Needle & Seat - Now with some real data!!

[29 Tudor]

Hi all,

I have a '29 with a stock Zenith on it. Periodically, (worse in in HOT weather) when climbing a light hill, it will act like it's running out of fuel. New line, new kit in the stock cast iron filter and it has GOOD flow out of the line at the carb. I pulled the carb apart and it has one of the old needles in it that is captured in the seat by a metal bar across the top (or bottom, depending on how you look at it) of the seat assembly. I was surprised to find that the drop of the needle is only about .025. I can BARELY feel any air when I blow through it. I'm sure that has a lot to do with the starving problem.

In looking at the various web sites for "A" parts, it looks like all the vendors are featuring the "Grose type" inlet assembly. I have read of a number of sticking problems with that design and would really be happy to find a good, HIGH FLOWING needle and seat. Does anyone still make a traditional needle and seat assembly for the old Zenith?

If not, I will have to try to disassemble the needle and seat I have and relieve the stop bar to get more, MUCH more drop on the needle.

Frankly, I'm amazed it runs as well as it does with no more drop than it has on the needle. There CAN'T be much flow through there.

Any thoughts out there?

Thanks,

Steve

[BRENT in 10-uh-C]

Steve, if you want to explore, remove the "cage" from the seat and see what you find. It is very possible the Viton material on the needle point is damaged or swelled restricting the flow. Maybe something can be fixed after opening up but realistically you have nothing to lose. I will tell you that a high-flowing Needle & Seat really is unnecessary in 99.9% of the applications you would ever bolt that carb on. It just doesn't take that much fuel to adequately supply the engine's needs.

As far as the Grose-Jet is concerned, Bill Stipe makes the best ones now. Several years ago there were some Chinese manufactured ones that did stick and give issues but I don't know of Bill having any of those issues with the ones he makes.

One other possibility would be to call Bill Coleman over in the Channelview area and see if he would have a core carburetor that would have a decent brass "original style" Needle & Seat that you could install in your carb.

.

[29 Tudor]

Hi Brent,

Thanks for the comments. I am going to do a flow test on the open line and then through the needle and seat and see what I find. I was surprised how little drop there was in the needle so it's probably good to see what that really does to fuel flow.

When I pull a "hill" (if you can call them that here in central Florida), it will typically pull well part way up the grade and then start to act like it's running out of gas. If I let off the throttle, it comes back and pulls again and then when I go full throttle, in a short while, it will starve again. I'm sure there is plenty of flow TO the carb so I suspect it has to be the restriction of the needle/seat but I'll report the flow results.

Thanks again for the thoughts and comments.

Steve

[cars56]

Amazing how much impact jets that are not factory impacts performance.
Solder them shut drill em out, makes a huge difference

[29 Tudor]

I have no reason to believe I don't have factory jets in this carb. In fact, the mixture control works as expected so I would be surprised to find the jet is wrong. The car runs excellent other that the full throttle performance on grades.

[flatford39]

Sponsored Links (Register now to hide all advertisements)

When you "pulled that hill" did you have a full tank of gas? If it was a quarter tank or less you may have had a problem with the gravity fuel delivery system that our A's have. I was told it was common practice to go up hills in reverse when the tank was low.

[29 Tudor]

Great suggestion but... I thought of that and tried it with about 1/4 tank and then about 3/4 tank. It made a SLIGHT difference but still happened with 3/4 tank of fuel.

Great thought though. Thanks for suggesting it!

Steve

[Pilotdave]

Originally Posted by flatford39

When you "pulled that hill" did you have a full tank of gas? If it was a quarter tank or less you may have had a problem with the gravity fuel delivery system that our A's have. I was told it was common practice to go up hills in reverse when the tank was low.

With outlet positioned as it is, would there be an issue with fuel flow from the tank going up hill?

[Kurt in NJ]

I thought it was with "T" that had the fuel tank under the seat that steep hills caused fuel flow problems.

[1931 flamingo]

Does the carb "open" all the way??
Paul in CT

[29 Tudor]

Yup, checked that too but if it wasn't, opening it more would only aggravate the problem. Thanks for the thought though.

Steve

[29 Tudor]

Okay, here's some data and some math....

I started with 1/2 tank of fuel. I took the line off the carb and measured the flow of fuel into a quart container. It took 50 seconds to get a quart or 200 sec/gal. That's obviously 20 seconds/0.1 gallon.

Next, I measured the flow out of the drain on the carb. It took 180 seconds to get a quart or 720 seconds/gallon. That's 72 seconds/0.1 gallon.

Now, I don't know what mileage an "A" gets at full throttle while pulling a grade but I'd guess it's between 5 and 10 MPG. So, that being said, if it's 10 MPG, we need 0.1 gal/mile. That means that at 30 MPH, we need 0.1 gal/120 seconds.

If it is getting 5 MPG, we need 0.2 gal/mile and at 30 MPH, we need 0.2 gal/120 seconds or 0.1 gal/min.

As you can see above, I'm only getting 0.1 gal in 72 seconds so, assuming my mileage estimate is anywhere near accurate, my needle will not flow enough fuel to support the motor at full throttle and it's simply running out of gas.

Now I need to see what I can do to increase the flow through the needle & seat... or build a better mousetrap.

Steve

[Farrell In Vancouver]

Originally Posted by 29 Tudor

Hi Brent,

Thanks for the comments. I am going to do a flow test on the open line and then through the needle and seat and see what I find. I was surprised how little drop there was in the needle so it's probably good to see what that really does to fuel flow.

When I pull a "hill" (if you can call them that here in central Florida), it will typically pull well part way up the grade and then start to act like it's running out of gas. If I let off the throttle, it comes back and pulls again and then when I go full throttle, in a short while, it will starve again. I'm sure there is plenty of flow TO the carb so I suspect it has to be the restriction of the needle/seat but I'll report the flow results.

Thanks again for the thoughts and comments.

Steve

Steve, this might be completely wrong, (probably is) but have you considered the engine might be running too rich at full throttle?
I only mention this because you said it comes back when you let off the gas. How does the car perforn full throttle on the flat highway?
Are you experiencing and lean back firing or is it loading the plugs?
Perhaps you could find a spot halfway up that hill chop the ignition, pull over, read the plugs and see what is happening. After disassembling a few carbs, I have found many jets too be "improved" by someone before. Worth a shot?
Interesting flow test you performed on the carb. I wonder if the time/flow difference was due to the fuel being slowed by the float seat and the bowl on its way to the quart jug.
Good luck friend, I am eagerly waiting for your solve on this problem.

[gweilbaker]

Steve,
The drain you refer to, is it the cap for the main jet also? If so, I would check a few things before heading to the trap store.

- Check the fuel line that it is not set too deep per the ferrule, like the line bottoming in the boss and a good length of line showing past the ferrule.

- How's the strainer screen?

- Make sure the in put path is clean and not obstructed with varnish, rust, crud, teflon tape or an old strainer. I have found all the above before.

Good luck,
GW

[29 Tudor]

Farrell:
I don't think it's running too rich... If I lean it with the GAV, it will clearly start to fall off so I have it on the "sweet spot". I also fly a plane (172) so I have experience leaning the motor. It runs well on flat ground and I have not held it at full throttle for a long period because I really don't want to rev it that high but on a couple of occasions at about 45 MPH or so, it has acted like it is trying to starve as well.

With the VERY tiny drop I get with this needle, (~.025) I do believe it's a significant restriction and with a little machine work, I can address that.

gweilbaker:
The drain I am referring to is the drain on the bottom of the Zenith bowl. It is a gasketed "bolt" that goes only into the bottom of the bowl. This carb looks new inside and out and is VERY clean.

I will check for bottoming of the line... that's a good point. I'll probably remove the needle and seat and redo the flow test. The line is new as is the filter in the cast iron sediment filter on the firewall. If there was a problem with the line to the carb or the filter, I don't think I would be getting the flow I am out of the open line at the carb. I think a gallon in 200 seconds will support a lot more horsepower than a basically stock "A" can make.

I do appreciate the thoughts and comments. Unfortunately, I will not have resolution to this soon as I am about to have to set this project aside for a while but I WILL get this solved... and when I do, I'll update this thread with the results.

Steve

[John S]

Sent a PM to you, Steve

[29 Tudor]

Hi John,

Sent one back....

[Purdy Swoft]

It sounds like you may be running a Simmons or Wizard improved cast iron carburetor. The original type Zenith carbs didn't have a drain under the bowl. The Simmons and Wizard carbs did look a lot like the original carbs and even produced a side bowl for the 31 models. All of the above carbs did have a bolt-plug under the main jet, just behind the bolt that holds the two halves of the carb together and this may be the drain that you refer to.

Here is a few thoughts about what could be causing the problem, incorrect float setting, add on fuel filters, micro filters, or any of the good suggestions that others have already given.

[29 Tudor]

You probably have more experience with the Zenith carbs than I do but this one has "Zenith 1" in the side of the cast iron bowl and the drain I refer to goes in a boss that is machined on the bottom of the bowl, next to the single bolt that holds the carb together. It looks just like Bratton's part number 14590 which is called "CARBURETOR LOWER DRAIN PLUG"

There are no add on fuel filters, the line is brand new (I just installed it). Also, as noted above, the flow FROM the line at the carb is good. It will flow a gallon in 200 seconds from a tank that is 1/2 full.

Thanks for the thoughts,

Steve

[Jim/GA]

Originally Posted by 29 Tudor

You probably have more experience with the Zenith carbs than I do but this one has "Zenith 1" in the side of the cast iron bowl and the drain I refer to goes in a boss that is machined on the bottom of the bowl, next to the single bolt that holds the carb together. It looks just like Bratton's part number 14590 which is called "CARBURETOR LOWER DRAIN PLUG"

There are no add on fuel filters, the line is brand new (I just installed it). Also, as noted above, the flow FROM the line at the carb is good. It will flow a gallon in 200 seconds from a tank that is 1/2 full.

Thanks for the thoughts,

Steve

The drain you are opening for your flow test is under the main jet. I do not believe that there is any other restriction between the fuel bowl and this drain plug, but it has been a while since I looked at it. I would say that this is a fair flow test.

I have also noticed that these needle valves do not drop very much. I always considered them to drop enough to run the engine, though.

What are you doing with your spark advance as you climb this hill at WOT? Did you ever try enriching the mixture with the GAV, not leaning it? You are asking for a lot of power.