Needle & Seat - Now with some real data!!

[quickchange]

Have you tried flow with fuel cap off??? as sometimes the air vent can get restricted alowing flow for most but under load will restict. Just a thought & once you find the problem let us all know, Derek from a frosty NZ

[29 Tudor]

Jim/TX:
I have tried retarding the spark a bit but it seems to make little if any difference. Of course, if I go too far, the power falls way off. At cruise, she seems to be happiest with the spark most of the way advanced. I have also tried opening the GAV a bit but, as might be expected, it seems to make the symptoms a bit worse and somewhat slower to recover. That might just be perception though as once it happens, its recovery is very predictable.

I'm going to pull the needle/seat completely out today and do the flow test again, just to be satisfied that nothing else is contributing to the problem.

quickchange:
Great thought on the fuel cap... that was one of my first thoughts as well. My cap LOOKS good but to be sure, I reached under the windshield and removed the cap when it started and it made NO difference in the symptoms at all. Then I drove with it off and (other than the gas smell) the problem occurred in exactly the same manner.

Thanks to both of you for the thoughts.

[29 Tudor]

Here's some more information:

I pulled the carb again (LOVE these simple things to work on!!!)and found that as suggested above, the drain plug is in fact, the access for the main jet and is not directly under the bowl. Pulling it does drain the bowl almost completely but it is through a passage that is roughly .125 inch in diameter. In order to be sure about the flow, I redid the tests I had performed before by leaving the bowl assembly off the carb and measuring the flow through the needle and seat with no float installed at all.

As before, the numbers were virtually the same with it taking 180 seconds to get a quart through the needle and seat. I then pulled the needle and seat assembly completely out of the carb to verify that there was no undue restriction at the end of the inlet fuel line and ferrule or through the in-carb filter, located behind the inlet port. Again, the numbers came out surprisingly close to the original numbers at 50 seconds to get a quart. By the way, there is a boss that prevents the fuel line from going in too far and getting blocked.

I did one more test to check the flow rate of the passage to the drain below the main jet by leaving the needle and seat out and putting the bowl back on. I turned the fuel back on and the flow out the “drain” was considerably diminished and in fact, in a few seconds, fuel started flowing out the vent at the top of the bowl. Therefore, it’s quite possible that measuring the flow through the needle and seat by using the drain plug is not, in fact completely accurate. However, by redoing the test with the bowl removed, I have no doubt about the absolute flow.

With that, I’m pretty convinced that the problem I have is related to low flow through my existing needle and seat. It will be a while before I get the new parts and get back to this but once I do. I will report on the results.


Thanks again to all who have commented and offered suggestions.


Steve

[John S]

Good Deal!

[29 Tudor]

Hi John,

I wanted to be SURE I had the data I needed and I'm convinced now that I don't have enough flow. I have ordered a couple of new inlet valves (two different styles) from Mac's so when I get back to the car, I bet I can get it fixed, once and for all.

Thanks again for your help,

Steve

[gweilbaker]

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Steve,
For Grins and giggles until the new parts arrive, what kind of flow rate do you get without the float valve body? Of course without the bottom half of the carb.

GW

[29 Tudor]

Unless I misunderstand your question, that's what I reported above. With no float valve (needle and seat) in the upper body and with the bowl (lower body) off, it takes 50 seconds to flow a quart through the upper body and into a quart jar. I was surprised that it was the same number I got out of the open fuel line. I had expected some restriction through the in-carb filter but that was not the case.

Steve

[gweilbaker]

WOW! I did not see that post. I think I was on the site earlier, returning after a few meetings then submitting my post before refreshing the site.

Sorry,
GW

[29 Tudor]

qweilbaker:

No problem! I thought maybe I misunderstood your question. You need to stop going to meetings!<grin>

.... retirement is great!

Steve

[29 Tudor]

Well, I got back to the car today and I have some NEW data!!!

As you might remember, with the needle and seat that I suspected was bad, it took 180 seconds to flow 1 quart of gas through the needle and seat. Today I installed a new "Viton" needle and seat from Mac's and did the same test again. This time, it took 75 seconds to get 1 quart through the new needle and seat. That results in 300 seconds to get a gallon, or 30 seconds to get 0.1 gallon.

As I reasoned before, if a Model A gets 5 MPG under full throttle, it needs 0.2 gal/mile so at 30 MPH, we need 0.2 gal/120 seconds or 0.1 gal/60 seconds. Clearly now, I am getting TWICE the maximum flow I would need if the car is getting 5 MPG.

This testing was done with only 2.5 gallons of gas in the tank since I emptied the tank (DRY!) to pull the gas gauge and overhaul that.

Hopefully this data will help someone else troubleshoot a fuel problem.

Driving the car now results in MUCH better operation under large throttle settings. It does have a new symptom that I need to work on and that is that it is running lean. Even with the GAV open 3-4 turns, it seems to starve and pulling the choke slightly results in MUCH more power but with the choke partially on, it now climbs the "hills" and never runs out of gas. MUCH better than before.

Tomorrow I'll verify timing and pull the carb again and look for some restriction in the jets as well as verify the fuel level.

[Purdy Swoft]

I'm going to guess that you've got a vacume leak. My thought is weak vacume is not allowing the engine to draw the fuel. When the choke is partially applied it increases the draw through the carburetor and the engine gets more of the needed fuel supply. I would check gaskets. I use the copper clad gasket between the carb and intake. It could be a small crack in the intake manifold or upper carburetor housing. The gaskets between the manifold and engine block could be leaking vacume at the intake manifold ports. If you are using vacume windshield wipers, there could be a vacume leak somewhere between the manifold and the wiper motor, or the wiper motor itself. Good luck with your search for the problem.

[29 Tudor]

I suspected a vacuum leak as well but inspection today didn't disclose one. I'll get deeper into it tomorrow. Thanks for the suggestion.

Steve

[Jerry in Shasta]

Could this be vapor lock.
JB

[Ron in Quincy]

Using a torch tip cleaner, mic the smooth end and hopefully you will come up with tips measuring .021, .037, and .039.

Make sure your idle jet is .021 ; make sure you have a type 3 removable secondary well (the one with the undercut at the entrance of the compensator jet).

Compensator jet: .037
Main Jet: .037
Cap Jet: .039 ( this will give extra fuel when excelerating.

When climbing a long hill, open needle seat 1/4 turn and slightly retard dristributor.

Also make sure you do not have the float level too low. If you get it too high it will run over the top of the cap and main jets. Sometimes you need to add a second gasket under the cap jet so it is at the same height as the main jet.

Ron

[29 Tudor]

Jerry, I wouldn't think vapor lock since the choke helps it. If it was out of gas (as with the old needle and seat or vapor lock), I wouldn't expect the choke to help it.

Ron, Thanks for the jet sizes. I doubt that's it since this carb ran fine before (until it became fuel-limited) but that's great info to have and it's on the list to check.

I appreciate it!!

Steve

[Tom Wesenberg]

Jerry might be right. That was my first thought when you said it is worse when hot. If I use crap gas with 10% corn squeezins then it acts the same way if the temps are over 80*. As soon as I switched to the regular good gas the problems went away.

[George Miller]

Originally Posted by 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

Are you sure it is a gas problem and not a weak spark? Have you tried pulling the choke a little to see if it gets better when it does miss.

[kenparker]

Experiencing a similar problem with 'le Coupe. Pulled the Screen out of the front of the carb yesterday and discovered the screen plugged with varnish. Blame it on sitting for extended periods of time (30 days at time) and this confounded ethey nol gasolene. Moral of story - check your screen. ken

[29 Tudor]

Tom: I have little doubt that the gas we have now is compounding the problem and I would LOVE to be able to get "real" gas but all we have here is the crummy gas. I sure wish they would leave the alcohol to Jack Daniel's!

George: Yes, I have tried pulling on the choke (slightly) and it helps considerably. By the way, the problem does not manifest itself by missing, the power just falls off and it helped considerably by applying a little choke.

Kenparker: It's not the screen... at the bottom of the post I posted the new data with the new needle and seat. It will now flow twice the fuel I calculated was needed at full power.

I have attacked the problem in several ways; I checked the timing and found it to be a little retarded (probably about 10 degrees) from where it was supposed to be so that is now corrected. I pulled the carb off and disassembled it to check all the jets for size (they were all correct) and obstructions and found them all to be perfectly clean. While reassembling the carb, I snapped off the Cap Jet so I have ordered a new one along with the required tap to remove the broken part. I have a spare (professionally rebuilt) carb that I intend to put on the car hopefully in the next few days.

The old carb to manifold gasket was pretty flat so the new carb will go on with a new gasket. I hope the new carb and gasket along with the timing being corrected will fix the problem. I'll report as soon as I get it back together.

Thanks to all for the suggestions!

Steve

[George Miller]

If the choke helped then it is a gas problem for sure.