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MG MGB Technical - Fuel Level in Carbies

G'day. I am trying to sort out a problem I have with the fuel level in the float chambers.

The rear carb as high fuel level which I can't rectify by adjusting the float. I have swapped the float chamber lids complete with needle and seats and floats thinking it would transfer the problem to the front carb. Trouble is it didn't.

The rear carb still has a high level, as observed by looking at the jets. Is there anything other than float adjustment that can affect the level in the jets. Personally I can't think of anything. Both carbs have the gasket fitted to the float lids.

Cheers

Tony
Tony Oliver

Whoops. It's a '67 model with HS carbs.

Tony
Tony Oliver

How are you measuring the fuel level? In the jets? How are you doing that? Liquids find their own level when they have the same atmospheric pressure both sides. If the rear jet is too low then its fuel level will be high compared to the other. You need to be comparing the depth of fuel below the top of the chamber in the two float chambers. If this is different is it leaking *out* of the front chamber through a faulty jet pipe seal and hence it is the rear level that is correct? If you are changing lids complete with floats and valves then the only thing that can be different is the depth of the two float chambers or the height of the top relative to the main body of the carb. Maybe you have a rogue float chamber.
Paul Hunt 2

Paul. I am doing a complete carb set up from scratch. Jets are both 8 flats from the top. I am looking at the fuel level in the jets. The front level is just visible in the jet, the rear level sits just underneath the bridge.

I swapped lids complete in case it was a needle/seat/float problem but the fault stayed with the rear carb. If it was a lid problem it should have moved to the front. I will check the level in the chambers but it's hard to see how it will give a different result to checking the level in the jets. I'll also investigate the possibility of a fuel leak but I test the levels with the pump running.

I'll go through the whole thing again. I've had problems for some time with 3 and 4 running rich which is why I am doing all this.

Cheers
Tony
Tony Oliver

8 flats seems a bit high, the normal starting position is 12 flats down, then fine tune from there.

The only reason I mentioned checking the fuel level in the chambers is because you are having a problem that swapping lids, floats and float valves over hasn't fixed. In that case you have to go back to basic levels and dimensions to try and see what is wrong. There has to be a disparity between the height of the bridge and consequently the jet, and the top if the float chamber between the two carbs. If the two float chambers are the same depth top to bottom, and the depth of fuel is the same, then the positioning of the rear float chamber has to be higher relative to the main body of the carb compared to the front carb.
Paul Hunt 2

Are your floats all-plastic, or do they have a metal
hinge pivot?

In anycase, check the pivot points on the float(s)
themselves, in particular - the bores through which
the pivot pin passes through.

They often wear themselves oversize or into an
oval shape. If so, then it's time to renew them.

With metal-plastic floats, you can simply bend the hinge very slightly to alter the float level.

In the case of all-plastic floats, you can shim the needle valve a small amount with aluminum
washers in order to tweek the float level.
Daniel Wong

Question from Hunt 2: "How are you measuring the fuel level? In the jets? How are you doing that?".

Take of the dashpots. Switch on the ignition so the pump fills the carbs. Look down in the jets. You should be able to see the fuel level. To check, blow air in each jet (no hose required), the fuel level will go down, back up and overflow a little, then settle.

Fuel level height doesn't have to be exactly equal, as long as it's there and it doesn't overflow. The difference in level you can compensate by the normal method of leaning/enriching the mixture (by adjusting the jet of each carb).

All imo,

Laurens.
Laurens CGT

Thanks Gents. I have measured the levels in the chambers and they are pretty much the same. As the levels in the jets are very different more investigation is required. I am taking the carbs off the car today to see if I can work this out. I will look into the possibility of the float chambers being different relative heights.

Tony
Tony Oliver

First prize goes to Paul. I owe you a few beers if I ever sell the house and go to the UK for a grand tour after 40 years absence.

The two carby bodies are a different part number. During a past rebuild I didn't take any notice of which float chamber came of which carby as I didn't think it made any difference because I didn't know the bodies were different.

The front float chamber was 5.0mm lower than the rear as measured from the top of the chamber to the base of the dash pots, hence causing the fuel level in the front jet to be lower. Swapping chambers from one carby to the other has resulted in the rear carby being 0.2mm lower than the front.

Problem solved.

Thanks Paul. I would never had thought of that!

Cheers

Tony
Tony Oliver

:o) I've only ever claimed to be a simple chap, and going back to first principles has stood me in good stead many a time. I hoping to visit Oz in 2010 ...
Paul Hunt 2

This thread was discussed between 27/03/2008 and 29/03/2008

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