refurbished carburetor - wild ride

[Gene F]

Question, A Model-B carb main jet should be what size with a improved cam, and H/C head?

[CA Victoria]

Later lower castings utilized a brass seat threaded in below the GAV needle valve for it to seat into. If you have this style and the seat is missing the GAV cannot be regulated.

[TomInCologne]

Originally Posted by CA Victoria

Later lower castings utilized a brass seat threaded in below the GAV needle valve for it to seat into. If you have this style and the seat is missing the GAV cannot be regulated.

i will check for this later today...


the GAV is turning rough and doesn't give a good 'feel', is there a sort of grease that i can put on the GAV thread to make it run more smoothly?

[nkaminar]

There is a steel ring around the end of the GAV housing. Take that off and it will turn easy.

[CA Victoria]

Tom. Check the fit of your new needle before you remove the ring.
Some replacement needles are a bit over size. Without the tension ring it may be to loose and turn on its own.

[Joe K]

Sponsored Links (Register now to hide all advertisements)

Originally Posted by CA Victoria

Tom. Check the fit of your new needle before you remove the ring.
Some replacement needles are a bit over size. Without the tension ring it may be to loose and turn on its own.

I.e. the principle of the thing.

The GAV housing "fingers" grab around the GAV needle and present friction against the GAV needle from turning on its own from vibration. The small ring "adds" to that friction.

Most "new" GAV needles/housings are mal-sized and the fit is "too tight." It's possible to use an ice pick and "expand" the fingers so they grab less tight. Either with or without the small ring.

Some I've left the ring off without using the ice pick. Others I've just stuck it with the pick and stretched everything. And another I replaced the steel circle with a small O-ring.

And another with the ice pick I've inadvertently broken out one of the fingers which didn't please me, but the GAV now turns properly and holds in place. So even broken, it can work.

And based on my self-created damage, I considered next time I might use small number size drills and "work my way down through the numbers" until I'm pleased with the fit.

The Model B carburetor invoked a different system to prevent the GAV from turning which is a small washer and a spring. But the GAV housing is "press fit" into the lower casting. Different altogether, but same intent.

Joe K

[TomInCologne]

Okay, thanks to a very generous member here on Fordbarn (Thanks Tim) i am now in possession of a set of calibrated jets. so fine, so good. I have to 'Helicoil' on of the threads on the flange to the inlet manifold, but i should be able to do this this evening.


One problem that i foresee is that i have two very similar looking jets, see attached photograph. I assume that one is the compensating jet but i don't know what the other is for. i also don't know which one is which...


Thanks for your input...

[nkaminar]

All the components, including jets, are shown here: https://model-a.org/components_full.html

[TomInCologne]

Originally Posted by nkaminar

All the components, including jets, are shown here: https://model-a.org/components_full.html

This helps, thank you very much!

[CA Victoria]

Not all carburetor bodies used the removable seat for the GAV needle (early carbs)
The seat has the large hole.
If a normal screwdriver will not remove it easily I would suggest grinding a screwdriver to fit the slot.
Have you removed the GAV sleeve yet?

[Marshall V. Daut]

That carb website is great, clearly presented and has helped many Model A owners over the years, including Humble Self. Just to be clear, though, in your picture of the two jets, the one on top is the compensator jet that screws at an angle into a threaded hole inside the lower casting's fuel bowl, while the taller jet is for some carburetors, whose lower bodies have a threaded hole at the bottom of the GAV tunnel. As "CA Victoria" points out, not all carbs have a provision for this jet. About late 1929 or early 1930 that jet was eliminated and a smaller hole was drilled in the lower body itself and chamfered to match the angle of the GAV needle. When laying out new jets, confusion arises because those two small jets both have the same size thread. They can mistakenly be interchanged, resulting in a poorly operating carburetor. You have to look close at the orifice size and chamfer inside the slot for the screwdriver, but the difference is not all that obvious. Just keep in mind that the taller jet goes inside the GAV passageway, if your carb uses that jet. It's easy to tell if you need that jet or not: if your carb's lower casting has been drilled for the needle only, you will not be able to screw that jet in. Very clear. If you do need that jet, however, be sure there is a little gasket beneath its head.
Marshall

[TomInCologne]

okay, i think i got the carb back together...


i had a worn out thread in the flange to the manifold. I drilled it out for a Helicoil / V-coil insert but there was not a lot of meat left in the body. Hopefully it'll work out, otherwise i have to use a nut... All passages were blown out using compressed air.



The linked page was very helpful, Thanks a lot. I did not use the valve seat for the GAV as there was a port drilled into the carburettor body for the needle. The carb comes from a '31 sl/ws, so that makes sense.


I think all is okay and ready for a test but i am afraid that i do not have time today and maybe no time tomorrow. But i will keep you updated...

[CA Victoria]

Looking good Tom.
If you do not have the capillary tube to set the float you can use this method with it bolted to your manifold.
Using the precautions needed with an open fuel container & I have a catch pan under my test bowl. It has a mark at my desired fuel level. On the car a second person would be needed to control fuel flow.
The copper carb to manifold gasket will seal well, take care to not over tighten your bolts with this type of gasket.

[TomInCologne]

Okay, at least a partial success:


- installed the carb and everything seems to be gas tight
- let the engine warm up and set-up the idle roughly
- test drive: engine was still reluctant to accept throttle and had many
misfires at half- and full-throttle. Changing the setting of the GAV did not
do much apart from when it was fully closed and the engine had a big
misfire.

- so i decided to go on my full speed test track, a 2 mile straight with 62mph
speed limit. She was still reluctant to accelerate past 45mph with lots of
buckling. Then i changed the setting of the GAV and it got much better.
She then accelerated past 50mph.


My thesis: the carb is now much better and i am getting forward. i will check the spark plugs tomorrow to see what they look like. I will also check the float level, i have the transparent tube setup. I will also check the ignition as far as i can...

[CA Victoria]

Things to check: would be your spark advance mechanism to be sure the distributor is moving fully with the lever. And to recheck for air leaks on the intake side.

[jeepguy1948]

If it accelerated past 50 MPH, I’m not sure that there is anything wrong at all now. If it runs smooth and will go over 50, what more are you hoping For? Every motor and every carburetor are slightly different. One might need the GAV open 1/8, another 1/4, and a third 3/8 of a turn. Depending on the humidity and air temperature the same carb may need to be adjusted different from one day to the next.

[TomInCologne]

Originally Posted by jeepguy1948

If it accelerated past 50 MPH, I’m not sure that there is anything wrong at all now. If it runs smooth and will go over 50, what more are you hoping For? Every motor and every carburetor are slightly different. One might need the GAV open 1/8, another 1/4, and a third 3/8 of a turn. Depending on the humidity and air temperature the same carb may need to be adjusted different from one day to the next.

The top speed is fine, i wouldn't go past 45mph often. The problem is that it doesn't run smooth, like some hickups. This may come from the carb and/or fuel/air mixture, but it may also come from the spark plugs or ignition. These items i will check this morning and then i will report back.


I have already checked for leaks on the inlet manifold with some 'jump starter spray', there don't seem to be any leaks...

[TomInCologne]

okay, found an issue with the connection coil to cable. The little bracket that clamps into the wire had disintegrated over the winter and the contact was only intermittent. i changed the whole wire against a new one and she runs very fine now. GAV is about 1/8 open.



I still have some issues with going down a hill at >1.400rpm with the throttle completely closed. It misfires badly, very loud. as soon as i open the throttle just a tiny bit, the misfire goes away. I assume a leak in the inlet, either between carb and manifold or manifold and engine. I did check tightness with starter fluid but only at idle. maybe there is a better way? This problem did exist before i changed the carb so i don't think it has anything to do with the carb.


Thanks to all, especially Tim.

[Bob Johnson]

To find a manifold leak I use a shop vac with the hose connected up to the "blower" side of the shop vac. For exhaust leaks connect the hose to the tail pipe and turn on the vac (blower). Spray soapy water and look for bubbles. For intake leaks connect the vac (blower) to the intake of the carburetor.

Bob

[1crosscut]

Maybe it was suggested in an earlier post in this thread but have you checked the condition of the lower plate wire in the distributor and it’s connections to see that they are good?