Got the flattie running Saturday, but she loaded the plugs after two thermal cycles (half hour of runnin' each time). Cleaned the plugs with a propane torch and ran it again... Here's the setup: 258 flattie ross pistons ported high-comp Weiand cheater heads (9.0:1, prox.) center dump headers Mallory dual point 6V system isky 400 Jr. cam Edelbrock SU 359 intake 3 - Holley 94s, progressive mount, blocked outboard power valves. 1/2 turn on the outboard idle jets 1-1/4 turn on the center idle jets stock power valve, stock jets Questions: 1. What does a stocker power valve cut in at (manifold vacuum)? 2. What vacuum should I be seeing on this setup at rated idle (1000 RPM)? Pic attached. I think the power valve is engaging, but wouldn't mind a 2nd (or 3rd) opinion. thanks in advance, Chuck.
Seems like no one likes to answer these types of questions. I've asked a few questions in similar fashion with minimal replies. The stock powervalves are typically set for 7.5" of vac. I've read a few articles on running 94's on flatheads and an easy rule of thumb to go by is to divide 7.5 by the number of primary carbs. If you're running a progressive setup with the center as the primary and the outters as strictly secondaries, with stock jetting for all three ... you might want to consider replacing the power valve in your primary carb with a 6.5 or 5.5 rating. From what I understand the only way you'll be able to tell is to hook up a vac. gauge and and see what kind of vacuum you're making under load and get a power valve that's approximately two values lower. Also, your jetting is probably too high for a flathead motor and you're exceeding the fuel demands. Run a search for multicarb setups and 94's. You'll find enough information to get you going with a good baseline, which will then allow you to make minor adjustments to get you going with minimal adverse effects.
Turn your idle adjustment screws on the end carbs all the way in - you don't need these for idle. Stock jets (for an 8BA ) are 51's. Try that first or if your using 59's or a different carb, start with the stock jets for the carb and go from there.
I corrected a problem with a friend's Holley four barrel power valve by having him drive it while I read the vacuum under different driving conditions. He had so much cam in the engine, the vacuum was all over the map. We changed out the power valve to one more appropriate with the average vacuum and the engine ran a ton better. No more black plugs or watering eyes from the gas fumes. I agree with the statement about the idle screws as well, you shouldn't need them on a progressive set up. I have read of flathead dyno tests that clearly show a dual carb set up to give higher horsepower and better throttle response than the treble carb set up, for what that's worth, there were alot of three carb flatties back in the day though... Flatman
Thanks, guys! I'll check the vac signal later this week when I get a hose barb to stuff in the side port of the manifold to get a reading. I agree with the power valve changeout - at idle, the motor should not be pulling anything thru the mains, so the plugs should remain fairly clean. in the meantime - I'll order up an assortment of PV's from Flathead Jack so's to tune the motor. Flatman... I got a good deal on the current setup (completely restored for $550; a pair of resto'd carbs run that much these days) so I'm gonna try to make it work. Three deuces is notoriously hard to set up to run right; I'm gonna fiddle with the linkages until it's 'right'. A healthy flatmotor really only needs two carbs; I'm thinkin' of limiting the stroke of the outboard throttle plates so as to keep the vacuum signal up. In the meantime - I'll be on the lookout for an alternate induction system for next year! As for the outboard idle screws... I had them in all the way - and it ran rough. A half turn smoothed the engine out; I suspect there's some air leakage past the throttle plates and the half-turn is a 'band-aid' until I tear into them.
Ian - Appreciate the response! There's a inverse correlation between valve overlap and vac signal; the signal goes into the basement with a lumpy cam. Your post jogged loose an old memory; I did a motor for Pop once with a pretty stout cam, and had to go to a 4.5 PV in his Holley 650 to keep the plugs from loading up.
I have just been setting up two holleys on my flathead and am running 4.5 PV's and 0.050 jets and it runs very rich. I am going to down jet them and see how she goes. I did have problems at the start with leaking power valves as they don't always seat right. I set it up on idle at about 18inhg vacuum and the power valves are opening at about 3/4 throttle.
Since this is a smallish flattie (eighth over, short stroke) fitted with a big cam, the intake manifold vacuum is waay low relative to a stock duration setup - if I had to guess, I'd say 7-8"Hg. If a stocker PV opens at 7.5 - then she's gonna load the plugs at idle. did some research, and it looks like I can run late model PV's with crush washers - if so, then I'll pick up an assortment at lunch and futz with it tonite.
Something else ( just what you need ) ----the Holley power valves do not seat quite right in the 94's. You will have to grind a little off them to get a good fit. Screw one in without the gasket and look at it closely---you will see what I mean. It makes a HUGE difference.
I read that - and remember the radius at the gasket base of the PV from other builds. I figured a fat copper crush washer would allow the PV to seat properly without dressing the mating surface of the valve.
I think a small piece of faced off tubing , with a notch or two would work in a drill press to machine away that radius on the power valves. going to try it today, im trying to dial my two in and starting over with a nice newer set of ecg hollys 94s 51 jets , and 4.5 valves
UPDATE: Pulled apart the carbs last nite; the outboard PV plugs were leaking. Fixed that; checked the PV on the main - it was an 8.5!!! the new PV's I bought did not fit, so I pulled the Q-jet trick and cut 2-1/2 coils off the PV spring to lower the enrichment point. Using a 4.5 PV as a 'checker', the cut coil PV came in at about a 5.5-6.0. Plugged it in, buttoned 'er up and... While it made a difference...it's still too rich. I've got to go to a 4.5 PV minimum - but - I'm pointed in the right direction. Jets were 51's, BTW.
I don't know what you mean by "don't fit", but I have found that the channel the plunger part of the pv sits in has to be relieved with some pvs. It's easy to do with a dremel. Just don't relieve the threads, though!
The radius area fit problem is a real wart in the design of current valves...looks like they might seal in a nervous kind of way. Rich idle could be caused by a leak at that radius allowing fuel down the vac passage, but NOT by the valve opening early...it feeds through same circuit as main jets and should not deliver any fuel til venturis reach critical flow...but if you are idling at 1,000 you are into the borderline area where flow through off-idle is reducing and venturis are just starting to pick up, so maybe on your engine. I think you will be able to slow down idle as engine breaks in and tuning gets closer. Obviously you do need a small-number PV. And they can jam against casting--upper part on 4 barrel ones is too big. Examine your nozzle bars--there are two air bleeds near front of each, one for idle channel and one for main. These are visually deceptive--there is a big hole with a TINY hole at the bottom, and the tiny hole is frequently blocked even on bars right out of the cleaning tank. Probe them carefully with copper wire strand or nylon from clothes price tag holder. I have size specs somewhere...those could richen idle.
thanks, Bruce! As a check - I reworked each carb body, filled the float bowls with fuel, set them on a paper towel - then went and ate dinner. No leaks - so they are sealing. I understand the air bleed issue; the carbs are super clean, so that shouldn't be a prob - but I'll check. As idle speed/transition... I crawled inside the carbs mentally last nite, and examined them in operation. Einstein called this a 'thought experiment', and it's really helped solve some knotty diagnostics over the years. anyway - as I sat on the edge of a throttle blade at idle, I noticed that the low manifold vacuum caused the engine to require the blades to be open further than they would be on a stock setup. This exposed a portion of the main bleeds, causing 'pull-over', or initial transition enrichment (from the idle circuit to the mains). In the past, I've fixed this by drilling a bypass port in the throttle blades so as to return them to the correct position - blocking the main bleeds. If I get a chance tonite - I'm gonna check the hypothesis by cracking the throttle blades on the outboard carbs, which will allow backing off on the center idle cam screw. If I'm right - she should lean out. Yah - it's unorthodox, but it oughta prove the point.
Another update: did the dremel thing to the center carb body so's to accept the newer power valve and installed a 4.5 valve. The motor ran better - and cooler, too! After about a gallon of fuel, tho - a couple of plugs fouled out. dropped to the 3.5 valve and it's been good so far - need to get it on the road to truly test driveability. Hopefully that's only a few weeks out...
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