Finally got my jalopy running, runs strong and smooth but acts like it is running out of gas. Especially up hills, but can be finicky to start and doesn't always settle into a good idle after slowing down. I was told it was marginal to use gravity feed with this manifold, some folks thought it would work and others didn't. So the obvious solution seems to be to go to a fuel pump. But I am suspicious because I still had the problems with a full tank of gas, and that seems like that would get the fuel level up plenty high enough for the gravity feed to work... Oh yeah, its a Holley 94 on a Model B engine. Just want to see opinions on what else it could be? Someone on the Banger meet suggested a different float - designed for gravity feed, anybody else have any experience with this? Maybe the gravity feed is fine but just isn't enough psi for the Holley 94? Father-in-law asked how the tank was vented, this would be simple enough to check (loosen the gas cap) but I was wondering if anybody has encountered this problem on a Model A gas tank before? Any other carb problems to look for or adjustments to try? Any other opinions on what could be causing my problem? If I go to a fuel pump, should I go with a pump and the low pressure Holley regulator? Any fuel pumps that would be safe to run without a regulator for this application? I am hoping to keep things as simple as possible. Thanks, Will Kimble
Will, I have that same manifold set up but with a LZ (97) carb, but on an A engine. Also a header and a dual point Mallory. I tried re-jetting it with .048's, .050's and then .046's. Nothing worked with the jets. Same problem everytime. I drove it on the same route after each change. The problem occurred at the same spot on the drive going uphill in the upper RPM's. I checked the gas cap vent, same problem occurred. I finally stuck a fuel pump on it and it cured it. I'm probably going to put .045 jets in it and call it good. You need a constant 2 1/2 to 3 psi. From what I've learned the new fuel pumps are in the range of 5 to 9 psi. Run a fuel regulator with it and you will be fine. Keith
I am running a banger with a Weber but have also run a Stromberg 48 and 97 on the same manifold. There is definitely not enough pressure from the tank to run with either carb. What I have done is run a line from the stock outlet through a solenoid cutoff and a fuel filter to a Carter Rotary fuel pump mounted on the right hand side of the frame. This pumps back to a Holley low pressure regulator mounted on the filewall and then into the carb. Notice also the T junction for a fuel pressure gauge in this line. I set the pressure to 2.5 to 3 psi while the engine is running, same pressure for either carb. Also included but unseen is an impact cutoff switch mounted on the inside right kick panel, this shuts off the fuel pump in case of an accident. Also I have an On/Off switch mounted on the right kick panel so I can turn off the fuel pump when required. It may not be the neatest setup but it works well and provides the same fuel safety mechanisms as in your modern car. And YES it is a Right Hand Drive.
I'll echo what the other two guys have said, Will. Y'need a pump, 2.5-3psi. I mounted mine inside the chassis leg under the passenger side, works a treat.
Will, I'm running the same set up, Scalded Dog mainfold and a gravity fed stock 94 with no problems. So it may be something else that causing the trouble.
Immediate suspicion...rubber tipped inlet needle?? I rebuilt a carb recently with one of those...it consistently hung up on the bench! If I closed it, it stayed UP and when I hooked up my dual compressors (cough, cough) I had to blow hard enough to hurt to get the damn thing down each time. I did not stop to figure out whether the rubber itself was the problem or whether the seat might have had a sharp edge grabbing the rubber, as there was plenty of room in the garbage can for both. I reseated the original steel needle right away. There are also numerous complaints of the rubber turning to black goo in current gasolines... The gravity system looks like it should be workable, certainly when full...try the olde Model T trick of BACKING up the same hill at full blast...also open a jet plug and see how long it takes for a gallon to flow out and if any funny declines in output during the flow. Never heard of any special float setup for these...?? Maybe someone relaying the info on the normal mods for the B based carbs once sold as A replacements??
I can back up the hills as fast as I want to go! That's how I got it home a couple times... And I'm not opposed to using the B fuel pump, that would be the simplest and cleanest way to go. But I don't have one in hand and have heard they are not as effective as the later V8 fuel pumps? Anybody have any good or bad experiences with a B fuel pump that they could share? Anybody have one they want to part with? Thanks everybody, I really appreciate it. Will Kimble
The hole in the needle seat is usually larger on gravity flow systems. Ran into a fuel starvation problem on my forklift when I made a gravity system for it. Tank had to be full to have enough head pressure to flow enough for it to run much above fast idle.
You can run the line from the stock tank down the inside of the fire wall under the floor board to your fuel pump and back up and out the stock filter. Works great- very hard to see- looks stock- amazes your friends. Remember it gets hot down there so I usually put a heat shield under the pump.
My 29 runs out of gas with 1/2 tank on a lot of our hills. It not out, just won't flo. I got a small electric pump and that fixed it. I have a B pump but it ain't goin any where. Found it on a B engine in florida, and the guy said (you can have it, cause i'll never use it) It was gone before he changed his mind. Will be using it on my next build. Ice man
Thanks for the input everybody, I really appreciate it. I am gonna try a B fuel pump first - stock appearance, no need for a regulator, only pumps while the engine is running. I talked to a a couple friends today who are running them and like them. But I do have an electric fuel pump kit on order, too, as a backup plan. I learned some things about a gravity feed system from my Dad and an engineer friend today that really impressed me, and I will try to share. I didn't understand the concept of a "column" of liquid with regards to pressure. It isn't the weight or amount of fuel in the tank, or the height of the drop from the tank to the carb that is important. The important factor is the height of the column of fuel directly above the outlet. So I have maybe 3" of drop from the tank to the carb, and maybe 18" of column if the gas tank is full, for a max total of 21" in my current setup. A Model A updraft carb would probably be at least 18" lower, for a total column of 39" - almost DOUBLE my setup. I told my engineer friend that 2.5 psi was recommended for the Holley 94, and he got out his textbooks and figured out that using gravity feed, with gasoline in 5/16" fuel line, I would need a column of around EIGHT AND A HALF FEET to get 2.5 psi. So... You may be able to get your Model A to run gravity feed with a stock tank and a downdraft carb, BUT it ain't gonna be 2.5 psi. Also, notice the stock model A configuration would give you a decent percentage of column, but my setup - especially with an empty tank - would not even be 10% of the necessary pressure... Will Kimble PS: I apologize if any of these numbers or concepts are wrong, I didn't do the math and am trying to understand the concepts myself. But I trust my friend who did the calculations.
How did they do back in the old days, when racing the flats. I assume they didn't want to have the tank full, because of the more weight. Did they perhaps use the B blocks more then the A's, because of the fuelpump? I run 2 81's with gravity feed and when You talk about it, i remember it run better with full tank than half tank. This is a good thread!
Evans's comment is the really interesting one up there...bigger hole equals more area exposed to pressure tending to open the float valve! On the pressure at needle, where it matters... You can see the flipside of the problem in the early Stromberg service bulletins for '34 Fords...Ford twice REDUCED the hole size at inlet to reduce total pressure at needle because of Stromberg flooding tendency. Now, if you drilled out that hole a bit, and fixed the needle advance by resetting float level...that would be a realdamncheap and interesting experiment here!
jalopy, the standard system for boosting fuel pressure while racing was the hand pump. The pump would pressurize the whole tank to push more fuel to the carb.
An update for those that are interested... We had the steel line jammed too far into the sediment bowl, apparently that can block some fuel and the car ran better after we fixed that. I took the best parts from three 94s and a rebuild kit and made one mongrel carb. Somebody along the way had added a "helper" spring below the accelerator pump and it was keeping the accelerator pump from moving. Ran a lot better after we took that out. But... There still seemed to be some irregularity in the fuel flow and occasional coughs while driving. Put a B fuel pump on it and it is running better than ever. It is also easier to tune, doesn't seem like it is such a moving target anymore. There were some great running cars at the SOSS picnic today with downdraft manifolds and gravity feed. It appears that it can be done, and done well. But my car seems to run better with a fuel pump, and I like knowing it should run the same regardless of how much fuel is in the tank. Will Kimble
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