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One year later, but better late than never...

I spent the winter months in trying to improve the dreaded off-idle lean stumble with little success.
I even drilled an additional progression hole next to the butterfly valve but the only progress was that I could go from 60F9 to 60F8 idle jets (with some better economy at cruising).

Ed's above comment on the air velocity over the holes is a good point.

My question is: when we measure the depression at a given rpm, say 1500, the depression is the same in a modified and in a non modified engine.
The depression is a measure of the quantity of air that is sucked by the engine (in my mind it should be a function of the rpms and the unitary displacement).

So why the same depression at low rpms is effective in creating the mixture in a non modified engine and not in a modified one?

My hint is that the flow in the modified engine is disrupted by the reversion due to the long opening cams. But how to confirm that?

Cheers to all
you opened a can of worms perhaps? :) nords are old hemi motors with large valves and big ports decent for flow not so much for velocity or swirl for low rpms. i reckon they are nice to around 160hp before there is a detriment to drivability compared to a stock motor.

im not sure how you measured your depression, you can have the same about of intake charge drawn the they cylinders but that dosent mean the velocities are the same between a mod and a standard motor especially at the carbs were your fuel metering point is.

mod motors typically include hotter cams, bigger ports and carbs. most of these increase more air in to your motor for that added hp. however they will most likely reduce velocity at low running speeds especially if you dont recoup the effective compression ratio with hi comp pistions. the increased cam overlap rather than reversion is playing of a role in your situation though they are linked.

try choking down your larger 45s and venturis back to 40's will make your motor more streetable at low rpms. you may loose up to 10hp at they very top end but imo its a worthwhile trade off for a smoother running motor.

my 2cents ... best of luck !
 

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None of you have mentioned the earliest and biggest cause of stumbling off idle. The stock dizzy sometimes advances too quickly - as in the (in)famous Shankle advance. Check your advancement curve, this might be able to help.

Some of the newer aftermarket dizzy's have programable advance curves. One of these might be just what you need.

Robert
 

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A bit late to the thread but better late than never.

I'm a Datsun guy but when I tried Keith Franck's 25mm recommendation for fuel height, I too and many other triple Weber Datsun owners experienced very rich cruise.

The solution was to lower the fuel to the recommended 29mm from Weber. 25mm may work on the Lotus engines (not sure of the engine's tilt) but it does not work on 510, 240z, 260z, 280z, or 280zx
 

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Joe,
I went through the same problems with my 2L Nord motor and finally set the fuel height to 28 mm

Great! I'm glad you found the same solution.

25mm was far too high and the 12.5 A/F ratio target was far too rich. I wonder if Keith Franck is really a bot from a petrol company that wanted to sell more fuel?
 
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