Big bore jetting - lx 50 big bore plug read help please!
Post Reply    Forum -> Project Reports
Author Message
Lx 50
Joined: 08 May 2019
Posts: 4
Location: Denver
Tue Jun 04, 2019 6:07 pm quote
Hi all,

been lurking here a while and finally bit the bullet and bought the big bore kit. 2009 Lx 50 2 valve. Did the big bore, cam, variator, and malossi filter this weekend.

So I live in Denver - 5280ft.

I put a brand new plug in and fully gassed it around the block with the stock #75 jet and then cleaned the plug up and put the #80 jet in and did the same. Here are the two. The #75 is the one with white and the other is the #80. I've never done this before - what do you guys think?

The white on the 75 indicates to me that it's way to lean - as expected. I know you want a mocha color, so is the 80 too rich?

When I watched the ScooterWest video where they install all the same stuff, they're in San Diego and put in a #80 . . . With that reasoning #80 would be too rich for me here in Denver.

Any help or advice from some more experienced folks would be much appreciated.



73 Rally, 74 Rally, 76 ET3, 80 P200, 06 PX150, 65 Li225 Silver Special, 86,95,96 Elite 80s, 2015 HD Road Glide Special, 2011 Ural Tourist
Joined: 18 Jan 2012
Posts: 3616
Location: Oceanside/ SF
Tue Jun 04, 2019 9:04 pm quote
I dont know about 4 stroke tuning only two stroke, but Iíll chime in for now. Iím happy to learn if things are different between the two.

With a two stroke plug chop, you need a few miles at wide open to accurately see how the engine is burning at its hottest point. Run it, cut the ignition, coast to a stop, immediately remove and replace the spark plug for each jet. Then you need to cut back the threads (hack saw/ Dremel tool) to see the base of the white insulator. Itís cone shaped and you want to see it at the widest part. That should be starting to form a ring. Look at the color of the ring. Perfect is coffee brown. If its tan, keep upjetting.

I understand your theory of reducing your points 5 for the altitude but would say richer is always better than leaner while breaking in. Leave the 80 in for break in and then go to 75 after a few hundred miles. If its too much and you foul a plug its only a few bucks verses running lean and seizing your kit. Disclaimer: Again this is two stroke talk.


GTS 250
Joined: 26 Apr 2008
Posts: 265
Location: California
Wed Jun 05, 2019 7:07 am quote
I don't know much about reading plugs, but the amount of fuel needed is not 100% based on altitude. It's all about air density, which factors in temperature & humidity. I would guess that the humidity is much higher in San Diego than in Denver.

This video is somewhat relevant:
Lx 50
Joined: 08 May 2019
Posts: 4
Location: Denver
Thu Jun 06, 2019 6:51 am quote
Thanks guys

At this moment the air density in Denver is 80% and in San Diego it's 98/99%.

This is factoring alt/humidity and temp.

With that being said - that's effectively a 20% difference.

If they're jetting the same exact lx 50 with the same exact carb and 80cc kit - it stands to reason I have to be smaller than a #80.

Actually cutting the plugs is a little too far for me . . . . I'm going to rock this #80 for a bit and pull it and see how it looks. If it's too much I'll drop to a #78

The #75 is definitely too lean - the carb when instantly white.

Thanks guys.

GTS 250ie Super, Italjet Velocifero, Scarabeo 150, S50,ET4
Joined: 22 Nov 2016
Posts: 469
Location: Winston-Salem, NC
Thu Jun 20, 2019 5:56 pm quote
On my 4 valve S50 I did the 80cc Malossi kit and was running an 84 main. I tried 86 and you could feel it was too rich. I pulled the snorkel and went to a 90, a bit to rich, went to an 88 and it was very good running. Plug chops are fine but a CHT and your butt will tell you when you are too rich or lean. Try an 82 and see how it feels, it will let you know if its too rich. A side note is that jets are not always the correct opening. I using a drill bit for an 88 I compared a generic 88 to a Keihin 88 and the Keihin was correct and the generic was just a little to small. Its all relative. Find what works for you and that's what you need.
Your Rally Here   Vespa Wasp Pin Badges   Scooter Parts Company
Post Reply    Forum -> Project Reports
[ Time: 0.1297s ][ Queries: 25 (0.0312s) ][ Debug on ]