Throttle cable vs. idle screw

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volt9

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Aug 10, 2009
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Mississauga, ON, Canada
I just recently installed an SBP expansion chamber as well as a new air filter (also SBP) and was mucking around with the needle settings in an attempt to minimize some 4-stroking. After some minor experimentation, the second highest setting on the needle seemed to provide the best overall drive. Runs perfect at 50%~100% throttle and only 4-strokes (ocasionally) when cruising at 1/4 throttle or under a light load (like a mild downhill slope). I don't think I can get it to run any better than it is now. But I am more than satisfied with what the SBP pipe has provided...well worth the money.

Anyways, while I was playing with the e-clip on the needle (and looking at the throttle cable) something occured to me. What's the starting point when you are setting the idle? What do I mean? Well, from what I can tell, there are two ways to raise or lower the idle. 1. you can use the screw designed for exactly that purpose, or 2. you could adjust the actual throttle cable (which effectively raises and lowers the needle) thereby achieveing the same result. Which way is better? And why?

The way I have it set now, is that I have the throttle cable adjusted 'just' to the point where there is no obvious slack in the cable, so any amount of movement from the throttle will result in raising the needle. If I adjust it any lower (the nut on the cable I mean), the cable becomes a little slack and the first few mm's of throttle rotation don't actually move the needle at all. (and you can feel it because there is far less tension until the needle starts to raise) I think the way I have it sounds right, but would like to hear how other more experienced MB riders do it.

So to get back to my first question, what's the starting point? Maybe what I should be asking is ; what to adjust first? Throttle cable? Or idle screw? And when you are turning the idle screw, what's actually being adjusted inside the carb?

BARRY
 
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volt9

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Aug 10, 2009
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Mississauga, ON, Canada
Thanks for the link. It sounds like my method was very close to that description. But with a lot more detail. Nice post.

However, I'd still like to know, how does the screw actually adjust the idle? What is mechanically going on in the carb when I turn the screw?

BARRY
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
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Screwing in the screw lifts the slide off the bottom of the carb. Never hold the dlide up with the cable adjuster, it's just for adjusting the slack out of the cable.
BTW- 4 stroking when under light/no load is exactly how one of these should run and is unavoidable.
 

volt9

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Aug 10, 2009
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Mississauga, ON, Canada
Screwing in the screw lifts the slide off the bottom of the carb. Never hold the dlide up with the cable adjuster, it's just for adjusting the slack out of the cable.
Please do not mistake this for ignorance, but I'm the type of person who does not give information without explanation. I appreciate that you are offering your help with something I am inquiring about. But simply telling me to 'never' hold the slide up with the cable doesn't teach me anything about why I should not do that. If it's for reasons that would cause wear or damage or something that would reduce efficiency or impact performance, I would like to know more as to why it should not be done that way.

The good thing is, that's pretty much the way I looked at it already, by using the nut on the cable simply to adjust the 'slack' and nothing more. Sounds good. :)

BTW- 4 stroking when under light/no load is exactly how one of these should run and is unavoidable.
That exactly what I wanted to hear. In fact, the reason why I felt that I had gotten as close as I could was because of what I had already read about people playing with the carbs, as well as many videos I had viewed on YouTube. I wasn't actually inquiring about how to make it smoother, rather just describing what I got when adjusting the e-clip. But as you said, I guess it's just the nature of these lo-tech 2-strokes. Thanks for confirming that.

BARRY
 
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bairdco

a guy who makes cool bikes
Aug 18, 2009
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to elaborate on Joe's reply, if you look at the slide itself, there's a tapered groove along the side of it that the tip of the idle screw contacts. as the screw turns in, it pushes against the groove, and the deeper the screw goes, the higher up the slide goes.

i think what Joe's trying to say is that everything has it's purpose, the idle screw should be used for idle, and the throttle cable adjuster should be used to adjust the cable slack, and not as a secondary means of adjustment. mostly because it's just not the way it should be done.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
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Well said, and to elaborate on your elaboration, if you hold the slide up long enough and the engine vibrates enough, the cable end will come off and be ingested and then you will have a bunch of scrap metal bolted to your bike.
 

volt9

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Aug 10, 2009
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Mississauga, ON, Canada
Well said, and to elaborate on your elaboration, if you hold the slide up long enough and the engine vibrates enough, the cable end will come off and be ingested and then you will have a bunch of scrap metal bolted to your bike.
Ahhhhhh, now that's something that would definitely deter me from doing it that way. Thank you for creating that mental image. With it, I'll have an easier time remembering why it shouldn't be done that way. :)

BARRY