Lotus Elan

Weber Idle Air Balancing

PostPost by: type26owner » Tue Feb 22, 2005 8:48 pm

Decided yesterday to go back and replace the butterfly I had to file on the outer profile inorder to get it to fit at all that I installed about a year ago. Bought two more 78 degree butterfly discs and proceeded to install them yesterday an immediately ran into a problem. They differed by .002" on the major axis so when one was tightly closed the other was hung open. I got had again by more official Weber spare parts being junk. My only recourse was to do what I advised just the other day should not be done and that is twist the throttle shaft. But this is not why I'm posting.

Went on and drilled the tiny air bleed holes in the butterflys to do the idling airflow balancing which must be done over and over again until you guess it correctly. What a royal pain! Realized what is needed is a jet that screws into the butterfly to make this tuning process even more user-friendly. Which led me to consider modifying my carbies so they have the air bleed bypass circuit with an adjustable screw just like the 151/152 version has. For the effort to make a selection of different jet sizes I can just build the bleeders. The air must be routed around the butterfly so the airflow can be measured with precision and the airflow made the same in all the throats. What this does for you is now you reduce or increase the airflow on all the throats together so as to exactly position the edges of the butterflies just downstream of the first progressive holes so there is no lean hole. Drilling the holes does not do that. You can only make them larger which closes the butterflies even more moving them way from the first progressive hole. One mistake and you start over again.

Got it fairly close with drilling the holes but it's a tremendous amount of labor to do it this way. It purrs now. I know with the adjustable air bleeders I could do this in a matter of minutes rather than taking all day. They're on my to-do list.
type26owner
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PostPost by: type26owner » Tue Feb 22, 2005 10:18 pm

A piece of 1/8" ID plastic hose with a simple pinch valve would do the trick. The reentrant port can be into the o-ring compliant mount if there is nowhere else to do it conveniently into the Weber housing. Can't be any lower tech than that. :D
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PostPost by: type26owner » Wed Feb 23, 2005 3:07 pm

I've been surfing the web looking for any postings on adjusting the air bleed screws on the 151/152 carbies. No one understands fully what they do for you. That is even out the airlfow between all the throats AND allow the angular position on the butterflies so at idle the first progressive hole is just about to contribute to the mixture if the throttle is opened by the tiniest amount. Most folks fear touching them and strongly advise others to do the same. If they do say to adjust them it's to just close all of them up. Sad there is such rampant ignorance associated with this carbies. Think about how many crappy running engines there are out now because of this foolishness! Why did Weber leave everyone in technical darkness for all those years? Kinda seems like a shoot yourself in the foot business strategy to pursue. Since they recently bit the dust kicking more dirt on them matters very little. :rolleyes:

Ordered some fuel rated PVC hose to make my own air bleed bypass circuits. The pinch valve I'll make out of a block of aluminum with some screws to collapse the hose. Piece of cake!
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PostPost by: type26owner » Mon Feb 28, 2005 3:05 pm

I stared at the carbies for hours this weekend trying to devise an easy solution for the air bleed bypass circuit onto each throat. Kinda decided the best approach would be to install a supply port for connecting the hose just upstream of the butterfly. The original o-ring soft mount plates are too thin to add the reentrant port so new thicker ones by an 1/8" are in order. Rather than collapsing the hose I'm leaning towards just popping in a pill into each hose which has an orifice in it. That way I can swap the pills out quickly without having to disassemble the carbies from the engine each time. I already have all the tiny drills to make the pills leftover from when I made up a complete set of main and air corrector jets. B)

That keeps it simple. Collapsing the hose is just asking for it to be unstable over time. This adjustment needs to be done in such a way so it has to be done only once and forgotten.
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