Much improved with help from you guys! Zoom zoom! (Teaser)

[teaser]

I live just north of Detroit, Michigan... I'm at 700 ft. above sea level - using BP premium 93 gas. When I got my 2005 GZ it was under-performing and could not be
taken on the freeway in it's original condition. I knew a 250 cc motor could do much better and I was willing to sacrifice a bit of gas mileage to get there.
I have been a technician my whole life, and I approached this as a "system"... first modifying the exhaust, (so it could breath but still providing some back pressure
as the engine was designed to operate with some back pressure,) then modifying the carb and using the K&N air filter. Thanks to Jamie (Squid) for his great pictures
and descriptions of his mods... a great help to me.
My current configuration:
Main Jet: 145
Needle Jet: lifted with factory washer plus two thin ones giving an additional 18 thousandts (.018)
Idle Jet: set at aprox. 3 turns out.
K&N air filter SU-5589 (washers behind the ears to correct for the depth difference.)
Exhaust: (see diagram below!)
Sprocket on motor changed to 16 tooth.

When I got this bike, it seemed to be geared incorrectly - first gear was only good for the first ten feet of travel... and the bike screamed at 55 mph and was difficult to drive.
The 16 tooth sprocket was an awesome mod and helped this a great deal, but it made a even bigger difference when I finished tuning the carb and got more power...(I'm down to
about 58 miles per gallon (city) but my top speed has gone up from 60 to almost 80 mph, (high speed shakes over seventy five but now I can do the freeway when I want to.)


I had to put washers behind the ears of this filter because it doesn't fit EXACTLY and I didn't want to ummm SCREW IT UP...ahem. (sixty bucks sooo..)


I got my carb jets from a website that sells parts for ATVS... I bought several for testing (only about two bucks apeice) thanks again Jamie (Squid) for the size and part number info.
It took seven tries to get the right needle jet setting and and a few more for the correct main jet... removing and examining the spark plug after each adjustment and test ride.
The main jet is a Mikuni type N102/221 (N102.221) <--this is the part number.
By the way, Jamies helpful post is under GZ250 Technical, How To's, GZ250 Performance Mods.



If you add up the size of the openings I put in the inner baffel plate, it is about the same size as the pipe that goes into it... but they're offset
still causing the exhaust gas to "turn around" before exiting. Sounds good and lets the motor really breathe with the K&N filter. (and the price was right,
zero... letting me spend in some other areas!) That trim ring around the end of the exhaust is actually a clothes dryer clamp I polished with steel wool.
I had to cover a notch I cut trying to remove the rear baffel plate before I decided to leave a part of that rear baffel to support the stainless screen I
wanted to add.



When testing the bike for the proper main jet size, I did the following... clear stretch of road on a warmed up bike, accelerate to almost top speed in FOURTH gear, then back off
slightly on the throttle... this chokes the bike slightly and if the main jet is too small it will actually increase in speed. Factory is a 120 jet and when I put the K&N on there unrestricted
it ran very jumpy and obviously out of tune. (wayy too lean.) First I put in a 135 jet to "ball park" the main jet from the numbers I got from other members on this site. Then I did the
needle jet adjustment first. When I got the spark plug to a consistent brown color at medium speed then I went for the main jet (high speed) adjustment. The 140 main jet did "work"
but the bike felt just a bit...starved for lack of a better term so I put in a 145 and so far everything seems fine.

I did install a tachometer, the one mentioned elsewhere on this site (50 bucks, yes the gz is a dual fire ignition because it has a spark during the exhaust
cycle,) and I wired it up to the horn circuit (orange wire for my american version,) and grounded it to the battery.) Too bad it only goes to eight thousand, I'm sure the gz motor can
hit 9500 probably redlines around this area. I would not recommend hooking the power to the same connections as the coil, or grounding near the coil either - (you guys complained
about needle bounce?) The only wire near the coil should be the one sensing wire that connects to it k? I made a little metal link to lift the right hand side of the tach to straighten
it out.
Makes more sense to my brain anyhow.





Now I can look down and tell what gear I'm in...(and of course it looks good,). Helps with the tuning of the bike too.

To match the exhaust and just for all around "coolness" I modified the airbox cover - made a "sharkmouth" intake. I drew the shape with a dry erase marker and
hogged out the opening with my dremel. Finish sanded being careful of the paint, and made sure my mounting hardware was polished, and that the holes were
in an area that had plenty of plastic "meat" around them...don't want to be close to the edges. Cut the screen and only put a slight bend in it - I wanted
a bit of tension on the screen when I tightened down the mounting screws but not too much. Came out real nice, and lets out some more sound when I get
"on it." say waaaa heh heh.


I could not have gotten this far without the help and inspiration of the people who posted to this website, not to mention the creators of it. Thank You so much really.
My bike is running great, I can take it anywhere and keep up with traffic, even outpace it sometimes but hey... RIDE SAFE Gentlemen and Ladies - like it says on my
tank as a reminder:



I just had to add an update, I just received and installed a 39 tooth rear sprocket (steel, JT) - I found it at WEMOTO (dot com) a store in England. I did have to drill the
mounting holes a bit larger (no big deal,) so the four bolts would fit. Since I already have a 16 tooth front sprocket I didn't have to shorten the chain and so far I'm liking
the new ratio. (I hit 65 in fourth gear, with a little room to spare - and 75 mph is more than do-able.) Very pleased so far. So originally I was 15 to 41, (2.7333...) and NOW
I'm at 16 to 39 (2.4375). In fifth gear my odometer is lining up with my speedometer, 4000 rpm is 40 m.p.h. 5000 is 50 and so on. Worth the change, I'm thinkin!

[Water Warrior 2]

Sounds like you have a good handle on things. Well done.

[blaine]

Nice job.Looks awesome!! Thanks for taking the time for the pics & info.Safe travels.
:cool: :2tup:

[mrlmd1]

Have you tried it on regular gas? What benefit do you think you are getting with premium besides lightening your wallet so you have less weight on the bike? (lol)

[alantf]

Originally Posted by mrlmd1

Have you tried it on regular gas? What benefit do you think you are getting with premium besides lightening your wallet so you have less weight on the bike? (lol)

These bikes are designed for the third world,where they can use the poorest fuel available - remember, it's a low compression engine. Like I always say - They can run on cats' piss if necessary. Don't waste your money, just get the lowest grade petrol available. :2tup: