Vacuum fuel pump vs Gravity feed

[bullybike]

Does anybody have opinions on any performance difference between a healthy 50cc running a vacpump from manifold vs a straight-up gravity feed tank with NO petcock or other engine vacuum draws ??

Considering electric fuel pump or tank relocation

[larryhobman]

I run a gravity fed system on my 50mm set up A9 cam and a 24mm carb off of a 150cc. Runs great,

[FrankenMech]

Use gravity feed if you can with a regular manual petcock. Vacuum fuel pumps are a PITA.

[bullybike]

I had a problem with seat tank where it would bleed down into carb and fuel spill eeerywhere. I ride my bike for transport and forget alot.
Using a plastic Patriot fuel pump made for lawnmower and NO Probs whatsoever.
Its def oversize compared to gy6 stuff but starts right up !
Just wondering how much air it steals hmmm
need MO Powah !

[bullybike]

i guess air is air tho

[FrankenMech]

Vacuum pumps do not consume air to operate. They operate off the vacuum (Pressure) pulses in the intake to operate a spring loaded diaphragm pump. They do not operate like a petcock and will let fuel drain through them in a gravity feed system. Use a manual or vacuum operated petcock with a gravity feed system and hope the petcock and float needle do not leak at the same time.

[cypher1982]

Aug 3, 2020 4:57:56 GMT -5 FrankenMech said:

Vacuum pumps do not consume air to operate. They operate off the vacuum (Pressure) pulses in the intake to operate a spring loaded diaphragm pump. They do not operate like a petcock and will let fuel drain through them in a gravity feed system. Use a manual or vacuum operated petcock with a gravity feed system and hope the petcock and float needle do not leak at the same time.

Do you happen to know how much vacuum is needed to open the peacock? At what point might one need to be concerned about vacuum loss at wot and it partially shutting down?

[GrumpyUnk]

I have used a MityVac equipped with a gauge to open the vacuum petcock on a Super 9. It seems to open with very little vacuum, around 1 or 2 on the scale on the meter, maybe 3. Don't know what the numbers are.
A vacuum fuel pump has check valves that SHOULD stop fuel from flowing from the tank to the float bowl. They are spring loaded to the CLOSED position, and it would be unusual for the gravity developed fuel line pressure to unseat the valves. Both IN and OUT valves would have to be unseated for fuel to flow, and I just don't see that happening unless one has a fuel tank that was set in the sun and has no vent of its own. That could develop some(enough?) pressure to unseat the check valves.
tom

[FrankenMech]

My vacuum fuel pumps would leak back into the tank and out of the fuel line to the carb with just a few inches of fuel pressure in the line to the carb above it. It does not seem to take much. I would not expect the pump check valves to hold against any pressure at all. I suspect that they leak quite easily. Maybe the seats are not really very good and they are just check valves against active fuel movement during the pumping action.

[GrumpyUnk]

Frank, is the pump the one on the Dongfang? I have a large 'victoria' that has a pump similar to the Donfang, and it seems to drool the fuel back into the tank within a few days of use. I figure the valves aren't that great. If they were new, perhaps they would not allow such seepage. Given this thing is about 7 years old, I expect that the petals are somewhat hardened and don't seal well at all.
A pump that is in reasonable shape should impede flow, at least as much as the two poppets springs hold them against the seats. Maybe these pumps are merde from their origin. I have a mower with a pump and it has worked fine for 5 years at least now, but it is 'lever' type. I don't know if these little machines develop enough pulse to operate a pulse pump used on riding mower engines. Those are pretty cheap. One other option would be to use the crankcase pulse of pressure as the piston moves back and forth. It may be stronger than the intake manifold pulse.
tom

[FrankenMech]

One pump was the OEM Dongfang pump, the other was a brand name pump, I don't remember the specifics. Both pumps leaked back into the tank in a few hours. I ended up installing an electric pump in parallel with the vacuum pump to prime the system when I wanted to ride and also as an 'emergency' pump. The electric pump draws enough power to run the battery down though, but it did get me home. I later found that the vacuum pump did not like a 'T' fitting in the vacuum line. The restriction caused the vacuum pump to quit pumping well at high RPM's.

Gasoline lawn mower engines are usually over 100cc. Cars used to use a lever pump operated off the camshaft. These vac pumps are extremely cheap and I would not trust them to hold against a gravity fed fuel system. An engine full of fuel can ruin your day.

[cypher1982]

Aug 4, 2020 13:35:24 GMT -5 FrankenMech said:

One pump was the OEM Dongfang pump, the other was a brand name pump, I don't remember the specifics. Both pumps leaked back into the tank in a few hours. I ended up installing an electric pump in parallel with the vacuum pump to prime the system when I wanted to ride and also as an 'emergency' pump. The electric pump draws enough power to run the battery down though, but it did get me home. I later found that the vacuum pump did not like a 'T' fitting in the vacuum line. The restriction caused the vacuum pump to quit pumping well at high RPM's.

Gasoline lawn mower engines are usually over 100cc. Cars used to use a lever pump operated off the camshaft. These vac pumps are extremely cheap and I would not trust them to hold against a gravity fed fuel system. An engine full of fuel can ruin your day.

I suppose that's part of my concern. They could potentially be remedied by a check valve in line but then how do you shut it off once done riding? Or would you recommend going to the manual style and bypassing the vacuum?