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Different Jet Sizes in my 550s, what does it mean? (VM22S)

roeme

roeme

Forum Mentor
Past Site Supporter
My daily rider, over the course of the last weeks, sometimes only stubbornly pulled, and was very weak at higher RPMs, which is not the natural behaviour of a 550.
I did start some threads asking about that in here, fixed some issues - O-Rings, loose vaccum inlet screws - but the issue kept coming back.

Just as I suspected the carbs clogged, she belched fuel onto the street repeatedly, so off the carbs came (Sure enough, needle valve had some varnish flakes, but that's for another thread).

Interestingly, the main jet size differs from the other 550 I'm rebuilding. On my daily rider, on the main jet I found a "80" stamped, the other bike sports "102.5". Pilot jets are 50 for both bikes.

My notes say that on the carb bank with the higher number stamped on the main jet, most carbs had/have its pilot jet needle screw 3.5 half-turns out, the pilot air needle screw 4 half-turns.

Soooooo - one of the has a way richer mixture? Wonder which one is closer to stock, and what the PO's motivation was to deviate. :-k
 
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Even if your bike is somewhat highly-modified, there is no need to turn the pilot fuel screw (that is the one on the bottom) out past ONE full turn.

The general rule of thumb is to set the air screw (the one on the side) to twice the fuel screw setting, I tend to start at a bit less than that, maybe 1.5 times the fuel screw setting.

That will provide a slightly richer mixture, when the bike is running and off-"choke", you can turn the air screw out a bit to lean it for proper running.

.
 
TxGSrider said:
By any chance was the one using the 102.5 mains running pods or modified exhaust?
Click to expand...

None of these, apart from the fairing. Unless the exhaust has been modified in such a professional fashion indistinguishable from stock.

The only thing I could think of would be bored out cylinders or living at a higher altitude :-k

Hoping for the former >:) but I think PO just wanted to have more oomph? Does one have more power available by simply putting in bigger jets?
 
Larger main jets would not offer more power on a stock bike in general. Odds are the previous owner had an aftermarket exhaust on the bike and sold it separately although it is possible to alter the stock mufflers by drilling out the baffle from the end. Higher altitude would require smaller jets to compensate for the thinner air by the way.
 
Ah, good input re. aftermarket exhaust.

Re. Altitude I should've spent a few seconds thinking before writing :) Smaller makes perfect sense.

Im thinking of rigging up something to measure displacement. Hrm...
 
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