Carb Tuning

[Capt_Zoom]

Question.
My bike: 1980 with stock 34 carbs and stock pipes that are without mufflers ending about at the brake pivot, pod style filters

Can anybody enlighten me on something with the carbs. If you go up a pilot and main jet, would the stock mixture screw settings be radically changed? Stock setting was 3-3.5 turns out.

Bike Idles perfect, with the throttle stop all the way out , but when I hit the street I'd breaking up when I apply throttle. Seems rich to me.

Just wondering if there is any sort of relationship between pilot and main size to mixture screw settings.

 

[5twins]

The mixture screw is for adjusting the low speed or pilot circuit but it's effect is limited. That's why jet changes are sometimes necessary. Your pilot jet feeds it's fuel/air mix into the main bore of the carb through 4 little holes. Three are clustered together at the top near where the butterfly closes. The fourth is slightly ahead of those by itself. The mix emitted by this single hole comes from the pilot jet through the mix screw and is adjustable. The three holes flow directly from the pilot jet. The only way to change their flow amount is to change pilot jet sizes.

So, your mix screw gives you control over roughly 25% of the pilot jet flow. Sometimes this is enough to compensate for minor mods but many times, it's not. A jet size change is needed. A larger pilot will increase the fuel portion of the fuel/air mix delivered by the pilot through all four holes. You should re-adjust the mix screw any time you change pilot jet size. Best setting now may differ slightly. Usually, it's not much, maybe a quarter turn or so, but it usually does change slightly. However, this isn't etched in stone. It may not require any change at all. But, you won't know this unless you adjust it and find out.

 

[Capt_Zoom]

That's what I thought. Thanks for the confirmation.

 

[Capt_Zoom]

So I messed with the mixtrure screws...didn't do much good.

Now I'm starting to think that its the sync. Since the carbs were apart the sync might have been thrown way off. I have a sync tool supposed to arrive next week so I'll sync it up first then try some more tuning.

 

[5twins]

You mentioned the "idle stop" being all the way out. If you're referring to the idle speed adjusting screw on the left carb then, yes, it sounds like your carbs are out of sync. If you back that speed screw all the way out so it's not touching the cable arm, the bike should die. If the carbs are in sync, that would completely close both butterfly plates and the bike would stall. If yours still idles that way then the right butterfly plate is open more than the left one.

 

[Capt_Zoom]

That seals it then...sync problems it is. Thanks. Now i just have to wait for my new sync tool to arrive. Good thing I have my own bike to ride. The xs is the wife's.

 

[Capt_Zoom]

Any idea how many turns that your supposed to have the throttle stop on the left carb turned in?

 

[jd750ace]

I'd start with at least one full turn past contact with the arm to crack the butterfly open on the left one. Might start a little rough and/or the rpm jump up initially, but should go from there.

 

[Capt_Zoom]

Thanks...I'll try that.