Amazon: Freshly rebuilt Zenith with some concerns

[samaron]

After more waiting, the carburetor finally has returned. So here is an update:

The gap between the float bowl and the carburetor body is now gone. Can't see any light through and unable to move the gasket around.

The throttle shaft now moves freely. No metal friction squeaking or excessive force needed to turn it. Can hear and feel a tiny bit of play in the shaft when I wiggle it up and down, but not nearly as much as it used to be before it got machined.

Checked the flanges for flatness before installation. The initial startup went fine. I let it idle until it reached operating temperature and performed idle mixture and idle speed adjustments then.

Had to turn the mixture screw in further than before, but probably not surprising since it doesn't have massive air leaks anymore. I could even stall the engine if I unscrewed the idle speed screw far enough, which it didn't before. Car drives nice too without surging at constant speeds. So far it seems like a win.

However, when I turned off the engine and tried to start it 5-10 minutes later, it wouldn't start. After an excessive amount of cranking, it tried to start. Eventually it managed to run on two cylinders or so, and would run perfect at idle again after revving it. Turning it off and immediately start it again, it would start perfectly by barely touching the key. If I left it for 5-10 minutes, it was hard to start again. I found that having the throttle wide open while cranking made it much easier to start.

The instruction booklet does not mention any special maneuver is required to start the car while warm. This makes me believe it isn't supposed to be like this.

Left the car for a few hours. Started without trouble and drove it until warm again. Upon some further observation, I found a droplet of fuel started to form near the base of the carburetor after turning off the engine. While running, it was completely dry and no droplet. It is coming from the circular part of the casting near the flange, see the attatched picture. The fuel runs down from there and soaks into the paper gasket.

If the weather permits it, I will start it up again tomorrow and pull off the air cleaner assembly and inspect inside the carburetor. Hopefully I can see where the fuel comes from when turning off the engine.

[142 Guy]

The fact that it starts easily when cold or if you immediately try a restart; but, is a problem if it sits for 5-10 minutes sounds like a heat soak issue of some sort. SU (at least the later ones) and Stromberg carbs had heat shields between the carb and the manifold to reduce heat soak. In the original installation, was there any kind of heat shield between the base of the carb and the manifolds or was there a thick phenolic insulator between the base of the carb and the manifold?

When you finally get the car to restart, does the exhaust smell like a rich fuel mixture? You might have the mixture too rich to facilitate a good hot start. Conversely, if it is a heat soak problem it might be running too lean.

One possible test that might confirm whether it is a heat soak problem is to have a hand towel soaked in ice water. After a 5 minute hot shut down apply the cold wet towel to the carb and let it cool the carb for a minute or so and then attempt a restart. If the restart goes more smoothly you have a heat soak problem of some sort. What exactly that heat soak problem is I can't advise.

The D jet equipped B20E and B20F have a miserable heat soak problem. On a really hot day, shut down on a really hot engine for more than a couple of minutes and the engine will be very hard to restart and then run like merde for about 2-3 minutes because the engine is running with a really lean fuel mixture. After about 3 minutes the engine starts to run fine because fuel flow through the injectors cools them off and everything returns to normal. Very similar to what you describe.

[Rustinmotion]

Check if the choke is open when hot, if it’s closed that will be a problem on a warm car
If it’s not mark the idle screw and turn it in half a turn making the mixture weaker, does it now start better when hot?
An engine that starts easy when cold and hard when hot could be rich or have the choke flap closed all the time

[Derek UK]

The Zenith has an accelerator pump. This will pump some fuel into the inlet manifold when you give the pedal a push down. This carb normally has a two position setting via a sort of small butterfly around the pump plunger. You just twist it to select the position. It sets the pump travel to give a large or small amount of fuel when the pedal is pressed. Small and large are relative but normally set to large. You can see how the fuel squirts by looking down the carb throat with the filter off. When starting from warm you may or may not need to give the pedal a pump or two. IF you give it too much and flood it, push the pedal to the floor and crank it until it catches. Don't pump the pedal when you are doing that or even more fuel will squirt in. If the choke cable is adjusted correctly it will give a small amount of revs increase for the first few mm of pull before actually enrichening the mixture. If you set it to this first position for your warm starts this may also work for you. Like that one pump of the pedal could be enough. It sounds as if the carb is now fine, you have to learn the knack of using it!
In your pic I'm not sure what I am looking at. What is the black and red thing. The pic is slightly blurred there.

[142 Guy]

Derek UK's comments twigged me to something. First off, heed the advice about not pumping the throttle prior to a hot start. That said, some carb / engine combinations benefit from additional 'air' during a hot start. This is particularly true if the carb tends to run a little rich during a hot start.

As a simple test, before attempting a hot restart, try opening the throttle a little bit with your foot before cranking the engine to see if that changes anything. If nothing changes then its time to look at other things.

[samaron]

Hello everyone and thank you for all your replies!

When setting up the carburetor, I used a Haynes manual for Amazon and P1800. Although it does describe a different carburetor model (Zenith 63 VN), the physical appearance is practically identical. Thus, I have assumed the same principals apply. Everything was adjusted as described, and the choke flap operation was verified. I used both a vacuum gauge for the idle mixture and a dwell & tach meter to check the idle rpm.

Reading through your replies I got a realization. I should have double checked the instruction booklet, and now I do feel a bit silly. It does in fact say the following regarding a warm start: "If the engine is warm; completely push in the choke handle and fully depress the accelerator pedal." Attached a scan of the page for good measure. It is in Swedish. I suppose the severe air leaks I had earlier compensated for depressing the accelerator.

This appear to be a classic case of user error! I will drive the car for a few more days and try to be more disciplined with the described starting procedures. As Derek UK put it, I need to learn the knack of using it.

To answer a couple of questions regardless:
The previous attached picture was an attempt at showing where the fuel comes out. Some fuel does seep out from the round portion of the body casting. I now realize it is where the throttle shaft is located. A tiny droplet bump does show, but I do admit it is hard to see. You can see the paper gasket around the flange start to become wet from the fuel.

The carburetor does not have any heat shield or phenolic insulator. Although none of the fuel and intake components were particularly hot as far as I recall. The warmest part were the fuel pipe between the pump and carburator since it is bolted to the engine, but wasn't uncomfortable to touch. I could do some more scientific measurements of the surface temperatures another day.

[Derek UK]

If the Haynes says 63VN that is a typo. B16A = 34VN. B18A = 36VN.

[samaron]

Reporting back on this subject as well after a few days of driving the car.

Cold start is flawless, and warm starts are much better after I started pushing down the accelerator pedal while cranking.

Sprayed some starting fluid around the carburetor base and the previous trouble spots, and do not get any RPM change.

The paper gasket at the manifold flange do get wet with fuel after a drive, but not enough for it to start dripping onto the manifold. The float bowl does become lukewarm as well after stopping, but not uncomfortable to touch. Can't hear any indication of boiling either. While driving it is cool and the paper gasket is dry.

Regardless, I suppose this can be called a win for now. The car hasn't been running this good in my ownership.

[142 Guy]

It is good that you have the driveability issues sorted out. I would keep an eye on that gasket and try to figure out why it is getting wet, particularly if that wet is from gasoline. I am thinking less about driveability issues and more about not having a local media story about an Amazon catching fire while being parked in a garage.

[samaron]

Hmm, so this is abnormal behavior? It is getting wet with fuel, yes.

If the car has been sitting overnight to let everything evaporate, it stays dry during starting and driving. It only becomes wet over time after turning off the engine. The droplet mentioned, and pictured, in my update post almost two weeks ago is the only place fuel comes from. The float bowl itself is dry, as is the carburetor in general above the flange. Fuel slowly seeps out from the throttle shaft bushing area and over time saturate the flange paper gasket.

I just assumed it is some sort of capillary effect happening, which is why the instruction booklet mention to fully push down the accelerator pedal when warm.

EDIT:
Attempted to highlight the droplet. See attatched picture.