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MG MGA - Fuel visible between jet and jet bearing top

I'd welcome thoughts on a problem with the front carb on my 1500.

The car is fairly new to me and I am working my way around various issues due to lack of use.

The engine was running rich. The rear carb set-up looked fine. With the front carb body and piston removed and ignition on I could see fuel in the carb throat. The needle valve is viton tipped and looks ok. I checked the fork was set at 7/16" - it was. I increased the height of the fork to lower the fuel level in the jet but I am still getting fuel in the carb throat when the ignition is switched on. The fuel pump is new and doesn't tick continuously but I can see fuel between the jet and the top jet bearing. When I dry the bridge and throat of the carb and leave the ignition on I then see fuel tracking from the jet bearing into the throat of the carb.

Could it be that a cork seal in the jet assembly has failed?

Many thanks in advance.
Richard Atkinson

Hi Richard
I am sure those more experienced than me will respond shortly, but if I understand correctly what you are saying, I cant see how a failure in the jet seals would cause that.
Are you sure the needle in the float bowl is behaving as it should?
Graham V

Richard

Brings back memories. I went through much the same as you and could never find the cause. Eventually I started looking at other things, and here I note you have a new pump. I had an aftermarket pump that was supposedly set up for the MGA delivery pressure. I was able to check it out with a pressure gauge on my Super King pressure regulator that I had installed when I fitted the pump. Pressure was way too high at about 4 psi. I lowered it down to 1.5 psi and the problem disappeared in an instant.

What pump do you have and what is its delivery pressure?

Steve
Steve Gyles

Thanks for your comments.

Graham, I am not sure about the needle. I have ordered a new one from Burlen today. Hopefully that will do the trick.

Steve, the fuel pump is a new SU pump correct for the car from Burlen. I guess it may be faulty but it would affect the rear carb too.

Cheers

Richard Atkinson

BTW what is the function of the hole in the side of the jet. And the two cork glands?
Richard Atkinson

Richard
Take a look, a picture says a thousand words.


Graham V

Your right. When you see how the fuel gets into the assembly it becomes clear. Cheers
Richard Atkinson

When fuel level in the float chamber gets to correct height, the float valve shuts off incoming flow. Pressure in the float chamber and in the carb throat will be one atmosphere (engine not running). Fuel level in the jet will be same as fuel level in the float chamber. This is a gravity function.

If fuel level in the jet is too high, then it is also too high in the float chamber, and you need to find out why and fix it. Maybe float is out of adjustment. Maybe the valve is not shutting off. That could be a sunken float, or sticky valve, or too much fuel pressure.

I have seen a couple of cases where the overflow vent in the float cover was completely clogged (small spiders can do that). Result there would be air trapped in the float chamber so fuel level cannot rise, resulting in fuel under pressure running through the jet (flooding) as fast as it can be pumped in.
barneymg

Richard

My over pressure appeared to affect just one carb. I guess it boiled down to the efficiency of the shutoff valves at the top of the carbs. I assumed on mine that a bit more fuel crept through one of them at the higher pressure, over filling the chamber very slightly.

I spent almost 15 years trying to balance the mixture. I could never quite weaken them off equally despite renewing just about everything during that period. i tried all needle options. You name it, I tried it. One tweak on my pressure regulator and it was all solved instantly.

I bought the pressure regulator at the same time as a Facet fuel pump. I got it from the MGOC and they assured me it was set up for the MGA. It wasn't but, as I said, it took 15 years to find out. Lesson learned. Don't assume. Check.

Steve

Steve Gyles

Barney - thank you for the explanation. I'll check the overflow vent.

Steve - I will start with the valve (hopefully it will arrive tomorrow).

Cheers
Richard
Richard Atkinson

Quick update

Fitted a new viton tipped needle and seat.

The overflow vent was clear.

Steve, I think you may be on to something. With the dashpot and piston removed you can see a spurt of fuel out of the top of the jet when the ignition is first switched on. The rear carb remains dry.

I'll take the carb off and fit new seals and check the needle etc before going down the route of fitting a fuel pressure regulator. One shouldn't be needed with a new standard SU pump.

Richard

Richard Atkinson

A quick update.

After fitting the new needle and corks seals I checked the float lever settings (7/16th), jet height (0.07" below bridge), carb balance and readjusted the idle. The overflow pipes are clear. The car ran well! The plugs are a dark brown rather than tan.

However, after a run last night I drove straight into the garage and cut the ignition immediately, planning to check the plugs. I also removed the dash pots and pistons to recheck the jet heights. For several minutes fuel was pulsing out of both jets. Has anyone else has seen this?

The fuel level in the float chamber must be too high, either because some dirt had temporarily obstructed the needles (there was some dirt at the bottom of the fuel chambers despite having been clean before the run) or the fuel pump pressure is too high. I have emailed Burlen for advice.

Cheers
Richard
Richard Atkinson

Richard
Somewhat frustrating for you. Be interested to hear what Burlen say. As the pump is new, that must surely be a possibility if everything was ok before.
Graham V

When fuel is pulsing out of the carburetor fuel jets, either the float valves are not closing, or the fuel pressure is way too high, pushing fuel past the closed float valves. Check/measure the fuel pressure.

Fuel level too high in the float chambers could make fuel trickle out of the jets by gravity, but would not be pulsing.

One more possibility. If the float cover vents are plugged, the float chambers could develop vapor pressure (or air pressure), in which case fuel could be pushed through the main jets under pressure with fuel level never getting high enough to close the float valves. Remove the cap nut from top of float chamber, and be sure the fiber vent washer is on the bottom side of the banjo fitting, and that the vent pipe is free breathing, not clogged.
barneymg

Barney, thanks for the advice.

I measured the flow rate. 18fl.oz. in 30s so almost double the minimum quoted by Burlen for the pump (7 gallons per hour). I have emailed Burlen.

The overflow pipes are clear.

Regards
Richard



Richard Atkinson

I spoke to Burlen today and was advised:

To assess the fuel pump it is fuel pressure, not delivery, that is required. The Workshop manual quotes pump delivery at 45 litres (10 gallons) per hour which is a little below what I measured. Still seems worth measuring the flow to me. They thought it was very unlikely to be the pump.

The pulses of fuel into the carb after a run was likely to be fuel vapourisation in the float chamber from the heat from the engine.

The castellated fibre washer under the over flow banjo was important as it locates the washer correctly to allow air to get to/from the over flow pipe. The front carb had a solid copper washer which cannot have helped (now replaced).

Richard



Richard Atkinson

Richard,
just been posting about Hardi electronic fuel pumps on Spridgets, I've had one fitted to my midget (well QH) for 13 years, another Frogeye owner 17 years, both fit and forget, rather than fit and fart about.

And I don't think it even needs to be "position outlet uppermost" but you'd need to check as I no longer have fitting instructions.

Text, lifted from Moss but I think you may be able get the exact same pump for less money elsewhere, but don't know -
"All HARDI-Pumps have a special control system that enables the adaption of the pumping capacity to the consumption of the engine. This means, it will pump fast at high consumption, slow or not at all at low consumption and therefore a great deal of energy is saved."

Fit and forget, get driving the car instead of fiddling about with it.

See Rover(!) M G A 1.5 - 1955-59
10 gallons an hour, 2-2.5psi delivery.
https://www.hardi-automotive.com/wp-content/uploads/Hardi-engl-Fahrzeuge.pdf
Nigel Atkins

's' removed from 'https'-
http://www.hardi-automotive.com/wp-content/uploads/Hardi-engl-Fahrzeuge.pdf
Nigel Atkins

Apologies for not updating this thread sooner. I finally resolved my carb problem by replacing the needle and jet. The needle was rather bent - see attached photo. Thank you all for your input.
Regards


Richard Atkinson

A bent needle really is trouble of course but would not explain why the fuel level is too high in one carbie. How about this idea. In 1998, early on in the ownership of my "A" I had one flooding SU with fuel pouring out of the overflow and a very rich tick over. On a run, I found giving the float chamber a clout or two temporarily fixed the problem. Back home, I found a load of (magnetic) dust in the bottom of the float chamber. Cleaned that out and the car was fine for a bit, soon more muck was found in the float chamber. I decided this fine dust that had come from the old steel tank had passed through the wire gauze strainers and was getting stuck in the needle valve seat somehow, preventing a good seal.
I decided to fit a cheap (£1.50) paper fuel filter and the problem was solved. That same filter is still fine today - same old tank too!
Pete T
PeteT

Richard
Good to hear your car is now sorted, but I agree with Pete that the needle should not have caused the problems you described.
But hey, if its fixed, just enjoy it
Graham V

some times people have fited a gasket thats too thick,The one that goes next to carb from float bowl, If too thick car will run lean and if too thin it will be too rich.
Best way is to remove piston-turn on pump and check fuel level at bridge. /blow at it a couple of times so it can give a true reading.
Sanders

Thanks for your comments/suggestions. Maybe the needle/jet were simply worn.

There was also some interesting observations on carb set up in the book 'Classic Engines, Modern Fuel' by Paul Ireland (no connection). For example, the angle of the float chamber relative to the carb body can alter the fuel height.

Regards
Richard A

This thread was discussed between 18/05/2020 and 14/02/2021

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