4100 Carb leaking raw gas onto manifold

[MN63Tbird]

Just to throw one more suggestion in before the new year. I had a similar gas leaking issue with my 4100 a few years back when I was also chasing down a vacuum leak. I found two culprits that I had assumed would be taken care of by the gaskets. They were the power valve cover and the front pump cover for the accelerator pump. Both if I remember correctly are aluminum. Over the years of checking the power valve after a backfire or changing out that little red valve in the pump area, I guess I would be torquing on the screws a little too much and eventually would bend the cover plates. The secondary cover plate was fine but could also be subject to similar abuse over time. The thin gasket of the pump diaphragm or secondary diaphragm would not correct the leakage issue.

The next time you take off the carb check each for how flat they are with a sharp flat edge. I used a ruler with a metal edge. They should be flat along each edge. I found with each cover plate one corner was slightly bent. It does not take too much bend to cause gas to leak. To correct the issue I gently sanded the face with 400 grit paper. I placed the sand paper on a very flat surface and gently rubbed the face of the cover plate against the paper. Remeasured after a few sanding motions to ensure my technique was correcting the issue. After doing this I have not had an issue for a few years. Good Luck

MN63Tbird

[mammentorp]

More info to report. I have now double checked float height and removed the floats and needles to make sure the seats are clean. Also blew some compressed air into the carb through the fuel line and installed an in line filter between the fuel pump and the carb.

Unfortunately, same results. I turn over the engine and the carb bowls fill up and then overflow through the vent holes. This is also happening the the same time in BOTH the primary and secondary. That last fact makes me feel as though this is not about debris in the seats - seems unlikely that BOTH primary and secondary would overflow in the same way at the same time.

I have a 0 to 20 psi pressure gauge coming in the mail in a couple days and will check pump pressure when I get it. I sourced my carb kit from Mike's Carbs. Is it possible I somehow got needles and seats that were mismatched? Also seems unlikely.

Adding a couple of pics here in case someone sees something obvious I may have missed.....??

[Alan H. Tast]

OK, folks, this has gone on long enough - we're running out of things to address. It's either the fuel pump is pushing too much or there's something FUBAR with the carb.

Just to be thorough, here's the Carburetor troubleshooting matrix from the '62 T-bird Shop Manual - note the comments at the bottom of the first part of the chart about fuel leakage including a cracked main body or the fuel pump is putting out too much pressure:

Carburetor Trouble Diagnosis Chart, pt. 1 (from 1962 T-bird Shop Manual)

Carburetor Trouble Diagnosis Chart, pt. 2 (from 1962 T-bird Shop Manual)

This is how to test the fuel pump pressure, along with the troubleshooting chart from the '62 T-bird Shop Manual - note it's comment about high fuel pressure and the fuel pump spring being too heavy:

Fuel Pump Pressure Test (from 1962 T-bird Shop Manual)

This is the exploded diagram for the fuel pump - the spring in question is part of the diaphragm assembly:

Fuel Pump Exploded Diagram (from 1962 T-bird Shop Manual)

These are the fuel pump specs from the '62 T-bird Shop Manual:

Fuel Pump Specifications (from 1962 T-bird Shop Manual)

62 SM pt 3-4 p3-20 Fuel Pump Specs.jpg (23.92 KiB) Viewed 499 times

Have you considered finding another working Autolite 4100 carb to swap out with your existing one to see if the problem still persists? If the fuel pressure is within specs, you may want to do that - it could be that your existing carb does have a crack in the body and the only way to fix that is to replace it with a good one.

Assuming the pump is developing too much pressure, resolving this might be as simple as changing out the fuel pump (two bolts removed and it can be pulled out) and either replacing it with one that's within specs, or rebuilding the existing one with a diaphragm/spring that's within specs.

Now maybe this question hasn't been raised yet - was the pump replaced previously, and did this problem start after the pump was replaced? If the pump can't be redone or replaced with one putting out lower pressure, then going the fuel pressure regulator route, and adding a bypass filter/line route to relieve the overpressure as a last resort, may be necessary. I'd really like to see this get resolved as this has impacts for a lot of people out there.


Just to make sure you're using the right specs, here's what's shown in the '62 T-bird Shop Manual for the carb - the second part has the fuel level/float settings:

Carb Specs - Jet Sizes (from 1962 T-bird Shop Manual)

62 SM pt 3-4 p3-20 Carb Specs pt1.jpg (24.69 KiB) Viewed 499 times

Carb Specs - Float Levels, etc. (from 1962 T-bird Shop Manual)

Pardon the pun, but I'm spit-balling here - do you recall dealing with any small steel balls when disassembling and reassembling the carb? They are check balls that should close off interior passages when filled with fuel. If they're missing they may be allowing unrestricted fuel flow.


Here's an exploded diagram from the '62 T-bird Shop Manual: Not there are several check balls shown. Are you missing any of these?

Carburetor Exploded Diagram (from 1962 T-bird Shop Manual)

Here's some illustrations identifying major components of the carb - sorry, but these images are from a very poor reproduction of the shop manual (see my question at the end of this message).

Carb Components (from 1962 T-bird Shop Manual)

Carb Components (from 1962 T-bird Shop Manual)

These illustrations/cut-aways show how the things flow through the carb - sorry, but these images are from a very poor reproduction of the shop manual (see my question at the end of this message).

Accelerating & Primary Stage Main Fuel System (from 1962 T-bird Shop Manual)

Power Fuel & Secondary Stage Main Fuel System (from 1962 T-bird Shop Manual)

Now, my big question: Do you have a copy of the 1962 Thunderbird Shop Manual? If not, get one - IMMEDIATELY! It has a whole section on the fuel system including the fuel pump and carburetor. Read it thoroughly to understand how the carburetor is supposed to work.

[claybreaker 144]

My recent fuel overflows were attributed to high fuel pressure and then a bad float in one of the Holly's on my M Code. I did have another flooding incident this past summer when I started the car to move it and after several run and stop episodes it flooded big time filling the exterior of the intake manifold. I was inside the car when it happened and didn't see where the fuel leaked from and also didn't notice the fuel pressure on the gauge mounted to the regulator I installed. I am guessing the regulator has gone bad after Alan's post from other forums where Chinese fuel regulators don't last much longer than 30 days. I'm rebuilding a 4100 with carb kit from Mikes and found their technical support to be responsive. They also advertise this secondary float spring (https://www.carburetor-parts.com/float-spring-66-454) that dampens the float a bit which is not a bad idea. Good luck!

[mammentorp]

Alan, thank you for the input as always! To answer the last question first yes, I definitely have the Shop Manual. (A purchase I prioritized over even Thunderbird, 1955-1966 from American Classics!) And I have been through the theory of operation section on the carb, although to really internalize this complex device you’d almost have to work with them every day. I’ve been through the troubleshooting guide, and am pretty confident of the float level settings. The carb has all new gaskets and a new accel pump diaphragm.

I have not seen any evidence of a crack in the housing, although I can imagine there might be something hairline that I missed. And again, the fact that both the primary and secondary bowls are overflowing the same way at the same time makes me think it’s less likely to be debris in the seats. (Plus, I’ve cleaned them out several times now just in case – never saw any evidence of dirt or debris.)

The two check balls associated with the accelerator pump circuit are definitely in place, as is the weight on the discharge check ball. I’m not aware of any check balls beyond those two.

I think my next step is checking the fuel pump for pressure and volume. I don’t know if the fuel pump has been rebuilt or replaced, although I don’t think so. Previous owner passed away, so I can’t ask.

[Cliff Rankin]

If this was mine , I would take off the power wire to the coil ( make sure it’s covered) take a remote starter button
With key off , turn the motor over with lid off carb. And see if I could stop the overflow manually by pushing on the
Needle / seat assembly. As that should be what happens.
But that’s me , respectfully , I don’t know how far your skills are , if you know what I mean.
Cliff

[mammentorp]

If this was mine , I would take off the power wire to the coil ( make sure it’s covered) take a remote starter button
With key off , turn the motor over with lid off carb. And see if I could stop the overflow manually by pushing on the
Needle / seat assembly. As that should be what happens.
But that’s me , respectfully , I don’t know how far your skills are , if you know what I mean.
Cliff

Good suggestion. Because that would help determine whether the needles CAN close off the fuel in the seats. I will give that a try. Although, being an old farm boy, the "remote starter" in my bag of tricks tends to be a pair of pliers at the solenoid. Understanding that sparks might be a negative in this context.....

[mammentorp]

If this was mine , I would take off the power wire to the coil ( make sure it’s covered) take a remote starter button
With key off , turn the motor over with lid off carb. And see if I could stop the overflow manually by pushing on the
Needle / seat assembly. As that should be what happens.
But that’s me , respectfully , I don’t know how far your skills are , if you know what I mean.
Cliff

Good suggestion. Because that would help determine whether the needles CAN close off the fuel in the seats. I will give that a try. Although, being an old farm boy, the "remote starter" in my bag of tricks tends to be a pair of pliers at the solenoid. Understanding that sparks might be a negative in this context.....

I performed this test this evening. Light pressure on the float tabs sealed the needles effectively. Then, after ceasing cranking, I let off on the float tabs and gas spurted up past the needles. So I conclude that the needles are seating correctly, and the position of the floats when I held them in place looked to be reasonable (not so high that the floats can't seat the needles on their own). So next step is checking fuel pump pressure.

[Karl]

G'day.
A photo can sometimes be deceptive but in your two photos it looks like the spring clip that holds the floats in position is not seated lower enough to hold the floats in place.
Can you lift the floats in and out without unclipping them?
Karl.

[mammentorp]

G'day.
A photo can sometimes be deceptive but in your two photos it looks like the spring clip that holds the floats in position is not seated lower enough to hold the floats in place.
Can you lift the floats in and out without unclipping them?
Karl.

Yes I can! Is this something I missed? It's not clear to me how the spring clips are supposed to hold the floats down? But if I don't have those installed correctly that would certainly explain the problem! (More research.)

[Alan H. Tast]

I think we have a 'Bingo' with the clips! Good catch, Karl - I didn't take the time to look at that. Those wires should snap into a groove near the top of the seat fitting. Will it solve the problem? Only the OP can tell...

[Rusty57]

I just watched Mike's video about setting the float level on a 4100. He makes a point to talk about hooking the back of that spring into the groove in the seat for the float valve. That holds the float down.

I'm not a 4100 expert by any means. When I looked at the OP's pictures I wondered what held the float pivot pins in place. I just assumed that there were projections on the cover that kept those pins from rising.

Once again, this proves the value of forums like this one. VTCI membership supports this valuable resource.

[mammentorp]

OP here.

And, like magic, snapping down those retainer clips fixed the problem! The funny thing is, I watched Mike's overhaul video probably ten times, and I even remember him making that comment about getting the clips in the groove. But when it came time to actually do it, it went right past me.

Thank you all so much for the help on this!! Another hurdle overcome.

Happy New Year!!!

[Karl]

Don't forget to re-set the float level.
Karl.

[Alan H. Tast]

Once again, this proves the value of forums like this one. VTCI membership supports this valuable resource.

Rusty, THANK YOU for the ringing endorsement. If not for dues-paying members to VTCI, this Forum would not exist. If you are reading this and currently not a member-in-good standing, here's the link to join VTCI and get its bimonthly magazine, "Thunderbird Scoop" (which this thread will be eventually transitioned into a tech article for future reference):
https://www.vintagethunderbirdclub.net/