10 replies to this topic

[Mito]

I've been trying to adjust the fuelling on my 91 998 carb mini using an afr gauge. I've got it set up using a length of pipe up the end of the exhaust with the sensor in - not the ideal way of seeing it up but I don't want to take the exhaust off yet so hoping it's good enough? 

 

I've been able to get the reading on the afr gauge between 14 and 15 when the pipe to the crank case is disconnected. As soon as I connect this pipe or block off this connection on the carb it drops to 10 and I cannot lean it off. This is without the air box on the car. 

 

I'll need to check the idle speed tomorrow but I think it was in the 800rpm range. 

 

I've checked the flatt bowl and there's no signs of overflow and the fuel appears to be shutting off okay. Float bowl is also level and the filter king is set to 1.5 psi.

 

I'm going to have to try to fix it tomorrow night or, in true top gear style, I'm going to have to take the BMW to the Elan Valley run.

 

Help?!

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Edited by Mito, 29 July 2026 - 08:49 PM.

[68+86auto]

Make sure a vented cap is installed. Make sure the jet isn't stuck down.

[Mito]

Make sure a vented cap is installed. Make sure the jet isn't stuck down.

Jet moves up and down freely with the choke cable

[Designer]

Hi,

 

I had this on our HS4 and it turned out to be the jet sticking very slightly open even though the choke was pushed fully in. This was due to having the carb tuned by someone at a garage who did not know what they were doing. They replaced the jet with an after market one. The length of the tube from the bottom of the bowl to the bottom of the jet was slightly too short thus causing the jet to be pulled to one side and therefore intermittently sticking in the slightly open position.

Having got a replacement jet from Minispares problem solved.

 

Paddy

[Mito]

Hi,

 

I had this on our HS4 and it turned out to be the jet sticking very slightly open even though the choke was pushed fully in. This was due to having the carb tuned by someone at a garage who did not know what they were doing. They replaced the jet with an after market one. The length of the tube from the bottom of the bowl to the bottom of the jet was slightly too short thus causing the jet to be pulled to one side and therefore intermittently sticking in the slightly open position.

Having got a replacement jet from Minispares problem solved.

 

Paddy

I fitted a new su jet a few years ago. I've just taken the carb off and the jet is moving fully up and down without restriction. 

 

There is definite play in the butterfly valve, so it definitely wants a rebuild. I'd have thought that would make it leaner though.

[Mito]

I've taken the carb off, round the jet fully up, then put the carb back on the car, drilled the air box and it's now running at about 18 at idle and mid 13's on the throttle. Getting there...

[Mito]

I've got it to idle at around mid 14's, it is all over the place, definitely wants a refurb, but still running rich when at higher revs as it drops to the 12-13's depending on the needle in it. It has an AAF needle in originally. 

 

Next thing I'm going to try is switching it for one of acdodds AFA 998 stage one profiles needless. If that doesn't work, it's going to have to be the BINI 

 

To minispares north I go...

[Ethel]

Sounds like two potential issues combined. Excess air through the breather, something with the mixture making it rich. There's not much you can do other than go through the float bowl assembly, jet needle & piston assembly and probably replace with known good bits.

 

Float not holed & not sticking on its "hinge pin"

Jet's not got bent from fitting the bulkhead - even very slightly can matter

Needle also not bent, fitting to the piston in order - right height, swinging freely

Needle & jet not worn together - "polished" spots on the needle, deliberately turning the needle through 180 degrees might make a difference

Piston drop test, "polish" on the piston and/ or dashpot contact surfaces (that shouldn't contact)

Piston spring not stretched

Damper plunger moving freely, also lacking "polish"

[Lplus]

I've got it to idle at around mid 14's, it is all over the place, definitely wants a refurb, but still running rich when at higher revs as it drops to the 12-13's depending on the needle in it. It has an AAF needle in originally. 

 

Next thing I'm going to try is switching it for one of acdodds AFA 998 stage one profiles needless. If that doesn't work, it's going to have to be the BINI 

 

To minispares north I go...

Note that peak power is achieved with an A/F ratio of about 12.6:1. For what I'd have a mixture between the high 12s and low 13s at steady state.

[Mito]

 

I've got it to idle at around mid 14's, it is all over the place, definitely wants a refurb, but still running rich when at higher revs as it drops to the 12-13's depending on the needle in it. It has an AAF needle in originally. 

 

Next thing I'm going to try is switching it for one of acdodds AFA 998 stage one profiles needless. If that doesn't work, it's going to have to be the BINI 

 

To minispares north I go...

Note that peak power is achieved with an A/F ratio of about 12.6:1. For what I'd have a mixture between the high 12s and low 13s at steady state.

 

Should it be at that under constant throttle?

[Mito]

Sounds like two potential issues combined. Excess air through the breather, something with the mixture making it rich. There's not much you can do other than go through the float bowl assembly, jet needle & piston assembly and probably replace with known good bits.

 

Float not holed & not sticking on its "hinge pin"

Jet's not got bent from fitting the bulkhead - even very slightly can matter

Needle also not bent, fitting to the piston in order - right height, swinging freely

Needle & jet not worn together - "polished" spots on the needle, deliberately turning the needle through 180 degrees might make a difference

Piston drop test, "polish" on the piston and/ or dashpot contact surfaces (that shouldn't contact)

Piston spring not stretched

Damper plunger moving freely, also lacking "polish"

It wouldn't surprise me if it was a bent jet but it is moving okay. It's a fairly new jet.

There's no sign of fuel getting into the float or sticking on it's hinge pin. 

The piston drops fine. 

 

I've given up on it now. I'm going to try and source a refurbished carb or get this one refurbished because this one is clearly worn out.