mark 20 starting issue

[ronboonville]

I am trying to start using gravity feed tank. Got spark and have cleaned the carb twice. It is not getting fuel to the cylinders. What have I’ve overlooked. I hooked direct to carb.

[RICHARD A. WHITE]

stuck float? I shake mine Merc carbs gently before installing…. Remove gas cap, remove gas and blow thru tank and air should go out the fuel line… if so it is in your carb, if not the screen in the tank is plugged

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[billw]

Gravity feed Mercs usually fed fuel into the bottom of the carb. The Mark 20 goes in the top. This has probably created a big loop or dip in the fuel line. I don’t know what the phenomenon is called but sometimes liquids will have a very hard time going down hill, then up hill, to the end of the run, even if the end of the run is lower than the gas tank.

Long live American manufacturing!

[jw-in-dixie]

Yrs ago I heard that Mark 20/25 would not run on grav tank but that ain’t so. I have a Mark 20 that runs on a tank mounted on the rear of the block.

[ronboonville]

Got it. I did not soak the bowl top in my carb cleaner, it has two vent holes that were partially clogged. Also I recently bought two cheap primer bulbs off ebay. They don’t work very well. I blew thru the fuel line and got gas in the carb. It starts now but only runs for a few seconds. Will readjust hi and lo needles according to manual and try again in the morning.

[ronboonville]

I have cleaned the carb again can only get it to run for a few seconds. Could my problem be behind the Welch plug?

[foot_doctor]

If I had this issue, I would install an AJ32A carb. It’s the right size for your 20 cu. in. engine, and designed for gravity feed. Gravity carbs operate with minimum head pressure, whereas fuelpump carbs require about 3 to 3-1/2 lbs of pressure.

You could try filling the bowl with fuel by simply pouring it into the bowl without the float installed. It will require about 1/2 fill to operate properly. See how long it runs in this mode. hi speed out ABOUT 1-1/4 turns. low speed out ABOUT 3/4 turn. If you could mount your tank a couple of feet above your powerhead while it’s in the test tank, the engine would probably respond better. just some idle thoughts. R.T.

[ronboonville]

Yes, I have that rube Goldberg thing going on. A sea king tank 2 ft above on a ladder gravity feed. I have also added an aerator pump with a cork and both ways only runs for a couple of seconds. That’s with squirting fuel into carb and full choke, adjusting needles in and out to no avail. Maybe carb is just junk.

[mercuryspecialist]

Hi Ron,
I have not followed your repair thread, but have you replaced the crankshaft oil seals? If not, then this could easily be contributory There originally is a wire mesh screen between the carburetor bowl cover and the elbow fitting that threads in, and it may need to be cleaned as well (or removed).

Regards,
Joe

[Anonymous]

What is the compression of each cyl. ? The reason I ask is, that low comp. will not pull fuel from the carb. or push the fuel to the plug. Also leaking crank seals (pressure or vacuum) will also stop fuel from reaching the plugs and on the same note a leaking worn center main will let the fuel charge be forced back and forth between the crankcases for each cyl. and not to the plugs.

[Author]

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