Jetting? Carb Sync?

mo650chopper · Posted: September 7, 2005, 9:29 pm Post subject: Jetting? Carb Sync?

On my 80 XS with Pod Filters and Open Drag Pipes I followed the Tech Tip on Mike's XS and installed a Main jet 142.5, Pilot Jet of #45, Air Jet of # 127.5
I'm still running a little rich, My bike starts and runs on startup without the use of the choke But I am averaging 45 MPG. I'm thinking of going down to a 140 on the main or should I go farther?

y next question would mixture or carb sync create a vibration? I have a vibration from about 1/4 to 3/4 throttle. It's really not a RPM vibration as if I decrease throttle or open in goes away. It also seems to be less prominate before the engine fully warms or on cool mornings which leads me to belive it's a rich mixture issue. Do out of sync carbs show at these throttle positions more so than others?

grizld1 · Posted: September 7, 2005, 10:41 pm Post subject: Re: Jetting? Carb Sync?

If you're too rich at idle the issue is probably the pilot circuit. Did you work with the pilot fuel screws? I suspect they need to be turned in a bit. I'd be really shocked if your bike was running too rich at full throttle with only 4 steps of upsizing on the main jets. Have you done a full-throttle plug chop to get a reading on the mains?

Re. synch: you bet an out-of-synch engine will vibrate, worst in the range you're describing and also at idle, worse at steady throttle than in transitions. A rich mixture would make things worse with warm-up, as you say. Did you shim the needles? If so I'd drop 'em back to the original position.

I'd do things in the following order. First: running the bike on one cylinder at a time, adjust each fuel screw to yield highest possible idle. Turn the throttle stop up if you have to, and work in 1/4 turn increments till you get the right position bracketed, turning the throttle stop down as the tuning gets better. Then synch. Then do a full-throttle plug chop, picking a long upgrade so you can hold it open or close to it for at least a couple of miles and the more the better. Put in fresh plugs at the start of the run and simultaneoulsly hit the kill switch and declutch at the end of it; don't let the engine turn over off the open-throttle position. Coast to a stop and have a look at the plugs.

If you still want to make a run out this way, we'll see if we can get things cleared up a bit. This weekend's clear for me, next one too so far. Send a PM and we'll see what we can schedule.

mo650chopper · Posted: September 8, 2005, 5:01 am Post subject: Re: Jetting? Carb Sync?

Didn't change anything on the needle jets. Does anyone have a good diagram of each circuits operation on an XS Carb? Doesn't the main effect the entire range of carb operation? Or at least off idle circuit? Let me see if I can get free one day this weekend grizld1, Kids sports, Weddings and?

grizld1 · Posted: September 8, 2005, 8:07 am Post subject: Re: Jetting? Carb Sync?

The mains affect overall operation, but the pilot circuit impacts a much bigger chunk of midrange on these carbs than most folks assume. A main jet increase of 4 steps is really small for the changes you've made. I run MAC 2-into-2's with street baffles and K&N pods on a stock engine, and I had to take more than 8 steps before the plugs stopped coming out bone-white off the chop. Standard practice is to tune carbs from the main jets down. You only really know what the mains are doing under full-throttle operation.

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