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Post by phatboy on Jun 7, 2015 4:48:18 GMT -5
Long story short, I had to put my old carb back on my 2T 49cc scooter. I finally figured out the problem all along seems to be that the previous owner messed with the float level trying to fix a problem, and I think he also got the wrong style of main jet installed in the carb, it threads in but is a different height. I got a correct main jet in now.
I am finding the float level hard to adjust right. The tab was already badly bent up and float was tweaked, but after much more bending and tweaking I finally got something close enough to work. Scoot runs great, great idle, great full throttle, just a slight bog in mid range. Float level is probably a tad low but I am afraid to mess with the float tab any more than I already have and break it or ruin an otherwise working set up.
Just wondering, can you compensate for the slightly low float level by moving the jet needle up? Or is my only solution to try to get the float level right at risk of irreparably damaging the float?
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Post by rcq92130 on Jun 7, 2015 8:35:20 GMT -5
You are absolutely correct - the level of the fuel in the bowl and the height of the needle interact. However, they work in entirely different ways, and trying to tune a/f mixture by varying the fuel level is not generally easy or accurate. It sounds like you are quite familiar with carbs, and probably you already know this, but just in case ....
Fuel level - set by float height - should be at a fixed factory setting. Normally there is a spec and with a small ruler you bend the tab so the needle valve fully closes with the float a specific distance from the lip of the carb. Unfortunately, there are dozens of carbs stuck into scoot engines, and i have yet to find anyone who knows the correct spec for any of the carb designs. You have probably done a good job of getting it "close enough" - not flooding at idle, and not starving as you go around corners. Since the needle valve doesn't really close all at once, a real accurate fuel height isn't really possible (it varies even though you don't change anything). You are probably in the "OK" range.
The idle mixture screw and diaphragm height are the 2 intended ways of adjusting fuel mixture (fuel height in the bowl not so much, though it does have an impact). Fuel height really is an engineering driven spec - the main air passage in the carb creates a vacuum because of the engineering design of the venturi. That vacuum needs to be enough to lift fuel the height of the main valve (say, 5 mm) so it reaches the air stream. Once that is accomplished, the QUANTITY of fuel dumped into the air stream should be regulated by changing the size of the main jet hole - i.e., by how far down the tapered diaphragm needle sits into the jet, or by upsizing to a larger (or down to a smaller) main jet.
Of course, that all is layered on top of the fuel dumped into the air stream by the idle circuit (and the idle mixture screw), which on our little carbs has a big effect.
So you want to first get the idle screw set best, and then correct for high speed issues with either a lift of the diaphragm needle or a larger diameter main jet. Messing around with fuel height in the bowl (via the float tab) is a pretty crude adjustment. So, you might want to consider your tab bending work done and tune from her on just using the things designed for tuning. There is a great tuning tutorial on this site from Alleyoop walking thru the process. A quick search will get it for you.
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Post by phatboy on Jun 8, 2015 1:24:37 GMT -5
Thanks that was very helpful! That is an answer I wanted to hear, as I did not want to mess with float level any more. Other than the occasional bog, the bike runs great! I rode it all over town today and no issues at all, not starving up hills or around corners or anything.
I can raise the needle but from what you are saying, the slightly low float level means the main jet may now be too small? Aside from the slight mid range bog it does bog when going from idle to WOT, but only for a second then it just screams. I should probably upjet?
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