Carburetor Jet Tuning

[Tinken]

Deciding which main jets to buy to tune your carburetor can be a difficult task and someone over the phone may not really know exactly which jet is best for your engine.

You may not know what jet size would be a good starting point for your combination. Why buy a bunch of main jets based on a guess when you may not ever use them again? By drilling out your stock main jet to larger sizes you will be able to narrow down what jet size your carburetor needs.

You will need a numbered drill bit set. This set from Harbor Freight. The brass main jets are easy to drill so you can get by with one of these inexpensive drill bit sets.



The first thing we do is match the stock main jet to its corresponding numbered drill bit. If your stock main jet is a 100 (stock size in a 22mm Mikuni) you will find this jet size falls between the #62 & #60 drill bit.
Run the engine with the stock main jet and take a plug reading. If it's lean and you need to increase the size of your main jet, drill the jet out with the next size larger drill bit, which in this case is the #60 bit. Run the engine again with the drilled out main jet and take another plug reading. Repeat this process until you get a good plug reading. Do not skip drill bit sizes because once you drill the main jet too large, you cannot go back to a smaller size.

You should use a drill press for drilling your main jet.



You will need a vice with jaws that will hold the main jet perpendicular to the drill bit. You do not want the drill bit to enter the main jet at an angle because it could create an oblong hole in your jet. Once you find the diameter which gives you the correct amount of fuel, I suggest you purchase the corresponding jet size to match. This will eliminate any drilling marks and yield better atomization.

[Spudgunman]

I do fell it hard pressed to say with out experience on plug reading this ... Rather simple guide is not for the faint of heart.. If your unfamiliar with plug reading or don't own tools that can aid in engine tuning.. It is likely best to buy those 2-3 new jets that if you experience problems - a reversal is simple as "plug and play" vs drilling your jets and then having no reversal.. If you haven't drilled jets - I would recommend buying a spare one anyway to test going larger or smaller with out wrecking a single jet you might own..

Understand that plowman isn't inventing magic however he is selling a service - a service of trust and knowledge that some might just say "I don't care how my engine gets there, I just want to pay and brag" vs understand what porting a jet for increased delivery of fuel preventing a lean engine and engine life vs starving out power, for a quick thrill of "30mph"

Another tip, dont just toss your old spark plug, take it to a real mechanic (meaning one who would rather piss on a odb2 tool rather then talk about how his odb2 is iPhone compatable)

[Tinken]

I agree, while not super simple, it isn't rocket science either.

Don's kits work great with Don's mods, but anything outside of his direct mods must be tuned specifically for that mod. Never the less, I wasn't picking on Don, nor Tom or any other Cart specialist. I am simply giving options. A cart owner may wish to use one of Tom's filters and one of Don's exhausts or some other combination which will otherwise require different tuning.

Ideally you could have a wide-band 2 sensor and a dynamometer to test jetting under load (which I have), but it is not necessary. You should have extra jets on hand just in case you go over on your tuning sizes. Look around for quality jets, they should cost around $4 each.

[panteramatt]

Where can u get jets online for around $4?

[Spudgunman]

Originally Posted by Tinken

Don's kits work great with Don's mods, but anything outside of his direct mods must be tuned specifically for that mod. Never the less, I wasn't picking on Don, nor Tom or any other Cart specialist. I am simply giving options.

and you are totally right, also when you work with Don that I personally feel that he is tuning for a tailored fit more so then a SKU for "ported jets"

but just yammering now, not to spin away from your orginal post which is a great guide to porting jets

[Tinken]

Here's a #61.3 pilot:
http://gilbertgolfcarparts.com/620-1...t-p-65053.html

And a #83.8 main:
http://gilbertgolfcarparts.com/3g2-1...n-p-65056.html

Those carburetors are used on various carts and water craft so the parts are widely available. Not sure if those starting sizes are correct. Most suppliers can resource close to what you may require.

[Tinken]

[panteramatt]

Arent thise stock size jets? Do they carry larger ones?

[Spudgunman]

Originally Posted by panteramatt

Arent thise stock size jets? Do they carry larger ones?

no these are OEM parts, the whole idea is that you port them (make them larger) to fit your use case. If you want to get larger ones you need to purchase them from a performance shop (they wont be $4) they will cost more because your paying for someones service to do exactly what the OP is detailing out for you here.

[Tinken]

Yes, these are oem parts. Manufactures usually list two or three sizes around the original size (83.8, 86 for mains and 60, 61.3 pilots).

Although, these are Mikuni jets and you can get them in almost any size. I was able to find a #102.5 (0.040") at TNT (a performance shop) for $6.95. I suspect that this is the larger jet size sold at most performance shops to cart owners with larger tires and modified intakes. I am still waiting to get pricing on for a full set from my supplier. I cringe in anger when I see these jets selling as high as $24.95 a piece.