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Old 03-01-2021, 07:33 PM   #1
kawagumby
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Default Fuel adjustment system question...

Hi all,
I have some questions regarding the fuel adjustment needle, orifice (jet), and housing. I have a 1931 roadster with the Zenith carb. The fuel adjustment jet is just an orifice in the casting, which is relatively round and seems flow-worthy.
When I got this car it was running very rich...the fuel adjusting needle was not coming close to being full-in due to corrosion in the brass-housing thread area. I cleaned that up and the car runs OK, but is still producing the gas-smell and sooty spark plugs common to that circuit being rich (around town driving).
I measured the needle length as it was fully threaded into the brass housing (while in the carb casting) and got a measurement. I then took the needle out and placed it in the casting orifice without the housing and took a measurement. The difference was 3/16", meaning the needle while fully seated in the brass threaded housing was out a full 3/16" from the orifice. Now, I've read where some needle tips are gouged by owners turning the adjustment full in too much, but that seems impossible with the setup I have. The brass needle housing has a stepped section held together with a small snap ring that prevents the wider part of the needle from going further in. I ordered another needle and it is mostly the same idea (some dimensions are inconsistent but I rack that up to typical model A replacement part variations), so I don't think the needle is the issue. As an experiment I machined the needle a bit so it would go another 3/32" closer to the orifice. This helped, and the gas smell is gone, and the car now starts almost instantly! But, I digress, as I am curious as to whether any of you know if the needle is supposed to insert completely into the casting orifice....as having to machine the needle to make it work better seems kinda mickey-mouse. Could the brass housing itself be a culprit?
Thanks for any input...
Tom

Last edited by kawagumby; 03-01-2021 at 07:48 PM.
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Old 03-01-2021, 10:53 PM   #2
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Default Re: Fuel adjustment system question...

You are on the right track. The needle must be able to be screwed in to the point that the needle will seat completely in the orifice and completely shut off the fuel from passing thru the orifice.

When the engine is first started cold, it may be necessary to richen the mixture a little by turning the needkle 1/2 to 1 turn out and then after it is warmed up, you may be able to close th needle completely to lean it back down. With some carburetors and todays fuel, it may be necessary when the engine is warm to run the needle a little open, 1/2 to 1 turn for the best mixture and a little more open while warming up.

My experience,

Chris W.
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Old 03-02-2021, 01:08 AM   #3
kawagumby
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Default Re: Fuel adjustment system question...

Thanks Chris,
That's what I thought would be the case. As with so many things with this particular car, it seems a lot of parts just aren't correct or not working and it is a wonder the previous owner could even drive it the few hours he did. The engine is model B with an A head, using an A distributor that was timed with the different B timing cover indent pin location, so the timing was too far advanced, as another example.

I guess for the time being I'll do a little more machining to get the needle to locate as close as possible to the orifice - then try to get another needle housing to see if that is the issue (I wonder if there is a length difference between the needle housing of the earlier carbs that had a jet rather than the casting orifice). The car is starting to run pretty good after about 100 hours of work, LOL, but when I bought this car, I thought the only thing I'd have to fix was the water pump..ah, the innocence...



Tom
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Old 03-02-2021, 08:41 AM   #4
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Default Re: Fuel adjustment system question...

Take the little snap ring off. It is not needed. It adds too much friction to the turning of the GAV (Gas Adjustment Valve).
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Old 03-02-2021, 11:03 AM   #5
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Default Re: Fuel adjustment system question...

Quote:
Originally Posted by nkaminar View Post
Take the little snap ring off. It is not needed. It adds too much friction to the turning of the GAV (Gas Adjustment Valve).

edit:
I took the ring off, and it doesn't make any difference as to the needle being able to travel further forward, as there is a limiting ridge that prevents the needle (stepped) from going further. I think the problem is likely due to the differences between the '28-'29 carbs and the '30-31 carbs which don't have a brass jet. If that is the case, the brass jet may project closer to the needle assembly. That is pure speculation on my part. However, I've noticed that the couple of parts dealers I've checked out only sell one needle design - I suppose I could tap the cast orifice and install a brass jet which would help, but I'm going to machine the needle a bit further and see if that cures the mixture issue.

Last edited by kawagumby; 03-02-2021 at 12:36 PM.
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Old 03-02-2021, 12:01 PM   #6
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Default Re: Fuel adjustment system question...

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The ring can be expanded with a scratch awl.
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Old 03-02-2021, 12:39 PM   #7
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Default Re: Fuel adjustment system question...

Quote:
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The ring can be expanded with a scratch awl.
Yes, but there isn't really a binding problem, just the needle not getting close enough to the orifice. I would love to see what the original factory carb had in it!
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Old 03-02-2021, 12:51 PM   #8
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Default Re: Fuel adjustment system question...

Sounds like the brass seat is missing.
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Old 03-02-2021, 01:20 PM   #9
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Default Re: Fuel adjustment system question...

Gumby - Brentwood Bob may be on to something. There were two "versions" (in a broad sense, not counting the Z1-Z2-Z3 castings) of the Zenith lower. The early lowers (I'm not sure how "early", but I kind of gather the switch was made in 1928) used a screw-in brass seat at the bottom of the GAV chamber. The later had the seat machined into the casting. Really the only ways to tell them apart are that the "early" had a threaded portion that allowed the jet to screw in, and the surface you see looking down the hole with the jet removed is flat. The machined in seat is obviously shaped to match the jet, and the surface just outside the machined jet is sloped. The hole in the version that takes the brass jet is also larger than the cast orifice, but that's not that obvious unless you have them side by side.

I just refurbished six Zeniths (in varying states of crudiness and rustiness), one had the screw-in seat and five had the cast-in seat. More, it was difficult to tell which was which on the used bodies until I had soaked in Chemtool and then carefully bead blasted down the hole.

Take a good look at what you have. The GAV needle should not need machining to fit. If you have that much gap it's sounding like you need the brass GAV seat but don't have it. It's also possible that somewhere along the 90-year history someone took a grinder and re-ground the seat, I've heard of that but not seen it.

BTW, one of the manuals I read says to remove the ring on the GAV housing. I've never seen a housing without that ring.

I live not too far from you (lower peninsula), I'd be happy to take a look or maybe show you examples of both.

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Old 03-02-2021, 01:35 PM   #10
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Default Re: Fuel adjustment system question...

Are you sure the adjusting needle seat is in place in the adjustment housing and in good condition ?
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Old 03-02-2021, 03:05 PM   #11
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Default Re: Fuel adjustment system question...

Thanks all, for the feedback.
I finally was able to get the needle to seat. I'm thinking the carb is the later type because I cannot see any threads in the orifice that would support the earlier carb type that uses the brass jet. (mine is a '31 that has been rebuilt by previous mechanics...) more than once by the look of it, helicoils and all.
I think the problem is poor aftermarket parts that vary from the original spec's.
After machining the narrower needle section further back it became obvious that the needle threaded section was the next limiting factor...so I was able to thread it just a bit further (metric pitch).
Then, I machined down the hex portion of the housing where you use the 7/16" wrench to tighten, and extended the 1/8" pipe threads to get the whole housing a bit farther down into the carb casting.

Whew...
The combination of all three mods allowed the needle to just touch where it should. So to test I revved up the engine with the GAV all the way in, and sure nuff, it lean backfires. Cool deal.
Something else, I have two aftermarket needles, and they vary in two ways; one is longer and one is of noticeably less diameter. I used the one that came with the car because it was longer. So who knows if the housing is to spec...I have another one ordered just out of curiosity.



Thanks again all,
Tom
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Old 03-02-2021, 07:14 PM   #12
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Default Re: Fuel adjustment system question...

Kawabunga (surfer term). Glad you got it worked out and thanks for letting us know what you did. We all learn from each other.
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Old 03-02-2021, 09:21 PM   #13
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Default Re: Fuel adjustment system question...

There were some needle housings made more then a few years back that the ring and way they were made it was suggested to remove the ring. Originals or ones made of late, the ring does not need to come off.
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Old 03-03-2021, 04:47 AM   #14
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Default Re: Fuel adjustment system question...

Yes, I too have come across a GAV needle that was too short by about 1/16 inch. (Careful measurment of everything - body , threads ,housing, seat etc comfirmed that the needle ws indeed he culprit ) The problem was solved by chucking the needle away and fitting a non-rogue one! I suppose a less dramatic finale could have been acheived by careful fettling of the threads.
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Old 03-03-2021, 11:06 AM   #15
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Default Re: Fuel adjustment system question...

Quote:
Originally Posted by kawagumby View Post
Thanks all, for the feedback... I'm thinking the carb is the later type because I cannot see any threads in the orifice that would support the earlier carb type that uses the brass jet...

Tom
Tom - the threads are not really visible. But another way to tell is to measure the diameter of the hole at the bottom. The earlier style (that needs a brass jet) will pass a 11/64" drill bit. The later style (cast in seat) will pass a 5/64" drill bit but not larger. BTW, the threads are an oddball size - M5 x 0.75 (the "normal" 5mm thread pitch is 0.8 mm). You can order the tap from a machinist supply company or from most of the Model A vendors.

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Old 03-03-2021, 11:38 AM   #16
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Default Re: Fuel adjustment system question...

Quote:
Originally Posted by JayJay View Post
Tom - the threads are not really visible. But another way to tell is to measure the diameter of the hole at the bottom. The earlier style (that needs a brass jet) will pass a 11/64" drill bit. The later style (cast in seat) will pass a 5/64" drill bit but not larger. BTW, the threads are an oddball size - M5 x 0.75 (the "normal" 5mm thread pitch is 0.8 mm). You can order the tap from a machinist supply company or from most of the Model A vendors.

JayJay

That's VERY good info...
Normally, if the area is unmolested, wouldn't you be able to see a machined seat also?
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Old 03-03-2021, 11:50 AM   #17
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Originally Posted by kawagumby View Post
That's VERY good info...
Normally, if the area is unmolested, wouldn't you be able to see a machined seat also?
Maybe it will look like a machined seat, maybe not. The seat may have gotten so corroded over the years, or so many gaskets with bits and pieces stuck, that it looks to be cast and not machined. That's one reason I've bead-blasted (carefully, and washed thoroughly afterwards) and scraped the GAV chambers, to be sure I'm looking at virgin iron.

Also - that's the same tap size as the other jets in the carburetor. Not specific to the GAV seat.

I'd be hesitant to assume that anything on a 90-year old carburetor is "unmolested", especially since you've already noted the presence of helicoils. And again, in 90 years, don't assume that the carb you have on the car is the original. Used to be that any auto parts store (e.g., Pep Boys, Western Auto, etc.) would sell rebuilt Zenith. You took in a core and you took away the rebuild. Maybe the rebuilder knew what he was doing, maybe it was a high school kid plunking in parts without any understanding.

That's part of the challenge of owning these things...

JayJay
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Old 03-03-2021, 02:57 PM   #18
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Default Re: Fuel adjustment system question...

Quite a challenge so far, I feel like I need to be Sherlock Holmes sometimes...
Thanks again for your experienced knowledge...it is appreciated.
Tom
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Old 03-03-2021, 07:31 PM   #19
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Default Re: Fuel adjustment system question...

The threads could be a tad stiff. Try shooting some penetrating oil in there. DON"T FORCE IT! YOU CAN RUIN IT.
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Old 03-03-2021, 07:39 PM   #20
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Default Re: Fuel adjustment system question...

My 28 has a Zenith (using a B carb at the moment) with no numeral. It has a double venturi. I think it has that screw in seat, now that I look at that parts diagram (above).
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