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Carburettors: Conversion from Injection to Carburettors

Article submitted by: Rich (adirtylittlecar<at>

Single Point Injection to Carb Conversion

IMPORTANT NOTE: An engine swap like this may consitute a notifiable modification to the insurers and licencing bod(ies).

A friend of mine has an M reg Single Point Injection mini. Since he’s had the car, he’s had nothing but trouble with one thing or another. The final straw was when it kept staling for no apparent reason. After asking a number of people for their advice, it seemed that the general opinion was that the ECU must be faulty, and the best cure would be to take it out and replace it with a carb instead (much more tuneable).

This is where I come into it, I was asked to help solve the problem. After an initial inspection I made a list of the parts required (at this point I’ll point out that we had a donor mini to cannibalise).

1 x Metro 1 3/4” carb
1 x K&N filter
1 x Metro/aftermarket inlet manifold
1 x Metro electronic dizzy
Male & female spade connectors
We also decided to change the plugs, leads, dizzy cap and arm too.

The carb, filter, manifold and dizzy were sourced from a local breakers for £35, while we bought the connectors and plugs, etc from a motor factors for around £30.

The first job was to remove the injection unit, which unbolts like any normal carb manifold. Because the inlet and exhaust manifolds are 2 separate units on the single point, unlike the standard carb engines where it is all one cast unit, we decided to keep the existing exhaust set-up for ease and to keep costs down. On removal of the inlet, the mating face on the head needed cleaning to remove the traces of old gasket, after which, a new manifold gasket was put on. Next we bolted on the new carb and manifold. At this point I’d like to point out that it is a good idea to remove the cover off the bottom of the carb and check the internals are all ok and clean before bolting everything down. (We didn’t and had to take it back off later to clean it)

Next was the distributor, the one for the single point is no good for a carb mini because it doesn’t have the little box on the side for the coil connections, it uses a control box which is linked to the ECU. The new dizzy was put in and the new cap and arm put on, along with the new plugs and leads too.

That was all we were hoping we’d have to do, other that a small amount of modification to the wiring, but after sitting scratching our heads for a while trying to figure out how to modify it, we went for a second option - take the front loom from the donor mini which had the same clocks and switch’s as the mini we were working on, so wiring it up was relatively easy. The loom from the SPI was a mass of wiring and took a bit of doing to get out, but looking back, we should have just cut it to bits instead of trying to save it. The only part of it we had to leave in was the exhaust sensor located in the manifold, this was only left behind to keep it’s hole blocked in the exhaust, though with the wires just cut off. While we were at it we replaced all the connectors that looked dubious so as to save any aggravation in the future.

Next was to make 2 new wires to go from the coil to the dizzy, there are ways to do it using the existing coil wires, but my way is easier! Firstly, use the original wire that supplies power to the coil from the loom, then find 2 pieces of wire around 8” long and put the relative connections on the end for one to go from the “ + “ coil terminal to the dizzy, and one to go from the “ - ” coil terminal to the dizzy. On the dizzy we used, it had a 3 spade connection on the little box on the side, one of the wire went to the top one and one to the bottom one. If the engine won’t spark when you turn it over try changing the connections over, so the top one is on the bottom and vice versa.

Next was the fuelling problem, we hadn’t realised that the SPI used an electric pump, but this was easily overcome by using the mechanical pump form the donor mini. But the SPI also has 3 fuel pipes too (2 large braided pipes, one with orange marking and one with green and a smaller black plastic pipe). We opted for the one with the green marking on it, so connected it to the fuel pump and the pump to the carb in the usual way, leaving the other 2 pipes free.

This was it, it was time to try it. Turning the key, it fired up first time! I was very relieved because I was unsure what the cam would be like with it being for an SPI, but it all seem ok. It was then taken for a test drive and after about going about 20’ cut out. “Shit! it aint gonna work, the cam must be all wrong” were my thoughts at the time, so we tried it again and the same happened. Every time it cut out there was a strange hissing noise, as if there was a build up of pressure escaping but we couldn’t find where. Desperation was setting in now, so I rang a couple of people I know who might be able to help. They suggested checking the fuel lines and filters that are in the tank and also that the exhaust was clear. We thought the best bet would be to change the fuel tank for the one out of the donor mini, but after trying to go for a test drive, once again it died with the hissing noise. Then we unbolted the exhaust at the cat to make sure it was ok. While dragging it!

out from under the car we noticed bits off black stuff falling out of it, this black stuff was the remains of the inside of the cat, which had broken down into bits and been blown back, blocking the rest of the exhaust solid! Back to the motor factors for a new rear section of exhaust, we decided to re-use the cat even though it was empty of it’s cat material because the motor factors wanted £140 for a new one!!

This seemed to cure it, it ran like a dream once I tweaked the ignition timing slightly, so off for another test drive. This time we got as for as the hill out of the car park, and it started coughing and farting again and cut out, but without the hiss. By now I was beginning to wonder if it was really going to work at all, or whether we should have left it as a single point. I messed with the timing and got a bit of an improvement, but it was still gutless on the hill and under load. I was running out of ideas by now big time. After about an hour of fiddling, I decided to take the carb back off and check that it was working ok wasn’t getting blocked. When I removed the bottom cover, I found it full of gunk and bits of dirt. Once cleaned and washed out, it was put back on and fired up and taken for a spin once again. FINALLY!! it worked it ran like a dream, didn’t cough, didn’t fart, didn’t cut out, perfect....

So, if anyone else has problems like this, hopefully I might have given you some ideas as to what the cause/cure might be and if your thinking about swapping from the SPI to carb, then maybe it’ll give you the incentive to go and do it - it's cheaper than a repliacemant ECU. In all it took 2 of us 2 days to do, but a day of it was spent trying to find out why it didn’t work, so 2 people should be able to do the conversion in one day if they’ve got a good idea of what they’re doing.

If you’ve got any questions email me:

Editors update: it appears in the UK that so long as the owner can prove the age of the engine the MOT tester may well be obliged to test the car to the standard required for the YEAR of the ENGINE and not the YEAR OF REGISTRATION of the VEHICLE itself. The owner will need to be well armed with the necessary information to substantiate the age of the Engine.

More from Rich:


When we took the car for the MOT, the guy doing the testing asked us if it was the original engine for the car (M reg). He said if it was, then we would have to sort out the emissions because of the lamba probe, which has i's own reading on the emissions test. I guess if you were to replaice the exhaust with an LCB and different system, then you may get away with it. But what he said to us, is what most MOT stations don't tell you is that if you put in an earlier engine - carb and pre CAT, the emissions are tested as a normal carb'd mini would be - up 3.5%CO, etc, and the lamba sensor is ignored. This is totally legal, just not mentioned by most MOT test stations and he showed us proof of this in the MOT test manual. So we just told him that it was an earlier engine because the engine number plate had been removed at some point.

We got the engine rolling road tuned today, it reached a max bhp of 45, which I feel is a little low for a 1300, but we didn't take it to our usual tuners, so variances in roller callibration have to be taken into account. Another factor that might play a part in this is the fact that all the gause or whatever it is inside the CAT has been taken out, so it is now hollow which could be effecting the back preasure in the exhaust. The stuffing broke down leaving the CAT empty, but we decided to leave the empty box on, just in case Mr MOT tester said it needed to be there.

Fatbloke's notes: Engines out of 1911 Delaunay Belleville HB6 Landaulette Rothschild's ( minimal MOT requirements) will not fit 1997 SPi Coopers :))

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