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carb tuning question..

  • Thread starter Thread starter Anonymous
  • Start date Start date
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Anonymous

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Hello all,
Longtime lurker firsttime poster I'm in the middle of ressurecting a 79 gs750e yoshimura 4-1, pods,115 main jets 17.5 pilot jet... I have the fuel screws turned out to 2 turns... carbs are clean, synched ,needles are at the lowest slot(richest setting)...bike idles great and pulls great until I hit 7000 or 7500 rpm..then sputters and pops...So should I put in one size bigger mains?Does that also mean I should raise the clip one slot if I do?

Thanx
Silverhammer
 
I could take your sputtering/popping description as being rich.
However, your main jet is only up 2 1/2 full sizes (from 102.5?) which is usually still lean in my experience for your mods. If the problem is happening during higher speed roll-ons, it can be a rich overlap effect from the needle circuit, even if the throttle is wide open or nearly open. But the bike should eventually clear and run better, especially if the throttle is held completely open.
If the bike is pulling well, no bogging, at wide open roll-ons in top gear, but starts the problem at 7,000+ rpm's, it's hard to believe your bike can't handle a 115 main. It may be a high float level, or poor spark at that point.
But if your sputtering/popping is actually a lean mixture, it could be the mains, but I would describe small mains as the bike just hits a wall. Other causes could be low float level, carbs not synched at the fully opened position correctly, poor fuel flow from a failing petcock or in line fuel filter, too small fuel line, dirty carbs, not removing the two floatbowl vent tubes (required with pods).
I would check my suggestions and go out and get some plug reads at 1/3 throttle and wide open throttle positions. Chop test the plugs. That way we can know if it's a lean or rich condition and remove any confusion relying on descriptions.
Remember, jetting is about throttle position, not rpm's. Always mark your throttle housing and grip before road testing. This way you have a better idea of what jetting circuit you're on.
 
Trying to set up these engines with pods is never straight forward, and it's always a good idea to make just one adjustment at a time and check the results and eventually it'll work out for you.
 
Thanx for your response... I tend to think that its lean because my needle was 2 slots from the top and the problem got progressively better as I raised the needle in other words, it was bogging at 3000 rpm and after raising the needle one slot at a time,eventually at the last slot the "wall"was at 7000-7500 up to that point, it goes like a raped ape :D Also, I didn't know that I needed to remove the carb vent hoses.. I'll do that too...So does 117.5 mains sound more typical for this application?
Thanx
Silverhammer
 
silverhammer said:
Thanx for your response... I tend to think that its lean because my needle was 2 slots from the top and the problem got progressively better as I raised the needle in other words, it was bogging at 3000 rpm and after raising the needle one slot at a time,eventually at the last slot the "wall"was at 7000-7500 up to that point, it goes like a raped ape :D Also, I didn't know that I needed to remove the carb vent hoses.. I'll do that too...So does 117.5 mains sound more typical for this application?
Thanx
Silverhammer
Click to expand...
Removing the float bowl vent lines will allow the bowls to draw fuel easier. That alone will help your lean condition some. Leave the nipples open of course (some people cap 'em. 8O ) The float levels are very important, especially high speeds. Be sure they're set correctly or your re-jet will be a worse PITA.
It's hard to just say what size mains, etc, will fix you up. I can't ride your bike and there's no plug reads to tell how close you may be. Off the top of my head, if you have K&N filters, I'd say 127.5 or 130 Mikuni mains. If you have the cheaper pods, I'd say 122.5 or 125. That's only 4 full sizes up from your 102.5 stock mains. I would still get some 1/3 throttle plug reads at least. If you're at the richest position using the stock needles, the stock needles may not richen you up enough. You may have to go with a jet kit such as DJ. Their needles are tapered differently and give better results most of the time. I've used DJ kits a lot.
Also, these carbs must be bench synched and then vacuum tool synched. Without a vacuum synch, plug reads can be inaccurate. A synch is an important part of the re-jet. All cylinders have to draw the same amount of mixture. You must set the ignition timing before the vacuum synch and the valve clearances should be correct. The side air screws have to be adjusted for the highest idle too. Your pilot fuel screw settings sound "ballpark" to me. They generally end up about 1 1/2 to 2 turns out for your mods.
 
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