Author Topic: Main jet size?  (Read 6547 times)

allanseth23

  • Newbie
  • *
  • Posts: 16
    • View Profile
Main jet size?
« on: December 27, 2014, 05:14:55 AM »
hello! Recently my stock airbox broke and is beyond repair so i decided to replace it with a cone type filter. My question is what could be the perfect main jet size? I tried 120 main still leans out on full throttle.

Has anyone experienced the same? Jets are quite expensive.

zombie

  • You never know do ya!
  • Hero Member
  • *****
  • Posts: 13870
  • Close enough to get the idea!
    • View Profile
Re: Main jet size?
« Reply #1 on: December 27, 2014, 05:25:11 AM »
There is no way to predict this.

I buy jets from Racing Planet for around three bucks each. If it is just lean at wot get your self a few from 122- 124- 126- 128-130.
 I would also step the idle jet up by two (ie: 38 0em to 40) This will also effect the WOT jetting, and will help you out in the long run.

There is also a shop in Cali that sells jets cheap... Treatland
"They have nothing in their whole imperial arsenal that can break the spirit of one Irishman who doesn't want to be broken."   Bobby Sands...

allanseth23

  • Newbie
  • *
  • Posts: 16
    • View Profile
Re: Main jet size?
« Reply #2 on: December 27, 2014, 05:47:13 AM »
Ok, its a guessing game. Thats bad news.
Yeah i forgot to mention that i have already installed 40 pilot.

Zombie thanks for your fast reply.

BettinANDlosing

  • Hero Member
  • *****
  • Posts: 2235
  • Carter Merz
    • View Profile
    • Columbia Scooters
Re: Main jet size?
« Reply #3 on: December 27, 2014, 08:27:57 AM »
Yeah the B&W 250 has a super restrictive stock airbox, allowing them to use very lean stock jetting. If I recall correctly when I tried open filter for a while i was around 130 but that was with an exhaust too. How do you know it's lean? I honestly never liked the way my B&W felt with a pod filter. The intake noise wasn't as cool sounding as gy6 either, kinda a bad "hiss" after 1/4 throttle. I usually like intake noise but i ended up just cutting my stock box up, a little less noise and still weatherproof.
2002 Kymco B&W 300; MRP 78MM "300CC", Naraku cam, Yoshimura rS3 exhaust, 17g Sliders, Yellow torque spring drilled airbox, stock carb #115 main #40 pj.

2001 "Yamaha" Zuma AKA MBK Booster; MHR OverRange, Dellorto 19mm BHBG, Polini "big" intake, RS-3 Rear shock, Stock cylinder.

mrbios

  • 2005 Kymco Grandvista 250cc
  • Hero Member
  • *****
  • Posts: 619
  • Just crusing along on my 2005 GV250
    • View Profile
    • My YouTube Channel
Re: Main jet size?
« Reply #4 on: September 15, 2016, 05:07:42 PM »
My thoughts on Jets & Air box....

1. I tried going from the stock #

The jet that provides the most benefit is the pilot jet because from sitting the passage that it delivers gas too is very small and difficult to clean so often the stock pilot jet is not able to deliver.

My Notes: 04-15-2014
Installed and tested three pilot jets: #58, 42, 52.  Stock is #38s.  #42 still hesitated, #58 too rich - exhaust smelled nasty like gas at idle - made my pants smell, #52 - no hesitation (pilot screw set to 2 turns out).  Then on the last test drive the Drain screw for the bowl fell out because I forgot to tighten it and the carb ran out of gas and stalled.  I had to push the scooter home ~ 1/2 mile.  Put epoxy in the carburetor bowl until the drain screw arrives.

Main Jet: 05-21-2014 Stock jet is Kehin #102
Scooter, changed the jets again. Experimented with #115 main - way, way too big.  Changed to #48 for the pilot jet.  Works great.  #45 was lacking in power a little.  #46 might work.

I also tried a #105 Main but didn't notice more power just reduced mpg.

I disagree that the stock air box is restrictive.  The air filter is HUGE for a 250cc engine.  It is close in size to the air filter on my '92 BMW K75S which was 750cc!  Even if it has some dirt on it it would take a lot before air flow was reduce to where the motor was starving for air.  For reference I read a test where a dyno shop took a stock k75s and ran it 3 ways: 1. no air filter 2. stock air filter 3. stock air filter 50% blocked with plastic.  All three tests returned identical results across the rpm range!  The reason by the way is the guy complained about a "flat spot" which showed up on the dyno.  The solution?  Remove the aftermarket exhaust and go back to stock exhaust problems solved. 

I agree that on small scoots 50- 100-150cc jetting can be fun and cool and yield noticeable gains in power but with the Kymco 250cc the aftermarket options are more limited, there is more than enough power to blow-away any 200cc and down scoot no matter how much it is jetted. 

Obviously, if the engine is modified with a "Big Bore Kit" and / or the exhaust & air box are modified to increase air flow - then the jets must be enlarged as the engine will run too lean.  Still, for the time and effort and poor mpg that will result if you really want more power you are still way better off going to a 350 - 400 - 500 - 600cc maxi scoot.  There is a place for the custom scoot with modified engine, exhaust etc but before putting the time, effort and $$ you should make sure you are doing it for the right reasons. 

Also, before modifying.... always tune it perfect stock first.  Ride it for a month then mod if that is what you really want.  Most people take a carb with dirty jets or a scoot that needs a proper tune up and instead of tuning it properly (stock) they start modifying.  My 2005 GV250 now has 20K miles and it runs perfect.  I'm ready for more power mainly for the highway so I'll get something bigger like an BV350 or a 400cc scoot.  I ruled out Burgman 650 because it is too big and bloated.

Read my post here:
http://www.kymcoforum.com/index.php?topic=11323.0


« Last Edit: September 15, 2016, 05:19:13 PM by mrbios »
PaulC

doublej

  • Newbie
  • *
  • Posts: 43
    • View Profile
Re: Main jet size?
« Reply #5 on: September 29, 2016, 12:10:43 PM »
You won't regret getting on a Burgman 400. I have both a 250 B&W and the Burg 400. Love them both....

mrbios

  • 2005 Kymco Grandvista 250cc
  • Hero Member
  • *****
  • Posts: 619
  • Just crusing along on my 2005 GV250
    • View Profile
    • My YouTube Channel
Re: Main jet size?
« Reply #6 on: September 29, 2016, 04:21:08 PM »
What about vibration on the Burgman 400 on the handlebars? Also can the clutch be modified so that it hooks up more like the gv250 I drove a couple Burgman 400 but they felt like they kind of slip excessively until they lock up when you first take off.
PaulC

doublej

  • Newbie
  • *
  • Posts: 43
    • View Profile
Re: Main jet size?
« Reply #7 on: October 01, 2016, 04:17:19 PM »
What vibration, none with mine. Maybe you had a unbalanced wheel situation. I still have the original 07 three shoe clutch in place, no issues also. Its the pussy slow riders that seem to burn out clutches. Most of my riding on the Burg is two up, yes lock her up fast and the back the throttle off. Easy peasy.... use original rollers also.  I also have a 250 B&W, great scoot but until I did the recall on the bi-starter it was just a pain in the a$$ on start up with the over reving, also had that one since new in 05.

mrbios

  • 2005 Kymco Grandvista 250cc
  • Hero Member
  • *****
  • Posts: 619
  • Just crusing along on my 2005 GV250
    • View Profile
    • My YouTube Channel
Re: Main jet size?
« Reply #8 on: October 01, 2016, 08:11:42 PM »
What vibration, none with mine. Maybe you had a unbalanced wheel situation....

Thanks for the info; you are in a very good position to compare because you own both.  The thing I have grown to love about the GV250 is the smooth drive belt / cvt / clutch & the low weight.  Last night I removed and reinstalled wheels on a friends '92 CBR600 motorcycle with over 50K miles on it.  I took it for a test drive around the block.  Obviously, much much faster than our automatic scoots but... it felt like I was driving a tank or a lead-sled.  It felt sooooo heavy and clutching especially when just starting out combined with balancing the extra weight is a real drag.  However, once on the open road it is a nice machine.

The few times I road the b400 I was impressed with the speed and the mpg that it can get.  So what about b400 vs BV350?  The BV is slimmer and almost as fast? 

PaulC

doublej

  • Newbie
  • *
  • Posts: 43
    • View Profile
Re: Main jet size?
« Reply #9 on: October 02, 2016, 11:53:06 AM »
GV250 is same drive line setup as a BW250 and weight. Very good transmission of the 250's power. I would still favour the 400 over the BV though. Have riden a BV 500.
Now as to the CBR etc. A motorcycle is a different beast and will balance diffferently then a sit upon scooter. I ride MC's also as I have a Kawasaki ZZR, and a pair of Honda's one being a new CB 500 and a CBR 125R. All different in your position but once rolling are very well balanced and responsive. It is all about getting used to the different bike feel.
Cheers...... get a Burgman 400 and enjoy!
« Last Edit: October 02, 2016, 11:55:11 AM by doublej »

mrbios

  • 2005 Kymco Grandvista 250cc
  • Hero Member
  • *****
  • Posts: 619
  • Just crusing along on my 2005 GV250
    • View Profile
    • My YouTube Channel
Re: Main jet size?
« Reply #10 on: October 02, 2016, 07:53:06 PM »
...
Now as to the CBR etc. A motorcycle is a different beast and will balance differently then a sit upon scooter. I ride MC's also as I have a Kawasaki ZZR, and a pair of Honda's one being a new CB 500 and a CBR 125R. A...
Cheers...... get a Burgman 400 and enjoy!

Again, I'm impressed so your a motorcycle rider too!  Well, given that you ride motorcycles, have a BW (GV) 250, and a b400 I must stay that is an impressive endorsement for the b400, one that I cannot easily ignore.  Once upon a time I had a FZ750, 250 road legal enduro, and old Honda ATC 200x trike AND a KDX200 two-stroke.... These days I strive to have two vehicles: car 4 wheels, scooter 2-wheels.  Recently, a neighbor dropped off a Chinese atv 110cc that was abandoned in an alley.  It needed tons of work.  I drive it on the streets around the neighborhood.    My point is to avoid having too many vehicles.  It is also not easy for me to part with the GV250 because I really enjoy it: storage including givi top case, , wind protection, automatic, low vibration, no chain to lube, etc.  So maybe the plan would be to sell the GV at some point and buy a used 2007+  b400.
PaulC

doublej

  • Newbie
  • *
  • Posts: 43
    • View Profile
Re: Main jet size?
« Reply #11 on: October 03, 2016, 10:57:09 PM »
You will have more leg room on the Burg 400. Mine is used mostly with a pillion rider. As a matter of fact I am off to NY/PA for a little R&R with it tomorrow.

mrbios

  • 2005 Kymco Grandvista 250cc
  • Hero Member
  • *****
  • Posts: 619
  • Just crusing along on my 2005 GV250
    • View Profile
    • My YouTube Channel
Re: Main jet size?
« Reply #12 on: October 04, 2016, 04:45:16 AM »
You will have more leg room on the Burg 400. Mine is used mostly with a pillion rider. As a matter of fact I am off to NY/PA for a little R&R with it tomorrow.

Have a great trip, I wish I was going!
PaulC

An Error Has Occurred!

Call to undefined function split()