Hi, hope I am posting this in the right place, my apologies if I am not - hoping someone here can help with a couple of questions... First little background, hope this is not too long winded or TMI but will try to explain what I have checked already: I have a model A with a 331 hemi, running 2 holley 94s. recently it started running really rich, took a look and one of the carbs was pouring raw fuel into the intake and fuel fouling the plugs. This happened a few years ago just after the carbs were rebuilt by Charlie Price, back then it turned out one of the carbs had a bad float that was filling with gas and sinking/not closing, replaced the float at that time and all was good. So this time I suspected a similar problem, first I tested the float on the leaky carb and it was good/floating. Thought back to what had changed just before the problem started - I had recently changed the fuel pump, I realized this might be causing an increase in fuel pressure and forcing fuel to blow past the float needles - I checked and the pump operates at around 7 pounds of pressure and I know the 94s can only take around 3 pounds tops. So I added a pressure regulator and adjusted to low pressure but even at 1 pound of pressure the carb was still dumping fuel. Then I checked the float level, it was close to correct but I adjusted it slightly to put it in spec, put it back together and it was still pouring fuel. I did some reading on the H.A.M.B. and found what I thought was the likely cause of the leak, I remembered that during tuning and testing and just before the overly rich situation developed there were a couple of nasty backfires through the carbs, I suspected this might have blown the power valves so I ordered new ones. I calculated 3.5 would be the correct size or close for 2 carbs on my engine. When I took the carbs apart to replace the power valves I saw there were plugs instead of PVs, I had known about this in the past and had forgotten. Put the PVs in and the car ran worse than ever, would only idle, any throttle at all and it died immediately and I found the bad carb was still puking fuel. Took the carbs off again, removed the power valves and put the plugs back in, took the top off of the leaky carb, held the float closed and turned the key to check for leaks, no leaks. Then I sat the top back on the carb body, turned the key, the fuel bowl filled, left the electric pump running for a minute or two, no leaks! Thought the problem was solved, screwed the carb top back down, turned the key again, carb was again dumping fuel. Took it apart again and checked, no leaks. Put it back together again, dumping fuel again. Pretty frustrating. Tried tracking down exactly what was happening, loosened the screws on on the top plate one by one and the leak stopped, but if I tighten down the screws in the front of the carb over the fuel/float bowl it dumps fuel. So, my questions... 1 (a and b). With the 3 front screws loose it does not leak fuel. How do I fix this so that I can tighten down the top plate properly without dumping? Is the problem a deformed/warped carb top or likely something else? Gasket looks good, don't see any cracks anywhere... it seems that tightening the top down is holding the float open somehow, but I am just guessing at this point. 2 (a and b). What is the logic behind removing the Power Valves and replacing them with plugs? It seems to work well, but why? The only info I can find on the internet says it is a bad idea but is vague regarding reasons why. The car is running perfectly with nice crisp throttle response in the garage at the moment, I haven't road tested it but it seems to run the same as it did in the past as long as I don't fully tighten down the top plate on the leaky carb. Any ideas on a fix? Thanks - Johnny
1-Take the carb top off and make sure the 5 screws can move freely up and down in the bosses, and if not, lightly ream/enlarge them to allow the screws to tighten the top and not "walk" up the top, resulting in a "tight" screw without good clamping. I have encountered this problem on some 94s. 2-Removing the power valves on a street driven car is not good logic, even though some "experts" preach this. Make sure you don't have a leaking inlet valve, caused by a bad Viton needle or dirt/etc.
Also...check carb top fit by setting it on body with no gasket...see if there are gaps. Carb metal can be sort of "body worked" with carefull support and limited violence to remove warp, on a 94 working on a plate with hole to clear float support legs. If no warping there...something has to be wrong with float drop tab, pivot pin, hinge part of float, or something thereabouts! Study this stuff carefully, if nothing figgered out swap some parts with other carb and see what happens. Running with no PV's means carb has to be set up rich, washing down cylinders and killing mixture at anything other than full throttle. Mix cannot be right at cruise and at full throttle without something to change it! Lack of PV's OR good diaphragms in PV's also does not mean no PV leaks...the plugs or the PV's themselves may be Holley four barrel parts, which seal in a different diameter than Ford 94 needs. When tightened into 94 some seal as center bottoms out, some do not seal. Look at PV area of carb...94 PV seals on a ring up close to threaded hole. Holley 4150 type parts have a sealing ring out too far to seal there and only seal accidentally when center bottoms against carb....
Thanks V8 Bob and Bruce Lancaster, this was very helpful. I think I've got it sussed, I swapped in a few different carb tops and found one that was better - tightening the top down didn't cause a gusher of fuel leakage like it did with the first top (I think that one might have had a small crack somewhere near the inlet that I can't see that was causing the leak, I am old and have poor eyesight) but with the new top I still noticed a steady drip from one side of the air bleed nozzle so fuel was still leaking past the inlet needle, took it out and cleaned it for the 50th time and blew through it, it was working perfectly as it was the previous 50 times I checked it... then I noticed the rubber inlet seat gasket was a little frayed, I think from installing it and removing it from the multiple carb tops. Replaced the rubber gasket with an old school paper one and put it back together, it seems to be okay now though after all of this I am paranoid it could start leaking again at any second... As for the power valve question, I think I understand that too now, thanks for the info - I think when Charlie Price rebuilt the carbs he set them up rich to compensate for the lack of power valves, and when I replaced the plugs with the power valves the combination of the enrichment from the PVs, the rich jetting of the carbs to allow them to work without PVs, the accelerator pumps being set to provide the biggest shot of fuel on acceleration, and the leaking fuel inlet situation was drowning the engine in fuel and causing it to sputter any time I touched the throttle. Also two other thoughts about power valves vs. no power valves: 1. I read this week that a single PV provides the proper amount of enrichment for an engine of around 250 CID - adding a second power valve provides the amount of enrichment necessary for an engine of around 500 CID. I am guessing this is the reason some people choose to jet the carb rich and block the PV. Perhaps this over-enrichment issue could be corrected by modifying the power valve to reduce the amount of fuel it can pass when engaged? By reducing the size of the opening in the PV maybe? 2. I am running a stock 54 Chrysler Hemi distributor with vacuum advance connected to a port on one of the 94s - I know the PVs open when vacuum drops, does the distributor see a different amount of vaccum when the PV opens as opposed to what it sees with the PV blocked? Could this be contributing to the engine*****ping out under acceleration when the PVs were installed? I am curious about this but sick of working one the thing at the moment and it is currently running great so I don't want to tear it apart again just to experiment with the PVs...
The two power valves do not provide twice as much fuel because the air flow through the venturis(which draws the fuel in) is halved because of the two carbs. The vacuum advance should be connected to manifold vacuum. The signal from that port on the carb is very low, something around 4 or 5 in. of Hg maximum.
Well, I thought I understood what was causing the problem when the PVs were installed but now I am confused again haha, I just took a look around at what I read before saying doubling PVs also doubles the amount of enrichment and also saw plenty of pages agreeing with what you are saying - what you are saying makes more sense to me. It looks like no one can agree on this issue but it probably doesn't matter in my case, the thing is running great at the moment with the PV plugs and that is good enough for me, I am moving on to other projects. Hmmm, interesting. The manifold is a U-Fab log-type with no place to connect the vacuum line, I suppose a fitting could be added to the manifold at some point in the future if need be. Thank you for taking the time to share this info - Johnny
"...1. I read this week that a single PV provides the proper amount of enrichment for an engine of around 250 CID - adding a second power valve provides the amount of enrichment necessary for an engine of around 500 CID. I am guessing this is the reason some people choose to jet the carb rich and block the PV." Doubling fuel---CROCK, from someone who doesn't know how a carb works, as FJ points out. If you enrich enough at mains car can have correct mix at full blast, BUT most of the time you are running on mains, which are now TOO DAMN RICH. This means sluggish performance and lowered engine life...a rich running car can wear out rings in a few HUNDRED miles. Really. Power valve woks in proportion with mains, and feeds through venturi IN RELATION TO AIR FLOW. Further, PV is an on-off...anyone who tells you to make its openings smaller has not even FOUND where the carb meters PV feed! Perhaps this over-enrichment issue could be corrected by modifying the power valve to reduce the amount of fuel it can pass when engaged? By reducing the size of the opening in the PV maybe? 2. I am running a stock 54 Chrysler Hemi distributor with vacuum advance connected to a port on one of the 94s - I know the PVs open when vacuum drops, does the distributor see a different amount of vaccum when the PV opens as opposed to what it sees with the PV blocked? Could this be contributing to the engine*****ping out under acceleration when the PVs were installed? I am curious about this but sick of working one the thing at the moment and it is currently running great so I don't want to tear it apart again just to experiment with the PVs... Port on carb is hooked to venturi vacuum (not needed except on***** Holley distributor) and to ported vac. Mallory conversions actually used this plug the inner hole that goes straight in, keep the downward one, you have ported vac. Engine will likely prefer manifold vac, but you could save that project for a heavier teardown...OR while you are messing with carbs...there is some room low in the throttle body where a carefully placed drilled hole could accept a piece of tubing.
Hi Bruce. I have been scouring the HAMB hard in order to find an answer to my current problem. I run a very stock 32 Tudor, stock, that is, apart from heads, intake and carb. So the heads are just later domed, the carb is a 94 with the intake it came with. I have set up a vacuum gauge on dash plumbed to the manifold, and a lambda sensor in the exhaust (2 into 1 down right hand side), again with read-out on dash. This is great set-up to fine tune carb. So with 5.5 PV in I feel the PV is opening at about the right time, but my air/fuel metre shows it is introducing too much fuel, going way too rich. So I know I can put in ,say, a 3.5 valve to delay PV opening, but the GREAT UNKNOWN QUESTION is.....what metres the flow of fuel in the PV circuit? If only I could find this out, I can fine tune my carb. If you look at an early PV that comes with a 94, the openings through which the fuel flows are small drillings. You could imagine them restricting fuel flow. The later PV's that you get nowadays have big ports which cannot be doing any metering. Any thoughts gratefully received. Tom
The actual power valve metering is cast into the carb body...the PV is just an on-off valve with flow metered by the 2 holes behind the PV hole that allow fuel from PV to join fuel delivered via main jets. The actual restricted part is slightly up the hole... The restriction is meant to add some extra in proportion to the noemal flow through main jets. I would in your case pull out the mains and see what size they are. Most true 94's are somewhere close to 050...if the various meterings are reasonably stock the PV contribution should be at least close to right. Look on the left side of carb as seen from diver's seat for actual Model...94 is a huge family, and many other carbs are routinely lumped into it as well. There should be a Ford part number prefix there like 91-99, 29, 59, etc.
Bruce, you are dead on as where the metering for the power valves are located. I agree also that vacuum still draws fuel as needed, it's actually the lack of good vacuum that power valves are necessary. All that aside you can control the metering of the power valve. After removing the power valve and look at the 2 angled holes where the fuel is metered, the outer opening measures (on bowl I checked) .142 dia. with .143 dia. starting. This dia. Is at least 1/4" deep, then as you say the metering dia. is at bottom of this, this one is .040 dia. I changed the amount of fuel the power valve allowed by making (2) .143 dia. brass inserts 1/8" long which I put .031 dia.hole through them. This needs to be done on a lathe as hole dia. must be accurate. I also polished the tip of them slightly so it starts straight. Tap them into the .142 dia. opening till they bottom out. Tom will be able to fine tune his air fuel ratio, it worked for me. I choose the .031 dia. that I used by checking the area of .040 opening and what percentage I wanted to change it. 2b-banjo P. S. I hope this is clear to everyone!!! Posted using the Full Custom H.A.M.B. App!
Thanks Bruce and Banjo, and I guess I hijacked johnynyns thread, but I've been looking for the answer to what sets the fuelling when the pv opens for some time. Now I have some pointers so thanks. I wonder why this is such a non issue? I guess its because you can't easily change the fuelling at the pv so no one bothers. I think I'm going to take this one further, maybe it could benefit the wider community? Thanks again and cheers, Tom.
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