Main Jet By speedracer (Read 2,526 times)

[speedracer]

I have a UNI air filter and just installed a 125 main jet. My question is when does the main jet kick in? Also running a 40 pilot jet. The main jet was a 112 and I'm not sure if I feel any difference at all when running from 40-55 MPH. Everyone says you must use a larger jet with the UNI. Well I am now but what is it actually doing?

[arigold]

Increasing the main jet size allows more fuel to flow which helps run the engine cooler. Keeps you from burning lean... At least that is my understanding.

[Bashan]

I can't believe I didn't get this in tech yet, there's a diagram in the article about how your jets kick in. This deals with a slide type carb but the basics still apply. See attachment, sometimes you have to click "allow" or continue. Rich

Attachments:

[speedracer]

Bashan, Thanks for your help. I have a much better idea of how the jets actually work. Who would think the MAIN JET kicks in so late in the throttle. Should have renamed it to high speed jet or something.

[Bashan]

Hey Speed, I'm installing a chrome manifold, Uni filter, and new jets on my Bashan (Peace, whatever) and I bought all three jets that Scrappy Dog offers: 115, 125, and "The Guzzler" 135. I talked to Rick Main when I ordered and he said I would probably have to tweak the A/F screw and needle clip to get the most out of the new jet. He said that would I need to start by moving the clip down one notch which would open the jet more, I guess to balance the increased flow by kicking in earlier. Maybe that's why you're not getting what you want at that speed. Just a thought, Rich

[mthomas]

Keep us posted on results and best practices...several peeps thinking about those exact same mods on our scoots...including me!!!

[wolfhound]

I have a different approach. I ride them like I bought them. My Tank 150, once the motor was broken in, cruises at 55, tops hills at 50, top speed is around 62. My TGB Delivery 150 was a
swap deal(my 250 Tank for his Delivery, even swap). I is not modified, now has 5700 miles on
it, was serviced just before I got it, runs all day at 55 tops out at 65. Fast enough for me. If I need faster I would go back to the 250. There is nothing wrong with modding a scoot if you
are into that and have the mechanical skills. Being a rider, not a wrencher(I envy you guys who are wrenchers!!!) I avidly read all the mod articles to see what the results are. We do plan a yet undesigned mod for the 150s this summer. If it works I will figure out how to post
pix. If it doesnt work I will tell you why.(this in journalism is known as a teaser line!! 8^) ).
I will say this: it will not be a motor mod.
Do not think that i am anti-modification, I am just an old guy who is stuck in the Model T age!!

[speedracer]

May 16, 2010 4:27:47 GMT -6 Bashan said:

Hey Speed, I'm installing a chrome manifold, Uni filter, and new jets on my Bashan (Peace, whatever) and I bought all three jets that Scrappy Dog offers: 115, 125, and "The Guzzler" 135. I talked to Rick Main when I ordered and he said I would probably have to tweak the A/F screw and needle clip to get the most out of the new jet. He said that would I need to start by moving the clip down one notch which would open the jet more, I guess to balance the increased flow by kicking in earlier. Maybe that's why you're not getting what you want at that speed. Just a thought, Rich

I have two carbs and neither one has any clips on them to adjust. Once the long spring is removed and diaphragm all I have is one long needle with no clips to move up or down. Am I the only one who doesn't have clips on the carb needle?

[zugzug]

Ive had carbs on MC that didnt have adjustable needle. I just put a #4 washer under it but I havent the need to try it on a scoot carb needle yet but Im sure its possible.
-Zug

[speedracer]

Dr J I will give it a try with my old carb. Don't want to break anything in the new carb. Now the question is with the use of #4 washers what will it actually do? Am I moving the invisible clip up or down by adding washers?

Thanks
Speedracer

[zugzug]

You are doing the same as moving the clip down/needle up. It will make it richer in the midrange.
-Zug

[Alleyoop]

Question, why are you messing with the Needle? Is it running Lean in Mid-Range? How does it act in Mid-Range or starting off when giving it throttle.
Alleyoop

[speedracer]

May 17, 2010 21:16:39 GMT -6 Alleyoop said:

Question, why are you messing with the Needle? Is it running Lean in Mid-Range? How does it act in Mid-Range or starting off when giving it throttle.
Alleyoop

Alley if your question was meant for me the answer is I have no need to change the needle position. I have full power from start to WOT with no drop off at all. I only asked why the two carbs I have came without clips on the needle. It has been running better since I put in a 125 main jet and that seems to kick in at about 6000 RPM's. My A/F screw is now set at 1.5 turns open down from 2.5. Does it run lean you asked. Well if I use the spark plug as a guide the answer is no. The plug has a nice brown color to it, it was black until I turned down the A/F to 1.5.