Is anyone running a multicarb setup with ALL the PV's blocked? reason I'm asking is the 3.5 I'm currently running has such a light spring that it doesn't seat all that well, wreaking havoc with idle quality and contributing to overheat issues. Seems like I can jet the carb a wee bit rich and ditch the PV - eliminating this issue. I'm not worried about fuel economy. anyone done it?
Yes, I have them all blocked on my Model A Pickup, and on DeathsDoorStep, I am running all six carbs on straight linkage on both.......My Truck runs rather well @ the track and on the street........Good-Luck Littleman
It'll work, but might be a little rough on your cylinder bores as you have to jet kinda fat if you want full throttle performance.
??? PV feeds into the main well, from which any idle or off-idle fuel has to go through idle jet. Any fuel from PV that actually gets fed into engine has to be drawn in at venturi, requiring considerably more than idle air flow. Do you perhaps have PV's with only the 4-barrel seating and a taper where it should be flat?? I think any mains rich enough to run well at full throttle will wash out your piston rings quickly.
I've modded the PV and the carb so the PV seats properly in the carb; the issue is the PV valve disc is not seating properly on its own seat for some reason; when it does, fuel leaks from the main body into the venturi...not a lot, but enough to enrichen the A/F ratio to the point where it idles rough. I caught it by accident last night farting around with the jets. I cycled the PV manually and the motor idled better and cooler simply by tapping the PV a couple of times with my finger - like a small pice of debris was lodged in the PV. Will add an inline fuel filter in the meantime...
My SBC has been running for a while now, its been run real hard and is more than due for a tear down and rebuild.........So I will see what the inside will look like real soon.........Alot people I know have six two's that seems like just for decoration, with mine I have alot of time and record keeping that goes along with them..........and for me the bottom line is reading your plugs and jetting your carbs to that info....Most of this stuff ,you do not know if it will work unless you give it a try..time slips along with great street performance and great plugs are hard to get and can be real frustrating, but all possible........anyone want a ride?, Littleman
How did you cycle the power valve manually? Also I don't see how it would run hotter with a little extra fuel. Usually hotter is caused by running lean.
Also...fuel from there can only get IN to the venturi if there is significant draw on main circuit, rarely possible below maybe 1500...that's uphill, and requires venturi action that shouldn't exist at idle. Have you checked fuel level? Is float level at lowest spec or a bit below?
Cycling it manually is accomplished simply by pushing the valve stem down with your finger. on '94's, the brass section of the valve extends into the float bowl - the diaphragm is down between the bowl and throttle body. as for pulling it in...All I can say is there was a difference in operation - and that's the only variable. Re: running hotter... Again, it was running rich; the original reason for pulling it apart was due to a stuck float valve - it had been rich all the way home, and ran hotter (15+ degrees) on the way back. Spent a fair amount of time thinking about this last nite, and what *finally* made sense is this: Stoichiometric A/F mixes burn the quickest; a non-stoich mix will have either an uneven burn rate (lean) or a slow burn rate (rich). the best example I can give comes from my tin can cannon days; you can add fuel 'till the cows come home, and all that happens is combustion is reduced to a flaming 'flumph' as the tennis ball rolls out of the muzzle. Get the A/F right, and the tennis ball goes outta sight with a big-assed BANG! Same holds true for a flattie - too rich, and the process of combustion will literally continue in the exhaust tract. Problem is, the first part of the exhaust tract passes thru engine coolant, which means the heat of combustion which is normally converted to work is rejected to coolant...thus adding heat to the cooling system. the same wouldn't hold true an an OHV motor, as very little of the exhaust tract heat would be exposed to the coolant.
I think for most folks running duals, blocking the PVs isn't a good option (although it's been done quite a bit) - I think when you start venturing into 4 or 6 carbs, it might actually be necessary to block the PV... This is because you're going to be running more fuel in through idle transfers & air bleed circuits than normal already...and you can't get PVs in low enough ratings to work as designed... ...just a gut feel with no science or explanation to back it up whatsoever!
On a stock flathead with one 94, feed through venturis begins circa 900 and doesn't fully take over from off-idle circuit til about 1250...1938 Service Bulletin on Ford carb operation. I would expect from lessened flow per carb on a multi-carb engine for these numbers to go up...so, as Ernie says, the off idle circuit controlled ultimately by the idle jet becomes important; control of gas vs. airflow here, basically not a real feedback system, is obviously MUCH cruder than the venturi controled main and power circuit, so might well be a cause of excess richness for much of the normal driving range! This is a very much inadequately examined area, along with the need to lessen the pump shot. On multis, venturi flow is LOWER than normal virtually all the time, so the notion of excess richness from multiple main systems is probably generally opposite to the real reason for bogs... power system feeds through main system, really just adding a couple of small extra main jets to system. If power system valve function actually affects idle, gas MUST be going where it shouldn't! On a 94 type, main feeds can dribble if float too high. Specs published after the '39 revision border on to high...highest acceptable level is the LOWEST level given in modern specs, that is the BIGGEST number as generally measured, or a bit lower than that! 1 11/32--1 3/8...If PV is feeding at idle, it either is dribbling (by which I mean venturi area feed without venturi draw) or leaking out vac port!
Bruce - Keep in mind I'm running a 400 JR cam; spec idle speed is 1000 RPM. I can tell you from firsthand experience the stocker PV was open (either an 85 or a 95) at idle and contributed to plug fouling...until I went to the 35, this was a prob. I was originally running a progressive 3x2 setup; when the engine started giving me fits, I went to blocked outer carbs and am t-shooting the center carb only at this point. When I get the center carb sorted - I'll add in the rear carb on the prog linkage and then finally the front carb - so as to contain variables. One 'other' thought I had was the position of the throttle blades relative to transition ports. A lumpy cam results in a lower vac signal - lower signals mean blade position is more open than stock; this would expose the off-idle ports prematurely, allowing 'dribble'. and finally... richness is not a notion here...it's a fact. I've had to burn carbon off the first set of plugs three times, have bought two more plug sets and have burned carbon off the second set twice. I've managed to get the A/F 'right', only to have it degrade without any setting changes - and that freaks me out, man! I'm currently hoping it's a debris issue, and a fuel filter will allow the system to run consistently. just becuase you built it all using new parts apparently does not preclude the fuel system from being dirty...
With idle at 1,000, and progressive, venturiis might well be feeding, then... off-idle would not itself be affected by PV, but might itself be too rich, but at 1,000 you probably are getting into venturi signal. Dropping that float if you aren't already there might postpone this tip in. Thoughts on that...maybe you are pulling enough at idle that you could support straight linkage and three carb idle...get them throttles closed down! Or...bypass a bit with holes in primary blade, but you'd still be runnibg 1,000 rpm worth of air through one set of venturis. I'm kinda thinking 2 or 3 carb idle...get the thing back onto just idle...then see if pv's respond to a tiny bit of shimming that spring. Get them toward halfway back to the last setting that sealed.
Interesting... motor DID idle better with a little air across all three carbs. After sorting out the motor on one carbie, I'll hafta try two together. Typical A/F ratio curves for carbs are curved - a dome shape. Rich in the middle - lean on both ends. Keep in mind the holley PV is reverse acting - meaning that a loss of vacuum opens the valve, and a strong signal closes it. therefore, to delay the opening of the valve requires a reduction in spring rate.
Chuck, Are you running your secondary and tretiary carbs with the idle ports and bleeds plugged? Flatman
Are you talking power valve or accelerator pump? No way can you operate the power valve externally. Also a lean mixture burns much slower than a stoichimetric and burns well into the exhaust cycle causing overheating. This is more critical on the flathead due to the long exhaust ports.
I had the ait horn off; PV is in the bottom of the float bowl. Rich mix also burns slow - you can see the result when dyno tuning as hp drops. more fuel, less hp= more heat going somewhere else.
On our HA/GR flattie (286 cid 400 jr cam) we have 3 - 1 1/16 bore 94s. progressive linkage, Navarro manifold, idles on center carb great. 8.5 power valve in center carb and 4.5s in outer two. 3 lbs fuel pressure and lots of flow- 3/8 line and fittings to each carb from large fuel block. filters in carbs removed. #51 jets in carbs. Plugs look like new, we may even have to step up a couple of jet sizes. Works great, idles like a dream at 1000 rpm. Transition from 1 to 3 carbs is a little rough but being a drag car it doesn't really matter to us. Brad
No doubt due to your PV selection... I'd be inclined to run a 4.5 in the center & ends...or maybe even smaller. But since it's a strip car, as long as it's running well & the plugs read OK - prolly doesn't matter much.
Chuckspeed, you haven't mentioned what vacuum you are pulling. A 3.5 is pretty low---is your vacuum really going that low?
When running multiples, a good starting point is to divide what PV rating you would use on a single by the number of carbs. So if you'd normally run an 8.5 PV, but have two carbs, a 3.5 or 4.5 is probably a better starting point...
yup. Along with what Flat ernie sez, the motor's a 258 with a 400 jr cam; long duration high lift. not much vac signal at all; have fouled three sets of plugs trying different PV's and carb settings.
Bleeds...each nozzle bar has idle and main bleeds near front: Biggish hole goes down to TINY hole, oft still plugged after long dip. Check with tiny piece of copper strand. Flosts: I charted all the 94 float recs I could find...Ford RAISED the float level circa 1940, and ever since then tuners (Including 1950's HRM tech writers) have valled for dropping it. The carbs can dribble or syphon gas through bars if float is anywhere near the higher specs that exist. How did you run a 94 with airhorn off??!? The float lives there!
I had the carb completely apart; the emulsion tubes are indeed open - I could blow air thru 'em. I didn't run it with the air horn off; I pulled off the horn to get to a stuck float needle...while it was off, I cycled the PV, as I'd noticed the fuel level was low. Motor ran different after that. I've checked - there's no leakage into the PV vac chamber, and I've checked for dribbling. It seems that some debris worked its way thru the fuel system and into the bowl, contaminating both the float needle and the PV seat, resulting in a double whammy erratic operation.
Not sure about those carbs, but on a four barrel Holley, when running a big cam and idle quality is poor, guys turn up the idle until it smooths out.....but then the throttle blades are opened past the transfer slots during idle, dumping more fuel to the mix......the trick was to drill an 1/8" hole opposite the transfer slots to add more air at idle....OR, adjust the secondary throttle blades open a little off the stops..... But on 94's not sure......Is the idle screw set so the blades are opened past the idle bleeds????
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