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Old 10-11-2009, 10:12 AM   #4
The Jerk OP
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Joined: Mar 2006
Location: Brooklyn, NY
Oddometer: 2,481
Carb Experts: Talk to me about jetting (I'm an FI guy).

Here's my situation: The W650 is pretty much bone stock except the PO punched out the baffles on the stock exhaust. I suspect that this is causing the bike to need a richer mixture than the stock jetting can provide.

I had the carbs apart yesterday (Keihin CVK) and found the stock jetting in place, that is a 118 main jet and 35 pilot jet. In fact, the carbs were so stock that I had to drill out the caps over the pilot screws before I could adjust anything.

I'm getting a bit of a lean miss/surge at steady throttle cruising, mostly @ lower rpms. It is worse when the engine is cold and improves a bit as the engine warms but still never goes away entirely.

My understanding is that the pilot circuit handles most of the fuel supply for that type of low-demand, low-throttle steady state cruising. I have opened the pilot screws up to 4.5 turns out and the lean miss has gotten better but has still not gone away entirely. I feel that at 4.5 turns out I'm probably pretty much near the max as to what the pilot circuit can supply.

Note that from the factory, one of the carbs had the pilot screw set at 2.5 turns out and the other one at 3.5 turns out.

An additional symptom of the lean running is I'm getting crackles and pops through the exhaust on decel. Part of that was due to the air injection system which I disabled yesterday by blocking the hose going to the air injection valve. This reduced the popping in the exhaust considerably but I'm still getting some, presumably from the mixture.

So my thinking is I might need to re-jet due to the now freer-flowing exhaust. My thought would be to just try and go up to the next size pilot & main and see what happens. I don't think I need to go crazy as I plan to retain the exhaust as is as well as retain the stock airbox. In doing a little research on the web, it appears that 120 main and 38 pilot would probably be the next step up.

Also I noticed that DynoJet has a kit for the W650 that has new mains and new needles (but they don't mention anything about pilots). Strangely, according to the PDF on their site, the mains are listed in various sizes from 108 thru 114. Unless DynoJet numbers their mains differently, that would seem to be a step in the wrong direction. Unless their needle changes the calculations versus the stock needle so that smaller main jets are used. Anyone know anything about that?
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