I installed a K&N lunchbox, Yoshimura slip-on, and rejetted my 2008 GS500F over the weekend. The jets I used, as recommended here, are 20/65/145, with 1 washer on the needle and 3 turns out on mix screw.
Had the bike out for some casual cruising yesterday and it seemed fine. Today, I pushed it hard from a stop for a 0-60 run; ambient temp around 92°F. It accelerated great, but when I upshifted and leveled out the throttle to cruise at 60 the bike started lurching/surging pretty badly. It felt like I was constantly getting on and off the throttle. It didn't feel safe, so I pulled over to the side of the road and let the bike cool for a few minutes. After no more than 5 minutes, the bike started fine and behaved normally for the ride home.
Now I would blame myself for ruining my bike, but I did experience this issue once before, previous to my performance mods. Same general behavior, after pushing the bike really hard, it started surging at normal throttle levels - though it was not nearly as pronounced as what I experienced today and I didn't think to much of it at the time.
So this seems to be an overheating issue of some kind, but I have no idea what. Is this an indication that I'm running too rich?
I'm guessing that there is a clog/leak in your vacuum system, petcock, fuel lines, or tank breather tube. Something is slowing down the flow of gas so that if you don't use much it can keep up, but if you push it and use a bunch of fuel at once you can't get enough to fill your carb bowls. Since you just put bigger jets in now it's even easier then before to use more fuel then the fuel system is providing.
Try switching to reserve as soon as it happens and see if everything levels out. Does it happen when in prime?
You may need to rejet, the suggestions are a good starting point, but each bike is a little different.
My guess is the breather needs to be re-positioned.
One more piece of info that I just discovered. After letting the bike sit for about 20 minutes or so I went back out and restarted it. Started without choke because it was still pretty warm, but I tried opening the choke as a test, and when I did so it cause the RPMs to drop and killed the engine. Closed the choke and the engine started ok again.
that should be normal once warmed, if I am not mistaken.
I second the fact that you used up all the fuel in the bowls faster than it could be replenished. Sane thing happens to my bike, I'm hoping that when i remove the inline fuel filters and put my nee lines on it will be fixed.
Ok, thanks guys. I'll check the tank breather line. It's also possible that I didn't open the tank valve all the way because it was late by the time I got everything back together and I didn't have the best lighting.
mine did this on the freeway one time. My petcock was original so I just bought a new one and it has been ok since. Well except when I ran out of gas a couple of times. That lurching sucks. (oh crap feeling) :o
I was having a bit of this issue at the start of the season this year. I cleaned out my gas cap vent and it went away.
Some easy things to try:
Gas cap vent
Carb vent hose out of wind and not kinked
Kink in fuel hoses
Air cleaner clean and air flowing correctly.
You installed the main jet in the main holder right? Make sure mid main goes in mid too...
the on frame petcock uses a vacuum signal from the carbs to open the flow of fuel to the bowls. ALMOST like a vacuum driven pump. sometimes on hard runs the vacuum signal isnt as robust at wide open throttle and it doesn't hold open as well as cruise speed or idle where the vacuum is stronger. its even worse when the petcock is kinda trashy. yes the GS can suck the bowls dray pretty fast, most racers ive known running them run on prime for free flowing fuel (or install a pingel)
Check the simple stuff, vent tube, fuel filter condition,hose routing, then check float level, petcock and even pickup tube/screen inside the tank
You guys were spot on. Turns out the tank breather hose was pinched between the tank and top of the carb. Routed it to the side, took the bike out, gave it a wristfull and it was fine. Thanks all.
The K&N and pipe is 147.5 mains - where did you get 145's from.
Cool.
Buddha.
Quote from: The Buddha on July 19, 2013, 08:02:41 AM
The K&N and pipe is 147.5 mains - where did you get 145's from.
Cool.
Buddha.
I plan to use the silencer baffle in the exhaust, and I didn't want to go too rich, so I figured I'd go with 145 instead of 147.5; shouldn't make
that big a different right? Bike seems to be fine since unkinking the tank vent hose.