Edelbrock Thunder AVS 650 Idle Channel Restrictor

I decided to perform a popular mod to my carb that I have read a lot about and that is increasing the size of the idle channel restrictor.

It is well documented that the Eddy carbs flow barely enough fuel in the idle/transition circuit to cope with big cams. I've experienced this through the various low speed running problems. I can see from the AFR that there simply isn't enough fuel at low RPM and there is a lean surge problem.

Closing the throttles and drilling holes in the throttle blades to make the transition circuit more active helped a little bit but the trouble is that the transition circuit is fed through the idle channel restrictor so this needs to be enlarged. In Holley type carbs, Quick Fuel, Demons etc, the top-end carbs have all sorts of changeable air bleeds and jets all over the place which can be easily swapped. On the Edelbrock however its a drilled channel so the only solution is surgery. It would be possible to drill and tap and replace with something adjustable by my approach is to make tiny changes until I notice a change and leave it at that. The risk of course is going too far then changes aren't reversible if you are drilling holes.

Here is the offending area of the carb as per the Eddy manual (note 5) :

It can be accessed by unscrewing the air horn and then removing the booster assembly that sits in the idle well. You will have to remove the throttle cable and choke assembly as well.

The channel restrictor is hiding behind the pressed-in brass jobby, labelled "idle air bleed" in the manual. Removing this is easy-peasy, get a self tapping screw and wind it in to the brass restrictor then just pull it out with some pliers, in fact one of them came out by hand.

You can now see the small hole that was behind the brass restrictor. This is the ICR that we are after.

Just so we are absolutely sure what it is we're drilling, here's a close-up with it circled in red:

It's now possible to get in with a drill and open up the orifice. I don't know what size it is to begin with but I ran a 1.5mm drill through which opened it up a small amount, it didn't remove much material. Its probably best to use numbered drill bits to 1) accurately measure what it is to begin with and 2) be sure of what size you are going to and make SMALL changes - I know its a bit hypocritical saying this when I just picked up a small bit and ran it through but the point still stands, when enlarging very small holes you can make massive differences in terms of % from what seems only a small increase in diameter.

Don't forget to do the other booster before you reassemble!

Anyway - the result, I'm pleased to say I can actually feel it and can see it on the AFR gauge. Without changing the idle screw settings, the idle AFR had richened up a little bit from 13.8:1 to about 13.5:1. This means that the idle circuit as a whole is now getting more fuel. 

When driving, the biggest difference was when plodding along under 2000rpm when the idle circuit is doing all the work. Before this mod I was creeping over 14:1 which was just about ok for constant throttle but if I made very small throttle adjustments of just a few mm on the pedal would send it into lean surge as it was clear the idle/transition circuit wasn't fat enough to give any extra fuel for small modulations on the throttle. These are pedal movements which are too small to get a fuel from the accelerator pump, just a couple of mm slow throttle movement.

Now that I've drilled the channel, the AFR doesn't spike lean and surge when making small throttle movements at low rpm and the general low speed manners have increased noticeably. I can give it a couple of mm throttle just to very lightly pick up speed and the AFR remains below 14:1 and lean surge is avoided so it was definitely worth doing - the way I see it in my head is that the transition circuit now has more "headroom" to cope with the many small changes in fuel demand that occur when you are cruising along gently at low rpm when the main circuit isn't fully active.