6-71 Fuel Return

[Freestyle]

Anyone know how much fuel a 6-71N returns to the tank?

Does it vary with rpm's?

Thanks

Bruce
Freestyle
1976 43 DC
Tampa

 

[SKYCHENEY]

I couldn't give you a number, but when we had our 43DC and tried to balance out the tanks, it only took a few minutes before we had too much in the other side. I guess my short answer is "a lot".

 

[MikeP]

I seem to recall reading somewhere - though I can't recall where - that the 2 stroke DDs return more than 75% of the fuel delivered to the pump back to the tank. I can't be totally positive about this but I believe it's right since it stuck in my head.

 

[REBrueckner]

My recollection is the same as Mike's.
If you really need to know, disconnect a return line and feed it to a five gallon pail for a fixed period of time at the RPM of interest. If the fuel lift pump is gear driven, then it pumps more fuel at higher RPM....

 

[Avenger]

A standard capacity fuel pump on a Detroit will return a minimum of 30 GPH at 1200 RPM per the 71-Series manual. Yes, it will increase as RPM increases.

My experience using a DD gear pump as a fuel transfer pump is that if unrestricted they will pump almost 60 GPH, so your return should fall somewhere between the two. Hope this helps.

 

[HATT 46]

How did you rig the DD fuel pump as a fuel transfer pump
THX MP

 

[Avenger]

MP,

I machined a coupling and attached it to an electric motor. Plumbed a Dahl 200 filter to it and Voila! instant fuel pump/polisher. It was all done with "junk" that I had lying around. Actual cost $0.00 if you don't count the price of filter cartridges.

No, it's not on an engine, if that's what you were thinking.

 

[HATT 46]

MP,

I machined a coupling and attached it to an electric motor. Plumbed a Dahl 200 filter to it and Voila! instant fuel pump/polisher. It was all done with "junk" that I had lying around. Actual cost $0.00 if you don't count the price of filter cartridges.

No, it's not on an engine, if that's what you were thinking.

GREAT idea. I have some old fuel pumps and I'll try your set up. What size motor? 12V 24 or 110V
Thank you
MP

 

[Finalee]

The 6v92s on my 43dc pump a lot too! Thought I read on this forum that they return 3x the amount used.

 

[REBrueckner]

This thread is a great example of why partially clogged fuel filters can damage injectors...by restricting the return fuel flow...even though the engine runs strong at WOT....the "extra" fuel returned to the tank cools and lubricates injectors...Hence, vacuum gauges are a good monitoring tool....

 

[Avenger]

GREAT idea. I have some old fuel pumps and I'll try your set up. What size motor? 12V 24 or 110V
Thank you
MP

Since you're so enthusiastic here's some "YachtsmanBill" type photos. The motor is 110V, 1/3HP and turns 3400 RPM. Again, what I had lying around. You may find a DC motor suits your purposes better.

The coupling was a piece of round stock that I drilled to match the width of the corners of the square drive of the gear pump. A set screw locks it on the shaft. I made a similar piece for the shaft of the motor and used a short piece of hose to couple them. This takes up any mis-alignment and will hopefully fail first if something locks up. I oriented the gear pump down so that any leaks will not end up in the electric motor. When I first assembled it I used a Dahl 100, but the flow was too much for effective filtration, so I had to scrounge up a 200. This works pretty well, but I still use a valve on the outlet to reduce the flow a bit.

I always mean to upgrade it, like making a real square-drive adapter and using a lovejoy coupling. Or bending tubing to connect the pump and the filter more neatly. Even a coat of paint would be an improvement. But there's so many other to-dos, as long as this works it will continue to be a "someday".

Hope this helps. If you want more info or pictures shoot me a PM or an e-mail and I'll do my best to answer any questions.

 

[HATT 46]

Thank you very much for the info. will give it a try.
MP

 

[yachtsmanbill]

Looks good Scrod!! I think the wood needs some TLC as well !! ws

 

[Brian Degulis]

This thread is a great example of why partially clogged fuel filters can damage injectors...by restricting the return fuel flow...even though the engine runs strong at WOT....the "extra" fuel returned to the tank cools and lubricates injectors...Hence, vacuum gauges are a good monitoring tool....

The problem with that is that if the secondary is partialy clogged your vacum reading will actually go down or look better even though less fuel is circulating. It's a realy good idea to put a pressure guage on the fuel manifold itself then you know the condition of both the vacum and pressure side. So you moniter the primary and secondary filters.

Brian