I have a 68 malibu, 327, th350, and a Barry grant speed demon (650cfm vacc secondaries/electric choke). i'm trying to get rid of my off idle stumble. After no results w/ adjusting the idle mixture screws, after asking some qustions around here..i then became aware that it might be a power valve problem. I put a vaccum gauge on my manifold( direct vaccum off my brake booster hose) Here are my results.

Car hot, idling, in gear:
11-12hg
700 RPM

The BG speed demon comes w/ a 6.5hg power valve. With the results up above, what PV should i get? Also, i've never taken a carb apart before, so i am not quite sure how to get to the PV. Do i just pop off the primary fuel bowl and it will be right there? This sluggishness/off idle stumble is really getting on my nerves, so any help would be great. thanks in advance- Eric

I doubt it's a power valve problem unless you blew the PV which I believe BG carbs have protection against. I thing you have a transition circuit problem and need to work on the accl pump, accl pump cam and squirters. It could also be the secondarys opening too soon...

It sounds like the idle circuit transition slot is uncovered at idle. Turn the carb upside down and adjust the secondary blades open just a little, then you will be able to close up the primary gap to cover the transition slot. Or you could drill 1/8" holes in the primary throttle blades to supply the extra air the engine needs at idle. The cam must be fairly aggressive to only provide 11-12" of vacume. Make sure your timing is set correctly.

after i make sure i get my timing squared away, i think i am going to pull off the carb and check the transfer slot. My manual said there should be .020" of the transfer slots exposed.

the transfer slot and accelerator pump shot are designed to help the transition from idle to main circuit. If the transfer slot is uncovered you'll usually end up with a rich idle and no fuel to cover the bog except the pump shot. If you crack the secondaries slightly you'll provide more air for the idle mixture while keeping the transition slot covered at idle. Most performance cams with a less than stock LSA tend to pump exhaust in the intake at idle during overlap at idle when exhaust velocity is low--------------the carb requires a leaner mixture or at least a greater volume of air at idle to combat this problem. Also if your idle mixture is right on your plugs will be nice and clean--------clean plugs accept acceleration/more fuel much better than a rich condition. Clean up the idle mixture, dial in the advance curve and initial setting, then pump shot timing and volume, then powervalve selection based on cruize rpm manifold vacume.

You'll swear it has fuel injection--at least till the weather changes :D

how do i crack the secodaries if they are vaccum secondaries. I thought the only adjustment i could make was by turning the idle screw to adjust the primary butterflys? :confused:

Eric

on a holley there is a small set screw that goes through the bottom of the baseplate, and makes contact with a stop located on the sec throttle shaft. you would have to take the carb off and look directly at the bottom of the baseplate near the sides (forget which side).

as far as i know demons use the same type of screw that is used for primary idle speed. you should be able to adjust the secondaries without even removing the carb. the screw should be located on the passenger side of the carb near the secondary throttle shaft.

I usually remove the stock setscrew and replace it with an allen type threaded in from the topside, this way you can make adjustment easy with the carb bolted on.