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.