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

[Warkton Tornado No.1]

Hi Adam.

I acquired a couple of race Dell'Orto 38/39 mm VHSC's with smaller float valves as a restriction in them but I don't know what size they are without checking. 150 rings a bell, though.

I don't think karts run a return loop, but, bearing in mind the fuel throughput on a race kart engine, you might need more restriction than a 150.

If your return loop comprised a double banjo on the carb with the return to the top of the tank, my thoughts are that you'd still be needing a restriction such as the 150 or less.

Have you seen the principle of Bell's example in his book?

Mark

[Adam_Winstone]

Hi Mark,

I've just returned from my maiden voyage and it seems fine with the 150 in it still, which I'd fitted prior to adding the return loop. So far the results are pretty encouraging, with the bottom end now seeming to be clean and without the flooding that I was suffering without the return loop.

Now all I need to do is safely ride to my dyno session tomorrow and I'll be better informed of what the impact is. Fingers crossed that it gets me there without issue and I can report back later tomorrow

Regards,

Adam

[coaster]

Can I ask what the thinking is behind using a fuel pump Adam? The only experience I have with return loops is on my Marcos which has a normal 1/4" or so inlet but the return is probably no more than 1mm

[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.

Cheers all

Adam



PS - Knowledge... this is the reason ...

It is not massively tuned but does run crankcase induction, which sees the carb sit much higher than on the normal reed manifold / piston port manifolds so the lack of header pressure at anything other than full tank does lead to fuel starvation.

Fitting a fuel pump has already totally changed the dynamic, for the better, at anything above tickover or very low throttle positions, which is what I'm trying to clean up now.

Fuel starvation is something that I thought I could work out of the mix with fast flow taps, drilled carb bodies, big float valves, etc. but it still caused fuel starvation and cost me replates

[Warkton Tornado No.1]

Good for you that you can think 'outside of the box' & seem to be getting sorted.

Fitting a pump makes sense to me in any case, especially if it stops flooding if you leave the tap on, dependent upon the type of pump.

I chased my tail years ago with blow-ups whilst racing @ Cadwell Park. The cure was a bigger float bowl, which I still can't get my head around entirely.

However, although light floats are the norm for two-strokes, that's another thing I've experimented with in the last few days on a road bike...

[Adam_Winstone]

^... cheers.

Yep, there are times when you need to diagnose and resolves issues yourself. It is so easy to assume that what works for others will work for you, however, as well as listening to good advice, you need to be prepared to listen to what your engine is telling you too.

Adam

[coaster]

............PS - Knowledge... this is the reason ...

Thanks for the explanation Adam but it's coaster

Colin

[Adam_Winstone]

Hi Colin (Coaster),

Not sure how I managed to read your response and a post by Martin (Knowledge) and cross my reply

Sorry about that... read one thing and typed another!

Thanks though

Adam