Another Weber Question;Idle Jets

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71rocket
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Another Weber Question;Idle Jets

Post by 71rocket » Sun Dec 27, 2015 11:26 pm

Can anyone tell me the difference between using a 50/1.00mm idle jet or a 60/1.40mm idle jet when both provide me with same idle quality and both mixture screws 1/2 turn out.
I'm guessing the air hole size difference is effecting vacuum acting on the fuel hole at idle,but what is happening during transition/driving?
(2x45dcoe on 5ltr V8)

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4sfed
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Re: Another Weber Question;Idle Jets

Post by 4sfed » Sun Jan 03, 2016 10:43 pm

It depends on the transition ports and their relationship to the throttle plate. You'll need a wideband O2 to get the jets right. You're close when the engine will run smoothly under no load from idle up to 2500-3000 RPM.

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Re: Another Weber Question;Idle Jets

Post by Runit » Mon Jan 04, 2016 10:28 am

At one time I thought I'd worked out a math formula for ranking DCOE idle jets by richness. (Fuel jet area/Air jet area)(Fuel jet area+Air jet area) I now think I was wrong!!! Evaluating jets in operation is probably the best way. Just be sure that you're making the fine points of the evaluation, not a test driver!!!

71rocket
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Re: Another Weber Question;Idle Jets

Post by 71rocket » Tue Jan 05, 2016 1:09 am

Thats exactly my point,surely it cant simply be a ratio thing.At idle maybe but light cruise is totally different.

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Re: Another Weber Question;Idle Jets

Post by Runit » Tue Jan 05, 2016 1:46 pm

A light cruise might also involve the main circuit. You can get some idea of where the circuits overlap by removing the main jets, e-tubes, air jets and holders and reving the engine. There won't be enough fuel flow for the engine to rev very high. Big venturis on a small motor might give 5000 rpm. Small venturis on a big motor might give 1500. The rpm reached will give an idea of where the main circuit comes in. Or, you can look into a trumpet and watch for fuel flow in the aux venturi.

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Re: Another Weber Question;Idle Jets

Post by modok » Sun Feb 14, 2016 5:39 pm

The idle air bleed can be thought of as similarly as altering the size of the highest transfer port.
When the throttle is closed it makes the least difference, as it is only one air bleed of many. When the throttle is open is the ONLY air bleed and the others are all fuel passages. Although it can be confusing to imagine what is actually happening..... it is easy to see it makes the most difference when all transfer ports are open, and the least difference at idle.

The transfer ports transition from being air bleeds to being fuel passages as the throttle opens. Generally, the transfer ports increase in size as they open, the air bleed tends to be is about the same size as the largest transfer port.

There are limitations tho, as the passage from the idle jet to transfer ports will act as a restriction past a certain point, so, there could be a maximum size of air/fuel that can be used before the passage becomes a restriction.
If the passage is a restriction the only way to get more fuel through the passage would be use a smaller air bleed, which would screw up the curve, requiring smaller transfer ports to match.
This could happen with the OE models of weber and dellorto that use a large air bleed and large transfer ports all in the 1.9mm + size range. If this is the case the transfer ports will need to be plugged and drilled smaller if it cannot be made rich enough in it's current form without screwing up the curve. OR, just drop the venturi size down until it works OK as-is.

the racing and universal versions of these carburetors the transfer ports are extra small, and this will situation is unlikely.
More likely is you need to enlarge the transfer ports or add an additional port, as they have been left minimal intentionally, because it's easier to dril than to undrill. If a hole added, or the top hole enlarged, then a larger air bleed will be needed to keep the curve the same.

Like the main and air jet combo, the idle and bleed is a combo. If you want to try a different size bleed, you will need to change the idle jet too to match.
The idle air bleed can also be used for fine tuning, since small changes make less than half the impact as fuel jet change the same magnitude. Can be easier to change the air bleed than to drill a odd-size idle jet.
Glen Urban

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