Inlet needle on OMC carbs

[ADAM GIBB]

When I have done early RD/big twin carbs, they usually end up so that you can squeeze a primer bulb until the bowl is full and then they seal so tight that it won’t force any fuel through, or leak out through the throat.
I have been through a carb on a 1967 33hp twice, and everything looks good, but when I squeeze the bulb, it will run fuel out of the carb thraot with very little resistance. It has the newer neoprene tipped needle, seat looks good. The float is cork and in great shape. Floats in gas for at least 24 hrs. Float is parallel to carb body when upside down. What could I be missing? It makes a mess in a hurry if you prime it too much.

Adam

[chris-p]

Adam,

Few things I have found on these.

-Sometimes certain gaskets are a bit too wide, and the float catches them when raising, not allowing the needle to seat
-Float too heavy, and wont allow needle to seat
-Minor imperfection in seat, not allowing to shut off fuel (did you try a replacement?)
-When I set my floats, I set it so that the unhinged end is slightly higher than the hinged end, when the carb is upside down and you are installing. This ensures that the needle fully seats, and the carb does not flood.

[frankr]

It would be hard to obstruct the bowl vent on that carb unless somebody did it deliberately. Did they? If vent is plugged, fuel will flow right through and out the main nozzle.

[frankr]

Read this—
viewtopic.php?f=2&t=7460&p=38186&hilit=float#p38186

[chris-p]

I must have missed that thread but I agree, if the drop of the float is set too low, it can/will catch! I personally don’t set a ton of drop on them myself, that is when I find the tab catches, or the float catches the bowl gasket. Just my 2 centos.

[beerman57]

I just went through too much float drop on the Gale 25hp. Never really paid that much attention to float drop on OMC motors, but I will from now on.

[1946zephyr]

The float is dropping too much. On these, you will find a tab on the rear portion. Bend this tab toward the front of the carb, so it is only allowed to drop 1/4"

[ADAM GIBB]

Well I’m stumped. I had at it again tonight. Tried adjusting the drop limit, swapping floats, swapping needles, all without improvement. If I leave the bowl off and apply light pressure on the float, it cuts the fuel flow well, even against the primer bulb pressure.

Frank, where does the bowl vent to? I am not sure that I fully understand what you mean about the bowl vent. It seems to me that the fuel is getting forced into the high speed Venturi through the high speed needle seat/valve. If I seat the HS needle, then the bowl will build pressure and force the float up hard, stopping the fuel flow.

Imp have done 20+ of these carbs, and none have given me this much trouble.

[frankr]

quote Adam1961:

Well I’m stumped. I had at it again tonight. Tried adjusting the drop limit, swapping floats, swapping needles, all without improvement. If I leave the bowl off and apply light pressure on the float, it cuts the fuel flow well, even against the primer bulb pressure.

Frank, where does the bowl vent to? I am not sure that I fully understand what you mean about the bowl vent. It seems to me that the fuel is getting forced into the high speed Venturi through the high speed needle seat/valve. If I seat the HS needle, then the bowl will build pressure and force the float up hard, stopping the fuel flow.

Imp have done 20+ of these carbs, and none have given me this much trouble.

Man, if I were a gambling man, I would bet that vent is plugged. Maybe a dirt dauber got in there. Look at where the low speed needle screws into the carb. See that big hole to the right and slightly below there? That is the bowl vent. The hole goes way in then down into the float chamber. If it is plugged up there is a bubble of air in the float chamber which cannot escape, which prevents fuel from flowing in and raising the float. Instead, it flows into the bottom of the bowl, through the high speed needle, and up through the main nozzle into the venturi, with nothing to even slow it down. It’s rare to see this on the Big Twin series of carburetors, but my goodness how many times have I seen it on Tillotson MD carbs where somebody has jammed a toothpick in the vent and broken it off to keep gas from squirting out. Took me all day to find it the first time.

[fleetwin]

I was going to mention a casting flaw/crack under the seat, but you say that no fuel sneaks by when you have the bowl off and prime with the float closed.

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