I would say yes from a logical point of view. But that is assuming a triple is logical........
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| Author: | Looney Cylinders [ Tue Oct 09, 2012 11:04 pm ] |
| Post subject: | Air screw effectiveness? |
A question: Does changing an air screw setting 1/4 turn have a bigger effect on the resulting idle mixture using a pilot jet of 12.5 VS the same AS adjustment change using a pilot jet of 30? Thoughts? J |
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| Author: | Ja-Moo [ Tue Oct 09, 2012 11:50 pm ] |
| Post subject: | Re: Air screw effectiveness? |
I would say yes from a logical point of view. But that is assuming a triple is logical........
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| Author: | Looney Cylinders [ Wed Oct 10, 2012 12:07 am ] |
| Post subject: | Re: Air screw effectiveness? |
I too think the smaller jet is effected more by given AS change. I would expect the amount of air being changed by an air screw adjustment by 1/4 turn to be a fixed amount. This fixed amount or air should dilute two very different amounts of fuel being introduced into the pilot system by the pilot jets.. 
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| Author: | usernameandpassword [ Sat Oct 13, 2012 6:03 pm ] |
| Post subject: | Re: Air screw effectiveness? |
I reckon youd also have to look at the size of the carbs as well and say yes. A 12.5 wouldn't be used in a large bore carb or vice versa, so I'd assume the air passages would be different size as well. Anyways a 12.5 you'd be lucky if it idled.
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| Author: | Looney Cylinders [ Sat Oct 13, 2012 7:27 pm ] |
| Post subject: | Re: Air screw effectiveness? |
usernameandpassword wrote: I reckon youd also have to look at the size of the carbs as well and say yes. A 12.5 wouldn't be used in a large bore carb or vice versa, so I'd assume the air passages would be different size as well. Anyways a 12.5 you'd be lucky if it idled. Actually, 12.5s working quite well in the range of 1.375-1.625 AS turns (depending on brand of fuel and air temp) in my VM 34s!!  There ARE little plastic thingys on the slide bottoms though  j |
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| Author: | mraxl [ Sat Oct 13, 2012 9:55 pm ] |
| Post subject: | Re: Air screw effectiveness? |
That is a good question! Airscrew 3/4 to 1 full turn in a stock configured carb is equal to about one jet size. With such a large change in pilot size it may well be that the same AS change might not still equate to one jet size since the fuel is metered by the jet and the air by the same screw/bore... proportions change a great deal. I would guess that the AS setting would be much more sensitive... but that's a guess. It would also seem that a slightly richer needle would be required... again guesswork. |
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| Author: | Jim [ Sat Oct 13, 2012 10:44 pm ] |
| Post subject: | Re: Air screw effectiveness? |
Dale, you're making my brain hurt. |
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| Author: | Looney Cylinders [ Sat Oct 13, 2012 11:19 pm ] |
| Post subject: | Re: Air screw effectiveness? |
mraxl wrote: It would also seem that a slightly richer needle would be required... again guesswork. Actually experiencing a few oddities with these UFOs. One thing is they lower the throttle range effected by the needle jet/jet needle all the way to full closure of the slide (rather than approximately 1/4 open). As such the airscrew setting/pilot jet determination must be done at a very slow idle with all three carbs on their stops (no dominant cylinder). Only in this configuration does the airscrew adjustment have any effect. If you try to set one carb higher in idle (to establish a dominant cylinder) fuel flow from the pilot jet circuit it is completely overwhelmed by flow from the needle jet. Just ordered more brass (another needle jet set and more $$$) as I work down in sizes to lean the lower range controlled by needle/needle jet while slowly dropping the clip to maintain/richen the upper end. Getting close now and hopefully this last order will get me bang on........just in time to put the bike away for the winter!!!  J |
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| Author: | Ja-Moo [ Sun Oct 14, 2012 12:42 am ] |
| Post subject: | Re: Air screw effectiveness? |
If you had one of my 1" "spacers", and Mikuni carbs, you wouldn't have to doodle with UFO's..........
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| Author: | scrambler73 [ Sun Oct 14, 2012 9:05 am ] |
| Post subject: | Re: Air screw effectiveness? |
this is a good post.....and something I'd never thought about before.  makes absolute sense though. I found that tuning my H2 (ported w/VM34's) to the UFO's was a piece of cake. Only had the carbs in and out a couple of times, which was basically just dropping the pilot size and upping the main. It helped that I read the instructions, asked others questions and did a little research on some of the snowmobile sites before installing. The bike carburates perfectly..... I might see if installing UFO's in my H1 would help get rid of the high speed "chopped throttle detonation" (since it richens up lower throttle openings so dramtically) The airscrew on the H1 is SUPER touchy.. In too far and its blubbery riding around, out too far and it wants to detonate. Almost like you are making huge needle changes? yet another winter project.. |
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