Quick Vacuum question (too rich idle circuit?)

[Anonymous]

Is it normal for the idle togo way up when you remove a plugged manifold vacuum port on the carb bsae? It's almost like the motor is starving for more air and when it's opened up, it gets it. The idle shoots up sharply when I unplug it. I always thought that by creating a vac leak, your idle and motor running quality should drop sharply. No?

I edited the title of the post because it seems maybe the cause of this is overly rich idle circuit. Maybe?

 

[mugsy]

Sounds like the "normal" idle is way too rich! By giving the engine more air and it is mixign with the excess gas and the engine speeds up. I bet the idle system in the carb is supplying too much gas.

I would re-install the plug/cap and see what can be done about the regular idle. And you are right, when a air leak is present a car should barely idle, stumble, and idle very rough.

tanx,
Mugsy

 

[Anonymous]

Yep, that's what i thought. I know the idle circuit is way too rich. I'm in the process of rejetting the carb now. I just added an Air/Fuel gauge/sensor so I can do a better, more educated job of it. I dropped from 73's to 71's but still need to go farther...enough about that, I have a whole thread going on this topic as well!

You guys would be bored if I didn't ask all these questions! :roll:

 

[mustang6]

Your work with the A/F ratio gauge and leaning this carb down will be interesting to follow. IIRC the main jets don't play a role in the idle mixture at all, so you may need to deal with that problem in other ways. it will be interesting to see what happens as you experiment.

 

[Anonymous]

hmm..now that you mention it, I think you are right. Main jets DON'T effect idle circuitry. What then can control that? Float level have anything to do with it? Maybe I gotta bad powervalve in there (or a bad vac gauge ) and the PV is open at idle!

more tinkering to do....fer sher

 

[Anonymous]

OK, so the powervalve is OK, so then how do i go about leaning the idle circuit other than the mixture screws? I can back them out to a point where my A/F shows just barely in the RICH lights, but any further than that and she sputters.

What in the world causes this strange manifold vac situation?! I'm stumped.

 

[addo]

I'd say it's irregular vacuum signal from those valve train issues.

 

[Anonymous]

That I guess is a possibility, but if the valvetrain issues were always present (meaning, things just didn't go wrong at higher revs), then would I still see a steady 12" ov vac at idle? Wouldn't it be bouncing all over the place and drop off sharply at times?

Could it be a clogged air bleed in the circuit? I have a good Holley tuning book and in reading that part about idle and mixture, it shows a small capilary that allow air in (not the mixture screws passage):

"...in cases where the main system has been leaned as much as possible, yet there is still an excessively rich muxture at low engine speeds, it will be necessary to reduce the size of the idle feed restriction or enlarge the diameter of the idle air bleed....(this) should be a last resort."

but, it does say this...

"...changing the main jets...affect afuel-air ration at virtually all engine speeds and loads."

So do the main jets meter the fuel in the idle circuit as well?

 

[Anonymous]

OK, I'm stumped. That's not big suprise though :roll:

all valve train issues aside, I uncorked that manifold vac port on the carb and the idle goes UP , the car idles better, BUT the A/F gauge doesn't change except that when I stomp on it, it goes a bit leaner for a split second then back to rich. How can that be? I create a huge vacuum leak and the car runs better? WTF? No only does it run better, but somehow the air/fuel ratio at exhaust is unchanged? I'm dumping a lot more O2 into the manifold yet it's not leaning the mixture. Somethin' aint right with that...or is it me?