Doing some carb tuning on our 1850 equipped 267 SBC in our '40 chevy p/u. Have a number of different power valves laying around with various opening Hg... is there any way to visually inspect if they are any good before I try them? Is pushing on the plunger and seeing the diaphragm move an accurate test (I am guessing no because the diaphragm could still be torn yet move back and forth???) Don't want to waste my time trying a blown power valve if I can weed out the good from bad.... thanks-
Usually when they blow it is obvious like in the guts poking out the sides obvious. There may be a way to hook a vac pump up to one and give it a pull. If the diaphragm is busted it won't pull.
**** on it (that didn't come out right) and use your tongue (still not coming out right) to block the hole and see if it holds vacuum...worth a shot.
to piggy back on this, how do you determine which rated PV to use. I have seen several versions from -half idle manifold vacuum -idle vacuum -2
The starting place for power valves is idle vacuum divided by two. The average Holly kit or new Holley comes with a 6.5 so I presume the average motor makes 14 inches of vacuum at idle. Let me reiterate the idle vacuum divided by 2 is just a starting place, you may have to bump up or down a size to get it right on.
Was idling around 15 in. Hg with horrible tune so am going with a 5.5 as I put it back together as a starting point...looking honestly at economy/driveability more than performance with this engine (imagine that with a 267 ) so am guessing if the power valve comes in a little later than sooner probably wouldn't hurt??
I'm not a professional!! This is how it was explained to me. Ideal circuit(fuel air screws) primary circuit(jets)secondary circuit(back ****erflies,secondary plate with jets or fixed plate)mechanical or vacuum. With a good vacuum gauge in your view when you are driving at a constant speed(highway) and on the primary circuit read what the vacuum is, it must be above the power valve number(or it will be letting fuel in the manifold)the power valve is to make the transfer from the primary circuit to the secondary circuit as the rear ****erflies open when you step on the gas hard,you will see on the gauge as you step on the gas.when the vacuum drops where your powervalve number is that when fuel comes in the help the jump to secondary circuit(perfect world you will not feel it but the car will accelerate. the accelerate pump(squaters make the transfer from the ideal circuit the the primary circuit and the power valve makes the transfer from the primary circuit to the secondary circuit) Most important are the jets have to be correct,get them right first then go from there. I explain like ****(sorry)hope you understand my rambling. Pete
Ok so I put a brand new 5.5 Hg power valve in cleaned everything from the carb body forward (metering plate, bowl, all the small parts etc....) put it all back together (adjusted the transfers slots on the primary and secondary side as well) Starts good but idles high, get the idle to come down alittle bit (with the screw on throttle shaft) but not to where it should be...turning a/f ratio screws in and out makes NO difference...suspect vacuum leak so I spray around the intake ports with carb cleaner and nothing happens, spray around the carb mount and nothing happens same with any vacuum fittings on intake. When I spray the secondary throttle shaft coming out the base plate the idle drops noticeably. I thought the idle would raise if I found where the leak was and sprayed it with a combustible, am I wrong?? Did I indeed find a vacuum leak at the secondary throttle shaft?? thanks-
Budget's technique can be modified by using a short length of 3/4" hose with a good square cut end to push against the power valve and giving the kiss of life to this.
You need a power valve checker to test a power valve. , about 30 bucks from summit. Tuning without one will drive you nuts because they can be off right out the box. If the idle screws don't stall the engine the PV is bad or the gasket is bad or there's some other problem in the general vicinity. If the idle screws going in don't stall it there's no sense in doing anything else till that's fixed.
IDK if this can help. Rebuilt 8BA carb with all fresh parts, including PV. It was running rich afterwords. Took it apart again and everything looked good to go. So the next day I started it and ran it until bowl filled up. I removed bowl section from the base and noticed fuel in the base cavity where PV rests. Turns out the PV was leaking fuel in to the cavity, (not the gasket). Luckily I had spares and put in another one and it solved my issue. As mentioned above you can get a PV checker from summit. Also, kudos to Bruce Lancaster for the the tip. You probably can't see it in the pic, but there was fuel in the cavity.
Also...if you a are a 94 or 4150 nut, there is a tool made by at least 2 different companies that allows testing... It is basically a lump with threads to accept a PV and a ****** to hook up to vacuum. As you apply vac (most controllable source would be one of those hand pumps with a gauge) you can see the thing move, see if it can hold vacuum, and if you have the pump with gauge you can check up on actual opening point. Here's one version. I have not tried mine yet since I am so utterly disorganized I cannot find it and the little pump at the same time on any given Saturday... (The aluminum testers are pricey...this article shows the commercial one, THEN shows a neat hookup that is more or less FREE. http://www.superchevy.com/how-to/74058-carburetor-valve-problems/ )
The Jalopy Journal
