Different idle mixture needles?

JohnJL · 09-18-2007, 04:10 AM

I was having trouble getting my #1 cylinder to respond to idle mixture adjustments. Even cranking it in all the way would not get the idle to falter at all. So I took them apart again and found the mixture screws' tapers are different. Sure enough #1 is a true taper while #2-6 are "stepped."

So I would guess #1 was running really lean already? I havent checked the plugs yet.

So, where can I get a "stepped" idle mixture screw (or are these worn or something?)

thanks all

JohnJL · 09-18-2007, 12:30 PM

Anyone know what the needles on an 40IDA3C are supposed to look like? Like #1 or like numbers 2 + 3?

Steve@Rennsport · 09-18-2007, 02:22 PM

John:

The left and center needles are from the IDT emissions carbs; the right one is the IDA one.

JohnJL · 09-18-2007, 02:57 PM

Thanks Steve...PO claims this was rebuilt by Parr...?

I checked the #4 cylinder also has the same style mixture needle as #2 and 3.

Whats the recommended course of action here? Replace the other 5 to the style of #1? Or grind down #1 to match the others?

I was able to get cylinders #2-5 to behave properly at idle though I was having some problems off-idle. @2-5 would react as expected (when turned in slowly they would bring the idle down and then would bring it back up when backed out per published instructions) however #1 never behaved correctly...all the way in or all the way out had no impact on the idle...

Steve@Rennsport · 09-18-2007, 03:05 PM

Are your carbs IDA's or IDT's?????

Since the progression hole positions must match the idle mixture screws for proper idle and progression performance, one uses hardware commensurate with the carbs in use,..

Personally, I would be VERY careful about modifying them to match,..................

JohnJL · 09-18-2007, 03:13 PM

They are definately labelled "40IDA3C"

Steve@Rennsport · 09-18-2007, 05:59 PM

Every engine's vacuum characteristics is a bit different so if the motor ran the best with the stepped needle, I'd replace or modify the other(s) to suit.

You may need to change idle jets as well as make idle mixture screw adjustments to get the engine off idle smoothly without hesitation.

Needless to say, the float levels must all be EXACTLY the same,...

JohnJL · 09-18-2007, 06:51 PM

Yes, after much reading here and consultation with a couple of local shops I suspect 60 idle jets were a bit rich so I have 57's on the way now.

Spoke with Topend today also, they suggested changing the "off" needle to another cylinder and see if I can replicate the issue there. They also stated they do not have any replacement mixture screws (our Host stopped carrying them and jets also...)

Floats are all spot-on too. Meniscus is in the middle of the upper two marks.