Jetting with upgrades

The original 2-stroke Genuine scooter and its 4-stroke manual and automatic offspring

Moderator: Modern Buddy Staff

Post Reply
zaynbidane
Member
Posts: 4
Joined: Fri Apr 26, 2013 2:31 pm
Location: Toronto

Jetting with upgrades

Post by zaynbidane »

Hello,
i have a 4t Stella 150 with a sito plus and ramair filter..
I had original jets until this week and she ran well but i was looking for a little added performance by upping the jets.. I ordered a 95 and a 98 main jet and a 42 pilot jet.. Installed both the 95 and 42 and went for a test run.. The bike sputters harshly from 3/4 to WOT.. I tried adjusting the air screw but i could at best lessen the sputtering. As i understand this means I'm running very rich. I now ordered a 92 main to test, but i was under the impression that the 95 would likely be the right jet with sito and ramair, am i doing something wrong or do i have to just drop to 92?
User avatar
Neurotic-Hapi-Snak
Member
Posts: 335
Joined: Thu Jun 05, 2014 2:56 am

Post by Neurotic-Hapi-Snak »

Take out the 42 idle, first, and see how it runs. The idle jet supplies fuel through out the throttle range. If it still sputters at WOT, put in a 92 main. Don't forget to do a plug chop, too, once it runs well.
zaynbidane
Member
Posts: 4
Joined: Fri Apr 26, 2013 2:31 pm
Location: Toronto

Post by zaynbidane »

Thank you, I will definitely try that first.. The thing I don't get about that is, all information that I have seen referencing the idle/pilot/slow jet says it only effects the bottom 1/2 of the throttle range.

http://www.carbparts.com/keihin/needles ... r_carb.htm
http://www.salocal.com/sohc/tech/carb/a ... tgraph.htm
http://www.4strokes.com/technical/carb-overview/

Also, I've seen multiple posts of people with similar setups running 95 or even 98 main jets.
zaynbidane
Member
Posts: 4
Joined: Fri Apr 26, 2013 2:31 pm
Location: Toronto

Post by zaynbidane »

I re-installed the original pilot jet with the 95 main, had to close the air screw a little, and it was a little worse at WOT. Then I put the 42 idle back in with the original 90 main jet, I opened the air screw back to where it was. It was better at WOT, not really sputtering, but seemed like a slight loss of power from usual. I think it's running a bit lean now because the air screw is more open than with the stock idle jet. I have a 92 and 94 main on the way, it seems like the 92 will be the best fit but I will let you know.

Summary:
90-40 - (Stock setup)Runs well through the range but looking for more power
95-42 - Fine at the bottom half but stutters at 3/4-WOT
95-40 - Stutters worse at WOT
90-42 - Decent but not as smooth as stock and seems underpowered at the top end
User avatar
Neurotic-Hapi-Snak
Member
Posts: 335
Joined: Thu Jun 05, 2014 2:56 am

Post by Neurotic-Hapi-Snak »

To be so rich that it stutters you have to have a main jet like a 5-10 points too big, so it might be a good idea to get an 88 main just in case. Only certain way to tell if the your fuel/air ratio is ideal is to do a plug chop, a butt dyno is only good for a close guesstimate.

What carb are you using? The stock Keihin PB18? Also, is it very hot or humid? When I was using the stock PB carb, I had trouble with the 42 idle jet causing bogging throughout the throttle range because the stock 40 jet is special jet with a different atomizer portion than the normal Keihin jets.

Also, don't just randomly set your idle mix screw, set it properly and independently of the main jet.

1. Get the engine nice and hot with a ride
2. Turn the idle screw in 1/8th or 1/4er turn so it's idling very fast
3. Turn the mix screw all the way in
4. Back the mix screw out 1/8th of a turn at a time till the engine will start
5. Turn the mix screw out 1/8th turn at a time and wait 10 seconds to see/hear if the idle speed increases or decreases.
6. Keep repeating step 5 till the idle speed decreases
7. Turn the idle mix screw back in 1/8th turn
8. Turn out the idle speed screw 1/8th or 1/4er turn to the correct idle speed
9. If the mix screw is less than 1 turn out, IIRC, go down an idle jet size, more than 2.5 turns, go up a size

A tachometer might help if your ear isn't calibrated well enough.
http://www.amazon.com/CyberTech-Digital ... tachometer
Last edited by Neurotic-Hapi-Snak on Sat Apr 25, 2015 7:34 pm, edited 3 times in total.
DanielDCB
Member
Posts: 17
Joined: Wed Apr 10, 2013 8:12 pm
Location: Phoenix, AZ

Post by DanielDCB »

FYI, the Stella uses an idle jet that is slightly different than the usual PB18 idle jets.

I tried a standard 42 and found that while the correct 42 is probably what I need, the stock 40 is smoother at idle, especially when not quite fully warmed.

I had no luck finding the correct jets at the usual online carb suppliers or the local Genuine and Honda dealerships. Nor did I order one from the Japanese ebay link in the following thread. Still running the stock 40, with a 92 main jet. Stock pipe, airbox, everything else.

topic27608.html
zaynbidane
Member
Posts: 4
Joined: Fri Apr 26, 2013 2:31 pm
Location: Toronto

Post by zaynbidane »

I received and installed the 92 main today, and left the 42 idle jet in. The air screw is around that 2.5 turns out I think. The top end is much better than with the 90. Top speed is around 5kmph faster and the top half of the rev band in general is really quick. When it was cold it felt a little sluggish getting off the line, and even warm it was not as peppy at the bottom of the range as it is at the top. I think i will try 92-40 when the smile wears off and I feel like tinkering. I'll let you know how that goes.

Couple other points.. to up jet from here, I have a 94 on order, but does there exist a 93 for the PB18? I have a brisk plug and hopefully if Tasso gets stock, a HT Hi power coil with cap should be on the way. When I get these, as I understand I can't do a plug chop, and have to use a temperature gauge. Any other way to do it? Isn't there a way to eyeball these plugs?
Post Reply