OS mixture control screw

[Jack Brady]

Hello, I was wondering what the main goal is when adjusting the mixture screw? Is it for mainly throttle responce, or for idling? Does it have any affect on full throttle running? I know how to adjust the needle valve but not sure what the goal is with the mixture screw. The manual is not very clear to me. Any tips would be great. Thanks

 

[Tim_Duggan]

Hello Jack,

it's for idle / low speed mixture. Many on here will argue that it's not needed at all, other than to block a hole! I like them personally, but agree that having this needle set too rich is far, far better than too lean.

On the shore from idle, the motor should hesitate for a second or two before clearing out when the throttle is opened fully. If it doesn't hestitate and has quick response to throttle input, wind the screw out anti-clockwise a few turns and repeat the same test.

Tim.

 

[Jack Brady]

I mainly ask because I have four engines and they all run differently. If I open the needle valve to say 1 1/2 turns open on each carb and put the flow meter on them they all flow quite different for the same needle adjustment. While the flow meter is on with carbs at full throttle I can adjust the mixture screw to get all the carbs to flow the same. So to me it seems like the mixture screw would affect the wide open running and needle valve settings. I was wondering if there is a optimum setting for wide open running. I dont care about idling much as long as it runs good enough to launch then it is pretty much wide open most of the time. Depending where the mixture screw is set you may have to open or close the needle valve to get the flow you are looking for and if one way is better than the other. I hope I explained what I am after good enough so you can understand what I am asking. Thanks.

 

[Tim_Duggan]

I was wondering if there is a optimum setting for wide open running. I dont care about idling much as long as it runs good enough to launch then it is pretty much wide open most of the time.

I assume we are talking the stock carb on the OS O/B here?

At WOT it won't matter if the low end needle is too rich, but it will matter if it is too lean. So not an optimum as such like a main needle, but a safe area. So just wind the low speed needle out to the point where it's too rich down low and forget it. No need for a flow meter value for low end on those carbs. Just set it to very rich down low and forget about it. I wound mine out til it hesitates after idling for a few seconds and I never touch it!

The main needle valve is the critical one. It's the one you should get a flow reading from at WOT after finding the optimum thru testing. Different motors of the same type will flow differently for various reasons.

 

[Jack Brady]

Yes, these are the stock carbs. I was mainly just using the flow meter trying to troubleshoot to see if I had a air leak or something with one of the carbs as that engine wasnt running right and just noticed how each carb if the needles are set the same flow quite different from eachother. I dont think the slit in the spraybar is real precise. Like you said I will just make sure the mixture screw is on the rich side and not worry about it. Thanks

 

[Jerry Wyss]

Yes, these are the stock carbs. I was mainly just using the flow meter trying to troubleshoot to see if I had a air leak or something with one of the carbs as that engine wasnt running right and just noticed how each carb if the needles are set the same flow quite different from eachother. I dont think the slit in the spraybar is real precise. Like you said I will just make sure the mixture screw is on the rich side and not worry about it. Thanks

The XM 20J carb is a VERY GOOD piece of work, I have 7 of them on 7 different 21 o/b powerheads with the spray bar and idle circuit intact. Engine not running right can be a dozen things including you. I had to change one needle after I dropped the powerhead and bent it,, I replaced it with the whole needle assembly.

Several needle assemblys of the same make and the same turns out would not necessarily flow the same. That doesn't surprise me since nothing man has made is perfect, specially considering mass production of any assembled component.