Losing fuel prime

[BugsandJeeps]

Hi All,
I have an issue with my M37 losing prime when sitting. I have to dump fuel in the carb to get it to start and once it starts, it runs perfectly fine. It will restart easily if I shut it off and does this for a few minutes but if I leave it for more than 30 mins or so I need to prime it again. It does have the winter fuel pump on the dash but it is not hooked up. Any thoughts on where to start on troubleshooting?

[just me]

Carburetor. That is the only place it can "lose" prime.

[Cal_Gary]

Agreed. Carb seepage, or if too hot is the fuel evaporating in the bowl?
Gary

[BugsandJeeps]

So carb needs a rebuild or something specific to the carb is not working correctly?

[ashyers]

BugsandJeeps,
I'd suggest first ensuring you have no vacuum leaks and then checking out the choke mechanism. If things are working it's capable of flooding the engine VERY easily when closed fully. The different carbs I've used on the M37 have all had their own "personality" as far as how much choke they required, but I have yet to see one that "lost prime".

If your float bowl is going dry that's a bit concerning. The bowl is not small and that fuel is either heading towards the exhaust manifold (external leak) or into the engine (internal leak). Next time you have a no start pull the plug on the bowl to determine if the bowl is going dry. That will provide some direction for next steps.

Andy

[BugsandJeeps]

Float bowl has fuel. Still need to throw a little down the air cleaner elbow. Fires right up after that.

[just me]

Then you need to work on your starting procedure.
Switch on
Throttle pulled out a few notches
Choke pulled out
Step on starter and push choke in while cranking.
Start every time hot or cold.
(The accelerator pump is vacuum operated. Pumping the pedal does nothing when the engine isn't running. You need to choke to enrichen the mixture.)

[BugsandJeeps]

I tried the starting procedure as suggested. Truck started fine a couple of times and now won't start at all. I have fuel in the carb, i have spark on the plugs but still no start. I am topping up the batteries but any other suggestions? Nothing else has changed. Thoughts?

[Rdinatal]

These are different beasts for sure.

I find that the choke is needed very sparingly. The hand throttle is needed to keep it about 1000rpm for a bit.
Too much use of the choke can easily flood.

I'm still getting used to everything.

[just me]

When it's warm, don't pull the choke out all the way. Close it quickly while cranking. All you want to do is pull a short slug of fuel to make up for no accelerator pump.
If you flood it, choke in, throttle to the floor and crank. It will clear and start with a pleasant cloud of black smoke and sound like a cammed small block

[BugsandJeeps]

ok. Bit more info to hopefully help me out here. I have tried every combo of throttle/choke and still no start without a prime after sitting for a couple of hours. So if i add a dribble of gas in the main vent tube it fires off no problem. I have fuel to the pump, fuel in the bowl of the carb but after sitting it appears no fuel makes it past the carb. Thoughts?

[just me]

Does the choke actually work?
Choking it will draw the fuel from the bowl quickly.
If the choke plate isn't closing you will not get the pressure differential you need to get the fuel to flow .

And I just keyed off of you saying that you dribble fuel into the main vent tube.
You are NOT running an ETW-1 carb. That carb is sealed and there is no easy way to even get at the main vent tube. (Without disassembly of the air horn elbow, piping and air cleaner.)

[BugsandJeeps]

The choke appears to work just fine. Moves back and forth when pulled. Here is a pic of the carb and the screw i had to remove to add fuel. According to the manual i have this is called the "main vent tube".

Image (1).jpg (243.73 KiB) Viewed 156 times

[just me]

So, what you don't know is if the choke plate is in the intake tract and if it moves.
But you are "lucky" that someone drilled an extra hole in the airhorn of the carb and plugged it with a bolt. You might be able to remove that and see if the choke is closing.
The actual bowl vent is along the side of the airhorn and in front of the choke plate.
So when the engine is turning and the choke is closed there is a low pressure behind the choke plate and a higher pressure at the vent. The higher pressure flows through the vent into the bowl and presses on the fuel in the bowl. This forces the fuel through the main jet and into the throat of the carb and into the intake manifold. (Low pressure area)
If choking doesn't cause a lot of fuel to enter the intake, some passage in the carb is blocked.

[BugsandJeeps]

pulled that non original bolt and used a probe to feel for the choke plate. It is there and it does move when the choke cable is pulled. I can't tell if it is fully closed and there is some slack in the cable if i pull the choke knob vs manually open the choke at the carb body. I assume the choke plate need to fully close for the vacuum effect you refer to? I really appreciate the help.