Plugged idle circuit

[Anker]

I have a plugged idle circuit on one of my Weber DCOE 40s. The car idles poorly and doesn't respond to one of the idle mixture screws being turned. I can turn it completely down without any change.

 

I would like to defer a complete rebuild to after the driving season and wonder whether I can clear the obstruction with compressed air. My plan is to remove the idle jet and give that a compressed air blast, but am concerned that the obstruction may be elsewhere and wonder whether any of you can advise me. One thought is to also remove the idle adjustment screw and blow compressed air into both cavities in the hopes of unlodging whatever it is.

 

It is not the fuel filter, because the other throat works just fine.

 

Another thought is to just remove the top cover of the carb and blow air through all passages that are in it.

 

Anker 

[SENC]

I would hunt for carb leaks and do compression and leakdown tests before jumping to conclusions.  I had a similar symptom, turned out to be weak valve springs on #3 allowing a slight internal leak that reduced vacuum at idle (which, of course, meant the idle mixture screw had no effect when adjusted in or out).  Bottom line, could be a vacuum leak (failed o-ring, throttle shaft shift, butterfly wear, leak around the idle mix screw, etc.)  just as easily as a clogged idle circuit.  The idle mix screws, in particular, can leak more than you'd expect - put some EZTurn or other fuel line lubricant/sealant on the threads to help with the seal.

[Anker]

Thanks for the suggestions, Henry! I'll keep them in mind.

[SENC]

Attaching a couple idle circuit maps that may be useful, if you don't already have them.  Does the engine transition from idle to cruise (at slow throttle increase) ok and does it cruise ok?  This key function of the progression circuit is fed by the idle jets and circuit, so if performing ok at those stages then I would think the only place a blockage could effect idle and not progression would be in the small chamber between.  A blockage further upstream from the progression holes should cause issues on that cylinder right up until the main circuit becomes the more dominant force (3k rpms, or so, I think).

 

Good luck, and keep us posted!

 

 

[Anker]

Hah! I found the problem. Not at all what I suspected.

 

I blew out all passages and jets, but no improvement. Then I got it running as well as I could, still popping and not running evenly. I then started pulling spark plug leads. The engine idled with the #1 and #2 cylinder leads pulled but quit when either #3 or #4 were. That cause a light bulb to go on, so I checked the balance of the two carbs. Bingo - #1 and #2 were pulling 5 and #3 and #4 were pulling 7. Got them balanced and the problem was fixed.

 

So what was happening was that the butterfly valves on #1 and #2 were almost completely closed. The two aren't exactly aligned, so #2 was pretty much completely closed and not pulling enough vacuum through the idle adjustment screw to make it effective.

 

The lesson I learned is not to mess with the carbs without checking the balance first!

[NSXguy]

@Anker curious what rpm do you idle at? 

[Anker]

I set it to idle at 1000 RPM

 

Anker