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MG Midget and Sprite Technical - Fuel Flow Rate Problem

I'm currently having a spot of trouble getting sufficient flow to get the best out of my Weber carburettor.

I'm getting 2l/min at the rear-mounted pump but only 1.2l/min at the pressure regulator/filter input and 1l/min at the output to the carburettor.

I was initially getting at least 3 PSI at the regulator but now it's reluctant to go above 2.5PSI.

I'm wondering whether there's a restriction in the pipe from the pump. Before the latest flow test I had blown the pipe between the regulator and the pump through with the air line but it still seems to have some resistance when I blow down it.

Before I started doing flow tests I took the tank out and cleaned it yesterday and got a good deal of grot out of the Facet pump's filter before checking that is giving the right flow of 2l/min / 26 gal/hr. All the hoses are quite recent.

Is there anything else I could try or should I just replace the original 1/4" metal pipe with 5/16" pipe?

Colin

C Mee

Colin, you mention 2l / min is 26 gals per hour. And also you are currently getting 1l per minute delivery, so presumably 13 gals per hour. Surely this is enough fuel? If you drove at 60mph there is no way you could use 13 gallons in that time!
Have I misunderstood something here?
GuyW

Guy

The Weber is still a bit of a mystery to me but I'm learning.

I took it for its/my first ever session on a rolling road on Monday. We never got to do a full power run as it was having fuelling issues which took a long time to track down. When accelerating hard it must have been gobbling fuel at a rate or more than 1l/min / 13 gal/hr because at high revs/full throttle there was just not enough fuel getting through.

For most driving, even hill climbs, it has been fine for me - at least when compared to the 1 1/4" SUs that I've been used to. I have always been progressive with the throttle but I gather I can be much more aggressive with this carburettor thanks to the pump jets - which, apparently, weren't giving everything they're supposed to.

C

C Mee

Well if you drove at 100mph for an hour and used 13 gals that is roughly 8 mpg. But add in wind resistance and it would be considerably less, though I suppose that does mean exceeding your pump capacity - but still within the bounds of possibility.

I know nothing of race engines or short frantic hill climb dashed, as you can tell!
What does Daniel say about it in his book?
GuyW

Hi Colin
Did you get afr numbers at different rpms wide open throttle? At what point did the fuel run out.
Sometimes I forget to switch someone's fuel pump on and it takes a while before the engine splutters with a full carb.
Peter Burgess Tuning

What is the pump stated output? The Facet Siver top I used was fine fuelling split webers on a full race engine
Bob Beaumont

Thanks for all your comments and queries.

Peter, no usable data as all the test runs were aborted. The fuel would run out mid-acceleration. I've been thinking of fitting a pump switch but it's still on the bench - the pump is on the ignition.

Bob, it's a Facet 'Fast Road' cube rated at 2l/min. I thought I might have found the problem when I flushed the pumps filter and caught a load of rust particles but it made no differnce at the carb end of things.

The less than 2l/min issue remains but I'm sitting here a bit red faced having just solved the main issue (I think!).

In all the switching of jets and floats and other possible problem items, nobody gave a thought to the brand new viton float valve. I had installed it to cure an annoying drip before the test.

After a very rough 5 mile test run this evening, during which it cut out several times, I have just switched valves and put the old, leaky brass float needle valve back in - MAGIC.

The Sprite has never felt so lively! Fiercer acceleration than I've ever had before - and not a single cough, fluff or stutter.

I guess we all over-think these problems some time.

I'll be very interested in the RR guy's response when I see him on Friday!

C
C Mee

Colin
Testing fuel flow on later fuel injected cars the normal flow rate ,even for 6cyl and v8 engines is 300ml per 10 seconds (at reg. pressure) so roughly 1.8ltr/min and that's enough to feed a 5ltr v8 at full hook.
1 litre/min should be plenty for you.
eg. I could get 6 laps out of a 5litre tank out of a full house B series engine--7mins on a circuit where you're on the throttle for around 80% of the lap.
I've been caught out with exactly your issue with a 50mm Weber, half way down the straight and brrrr,out of fuel, and it turned out to be that the weber had a very fine fuel strainer inside the banjo fitting on the carb. for the fuel supply line-It was a little nylon cylinder shape, it looked ok but if you held it up to the light there wasn't much getting through. Had a run without the filter and it was fine-gave it a clean with carby cleaner and couldn't believe how much crud came out of the tiny filter that looked ok really--
Might be worth having a look-see if you have one in there blocking the flow.
Also, but probably not the issue ,but you never know, you can get different sizes in float valves (needle/--seat) If you have a flow issue going to a larger capacity valve could be a help.
William Revit

Thanks Willy

It seems as though the flow I'm getting now is good enough. After I'd switched back to the old needle valve I gave the Sprite the same 5 mile test run without any hint of fuel starvation.

I had found the filter in the banjo. It looked OK but I'd swapped it for a new one anyway when I changed the float valve.

Thanks for your advice.

Colin
C Mee

Did you set the float height with the new float Colin?
Peter Burgess Tuning

Not yet Peter.

I just did a quick switch-over to the old valve last night. The RR guys put in a brass float as they thought the plastic one might be sticking.

You'll probably want to hit me when I say 'the float level looked about right' - but I will take a much closer look and check the level more carefully over the weekend.

C
C Mee

Colin
My apologies, I hadn't read through your post about switching the float valve before I posted about different valves etc, don't know how I missed that but there you go, you've fixed it anyway.
It'd be interesting to compare the orifice size of the two valves you have.
William Revit

This thread was discussed between 14/08/2024 and 16/08/2024

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