Tuning my carb - which carb model, and types of jets?



  • Hi!
    Haven't been very active on the DT since I had the engine rebuild in the winther - i'm about to tune the carburetor since I added the Gianelli full system exhaust, de-restricted the airbox and had the barrel re-bored to oversize. BUT - i'm having a hard time figuring out which jet is what, and precise what model my carb is - in the Haynes manuel the VM26SS are described, but I don't think it is the same as mine, when I look at the illustrations/pictures - as far as I can see, mine is a VM28, can anyone confirm this?

    And about the jets - in Calums guide, there is described a needle jet, pilot jet and main jet. I can only seem to find 2 types of jets, the main jet and a pilot/needle jet (which one is it?) I can't follow the guide before I figure this out, so I really hope you guys can help me out 🙂

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  • FYI not my guide copied and pasted it from the Web. I thought it was very useful.

    Very early DT'S will be 26mm.

    Later ones, the majority are miking 28mm flat slides

    The needle and main jet. The needle attaches to the throttle slide and it falls I to the main jet. The needle controls how much of the main jet is exposed. The needle will do the majority of throttle openings.



  • @Calum Thanks - but I'll have to admit that i'm still a tiny bit confused, the first step on the guide you posted is to "DETERMINE THE CORRECT NEEDLE AND OR NEEDLE JET." - I understand what the needle is and what it does, but what about the needle jet - where is it placed on my carb? The only jets I have bought to choose from are the pilot jets (blue on my pic) and main jets (red on my pic). These rest of the guide I think i'll manage, but the first bit confuses me. I wish I could find a specific guide for my carb, since most including the one in the Haynes model are about these older types of carbs, I guess.. And again, thanks for your help..



  • It doesn't make sense for a reason mate. It probably doesn't apply. Visually I'd say the needle jet has been substituted for main jet. The principles in that guide will help you tune any carb.

    But you don't start with the needle. I'd have to reread the guide.

    But can I ask what you are trying the achieve. Your mods won'won't change the jetting that much. There is no airbox restriction. Hacking away at the air box is not a derestriction, as it upsets the air flow andn stall the flow. Hence why they run a smooth snorkel to allow safe passage into the their box. Remember even GP bikes use airbox and snorkels, for a good reason.



  • Ah - thanks, now it starts to make sense. Been fiddling with it this afternoon and it seems to run great with a 260 main jet and 30 pilot jet - the only thing I wanted to achieve is reliabilty, making sure it not runs to lean, not to maximize performance..


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