Aisan SD40 Carb Question

[Nothinghead]

When I rebuilt the SD40 big window carb on my '65, I noticed there was no step-up-rod in the kit but there was one in my ash tray. I haven't got a clue why it was there but I put it in as part of the rebuild.

Since then, I've noticed if I have it adjusted properly as per the FSM, it hesitates and sputters (no backfire) whenever I depress the accelerator. If I pull the choke out about 3/4", the problem goes away.

I've attached a video and, yes, the timing is set right on the BB with Pertronix in the dizzy. The engine is warm:

http://youtu.be/Oc_8apP4gXU

This seems odd to me, why I only run smoothly with the choke pulled. Is this a problem? Should I be looking for other things to fix or adjust or just keep driving it?

 

[nuclearlemon]

our d40 is like that too. just a touch of choke, but still some required.

 

[Nothinghead]

our d40 is like that too. just a touch of choke, but still some required.

So I guess the choke is helping it to not run too rich with the step-up-rod plugging the jet at slightly open throttle. Without seeing what the standard step-up-rod looks like, I believe I have the larger diameter one installed that is supposed to get slightly better economy.

If so, what about WOT? Will it be too lean?

Please forgive my ignorance. This is the first carburetor I've ever messed with.

BTW: with the choke cable 3/4" out, it runs fairly smooth except it does sputter a bit when letting off the accelerator back down to half throttle.

 

[cuencanolenny]

Pulling on the choke actually makes the mixture richer as what you are doing is closing the butterfly valve and thereby reducing the volume of air that can move through. So this would indicate the mixture is too lean and you need a larger main jet. Since the effect is immediately off idle that would be the low speed main jet.

 

[sggoat]

When I rebuilt the SD40 big window carb on my '65, I noticed there was no step-up-rod in the kit but there was one in my ash tray. I haven't got a clue why it was there but I put it in as part of the rebuild.

Since then, I've noticed if I have it adjusted properly as per the FSM, it hesitates and sputters (no backfire) whenever I depress the accelerator. If I pull the choke out about 3/4", the problem goes away.

I've attached a video and, yes, the timing is set right on the BB with Pertronix in the dizzy. The engine is warm:



This seems odd to me, why I only run smoothly with the choke pulled. Is this a problem? Should I be looking for other things to fix or adjust or just keep driving it?

Obviously not getting enough fuel at idle if you have to use the choke--slow/air correction jets partially plugged-but then again, maybe just an idle screw adjustment?--Try that first before carb disassemble--