9 replies to this topic

[daddums]

Hi, I've been having trouble figuring out why my engine's always running too rich.

 

Watching an AC Dodd video recently put me onto the idea that the fuel pump might be supplying the carb with fuel at too high a pressure.

I bought a cheap fuel pressure gauge off ebay and attached it with a t-piece before the carb inlet and it's reading just over 6 psi. I understand it should be around 1.5-3.6 psi.

 

Questions:

1. Am I right in thinking that could explain why I haven't been able to lean off the air fuel mixture?

 

2. AC Dodd suggests fitting a fuel filter with built in pressure regulator (FPR004 on MiniSpares I think). I've read on here that others are skeptical about a pressure regulator and suggest replacing the pump with an electric one which supplies fuel at the correct pressure (such as the ECCO 1LM12). Cost either way seems similar. What are the advantages and disadvantages of each option?

 

1991 Mini City E

998cc engine with 12g940 head, hs4 carb, maniflow exhaust

Currently has mechanical fuel pump which looks like MiniSpares AZX1818MS

 

Thanks in advance!

 

 

[lsto]

I guess if the fuel pressure is too high it could be forcing past the needle valve in the carb, I have no personal experience of this but it does sound like it could be your fault.

As for mechanical with regulator or electric, I would suggest it's personal preference. If you go for a regulator you could get one with a built in filter which would always be a bonus.
If you go electric you'l have to wire it in somewhere into the ignition and ideally fit an inertia cut off switch for safety's sake. I guess ultimately it depends how much effort you want to put in. Regulator would be easier to fit, but you can hide a fuel pump away under the car and if you remove the mechanical fuel pump it eliminates a potential oil leak and frees up a bit of useless space behind the engine.
Good luck

Edited by lsto, 20 August 2025 - 03:45 PM.

[Spider]

Replace the pump.

[Rubbershorts]

From my recent dealings with Mr Dodd. My carb is worn so can't control it's fueling. For a temporary measure until my new HIF44 is ready, and I fit it and have it tuned, he suggested I fit a filter/regulator, set at 2 psi. This would be really useful for running with the new 44 anyway he said, so wasn't money spent unnecessarily. Is there perhaps a problem with your carb?
On the pump front, if you change to some kind of electric gadget, you eliminate the risk of the mechanical pump letting go and filling the sump with fuel.

[68+86auto]

Something to think about first is if the gauge is accurate. I have had very bad luck with cheap gauges.

You could set up a gravity feed system and see if you can then lean out the mixture.


You have obviously read one of my posts in regards to regulators so you know my opinion - fit a pump which outputs the correct pressure. That could be mechanical, it doesn't need to be electric.

[floormanager]

Watch AC Dodd's videos.  On HS carbs, I believe the pressure should be 1 to 1.5 psi max.  I'd thought the needle would be forced open at such pressure.

http://https://www.y...h?v=VodAnLccjB0

Edited by floormanager, 21 August 2025 - 05:51 AM.

[Spider]

Watch AC Dodd's videos.  On HS carbs, I believe the pressure should be 1 to 1.5 psi max.  I'd thought the needle would be forced open at such pressure.

http://https://www.y...h?v=VodAnLccjB0

 

BMC / Leyland / Rover state the pressure should be 1-1/2 to 3 PSI.

I've consistently found pressures as low as 1 PSI is too low.

< EDIT:- Factory Data from various years;-

 

 

 

I stand corrected here, the maximum pressure in later years was 4.0 PSI

 

Edited by Spider, 22 August 2025 - 07:26 AM.

[floormanager]

Don't shoot the messenger but Mr A C Dodd says at 1min 10 secs it needs to be below 1.5psi

[Cooperman]

I run twin Facit pumps with in-line filters piped in parallel with a pressure regulator set to 3 psi on both of my 1275 cars. This I have done for over 40 years and I have never had any fuelling issues. One of my cars has twin H4 carbs and the other an HIF44.

[Ethel]

A 4 foot head should be in the right ball park with petrol

 

41.5 inches of water is 1.5psi if you wanted rig up a test of the gauge.