Topic: Holley Le Mans Primary Fuel Bowl Bypassing Accelerator Pump Inlet Check Valve A

[Dan Case]

Holley(R) Le Mans Primary Fuel Bowl Bypassing Accelerator Pump Inlet Check Valve Assemblies


If you use a Holley Carburetor Company(R) 4V carburetor model, an original assembly line or genuine factory built service unit, that used the original design racing "center inlet" fuel bowls (a.k.a. Le Mans bowl at a later date) this subject may be of interest to you.





Multiple Ford and a few Chrysler racing carburetors used these type bowls 1963-1970. The primary fuel bowls of the original center inlet design (a.k.a. Le Mans bowl design) created for Ford racing has a modular or cartridge design press in check valve assembly in their floors for the accelerator pump system. The valve assembly was pressed into a passage bored into the floor of the bowl. No sealer was used. A tight press fit was the only protection against fuel going around the insert and instead of out to the engine.





Over time valve inserts that were leak free for the first few decades of use may become loose enough to start letting fuel bypass their valve inserts. They can get loose enough to rattle or worse. My theory is that the zinc alloy of the bowl casting and the brass alloy of the insert have such different coefficients of thermal expansion and contraction that the relatively hard brass part stretches the bore in the zinc floor wall slowly over time. I have come across inserts that became loose enough to easily pop out of the bowl floors. Just because any given assembly was leak free last year does not mean that is still leak free this year. In the 1980s I developed fine tuning for the R-3259-1AAS unit installed by a prior owner’s Shelby dealer in our black car in the 1967ish time frame. One spring I noticed that the engine was having a brief transient lean condition during every acceleration period; a lean stumble. I removed the carburetor and quickly found the problem. The accelerator pump inlet check valve insert was now leaking back to the bowl, a lot. It was not leaking when I took an afternoon to install an ethanol tolerance accelerator pump diaphragm a few years before. I have been testing Holley inlet check valves since the mid 1970s. If I have a primary bowl off for any reason I will test the valve. Of course I test the valves in every Holley 2V or 4V carburetor I have ever serviced since the 1970s.  Carl’s Ford Parts sells replacement bowls with a Ford style one piece silicon rubber valve (a.k.a. umbrella valve) that is practically failure proof. I bought one of Carl’s bowls and the performance of my car’s carburetor was back to excellent.


In my contacts with others that service, repair, or restore carburetors with these type bowls I found that:


a) many did not care as long as the insert literally did not fall out (which has happened). Either they did not test such things or if they did, they did not act on what they found. I find that attitude appalling. Contracted to repair somebody's carburetor, often at significant cost to the owner, they just ignore the issue. I have found this loose leaking insert issue with units “restored” (made to look pretty) by world famous shops! In one case I was asked to help with a freshly restored carburetor that was as done better than typical finishes and correct parts wise but the owner reported a horrible acceleration stumble once the carburetor was back on its engine. The restorer indicated that the owner was doing something wrong or the engine had some vacuum leak or ignition problem. The owner asked me to find out what was wrong. The accelerator pump check valve insert was very loose. At least half of each pump stroke volume just went around the valve and back into the bowl. I swapped in one of Carl’s bowls with an umbrella valve and the owner reported back that his engine now performed great.


b) some have resorted to some type non-metallic filler to "glue in" the insert and or just seal it. "Red" grade thread locking compounds, fuel tank sloshing type sealer compounds, and various two component hardening organic compounds have been used.


c) some of us have purchased new replacement bowls from Carl's Ford Parts. Carl is the go to guy for many parts Holley® will no longer service. Call or e-mail Holley and they will redirect you to Carl. The new replacement bowls have updated the accelerator pump system to include the Ford 2100/4100 silicon rubber "umbrella" valve. It was a Ford carburetor exclusive for decades. The umbrella valve is almost idiot proof and chemically nearly indestructible. I have never heard of a single performance issue with them with any kind of fuel mix. Ford did a real good job in 1956 including the design in its carburetors.


d) repurposing a rear bowl is another direction to arrive at a permanent solution. Secondary bowls, originals or reproductions, were made with only semi-finished accelerator pump features. A little bit of machine shop time to drill the passages required to install an umbrella valve and a path to the main body and presto a genuine 1960s secondary bowl with appropriate patina retasked to be an upgraded primary bowl.


e) the other idea I have had, which would be relatively expensive time and money wise, would be to replace the insert with a piece of wall cut from a junk fuel bowl, get it welded in (Not everybody has the materials and skills to weld on thin wall zinc die castings but it can be done.), and then machine new chamber floor to the umbrella valve required configuration. Thermal masking of most of the bowl should preserve its color and patina.

[Dan Case]

Typical umbrella valve installation.

[J_Speegle]

Thanks Dan for putting in the time to make this post and share your research/knowledge in this thread and others

[Dan Case]

You are welcome.
It takes a little forethought for me to remember to have my cell phone by the work bench and then remember to stop at key steps for capturing images.  Carburetors are complex devices. The variability in tiny details makes the difference in okay running and world class running engines.

[RoyceP]

Word from Carl is that Holley is not going to renew the agreement with him; therefore his current stock is all there might be if Holley discontinues the period correct looking carbs / parts.

Holley management is not what it once was and we may or may not like the changes that they make.

[Dan Case]

Word from Carl is that Holley is not going to renew the agreement with him; therefore his current stock is all there might be if Holley discontinues the period correct looking carbs / parts.

Holley management is not what it once was and we may or may not like the changes that they make.

Good to know, thanks. Buying a new made primary bare bowl or assembly from Carl has been the easy way.

Not every carburetor shop tests for such things as leaks around that insert but some that do test have been dealing with the bypass leaking issue for years.  Randy Gillis mentioned that is real easy to machine an original 1960s rear bowl to use a rubber umbrella valve making it into a front bowl.  Several companies sell valves as singles or in groups.

I am working on a slide show that includes sketches I did of Ford Motor Company's two hole pattern under their 2100/4100 carburetors umbrella valves and Carl's Ford Parts three hole pattern.  Good used secondary bowl cores are easier to find than good primary bowl cores. I see more new old stock rear bowls for sale than fronts also. (Tip: Some rare Chrysler race carburetors used center inlet bowl systems.)

Just because the insert in your carburetor was not bypassing the last time somebody inspected it does not mean it is still not leaking.  Our black car has an R-3259-1AAS assembly dated 712 that a previous owner had installed. He bought a parts catalog from Shelby American and had his local Ford-Shelby dealer order a new "CS BLUEPRINTED" ignition distributor and a single 4V "COBRA" induction kit. I used the carburetor after buying the car in the early 1980s to develop float levels that worked well for all conditions, street and track, for an original Cobra.  (Nominally the R-3259 family of carburetors sit level to the horizon in most stock chassis Ford vehicles. In a Cobra they pitch on the order of 2? downward in front. That angle change and the dynamics of the short wheel base light Cobras makes engines in Cobras using an R-3259 family carburetor with float settings like any other Ford produce a miserable performing engine that often has fuel escaping out to the intake manifold.) Once developed circa 1985 the carburetor made it to 2006 (I log service work and repairs.) before requiring any work.  Then multiple gaskets and the accelerator pump diaphragm started leaking.  The diaphragm failed with tiny little fractures in the rubber.  I assumed it was the ethanol introduced into the local fuel supply that was the new problem. (The marine maintenance shops along the river were choked, at the time, with fuel system problems work as boat owners suffered failures as ethanol was introduced to 'gasoline' locally.  One large volume shop paid an engineering test company to gather samples of fuel from all the local automotive and marine stations close to the river and send the samples off for testing. They found 'may contain up to 10% ethanol' signs on pumps actually meant customers were getting everything from 0% ethanol to 28% ethanol depending on where they bough fuel when.)

Anyway, by 2006 it was possible to get carburetor service soft trim parts that were ethanol resistant. I cleaned up my carburetor and did all my normal tests. The accelerator pump insert was not leaking.  In early 2017 while the weather was still cool I noticed that a hesitation on any acceleration. Tip in performance had always been fantastic so any fall off in performance was noticeable. Cool weather made the issue more detectable especially until the engine fully warmed up.  I removed the carburetor and started testing, I bench test everything I can before disassembly and can test some things during disassembly. It was really obvious that the accelerator pump shots were low volume and ended way too soon.

The only issue I found was the accelerator pump check valve insert that was fine in 2006 was now bypassing significantly.  I ordered a new bowl and umbrella valve from Carl and installed it in April 2017. My engine's old tip in and all rates of acceleration performance were back.

My theory is that the rates of thermal expansion of the brass insert and the zinc bowl are so different and that the strength of the metals are so different that over time and many heat cycles the brass insert enlarges the zinc bore it was pressed into long ago.  A Cobra owner friend sent me his poorly performing carburetor and the insert was so loose it literally fell out while I was washing the bowl and scrubbing it with an old tooth brush. (I have seen used primary bowls for sale on eBay(R) totally missing their inserts.)

Roughly three in four used primary bowls I have tested since April 2017 have leaked to some degree or another around the check valve insert. That includes the primary center inlet bowl in a carburetor that got a very expensive "restoration" by a world famous shop. The owner wanted me to test check the unit out before he installed it on his 427 Cobra's engine.  A leaking check valve insert (whatever they do leak wise now they will not fix themselves and will probably get worse if used as is) was not the only problem the primary fuel bowl had but finding that issue alone made it worth the side trip to my work bench.  (The carburetor looked great but would have worked poorly. I even had to get some parts and help from Drew Pojedinec on that unit.)

If you have an engine that uses any of the Ford-Holley carburetors with the center inlet bowls this is a subject you should be aware of.   Sound like repairing original bowls or reworking rears is going to become more important.

[Dan Case]

Valve Update.
I recently made numerous functional repairs to a freshly "restored" R-3259-1 Holley(R) carburetor originally made in 1967 for a friend. The cartridge valve (brass insert, check ball, and ball retainer clip) pressed into the floor of the primary bowl was a new replacement. I was surprised that the brass body of the valve appeared to have been media blasted or tumble deburred all over and any surfaces I could see with it installed including down in the pocket the check ball fits into. The rough side walls of the bore created a little bit of resistance to movement of the ball and the ball was not sealing liquid tight against the rough seat. There was also some slight by pass leakage around the perimeter of the insert.

Interesting concept, expect a smooth round chrome plated ball to seal against a rough seat.  There is not much room to mess with here because anything you do to increase the gap between ball and retainer when the valve is closed changes pump functionality a little.  If the ball and retainer gap in enlarged, more gap would mean more fluid will escape before the valve seals during each pump cycle.  I have never tried to adjust the gap in a center inlet bowl assembly but that is a variable to check in all other 4150/4160 fuel bowl  with accelerator pump assemblies with a little metal bar acting as ball retainer. 

For my friend's carburetor, I sealed the perimeter of the insert and very carefully polished the ball seat just enough to get a leak free check ball seal.  The polishing also made bore walls smoother and ball movement easier. The final assembly did not leak around the insert or by the check ball.  (Anything but very slow primary throttle opening should cause a discharge of fuel out of the pump jets (a.k.a. nozzles).  Any delay in pump shot for any reason will allow a lean air fuel mixture engine stumble but do not confuse a lean idle mixture hesitation with a pump shot problem. Most 1960s engines require a slightly rich idle air fuel mixture for engine temperature control at idle and smooth drive-away operation.  Some vehicles had ancillary systems to increase engine rpm if some temperature trigger point was reached. That cools the engine in different ways.  If you jab the throttle pedal from any road speed and the engine hesitates there is probably an acceleration enrichment problem of some kind. If you are easing up in line behind stopped traffic and the engine hesitates or might even stall then your idle air fuel mixture is probably too lean.  The idle circuits gets the car moving in slow starts, then the transition circuits come in until engine load and speed are sufficient for the main system to start.  The accelerator pump system takes care of more rapid accelerations.  The one you adjust with a screwdriver is the idle mixture.)

I subsequently purchased two new valve assemblies and the brass parts had the rough texture all over also.