Fuel Pump + float valve size with return loop?

[Adam_Winstone]

Right guys (and gals),

So, I've been trying to set up a fuel pump to run with the 150 float valve and needle (VHSB) that some suggest is sufficient restriction to allow the carb to operate without the need for a pressure return loop to tank or tank side of pump. However, the pressure is still too great and I have not been able to eradicate it pushing too much through at tickover and low throttle openings. Once on the open road then the motor can handle any surplus and burn it off... or at least push it through.
Anyway, I've opted to change to the carb-side to tank-side recycling loop to reduce pressure but need to hear from those with experience...

Now that I've got a pressure return loop, do I stick with the 150 float valve or should I be increasing this to somewhere between the direct pump 150 and the normal gravity feed 300-350? What I don't want to find is that the return pressure release now leads to fuel starvation (!).

Your thoughts please?

Adam

[holty]

hi adam, is this a vacumn operated pump or an electrical pump, if it was electrical you could fit an adjustable fuel regulator and dial it in to suit your fuel delivery needs, although i know a few people have these on 350 conversions, and they have to start the engine as soon as the pump comes on or they flood the engine.

[cartmel808]

The system I have (this is on a Vespa) uses a small header tank which sits in the void in front of the fuel tank - this is fed & primed from the main tank via the vac pump.
The header tank is constantly primed by the pump & then gravity feeds directly to the carb from its elevated position & without the need for any type of flow restrictor - there is a pressure return loop from the header back to the main tank to regulate flow between the two...
No reason why you couldn't fit similar to a Lammie & the header could be placed in the area that the toolbox sits (if you don't mind loosing the toolbox)...



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[Adam_Winstone]

Hi all,

Back from my day out, riding there and back, with no changes needed to the carb, however, it was decided to fit a 250 float valve as it represents a happy medium between the typical 350 gravity fed valve and the 150 no-return pump valve. TBH it didn't seem to make much/any difference to the running but did give peace of mind for me to be able to hold it open as long as I wanted... no hiccups to report.

In response to previous comments (thank you):

- Mikuni vacuum pump, rather than electrical. Yes, mine would need to be started without choke as it instantly pushed through fuel.

- I've been trying to avoid a header tank but totally understand why it would help, as with fitting a remote float mechanism to early carbs. Thanks.


Cheers all

Adam

[B-Race Tuning]

Hi Adam, I've always used two Tee's, with a short piece of pipe between them with the nipple from the top of a spark plug inside the link pipe. that way it always feeds the carb by preference but has a pressure by-pass if float wants to shut. Never had a problem since doing this over 25 years ago using many various vac pumps. Keeps standard 350 float valve. Simon.

[Adam_Winstone]

Simon... guess what? That's exactly what I put in mine last night before going on today's journey!!!!

I'd noted the restrictor on the Dellorto illustration that is floating about on-line and read about fitting jets (or other drilled items) on various threads and it immediately occurred to me that a plug nipple would be about the right size, which it is. I didn't have time to start varying the hole size or doing any testing so pushed the nipple into the link pipe and ran with it.

Well, in my case it was more luck than judgement but thanks for the feedback, which it appears is spot on advice.

Ta

Adam