Carburetor tip

[carbking]

I have posted many times in these forums about the importance of using the proper orifice fuel valve in a carburetor. This is true of ALL carburetors, and the size valve to use depends on many things; but the primary consideration is the design fuel pressure, and the supplied fuel pressure.

 

Those that add electric fuel pumps to vehicles not designed for electric pumps often run into this issue.

 

Even those that simply order an inexpensive generic rebuilding kit can unknowingly run into the issue. A classic example of this are most aftermarket kits for the Carter YF used on the 134 CID 4 cylinder Willys Jeep. I do not know that ALL of the cheap kits are this way, but we keep getting calls from folks that just rebuilt one of these with a FLAPS kit, and the carburetor floods. First question we ask is "what size orifice is the fuel valve". Generally the customer does not know, and later calls back with the orifice. Seems the numbers we keep getting are: spring-loaded fuel valve 0.097 inch and solid fuel valve 0.101 inch. The spring-loaded was designed for marine YH and small marine AFB carbs (yes, it does have the same thread size) and the 0.101 was designed for the Ford 300 CID 6 cylinder. The correct size for the Willys Jeep is 0.076 spring-loaded. If you do the math (remember pi Is involved) you can see how much larger these valves really are, when compared to the correct size.

 

The bill-of-materials for most of the generic aftermarket kits is designed to reduce the number of kits the dealer must stock; NOT so the customer gets all the correct parts.

 

The trigger that brought on this post are the early Stromberg spec cards that I advertised a few days ago. Have been reading some of the cards in the evenings. While these cards are for updraft carburetors, I thought some might like to know that THE PLACEMENT OF THE FUEL TANK OR VACUUM TANK IS PART OF THE CRITERIA FOR CHOOSING THE PROPER ORIFICE! Take 2 different 4 or small 6 cylinder cars using an S.A.E. size 1 or 2 carburetor, with the same exact engine, just different fuel source placement. Those with a "gravity feed 8" head" would receive a valve seat orifice of 0.140 inch, and those with a "gravity feed of 18" head would get an orifice of 0.113 inch. Same engine, same style and size carb, just different fuel tank placement.

 

For those that use different carbs or fuel delivery systems, thinking about the above, might save some headaches.

 

Jon.

[dl456]

Jon,

Please excuse my ignorance, but are you referring to the opening diameter of the "needle and seat" valve?

Thanks,

Dennis

[carbking]

Dennis - the diameter of the orifice in the needle seat, which is closed by the needle seat valve. This is the entry point for fuel into the carburetor.

 

Jon.

[dl456]

Jon, 

That's what I thought. This is great information and more often than not, overlooked.

I am well aware of head pressure variations just due to the placement of vacuum tanks.

I was unaware that it may require a different carburetor part.

And further evidence why you shouldn't expect your (gravity feed) carb to run correctly with a electric pump!!

Thank you

Dennis

[WPVT]

Jon,

   That's a good tip to keep in mind. It's amazing that a few inches of head could make that much difference.  As you may recall, I was having trouble with a 1941 surplus Type 63 Zenith updraft being used on a 1929 White 260 cu in 6 cylinder. The carburetor was running dry after a few minutes at high idle. The truck originally had a vacuum system but is currently using a 3-4 psi electric fuel pump. So I took the carb apart and recorded all the jet sizes, etc. Here's what I found:

Venturi size: #28
Inlet valve: #40
Main jet: #25
Discharge jet : #25

That seemed like a small inlet valve orifice, even for an electric pump, so I drilled it out with a #36 drill (.1065"). Now it runs fine. I intend to rebuild the vacuum tank, so we'll see how it does when it runs on gravity. 

Trial and error worked for me, but  I'm wondering if there is a more scientific way to size the jet when a carb is being used in a different vehicle than it was intended for?  

                 Walter

 

[Steve Hammatt]

I was having difficulty in feeding my 1914 Overland 79T, running a later added-on Carter BB-1 Carb.  Fortunately before we were about to obtain an original carb for testing,  we stumbled across some Carter literature and discovered that Carter BB-1s could come with two different main inlet jet sizes (one for pressurized and the other (larger) size jet for gravity feed.  I may be off slightly, but the numbers were something like drill size 31 (.120" dia.) vs. drill size 39 (.0995" dia.) for the main jet.  We chucked up the main jet in a lathe and carefully enlarged the main jet to the larger size, since we were running a gravity feed system and then never looked back.  IIRC, there was something like a 40% increase in cross-sectional area for the fuel flow.  Suddenly no more problems running up hills or having to maintain a 3/4 full fuel tank.  I could now make full use of all 13+ gallons in the under front seat fuel tank.

[Bob McAnlis]

Guys.  I am very interested in this topic and thanks for putting it out there.   I need to be sure I am okay with a gravity feed to a newly installed BB1.    Carbking I am pretty sure you are talking about the inlet hole size for the float operated inlet valve.     While Steve,  you seem to be talking about the main jet orifice.  I’ll be looking at both this next week      

Thanks

bob mcanlis 

[PFitz]

I'm pretty sure everyone means the same part - what many call the "float needle and seat" - just different names for it.  Key word in all this being the word, "inlet".

 

Paul  

[Steve Hammatt]

I wrote my reply (above) without some needed detail.  I'm talking about the float needle and seat.  Not the adjustable (on some models) main jet.  Here's what I did to make sure, once and for all that my problems would go away.  Due to the low head pressure offered in my Overland, the incoming fuel pressure at the inlet needle and seat was quite low.  In fact, there wasn't enough pressure to push open the HORIZONTAL needle.  The key word, in my  mind, is horizontal.  If it was a more typical vertical needle and seat arrangement, the needle would open by the simple gravitational pull of mother nature (grin).  Due to the low pressure, combined with the small cross-sectional area of the seat opening diameter, there was inadequate force to push the needle open.  Here are my fixes, all made at the same time:

1.  Open the seat diameter to drill size 31 (.120" dia.) vs. the original drill size 39 (.0995" dia.). 

2.  I also replaced the original needle with a viton tipped needle pulled out of a friend's carb junk box. 

3.  In addition (here we go with the belt and suspenders approach) the newer viton tipped needle had a groove machined around the o.d. at the other end of the needle, up near the end where the float tap pushes the needle closed.  I found, in that same junk box, a small ss wire designed to engage into the groove and at the same time, hook over and behind the float level tab; this arrangement would literally "pull" the needle open when the float dropped. 

So now, I was assured of two ways to open the needle, the metal spring wire and the increased pressure offered by the larger diameter (+/- 40% increase) in cross-sectional area.  Remember, the increased flow opening also allows for added flow into the carb.  

If you have any questions, drop me a note gsteve@hammatt.com or call 360-661-6060.