OK, so here’s the deal. Last year, bought the GS500, drove it home. It has two 34mm Mikuni vacuum operated flat slide carbs. It has a free-flow Vance and Hines Supersport system.
Pig rich and fouling plugs at highway speeds. Barely made it home. Running poorly in the midrange and shitty/bogging throttle response. No nuts down low at all.
Someone has installed a Dynojet kit for it. 122 main, pilot looks stock. Idles fine, but fouling plugs after highway riding, within say 300 miles. Plugs are so bad I had to swap them to limp it home. The kit says this is the correct configuration…except the person that installed the kit left out the spacer below the needle. That would, in theory, make the carb lean in midrange until the slides open fully. At WOT, with slides open, it then would enrichen normally. The inside of the carbs are clean as a whistle when I disassembled them.
So, I tear it down. I installed a 118 main jet, needle is seated where they said (second groove), but I omitted the spacer in the slide/needle assy below the needle as this is the way I took it apart.
Idle air mix screw is set to 2 turns out from light seating.
It idles fine at 1500 RPM (basically where the manual says it should be) with the air-box off, but the exhaust gases were cold to the touch. I then rev it and it falls flat on its face. Stalls out just revving it barely to 3000 and letting it come back down. Slides are operating freely and equally.
I let it idle for 10 minutes, pulled the plugs. White as a ghost. Definitely lean at idle.
I am planning on
Pulling it apart again and installing the stock main and stock needle with the stock under-needle spacer. The stock main is 122.5 with a different shape than the Dynojet main. The stock needle is not tapered at all.
Setting the idle mix screw out to three turns instead of two.
Q’s:
It should be set up slightly lean (just a slightest hint of white on plugs) with the airbox off while getting it set up, correct?
Does the return to stock needle/jet to establish no other problems exist sound like a good idea?
did you clean the carb out? disassemble and clean it up and reinstall. basically rebuild the carb. sounds like bike might have sat for a while before being ran again. just a suggestion
Update: obi-1 and I reset the bike to stock carb jetting. I set the idle mix to 2.5 turns out. Bike started idling funny, shooting up to 5K. I figured the idle set screw was too high, so we started backing it out…no effect. obi-1 says “take into account the throttle plates”. I’m like “I may have over tightened the throttle cable”. We reset the throttle cable and it starts idling great. Exhaust feels warm to the touch. Exhaust gases start warming up, which is a good sign we’re in the ball park. I think we got the idle OK. Thanks obi-1!
Idle circuit is set correctly…or nearly. I’ll do the “turn the idle mix screw in and see when it stalls that cylinder and open it back up” testing…then pull plugs and look to see where’s it at.
Next, check the mid range and pull plugs. I think with stock equipment it’s gonna be spot on probably…I’ll do a final check of a/f/r with full throttle 4/5 gear runs with shutdown and pull plugs to verify it’s in the proper range.
Plunked a new battery in it this afternoon and went for a very short ride. It’s running very well, not stumbling or fouling.
I’d recommend that you never put a dynojet needle and on a stock-airbox’d bike. The kid I bought this off of didn’t know what the hell he was doing by putting a jet kit on that bike apparently.
The only problem is now that the slides don’t close a fast as I’d like them to…RPM’s don’t fall fast enough for my liking. I may get new ones and see if that makes a difference. or grease’em…
It started loading up on me again last night after a few mins of riding.
During mixed riding, the RPM’s not falling fast enough/slides staying open while throttle plates are closed may be the culprit. I may experiment with greasing the vacuum operated slides & see if it clears up. They may be hanging up due to friction. In that case, I will probably get a complete rebuild kit and start from there.
I was going to buy my next brand new HD off them if the GS came back right, definitely not now. Got bike back and after 426 dollars and the service manager saying they rebuilt the carbs to stock, replaced plugs, general tune up, etc. and it’s still fouling plugs. I got it to foul the plugs within 1 mile of riding it.
I’m just going to complain and try to recoup some cash. They said they rebuilt the carbs to stock but they didn’t…the slides should have been replaced with new ones…
I read some documentation that the Dynojet kit apparently tells the user to drill out one of the slide openings and plug it and then drill out the other side to put a reducer in the remaining slide port to make the slides more “responsive” to the main butterfly opening.
I think I got this figured out: I changed the slides’ air openings’. It’s the Bernoulli principle: low pressure in the top of the slide housing and high pressure in the carb venturi = slides raise -> fuel enters the carb for atomization and is then passed to the engine.
I’m pretty dumb sometimes…it basically means it’s going to open the slides more quickly with the holes plugged (slower equalization of air pressure above and below the slides) and that means throttle-opening-opens-the-main-needle-too-fast in this case causing a “there’s too much fuel and the thing bogs out and fouls the plugs” condition.
So, I figured out why the thing was so rich…the slides were opening very quickly and staying open too long after I shut the throttle. And that translated to flooding the carb and way too much fuel. That’s why the idle circuit was fine, but any riding action and it would load up. The main was engaging way too early and staying on way too long.
All I need to do is mess with it a few more days and I think I got it now that I understand the root problem…it wasn’t a needle, seat, or any other problem other than slide action which is directly affected by the little holes in the bottom of the slide.
Also, if anyone has vacuum slide operated carbs, I’m pretty much on a path to become a guru at this now…if you need help I probably can tune it close to perfection without a dyno or wideband at this point.
