Help w/ Power Valves

[951mat]

I recently dyno'd my car at 400 rwhp. It has a 427 center oiler, Holley 750 dual feed w/ electric choke.

The problem was that it was running off the chart rich. I will have them retune on the dyno again soon. One thing I have investigated was the power valve. It has a 6.5 in it (stock). I measured my manifold vacume at idle at +/- 7". According to Holley manuals, this would call for a 3.5 power valve. I want to replace it today. Just curious if this could be the main source of richness. FYI the primary jets are 72. Thanks for any advice.


Mat

[Curt C.]

Lowering the PV to a 3.5 will help clean up your idle, but will be open just like the 6.5 at WOT and result in the same rich Air / Fuel ratio. Sounds like you will need to lean out the jets some

[Excaliber]

Good point Scott, that hadn't occured to me. But of course ANY PV will be open at WOT!

When moving jet size I like to go about 4 steps at a time. From 72 to 68's maybe. I was running 64's, but then again, I had TWO carbs.

[StanJ]

Mat, does the engine seem rich only at idle, or all the way through the RPM range? Curt C.'s comments are correct; a power valve(s) with the proper vacuum rating (with 7" of manifold vacuum at idle, I'd suggest 4.5 - 5.0 in your case) will help clean up the A/F ration at idle/low load conditions but will not affect WOT...unless your present power valve is damaged. How old is the carburetor? Most Holley's come with power valve protectors these days, but this hasn't always been the case. If the carb is 5 years old or more (regardless of recent rebuilds)...and if the engine has "sneezed" (backfired through the carb) hard since the power valve was last replaced, the power valve diaphram might be ruptured, which would allow a considerable amount of raw fuel to drip into the engine under all conditions. One tip-off is that engines with this problem are usually very difficult to start (a prepetual state of "flooded").

Also, is the carb clean? The (stock) main circuit air corrector orifices for your caburetor are only .028 I.D., and are exposed to all the potential "road grit" that might be getting by your air filter (you are running a good, clean filter on this engine, aren't you?). If even one of these orifices gets plugged with dirt, your carb's A/F ratio will dive toward the rich side.

[951mat]

The car's A/F ratio is very rich throughout the rpm range. I'll start w/a PV change and probably do the jets on the dyno. It was also rich w/ the air cleaner off as well as on. I probably could use a filter anyway. Thanks for the advice. Not exactly sure how old the carb is. It could be +/- years old.

mat

[392cobra]

Be sure and check your float levels. They could be set too high.

[Excaliber]

When changing jets on an older Holley be careful to NOT tighten the float bowl screws to much. Generally in selecting different jets you have to tear down the carb bowls several times, to replace the jets yet again. The threads WILL weaken each time you do so and the screws eventually WILL fail, thus your left with a damaged perhaps fatal problem for the carb.

I ALWAYS use new float bowl gaskets and tighten 'just enough' to HOPE it won't leak. If it DOES leak I am forced to tighten down the bowl screws 'just a little more' to stop it (which can be done on the car).

If the screw holes DO 'strip out' you can 'heli coil' them but I would recommend it be done by a competent machine shop as it is really a bear to get the new heli coils properly placed.

Been there, with a NUMBER of old Holleys!

[Michael C Henry]

Also check the air bleeds on top of the metering block and make sure they are not blocked or clogged .Some performance carbs have replaceable airbleeds. which is handy if you have tried to open them up and went too large. They help break the venturi vacume pulling the gasoline through the jets and start the emulsification prosses.