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MG MGA - MGA SU Carb Fuel Level

I've been struggling to get my 1960 MGA project car to run properly - it always seems to run very lean. Today I pulled the dashpots and pistons and cleaned everything up centered the jets, etc and set the mixture at 13 flats from lean. I read that the fuel level should be at the tops of the jets, but mine is way down there - I have to put the choke full on to get to the fuel level. I checked and adjusted the floats, which were a hair off, but not much, and cleaned the passages from the fuel bowls to the jets. This has been a long-standing source of frustration and any advice would be greatly appreciated. The fuel pump, lines, and filter have not seen much use but everything has been sitting more or less for a few years. Thanks in advance for any leads..
Paul H

In my recent thread on fuel levels Mick Anderson posted an extract from the SU manual that said the fuel level in the jet should be between 1/8" and 1/4" below the top.

Other than that I am going to sit back and learn from this thread. I got myself into deep water with theories before the true cause of my incorrect fuel chamber level was sorted - too much fuel pressure from my after market Facet pump, combined with an incorrectly set fuel pressure regulator. The H4 only likes about 1.5psi. Mine was running at 3psi.

Steve Gyles

Just repeating the extract from the SU Manual.


M F Anderson

I had never paid much attention to the level of the fuel in the jet, but then I was told about adjusting the floats so that the level was just below the top if the jet.

It takes a bit of trial an error and several removals of the dashpot cover and float chamber cover. Once the car is up to temp and you can run it without the choke, remove the dashpot covers and check your levels. If they're low, remove the float bowl cover and bend the tabs up a bit. Re-install everything, run the car, then redo the fuel level check. Just keep repeating until both jets have the proper amount of fuel showing.

I had always just adjusted the floats according to the measurement in the manual and left it at that. Now my mixture is much easier to set.

Mark J Michalak

I too, for years have assumed that SU fuel level should ideally be at the top of the jet, and never achieved it! It's always been several mm below. So, some time ago I spoke with a technical guy at SU/Burlen about this who said that people get too hung up about this issue and should relax on it. He reckoned that it's quite adequate to just follow the float setting instructions for any particular SU carb, ie. the 7/16" or 9/16" rod trick, or whatever recommended) and leave it at that. Tuning will then be possible. Certainly, I have never seen the fuel level get right to the top of the jet and I no longer care!
So, for what it's worth that's what I know courtesy of the experts. Hope it helps Paul, and good luck.
Bruce Mayo

Thanks for all the replies and insight - considering I have to back the mixture adjusting nuts out much, much further than usual to get it to run I think I will try adjusting the float level off factory spec and see if that helps. I was getting so desperate that I cheated the needles up a bit, but the fuel is really down in the jets that didn't work - no idea what else to try, so as a culmination of all the posts I'm going to not worry about it too much, but try to get it closer using the suggested method. Thanks again!
Paul H


I know I said I was going to sit back on this one. However, the thread has not gone any further than a mild discussion on the ideal fuel level in the jet. You said your level was ".....way down there". Your plan to alter the fork is exactly what I did albeit for a different reason. Whilst this will work it is a case of dressing the wound rather than removing the bullet. i.e. you have not sorted the cause of the problem. I did the same until eventually I discovered the root cause and returned the forks to the standard setting.

Can you give us an idea of the measurement of your fuel level "....way down there"? The other obvious question is why is it down there? Are your floats standard? These can be the only other variable to the fork measurement in setting the fuel level in the chamber? The normal problem suffered by some is a sinking or heavy float (many repairs), but these give too high a fuel level (rich) in the chamber.

Setting the 7/16" on the forks in the lid takes out any variations in shut-off valve types, so it all boils down to the floating characteristics and measurements of the floats. One other small variation would be the gasket between the lid and the chamber. Do you have it in place? Leaving it out would lower the fuel level by its thickness.

Steve Gyles

I wish I did not keep tampering with my car to investigate other people's issues! I am currently overhauling my wiper brackets from another thread!

Following on from my last post I thought I would go and peer down my jets to see where my fuel level was, bearing in mind that my car is working fine and the use of choke is minimal from cold. Well, I could not see the fuel level. To quote Paul it was ".....way down there". I pulled the choke fully out to lower the jet assembly and still no sign of fuel. To get the level up to within the quoted 1/8" to 1/4" of the top of the jet bearing I had to fill the float chamber to within 0.41" of its top. To give you some idea of this in your minds, this meant pushing the float fully down to raise the level and then add a few more teaspoons of fuel. Seems very odd.

Steve Gyles

Steve, I've found exactly what you report and wondered if by taking the needle out of the jet the level is bound to fall. Another thought was that surface tension plays a roll and only when there is vibration from the engine running does the fuel level creep up near to the top of the jet
J H Cole


That makes 3 of us. You can only measure the level with the piston and jet needle removed and the choke pulled out. The needle would make no difference to the balancing of fuel levels. I watched the level rise and fall in jet bearing as I lowered and raised the float. It must bring the SU instruction into question as to precisely what they meant.

Steve Gyles

This thread was discussed between 25/10/2012 and 29/10/2012

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