Classic Mini runs rich/stalls when idling

Hey all,

This is my first post here, so naturally I’ll take the opportunity to whine about an issue with my car rather than giving a proper introduction. I’ve got a 1984 Austin/Rover Mini Mayfair saloon (automatic, right hand drive) that I picked up a few months ago from a guy who had it imported from London to Manhattan. He had done the normal upgrades one would do to a classic mini (upgraded exhaust, replacing the valves for unleaded gas) but he kept it garaged for most of the time it was here (about 4 years).

So here’s my issue: About a month after buying the car, I noticed that it started to misfire/chug at idle when in gear. If I was stopped at a light or even in slow traffic it would do this, and even started stalling out on me. (If you’ve ever been on the Gowanus Expressway, you probably understand how nervous I was when I stalled out in the center lane. Twice.) It also has started to run very rich and has poor gas mileage (not good when you only have a 9-gallon tank.

I’ve realized that the problem is worse as the engine gets warmer - in fact, the first time the issue presented itself was after a trip to Delaware, where it performed wonderfully on the New Jersey turnpike but then stalled at the first stoplight I hit in Delaware. I pretended it didn’t happen. Putting it in neutral at a stop keeps the car from stalling, but once in neutral the engine starts racing. The longer it’s driven, the harder that engine races in neutral. I’ve heard the cartalk guys always drawing a correlation between heat and oil viscosity playing a role in this, but I can’t see how that makes sense in my situation. Do I have some kind of gasket leak that only rears its head in warmer conditions? Where do I even begin to look?

I bought this particular car for several reasons. Besides being in love with the look of the mini, I needed an inexpensive daily driver and a simple set of components on which I could learn how it all works so I can service the car myself. I’ve already learned a lot from enthusiast forums but this issue seems to be eluding me. Please help!


1984 Austin/Rover Mini Mayfair (auto, RHD)
998cc single SU HS4 Carburetor
20650 miles
1 frustrated owner

It could be a million things but vacuum lines is a good area to start.
A cheap way to check vacuum lines is to get some starter fluid and carefully spray it around vacuum lines. If the engine speed briefly chances after you’ve sprayed in a particular area, have a closer look at that area.
One other one of those million things it could be is maybe the idle control valve, which I assume that car has. Not sure why it would do it only when hot, though.

A mini really is considered a special interest car so stick close to those forums. Those guys no doubt know all the ins and outs.

I haven’t worked on mini’s since the mid-90’s, but if I remember correctly, that SU carb has a center plunger that sits in an oil reservoir. Unscrew it and pull it out, then push it back in. You should feel resistance almost the whole way in. If not, you can use regular motor oil (30 weight, I believe is preferred) and put some in (it doesn’t take much) until the resistance is right. While it may not solve the problem, I’ve had problems with weird running Mini’s and this has settled down a few.

Chaissos, is a mid 80s still carbed, then? No doubt you’re right - that hadn’t occurred to me.

“998cc single SU HS4 Carburetor”

From what I remember, standard 1 Liter engine, single carb, engine mounted with the rad on the “curbside” as they say in the UK (although I do mean the curbside now, since it’s on the right hand side of the road).

I did not think it was possible to import cars like this into the United States…The idle mixture and float set-up on SU carbs goes back into the 1930’s…Crude stuff…getting these “instruments” to operate properly is more art than science…You need to find an artist who knows how to set one up and adjust it…These cars are out of their element on the Jersey Turnpike…

Well a vacuum leak would cause you to run LEAN…so it wont be that. It sounds as though your choke may be stuck on or maladjusted. You could also have clogged passageways in the carb that have now over-rided the mixture screw adjustments…making the adjustments null and void due to passageway clogging.

Pull the air cleaner and look at the carb…make SURE your choke is working properly and more importantly…disengaging properly…methinks you have a stuck/malfunctioning choke. There aren’t too many other things that will give you an over-rich running condition…aside from a loose or fallen out jet…I’d start at the choke if I were you and work your way twd more difficult things.

So check out the choke…the choke cable if equipped, The air filter…is it dirty/clogged? If after you check these things it is still doing it…then you would need to do a carb teardown… However your carbs aren’t that complex… They are SU carbs, no? LOOK AT THAT CHOKE…


The “choke” on these carbs is a linkage that pulls the main-jet out of place, intentionally flooding the engine. Owners develop a “touch” to achieve the proper degree of flooding for a cold start…

I’m with Caddyman - how did this car get into the US? How was it ‘Federalized’? There are many opportunities for that process to have messed up something.

The OP needs to find both a classic Mini forum and mechanic.

Thats Correct Caddyman! I thought about the mechanism of the “choke” for those SU carbs a little while after I wrote my post. I was thinking…“hey how the hell does that choke work on those carbs again?” I recalled exactly what you said…the mechanism is moving that main jet out of place… Those are the only carbs that have a moveable main jet like that…I think…I distinctly recall that rubber hose going to the bottom of that Jet…it is wrapped in a long coiled steel spring to protect it. My MGB had 2 of those things and back then I had no idea what the hell I was looking at.

In the OP’s case…he needs to check to see if the Jet has returned to its “home position” …if it hasnt then it is going to continue to enrich his mixture no matter what he does. The are infamous for being gummy and slow to move. Good call… I was going to do a follow-up on that one, thanks.


As I recall, that critical area is also the idle mixture adjustment. A big gland nut on the bottom of those carbs adjusts up and down, setting the position of the main jet tube and controlling idle mixture IN THEORY anyway…But every time you use the choke, the movement of the tube disturbs the adjustment…The gaskets and seals that keep gasoline from leaking down the tubes and dripping require frequent attention…All part of the fun of “Maintaining the Breed”…

Yeah true…not to mention…the viscosity of the oil in those dampers on top of the carbs… LOL… Talk about black magic man… You could fill those dampers with different oil to obtain a different characteristic of throttle or enrichment… Like the modern CV carb…just with a well of oil instead of a spring I guess…Right?

I am so proud/happy/lucky to have been educated in the old school first…and had the “wherewithall” to stay into the computer age. Most people are one school or the other…but I can tell you…You will get no where without a FIRM grasp of the fundamentals…and those fundamentals almost always come from the old school.


Hey guys,

WOW this got a lot of comments while I was away! Thanks for the quick feedback - I’ll toss some answers in here ordered by how much I know what I’m talking about :slight_smile:

  1. Any car over 25 years old is exempt from passing federal emissions and DOT laws. That includes imported cars. The only place this is not true is California, but CA does honor titles from other states so that would be the way around that. The newer classic minis that were built in the 90’s/00 with SPI/MPI and all those groovy things will not be legally available in the united states until they’re 25, but people re-VIN these cars with older numbers and illegally get them registered in the states. Sort of claiming that it’s a classic car with all parts replaced to modern specifications - shady. Mine is legitimate though.

  2. I’ve definitely looked at the choke. This was happening before I removed/cleaned the carb and it’s happening after I’ve replaced it. From what I hear, a carb that does not fully disengage on its own after letting go of the throttle sign of a rich mixture, which I can already tell by the smell.

  3. I’m actually on a bunch of enthusiast forums and speaking to one or two people about the issue, trouble is most of those guys are in the UK, where it’s a helluva lot easier to just take these issues to a mechanic. Also, they’re not nearly as active as CarTalk.

  4. I thought about the oil in the dashpot, replaced it with what was recommended in the forums. I had really thought that would work and was pissed when it didn’t.

  5. I tested for a vacuum leak using a can of WD-40 as suggested by a rare yank in the mini forums. No leaks to speak of but I’m glad I did it because there are some spots in the engine compartment that really needed the stuff.

As a little update to my story, a guy I took it to had suggested it was the contacts in the mechanical distributor that caused my problem. After waiting for the part to arrive and installing it, I was very disappointed to notice no difference.

Thanks again for being so active in here. I’m going to try to forget about the engine troubles for the time being by installing this new stereo. I mean, what better way to get rid of the puttering sound?


Cool…yeah that oil in the dashpot was just me thinking out loud… sorry… The only way that would be your issue is if you didn’t have any or a near empty dashpot…that would allow the throttle slide to rapidly go up and down…permitting fuel to enter the engine too easily.

I am still stuck on you having a stuck choke… Are there any removable jets in this carb…I dont recall…make sure all the jet screws if there are any are screwed in and tight… Once while listening to my buddy describe a too rich condition on a 2cylinder 2 stroke Yamaha Banshee…After hearing his description I said it sounds like your main jet is rattling around inside your float bowl…Lo and behold…he pulled the carb and there was his main…rattling around in the bowl…causing his over rich condition…he near hit the floor when he saw that…and wondered aloud how the hell I could call that one, just by hearing his description…I get lucky sometimes I guess…an Educated sort of lucky, but lucky nonetheless…lol


“1) Any car over 25 years old is exempt from passing federal emissions and DOT laws”

This is NOT TRUE. Per the EPA:

"1. Excluded Vehicles
Excluded vehicles are those vehicles that have been excluded from the emission requirements of the Clean Air Act. These vehicles are excluded by either their age (i.e., manufactured prior to the regulations), the type of fuel the vehicle uses, the maximum speed of the vehicle, or the lack of features associated with safe and practical street or highway use. Excluded vehicles may be imported by anyone. No bond is required by EPA.

a. Exclusions Based on Age
The vehicle is a Light-duty gasoline-fueled car or truck built before January 1, 1968"

There may be a NHTSA exemption for 25 year + vehicles, but not the EPA.

Just think - if one could import anything older than 1987 - the streets would be crawling with all kinds of odd stuff.

Texases - all I know is that my car is exempt and that it was not manufactured prior to the regulations but built according to the regulations of another country. I have a feeling that the streets are not crawling with all kinds of odd stuff because even people in the CarTalk forums don’t know it’s legal to own one of these critters in the states.

Rich, it’s not legal. Yours may have been ‘federalized’ and that way it passed the current regs. There’s no legal way around the EPA requirement. My point is that the federalization process can create problems when trying to modify the car to meet EPA requirements.

You have been told your car is exempt. The person that told you that is wrong.

I have an exempt sticker from the State of New York. For my car. On my car. I now realize I should’ve said ‘inspection’ instead, which I understand is completely different. However, now that I think about it, I wouldn’t be surprised if this car actually met EPA requirements. Can you post that link?

Blackbird - I’m pretty sure the choke is fine. I’ve even disconnected the linkage and it runs the same. The jet is moveable and adjustable by a nut connected to the housing. Even turned all the way in the leanest direction it still runs too rich. Maybe the mechanical fuel pump?

Possibly you have a leaking needle and seat. I don’t where it is located in this carb, but as far as I know, all carbs have them somewhere.

I had a Datsun with twin SU’s once. I filled the top chamber with 3 in 1 oil monthly and never had any trouble, so I never had one apart.