HD/TD Float Height

[steveh]

Guys, I know this was covered shortly before the website crashed, but if you don’t mind revisiting it, how high should the level of gas be in the float chamber of the HD/TD motors?

My HD-25 seems to have a flooding issue. The fuel does not come out of the top of the float chamber. However, if I shut the motor off, gas will drip from the low-speed needle assembly. To restart the motor, I have to close the low-speed needle, pull 3 times to start the motor again, then open the low speed needle.

Any thoughts?

Thanks,

Steve.

[aquasonic]

Steve, glad to see you’re back on the forum! Although I can’t tell you the design height of your HD-25 float, I do have a few thoughts for what it’s worth.

As you know, the length of the float assembly is dictated by the groove in the float needle, the clip, and the brass screw that secures the clip to the top of the float. So if you are experiencing flooding, then two things come to mind. One is that the float needle is not achieving a good seal with the float needle seat thereby allowing excess fuel to seep into the bowl. Maybe the needle and seat just need cleaning. The other is that the float is just not buoyant enough to lift the needle up to where it can make a seal with the seat.

It seems to me that Lloyd had posted something about running the motor with the top of the carb off and there is a port inside the float chamber that the float height can be referenced to, but I don’t remember the specifics.

[legendre]

Thing is.. The bore in the low-speed assy. is a full-inch or more above the top of the main carb body. This would place the actual fuel level almost two full inches below the low-speed bore. It’s from the bore that the fuel drips, correct?

Unless I’m way off, to have fuel leaking from the low-speed bore, you’d need more than just a simple flooding condition – as the fuel would find other exits (like the top of the bowl cover) before the level could ever rise to that of the low-speed unit.

I’d say a very thorough disassembly and inspection of that carb is in order. Because ultimately, the float level in the TD/HD carb isn’t designed to be adjustable – all you can do is ‘move’ the groove for the clip to an area the factory never intended it to be. For starters, fill the carb with the top cap removed, and see if it actually overflows or not.

BTW – what are you using for a float in there? Hand-made, store-bought, ancient original..?

[aquasonic]

quote legendre:

Thing is.. The bore in the low-speed assy. is a full-inch or more above the top of the main carb body. This would place the actual fuel level almost two full inches below the low-speed bore. It’s from the bore that the fuel drips, correct?

Unless I’m way off, to have fuel leaking from the low-speed bore, you’d need more than just a simple flooding condition – as the fuel would find other exits (like the top of the bowl cover) before the level could ever rise to that of the low-speed unit.

I’d say a very thorough disassembly and inspection of that carb is in order. Because ultimately, the float level in the TD/HD carb isn’t designed to be adjustable – all you can do is ‘move’ the groove for the clip to an area the factory never intended it to be. For starters, fill the carb with the top cap removed, and see if it actually overflows or not.

BTW – what are you using for a float in there? Hand-made, store-bought, ancient original..?

I agree with that. Maybe focus on the low speed needle, seat, and packing. Could it just be residual fuel that is leaking past the needle and packing?

[legendre]

quote aquasonic:

I agree with that. Maybe focus on the low speed needle, seat, and packing. Could it just be residual fuel that is leaking past the needle and packing?

I’m ready and willing to learn, but I can’t come up with any scenario where the carb bowl is filled (and stopped at) the correct fuel level, yet liquid fuel continues to issue from the low-speed bore on a stopped engine.

The way I see the system, fuel only travels up and into the low-speed unit when the engine is running and a vacuum is produced in the crankcase. Otherwise, the fuel level (below the fuel tank) can never exceed the level set by the float valve in the carb. In other words, the feed pipe to the low-speed unit will be about half-full of fuel in a static condition. Only when the motor is producing vacuum, will that fuel level rise high enough to enter the bore.

Well OK, or when the primer pump is depressed – but that’s a different matter.

ETA: Wait a moment.. is there a the possibility of siphon being established? How would the float system respond to that? Hmmmmm….

[legendre]

For the sake of discussion, what is it that brings fuel up to the level of the low-speed device, during normal operation? Seems it would have to be drawn up by engine vacuum, as the float system in the main carb will determine the maximum height for (and volume of) any fuel below the tank when the system is equalized when all points are at equal (atmospheric) pressure.

Otherwise, I don’t see any way for fuel to rise to the level of the low-speed bore if the float valve assembly governs the fuel level at the correct height. If the float valve stuck open, fuel would flood from the top of the carb before it reached the low-speed device.

Even the fuel and needle valve fittings on the low-speed are still above the level of fuel in the bowl, so I don’t see how poor seals in that area could result in a leak. Though these areas are the most probable, of what’s been discussed so far – as they are the closest to the fuel level.

[farmboy71]

Just a thought. Could the fuel tank vent not be working causing pressure to build in the tank and forcing gas through the system?

[jeff-register]

If by chance the needle & seat is leaking use valve lapping & finishing compound to reseat the valve.

[garry-in-michigan]

You are all forgetting about the drain line from the top main bearing. Excessive gas/oil from their would indicate excessive bearing clearance. Plug up that line and the magneto gets flooded.

[legendre]

Hi Garry,

quote Garry in Tampa:

You are all forgetting about the drain line from the top main bearing. Excessive gas/oil from their would indicate excessive bearing clearance. Plug up that line and the magneto gets flooded.

Can you cite anything in the literature which describes the function of that ‘drain’? I’ve looked at it while I’ve had motors apart, and I can’t see how mixture would ever accumulate outside of the case, above the main bearing and beneath the magneto.

I’ve always assumed it was part of the lubrication system – vacuum from the low-speed bore would pull mixture upward, into the helical grooves in the main bearing, to provide lubrication.

Otherwise, how would the upper main be lubricated? Gravity takes care of the lower, but the upper would need a little more help in that department, I’d think..? Or is the primary compression pressure enough to push mixture into that area?

[Author]

Posts