Re: Needle shim modification
 

After the needle shim is it nessesary/helpful to modify the air filter? or shoul a complete rejeting be done first?

Re: Needle shim modification
 

You need to go back and read the carb modification threads carefully.
Those who have actually DONE it know more than those of us that have NOT !!! :roll:

IIRC, if you ONLY do the needle shim, you don't really need to mess with anything else.

After that, the jet change and idle air screw adjustment goes along with better air flow; you should consider those as a package and be prepared to do both together.....if you do any part of it.

Re: Needle shim modification
 

Well I shimmed the needle the other day and noticed alot more throttle overall. I also did the 16t sprocket mod the day prior to shimming the needle. Best results came from going back to the 15t sprocket and keeping the needle shim mod. I am much happier with the bike. It is more confident in all gears. I am still toying with doing the other jets and installing a K& N filter. my goal is to see if I can maintain the speed (or gain some) while going up a hill. i live in the foothills of NE Georgia and go up and down many hill in my commute. The only other hope is to just get a bigger bike. :??: :??:

Re: Needle shim modification
 

Easy, I have been riding as you suggested ( accelerating to a high speed before I approach the hill) for a while now and my complaints/ questions reflect still riding the way you suggested.

Re: Needle shim modification
 

The GZ is what it IS. No matter what you do to it, it will NOT accelerate UPHILL in 5th gear; it just won't.

OTOH, it won't completley choke either so you have 3 basic choices:
1) Learn to live with the limitations.
2) Put the 16T back on and shift to 4th for going up hills.......and #1 too.
3) Get something different that will hold the hills.

Re: Needle shim modification
 

Have learned to live with it, trying to like it.

Re: Needle shim modification
 

The real solution to your problem is another 250cc or so. Maybe a Kawayamazuki 500cc (or larger) would be better for your needs.

There are places where a GZ is just too damn small. Being 6'-1" and 280 lbs, I run into that occasionally here on the Central Florida hills.

If this is ongoing daily really-piss-you-off issue, start looking. With the economy in the crapper, this is a good time to buy a used bike.

I would suggest keeping the 'ol GeeZer though, as you can never have enough bikes.

:rawk: :lol:

Re: Needle shim modification
 

Okay. Post needle shim modification, I tracked my gas mileage. After many miles and several fillups, here are my results:

MPG per fillup:
==============
1. 68
2. 65
3. 65
4. 64
5. 65
6. 65

Average = 65.3

I was averaging between 65 and 70mpg before the modification. I am seeing about a 7% drop in gas mileage now.

I drive a regular commute each day and take joy rides over the same roads. Most of my driving is at speeds between 40-50 mph. Maybe 20% is open road, 50-55+ mph. The terrain I drive on is generally flat with some rolling hills.

I am very happy with the performance increase - the slight drop in mileage is small price to pay.

Re: Needle shim modification
 

Buck GZ ...
I've done the needle mod, the air intake, the jetting and exhaust. I also put in a NGK Iridium spark plug. That coupled with the 16 tooth sprocket makes this a sporty bike. I know of no one that has done all of these mods and regretted it. My bike Raven has never ever complained to me. I still have a couple of add ons to do to make it my special buddy. Go for it! :rawk:

Re: Needle shim modification
 

My miliage seemed to improve after the needle shim mod??? Before i would have to switch to the reserve tanks at about 185 miles now I switch at 200???? (and I know I shouldn't run it dry just a trial test)

Re: Needle shim modification
 

I did my needle shim and installed a 140 main jet and my milage droppped 10mpg. Don't know what the deal is. Thinking about reversing the procedure to gain back my mpg.

Re: Needle shim modification
 

A few others have had a similar experience but don't give up until you've run at least 3 tanks through.

What ELSE have you done?
16T
K&N
muffler
????

Re: Needle shim modification
 

The deal is that by increasing the jet size you are letting more gas into the engine than before. It's very commong to experience a loss in gas mileage when rejetting for more power. Try backing off just slightly on the jet size and see if you can maintain a proper mixture. You can find a balance between power and economy. Selecting jets isn't a written-in-stone procedure. If they make a 138, for example, I would try that. If you're get a lean mixture with a 138, then you need to stay with the 140 and you'll just have to live with the loss in mileage.

I just recently down jetted my scooter and I saw a 17MPG increase. Talk about exciting. Top speed and engine "smoothness" stayed the same. Some clown before me had it over jetted. I must be a clown too, for not checking it sooner. I dropped from a 125, which was only slightly rich, down to a 115, which is perfect.

Re: Needle shim modification
 

By the way, for those who don't know their way around a carb, like me, page 11 of the Parts manual (see the first post in the Troubleshooting section to download it) has an exploded diagram of the carb. It really clarifies the instructions at the beginning of this thread, especially about the springs and plastic "thing". ;)

Re: Needle shim modification
 

if you change the intake and exhaust to get more airflow, you are just helping the engine breathe better. carb mods are only helping to correct for that change. if anything, there should be an increase in gas mileage. if it decreases, i would wager that it's because the new power encourages more aggressive riding.

Re: Needle shim modification
 

No doubt that's part of it but making the mixture RICHER and allowing more of that richer mixture to "flow" through the engine pretty much automatically means that you are gonna use more gas in the process.

None of the changes described here makes the engine any more efficient.

Re: Needle shim modification
 

how can freeing up the intake and exhaust not make the engine more efficient? getting more air into and out of the engine is the definition of increased efficiency. no 4 stroke engine needs exhaust backpressure to run properly. the illusion of needed backpressure is just a sign of a poorly tuned exhaust.

Re: Needle shim modification
 

What ??

No it isn't.

The definition of "more efficient" is getting more power or mileage out of a given volume of fuel.

Real world reports from riders who have done the mods outlined here indicate, without exception I think, that the engine "feels" like it has more power and runs "better" but not a single one has reported an INCREASE in fuel mileage.......or even exactly the same. The difference isn't much but it is always a bit lower. That tends to indicate that the extra power they achieve requires a little extra gas to get it.......and that is not increased efficiency.

Re: Needle shim modification
 

:plus1:
Increased performance-Decreased efficiency.
:2tup:

Re: Needle shim modification
 

Not looking for a war of words here but you are so wrong. Some high end cars actually have a variable back pressure system to promote better scavaging at different RPM ranges.

Re: Needle shim modification
 

i could see having some type of system like that to make a car with a very narrow powerband easier to drive in the real world. but it's a crutch, not a power maker. they would need that extra scavenging cause the engine is out of its happy place.

maybe most or all people here that have changed the muffler and/or air filter on their bikes have then lost some mileage after retuning the carb. how did they tune it? on a chassis dyno? how else are you gonna REALLY tune the engine?

Re: Needle shim modification
 

You "mess" with the needle and main jets until it runs better............or take the recommendation of someone else who has already done that.

We may have had one nut....mmm..err..rider here who did some dyno runs but I doubt it.

Re: Needle shim modification
 

i guess i'll live with my muffler for now, but that stupid $40 air filter has to go.

Re: Needle shim modification
 

Talk to the designers of BMW car engines. Their variable back pressure system keeps the engine in it's happy place over a very wide RPM range. This is actually old technology now and I would not be surprised if a large % of cars have this to meet emissions while still providing performance and economy

Re: Needle shim modification
 

A good replacment is the Hiflo-Filtro-HFA-3503 at less than $20.00.It is listed for the GS 500 but is a direct fit.

:lol: :roll:

Re: Needle shim modification
 

And does NOT require re-jettting, if I remember correctly ????

The stock filter has enough filtering area that they should be good for MANY years of light to medium distance riding if cleaned occasionally. At $20, I'd say that's a good buy if you don't have to screw with any other settings. :tup:

Re: Needle shim modification
 

Does not any rejetting,drop in and go. :2tup:

Re: Needle shim modification
 

After 5 years and 50,000 kms my Vstrom air filter is still good. I don't look forward to replacing it because it is a one of a kind and only Suzuki has it. Waiting for Hi-Flo to make them at a cheaper cost and crossing my fingers.

Re: Needle shim modification
 

Would the needle shim mod work on a GN250?

Re: Needle shim modification
 

Yep,same carb. :) :cool:

Re: Needle shim modification
 

Too funny I just read this tip, and one of my first mods to any bike that has a slight lean stumble off idle and into the mid range is to shim the needle. Modern bikes come so EPA lean with respect to low speed jetting they run like absolute garbage in cooler temps (like we have up here in New England). I drove our new GZ for about 1/2 hr in colder temps and knew right away my carb need some richening. I have a bunch of 3mm X .020" washers I got from Radio Shack, and they work perfect for shimming needles. I also pulled the "epa" plug for the air fuel screw, and added 1/2 turn CCW for some much needed cold starting ease. Now even if it's 40F, our GZ fires right up on 1/2 choke as soon as I hear elevated clean rpm's I shut the choke off and the engine purrs perfect. No mid range stumble either. I've shimmed needles and many times also have to bump the pilot jet(s) one size richer too, but so far doesn't seem like the GZ will need a richer pilot jet. Great tech thread !

Re: Needle shim modification
 

Easy. The cylinder head port design is most likely the "bottle neck". A combustion engine is simply an air pump. In theory the more air you let in, the more air you need to let out. So one would think that adding a free flowing exhaust, a K&N filter and re-jetting the carb would yield flow and hp gains. BUT if the valves and the intake and or exhaust port design are already at max flow, all your other intake and exhaust flow performance mods are useless. I would bet my left *** that if a GZ owner sent their head out to be velocity ported, AND added a exhaust, filter and re-jet, now I would bet you would feel a substantial difference. But again, the GZ was not engineered to be a stop light to stop light beast... LOL

Re: Needle shim modification
 

Exactly the same with mine.Couldn't believe the difference in cold starting.You wouldn't think it was the same bike.Oh.Welcome to the family.
:) :cool:

I did the needle shimming yesterday and today, while riding, my bike stopped for no reason, it just started to make less noise then stopped all together. I was able to start it again after a couple of tries and get back to work. Do you think it might be related?

While doing the shim, you mess with a couple of things that could cause an air leak. A big enough air leak and you loose vacuum to draw fuel, which would eventually kill the bike.

Make sure your vacuum hoses are connected.
Make sure your carb boot is secure
Make sure you didn't kink the fuel line
Make sure you didn't pinch the rubber diaphragm under the black plastic cover on top. (That happens a lot)

Ok thank you !