Finding the source of a gas leak

[Dave Neptune]

Eeek

Is the scavange tube hooked up?

Dave Neptune

[Minnesota_Slick]

Is the scavange tube hooked up?

Dave Neptune

Is that the hard, copper line that screws into the block close to the carb? If so, yes, it is connected and dry.

I forget what the purpose of that tube is now, but I remember reading about it in the Moyer manual. I will double check when I'm out at the boat today.

I'm old enough to remember cars that used carburetors, but I've never actually seen the inside of one. My A4's carb will be the guinea pig.

[Dave Neptune]

Scavange tube

Slick, make sure the copper line ~~the scavange tube~~ is clear!! It's purpose is to "suck" (via vacuum) any dribbling or puddling fuel in the throat of the carb back into the manifold, a very important SAFETY feature.

Dave Neptune

[Minnesota_Slick]

Slick, make sure the copper line ~~the scavange tube~~ is clear!! It's purpose is to "suck" (via vacuum) any dribbling or puddling fuel in the throat of the carb back into the manifold, a very important SAFETY feature.

Dave Neptune

Will do, Dave. I disconnected the end of it that connects to the bottom of the carb, and will check it for flow next time I'm on the boat. I have a can of compressed air that I'll use to clean it out(once the fuel is out, of course.).

This is probably not the only culprit, but I found this when I removed the flame arrestor today.


I was able to source a new one at a local marine parts store. I also replaced all fuel lines and the glass in-line filter with a metal one.

I brought the carb home with me, and will open that up tomorrow. The old-timer who sold me the gasket told me that the reason the fuel leaked is because either the float valve stuck, or the choke doesn't close all the way.

Since the mechanic replaced the carb kit last summer, do you think I can get away with re-using the gaskets? Or should I purchase new ones just in case?

[Al Schober]

Hey Slick,
If you have fuel overflowing the carb, it can be ONLY one of two things - a) bad float valve, or b) crack in the bowl.
Personally, not sure about that suction line and it's function. Let's see - no suction line and float valve doesn't close - engine runs rich and finally dies. But with the suction line, engine runs rich then finally dies. Am I missing something? Hey, let me know - I would like to know this stuff! How does that suction line make the engine safer???

[Minnesota_Slick]

Hey Slick,
If you have fuel overflowing the carb, it can be ONLY one of two things - a) bad float valve, or b) crack in the bowl.
Personally, not sure about that suction line and it's function. Let's see - no suction line and float valve doesn't close - engine runs rich and finally dies. But with the suction line, engine runs rich then finally dies. Am I missing something? Hey, let me know - I would like to know this stuff! How does that suction line make the engine safer???

Suction line? I'm afraid I don't know of that. Are you talking about the scavenge line?

Let me re-iterate - The engine runs great, and no starting issues. Only leaking when stopped.

I forgot to mention that I also replaced the fuel pump filter and cleaned the magnet on the bottom of it. There was a lot of crud attached to the magnet.

This is a pic of the cleaned magnet and new filter & gasket.

[ndutton]

What you're calling the suction line is more commonly referred to as a scavenge tube. It's a USCG requirement for updraft carbs. Here's the language of the requirement:

Code of Federal Regulations > Title 33 - Navigation and Navigable Waters > CHAPTER I--COAST GUARD, DEPARTMENT OF HOMELAND SECURITY > PART 183--BOATS AND ASSOCIATED EQUIPMENT > Subpart J—Fuel Systems

§ 183.526 Carburetors.

(c) Each updraft and horizontal draft carburetor must have a device that:

(1) Collects and holds fuel that flows out of the carburetor venturi section toward the air intake;

(2) Prevents collected fuel from being carried out of the carburetor assembly by the shock wave of a backfire or by reverse air flow; and

(3) Returns collected fuel to the engine induction system after the engine starts,

[CGD 74–209, 42 FR 5950, Jan. 31, 1977, as amended by CGD 77–98, 42 FR 36253, July 14, 1977]

By design residual fuel upon shutdown pools in the air intake horn. At the next startup the manifold vacuum draws the pooled fuel into the intake where it is burned in the cylinders. Note that downdraft carbs have no such requirement. Their residual fuel naturally pools in the intake manifold.

[Dave Neptune]

Another tack

Hey Slick, how old is the exhaust? If the exhaust is plugging up it can via the back pressure slow the intake (volume & velocity) and fuel will precipitate (gather) in the intake by sticking to the runners and such. Once shut down it leaks out of an UPDRAFT manifold and into the carb. It will be dirty stuff as there will be "exhaust" (burned and unburned gases) forced back into the intake too.
A possibility.

Dave Neptune

[Minnesota_Slick]

Hey Slick, how old is the exhaust?

Dave Neptune

Hello Dave,

The PO rebuilt the exhaust, along with most everything else, within the past two or three years. I would estimate that it has less than 100 sailing hours on it.

Is there a way to test the exhaust? Btw, I get a huge amount of water out the exhaust in the rear of the boat, so I don't seem to have any obstructions there. Unless I'm confusing water exhaust with internal combustion exhaust.

Surgery on the carb is scheduled for later this morning. I will post pics while the patient is under general anesthesia.

[Minnesota_Slick]

Exactly how does one remove the Venturi tube?

The Moyer Manual simply says to remove it, without specifying how. It looks like it can be popped out with a flat head screwdriver, but I'd rather not experiment.

Btw, the float valve needle seemed to open and close just fine, and the float valve valley looks pristine, with no crud or obvious scratches.

The float looks good, too. No obvious cracks or holes, nor any "sloshing fuel" feel when shook near my ear.

I don't think I'm going to risk removing the idle jet, as I'm afraid of breaking it. The small screwdriver I have is a little thin and I think it might strip the groove.

[sastanley]

MS, I agree with your cautions. I've mangled jets when the screwdriver was not the perfect size. As long as you can blow it clean with carb cleaner (and air if you have it) and it comes out somewhere else (where safety glasses!) I find that is usually sufficient.

Also usually you need a small piece of wire to get the idle ports clean in the throat..they are on either side of the main butterfly and very tiny.

[Dave Neptune]

Bit of twist

Twist the venturi with a rag around it with a big pair of pliers. If you nick it just file or sand smooth. The fit of the venturi is LOOSE as the idle air for the idle screw feeds from the area between it and the housing and is the vent for the fuel bowl. All internal for the marine updraft!!

Dave Neptune

[Dave Neptune]

Part II

Slick if you have the carb apart BE SURE to remove the fitting that feeds the fuel into the carb AND BLOW ANY ACCUMILATED KRAP out of the cavity before reassembling. The cavity is rather large and has areas that can trap chunks O"KRAP.

Dave Neptune

[Minnesota_Slick]

Twist the venturi with a rag around it with a big pair of pliers. If you nick it just file or sand smooth. The fit of the venturi is LOOSE as the idle air for the idle screw feeds from the area between it and the housing and is the vent for the fuel bowl. All internal for the marine updraft!!

Dave Neptune

I was out in the garage experimenting when you posted, Dave, so I ended up gently prying it off with a small screwdriver. It was not loose, it was on there fairly secure. I will sand around it and see if I can make it loose.

[Dave Neptune]

Loose but not to loose

Slick, as long as the venture fits in without binding it will make assembly easier. No real need to make loose as the idle air passes through the gap in the venturi.