Needle shim modification
 

The GZ in its stock form comes ridiculously lean from the factory in order to satisfy the EPA. Here is the modification that I, as well as Pat Henry, documented over on the Yahoo board. Hopefully it will be a help to those who want to fatten up the mix just a bit.


How to remedy the lean carb setting:


Original Post:

After reading a lot about shimming the needle on the carburetor to counteract the leanness of the bike, I decided to do just that this afternoon.

Went to the hardware store, bought the appropriate washer and headed back home to tear into my little project.

On the GZ, the carburetor sits directly beneath the frame and where it makes a "Y" to split down both sides of the bike. The two screws that hold the top on the carburetor are easy to get to but watch which Phillips screwdriver you use, as a #2 doesn't like that screw.

Once you have the screws almost removed, grab hold of the top of the carb. The reason for this is when you remove that top there is a spring, approximately 5-6 inches long that will come flying out of the carb. It takes a little finesse, but if you will hold the carb top with your left hand, and take two fingers and compress the spring down into the carb then you can remove the top between the forks of the "Y" that I spoke of earlier.

Removing the diaphragm is a delicate proposition. I sincerly think that body prophylactics are stronger than this particular diaphragm. :D Again, you can remove it between the forks of the "Y", just don't manhandle it. It is a VERY delicate membrane and will tear easily if handled wrong.

Once you have the diaphragm and the needle body pulled, there is a plastic "thing" -- for lack of a better term, that has a rubber washer on the end of it that holds the needle assembly in place. Be careful when you remove this (with needle nose pliers). It is not threaded, and simply pulls straight out. The caution comes because there is a tiny spring underneath that plastic piece, which allows the needle to move up and down. It can be lost VERY easily. Also, there is a tiny washer that sits on the very top of the needle beneath that spring I just spoke of. Again, easy to lose if you aren't expecting it.

Once that plastic piece is removed, you can then lift the needle out of the slide assembly. You will then slide the washer up the needle and against the plastic shim that is already provided stock on the needle. [please read the beginning notes for suggestions before continuing] Once you have done this reassemble carefully, taking care to make sure everything is free, with no binding, and that all parts are replaced in the order you removed them.

My result from this. I took a test ride afterwards, and the bike does seem more confident moving up through the rpm range. I occasionally would notice a bit of lag in spots during acceleration, and this seems to be gone. I'm going back out in a bit to ride it for several miles and should have an even better feel for this mod then.

It is a modification that is easy to undo if you decide you don't like the resulting effect.

Thought I'd share this with the group if anyone has been toying with the idea and wants to try it.

Tom


Here is Pat's addition to the needle shim modification...

After Tom's excellent description, I finally got the chance to go out and do this mod today. What a pleasant-to-ride bike this has become after the modification (not that it was bad before). But it now has a much more linear and even throttle response - at least under the cool/humid conditions that I rode under today! Very nice.

Some important notes in addition to Tom's excellent comments (which should be read carefully). Suzuki makes it really easy to do this modification on the GZ250: you don't really need to buy an additional washer! Just take the washer that's already there, the one directly above the needle circlip and underneath the spring, and move it to just below the circlip (where it will raise the needle slightly). The spring will seat directly on the circlip after the mod., which I figure is OK since there is no movement - the spring just keeps everything seated in place.

Some additional notes in addition to Tom's: It's probably best to put the washer directly underneath the circlip and above the longer plastic spacer that's alread there (remove the plastic spacer and place the washer above it), rather than below the plastic spacer. Reason? This long plastic spacer has a flat spot at the bottom on one side that must match the mating flat spot in the slide/diaphragm that it sets on in order to seat properly (I almost didn't see
this). If you insert the washer at the bottom of the plastic spacer, it could sit a little cock-eyed on this indentation causing an alignment problem.

All in all this is a simple mod. to do provided you take your time and are careful not to lose or damage anything. For those that believe a picture is worth a 1000 words: take 9 in this picture and put it between 10 and 11. You've done the mod!!

http://www.ronayers.com/fiche/300_0304/ ... arent=6620
or
http://tinyurl.com/8s4qv

This Suzuki washer seems to be slightly thicker than an equivalent hardware store version. And it fits perfectly without modification.

Amazing, I've had this thing (GZ250) 5 years and am still learning!

Pat

Good deal, was hoping one of you guys would post this. If you want, you can put a copy of this in the HowTo section as well.

If you want to move it, feel free to. I guess I overlooked the How-To section! :D

That's all you have to do? Any noticable gain after doing this? Also, is there a way to get it even more richer?

Yes, look in the how to section for my muffler swap. I have in there how to get the plug off the carb to gain access to the mixture screw.

Do you need to remove the tank to ba able to access carb better?

Yes.

Flat Spots Are Gone
 

I did this yesterday and the flat spots completely vanished. I used the move the washer method described above. It worked great. I removed the clip and moved the washer to the other side and reinstalled the clip. At some point I'm going to get a washer and put the stock washer back where it is supposed to be.

To do this I had to remove the gas tank, both side panels, air box bolts, and the carburetor. I tried getting the diaphragm out with out removing the carburetor but found this difficult. In my garage dropped parts vanish. I suggest removing the carburetor and doing the disassembly in the bottom of a clean white 5 gallon bucket so when the spring pops out it will be in the bucket. Working inside a clear plastic bag also works well.

I think my wifes bike has already had this done when we got it because it has no flat spots. It's still smoother and quieter than mine.

Hmm, diaphragm came out pretty easy on mine. Putting it back on was a 'little' tricky but went ok. Its good knowing about the spring BEFORE you take it off, so i was careful lol.

in the tight space when the cap didn't come right off I stopped and considered that there are springs in there I don't want to bend tiny little parts I don't want to loose. Safe is better than sorry in my book. Anyway I got to look in side my carburetor after 13,000 mile. Very clean. Shiny.