.marauderos.org. I think that you got your workshop manual from there.

Related to this topic, I've purchased several main and pilot jets and a new jet needle to try it. I've already removed the air filter box (mixture becomes so lean) and I'll replace it for a "powerfilter" (our carb's mouth is 50mm diameter) and then I'll carb the bike. By the way, there's no need to remove anything to gain access to the carb and the jets.

You can view the carburettor config of my GZ250K6 in the last column of this chart, extracted from the workshop manual.
http://img.photobucket.com/albums/v2...ker/chicls.jpg

Also, the product codes of the jets are the following:
Main jet - N102.221
Pilot jet - N224.103 or VM28/486

I'll share with you my experiences as soon as I'll test the stuff.

Sorry about my English. :blush:

Don't apologize!
 

I wish half of my students could write as clearly as you did! I hope you continue to tell us of your experiences with jetting. I am on the fence about it myself. I do have the sixteen tooth sprocket on order though -- this forum sold me on that mod!

Buena suerte!
-Bob :tup:

It's nice to see that we're getting some international attention here. Best of luck to Jaime with his jetting as well. I would ask that you please make a separate post, perhaps called "Jetting: PART 2", so we can have varying options and advice on jetting without confusing the reader.

Also, I appreciate your compliments about writing. It's important to speak and write as clearly as possible so that people don't have to ask a hundred questions in order to understand what it is that one is saying.

The filter that was sent to me was the wrong part number. I ordered the correct number but received a different one.
So, plans are on hold for a bit longer.

We'll get there,
Jonathan

PS: Check the How-To section for a walkthrough of changing the sprocket.

Side note to Jonathan (off topic)
 

I did see the guidance on the sprocket change -- in fact it was that excellent "how to" and pictoral that convinced me that even someone with two left thumbs (like me) could do it. I enjoy my daily ride too much to miss it because the poor girl is scattered all over the garage floor while I try to figure out what piece went where. :cry:

-Bob

HAHAHA! Or becuase you were in a hurry and knocked your bike over on the floor... :blush:

I've installed the filter above, but it's nearly as restrictive as the original one, so I'm going to buy another with bigger filtrating surface. Then I'll try the jets and maybe I'll start the "Jetting: PART 2" section. :)

That I could live with...
 

Clumsy is as clumsy does Forrest...

...but it always galls me that when I reassemble something, there are always a couple of "installed spare parts" left over or a freshly bent bracket with half of one of my knuckles smeared all over it.

So my hat is always off to those of you who make it look so easy! :bow:

If your bike was sold new in Spain, you may be in an ENTIRELY different situation than those of us in the US and Canada. You probably have bigger jets already.

Well, I just TRIED to look it up in my service manual. Alas, I can't really tell what the carb. specs. are for Spain. It depends partly on what year model it is.

At any rate, the carb mods are only required if you open up the air intake, open up the exhaust a bit and notice a DECREASE in performance (or no change). If you notice a slight INCREASE in performance, then tinkering with the carb. is probably a waste of time.

I posted my jet sizes in this post's first page. I think they're very similar to yours.
About the need to recarb, replacing the original air filter makes the mixture too lean that you can't accelerate the motorbike above 1/3 throttle. That's because the original filter is as intentionally restrictive as it looks. The inner filtering surface is partially covered with plastic!

http://img.photobucket.com/albums/v2...er/filtro2.jpg

Exactly.

The reason you cannot accelerate above 1/3 throttle is because the mixture is now too lean. There is not enough gas getting into the mixture to produce enough boom to make the bike go faster. You are basically breathing too much.

I've got my filter, jets and some know-how. Now, I just have to find time to go outside and get it done. Hopefully, it will happen before the Franklin, GA meet next weekend.

EDIT:
In response to what Jaime said, even removing that little cover over the filter mouth makes a difference. Image what happens when the resonance of the air box is removed from the system entirely. On top of that, you are increasing the amount of air 10 fold.

Remember, LEAN = TOO MUCH AIR
RICH = TOO MUCH GAS

I'm glad that someone else here is trying the same thing. Hopefully, between the two of us, forum readers can get a pretty solid idea of what is involved in re-jetting their bike.

From the outset, it can seem intimidating. However, as long as you can follow instructions and have a little mechanical know-how, this isn't all that big of a deal.

Take care,
jonathan

I have great respect for the effort you are putting into helping your fellow GZ riders get the most from their bikes.

I cannot, however, let the above statement pass without comment.
It just simply isn't true.
The throat of the carb is only SO big. The only way to "increase the amount of air 10 fold" is with a supercharger; even a turbo doesn't provide that kind of boost! :roll:

Keep up the good work.......but let's try to keep it "real". :)

Ok.

So let's pull out the phrase "10 fold" and say "by an exaggerated amount" or "by a lot".

I don't know the flow capacity of this carb or I would attempt to give some real stats.

The point is that the removal of the compact air box and the replacement of the restrictive cottom element with a pod filter will significantly increase air flow.

:tup:
Jonathan

Not sure I completely agree with that .......but don't completely disagree either. :cool:

You've made your case; I'll make mine and then we can move on. Probably won't be able to "prove" it one way or the other with any scientific certainty. :roll:

Air box:
Is HUGE, compared to the carb throat. Can't see how that the box itself would offer any significant restriction. (And I think at one point you said you weren't sure that removal was worth the hassle).

Filter/air inlet:
Some restriction both these places. Some easy (and reversible) changes can be made here.

What I plan to do: (hopefully in the next week or so):

Borrow or buy a moto-tool so that I can remove the center of a stock filter (while leaving the mounting tabs for the diverter). This should remove a lot of restriction, with a much bigger hole and access to more of the filter element. I might then rotate the filter 90 degrees and put the diverter back on facing forward; will try it both ways.
This change can be reversed simply by getting a new stock filter.

Since it seems that nobody has done it (this way), I plan to drill 4 each 1/4 inch holes in the muffler back plate, INSIDE the ring. This change can be reversed by tapping the holes and screwing in 1/4 inch machine bolts (stainless, probably).

If these two changes achieve the desired result, I may then have a LOT MORE interest in your jetting project !!!! If not, nothing lost.
:tup:

Sounds good.

I can give two pieces of advice for personal experiments/mistakes.

My first experience, which was not a mistake, was to rotate the air filter housing 90degrees, as you have mentioned. I couldn't detect any noticeable change. Now, granted, I didn't go to lengths that you will in opening the filter mouth.

As for the muffler drill, I would consider this a good idea. It should open up some deep tone without affecting bike function. And, as you said, it's reversible.

I will warn you against drilling or punching the baffle that is down inside the muffler. I, on a curious afternoon filled with stupidity, decided to drive a punch down into my muffler and open up a louder exhaust note. THIS WAS A $600 MISTAKE. It makes the muffler sound like....well it made it sound like it had a hole in it. I was so obnoxious that I shelled out the extra cash for replacement muffler.
Bottom line: If you get a whim of curiosity and decide to try it out.....DON'T.

Let us know how your experiment goes.

-Jonathan :)

The air box isn't restrictive, but we must remove it if we want to use an universal filter. You can check this removing just the filter and running a bit; you'll get the same unusable bike than also removing the air box.

So, modifying the original filter is a good option too and requires fewer tweaks, but maybe you'll get better results with the full mod and it'll be more expensive if you want to undo the changes (original filters are quite expensive).

Yep, will do. Looks like it won't be done as soon as I hoped.
Too hot and humid here to do much of anything right now.

I'm really content with stock performance with the 16T sprocket..........until I hit that wall at the top end. That's all I really want to do is get just a tad bit more on the highway in top gear. I figure that's the only place that rotating the air scoop would make any difference anyway; a ram-air effect. Might not even do that since it's behind a cover.

Thanks for the advise! :tup:

The "ram-air effect" is achieved at very high speeds throughout a well designed ram-air system.
Let's compare it to the stock ram-air system of a sports bike like Kawasaki Ninja ZX-10R that boosts power from 128.7kW to 135.3kW. It's about a 5% increase. We could get the same 5% in our GZ250s (from 15.1kW to 15.87kW; such tiny increase) at a speed that we'll never reach (around 150 mph) and considering that our design is good.

Conclusion, you'll get an improve only if the original design causes turbulences and rotating the filter eliminates them, but not because of the ram-air thing...

Very likely that you are 100% correct.

I think, however, that my chances at a (significant) improvement doing that are just about as good as removing the air box to install a marginally less restrictive filter !!

:tup:

So we're on the same page. The only draw back of the 16T sprocket is this dead end around 65-70mph. Once you get up there, the larger sprocket makes the bike feel like it's too heavy to go much faster.

This is the only reason that I am altering the filter to begin with. I rode my buddy Mau's bike, which still has a 15T sprocket, and it feels more peppy and free spinning. He can also pull away from me on the open road.

Hopefully, with the correct carb adjustments and the pod filter, this dead spot will be corrected. If all goes according to plan, the added boost will be beneficial to everyone, sprocket change or not, but will allow those of us with the larger gearing to run at higher freeway speeds.

Right.

The air box isn't restrictive. Actually, it is only there to create a resonance effect and help in producing torque on the low end and give an area to catch blow-by oil from the crankcase and recycle some nasty engine fumes.

The restrictive part of the system is the filter and the filter mouth. (the area where air enters the filter)
As Easyrider has said, it might be possible to simply gut some of the filter and open up some air flow.

Since removing the D@#% air box is so frustrating, this may not be a bad idea. However, if you want to get the most out of the alterations, I'd suggest just pulling the whole system for a pod filter, as the paper filter element is the flow restrictor.

Expect results this weekend.

I didn't get as much accomplished this weekend as I had planned.

I did manage to get the oil changed and a fuel filter installed.

For what it's worth, the switch to synthetic makes the bike seem to rev easier. This may be my imagination, but it feels really free, responsive and willing to rev up.

I'm at a crossroads now, with this Franklin, Ga. trip coming up this weekend. I don't want to rush my results with the jetting, but I would like to test it over a long trip. However, I don't want to encounter a problem 200 miles from home. The boys in Franklin will be the first to know if I got it done.

The Final Word
 

Here it is. The final word on this.

I spent 2.5 hours working on this thing this afternoon and it's not worth it.

The jetting wasn't as hard as I made it out to be. The midrange was spot on and the main jet only required an increase to 125. The idle mixture only needed to be slightly tuned.

Even then, aside from some AWESOMELY raunchy noise under full throttle, the bike makes no more power. I did notice a loss of power taking off from a stop, but it was negligable. As Badbob mentioned in another post, it was, unfortunately a waste of time.

I'll post some photos from the process tomorrow morning.

I'm a bit discouraged, but I've got the bike back together and she's ready for the ride down to Franklin tomorrow.

SEe you then,
Jonathan

Re: The Final Word
 

Which filter are you using right now? I bought a small one and the jetting is similar to yours, but without filter even a 130 is still lean at full throttle.
Now I'm waiting for a big K&N filter and I spect a higher jetting than 125... Keep in mind that a "richer" jet needle could increase power too (the stock needle limits the maximum fuel flow of bigger main jets, even shiming it) and a bigger pilot jet will remove the power loss at taking off, as I've checked.

Also, now it's possible that the exhaust system would be the limiting factor.

I stopped taking pictures the second time that I pulled the carbs. The sun was starting to go down and I wanted to get the work done before it got too late and I couldn't see the important bits.


http://upload2.postimage.org/312737/DSCN8508.jpg



http://upload2.postimage.org/313006/DSCN8504.jpg

http://upload2.postimage.org/313008/DSCN8511.jpg



http://upload2.postimage.org/313009/DSCN8514.jpg




http://upload2.postimage.org/314024/DSCN8510.jpg

http://upload2.postimage.org/314030/DSCN8506.jpg

Wow, just the same filter I use... Definitely, it's not enough to improve performance. Too small.
Anyway, it's not all lost. I tryed my car's big Green filter and it has more potential than the tiny filter. There are still chances.

http://img.photobucket.com/albums/v2...cker/filt2.jpg
http://img.photobucket.com/albums/v2...cker/filt4.jpg



I started removing the fuel tank too, but it's not necessary for any task about the carburetor.

WOAH! What is that electronic device on the side of your carb? We don't have those....


http://upload2.postimage.org/313975/DSCN8507.jpg

It's a Throttle Position Sensor (TPS) that sends a signal to the electronic ignition. It's installed in latest years models, but I don't know what it does. It probably reduces contamination.

In injection vehicles, the TPS determines the amount of fuel needed, but in this case it isn't possible.

Wierd. I have a 2006. That's pretty new.

Thanks for the info.