2 replies to this topic

[JHN_mini]

Hi All! Just joined the mini forum as I thought this would be the best place to find help.

I bought my first mini in June this year, it’s got a 1275 A plus MG metro engine in it. We’ve always had abit of smoke with it from the start but we put it down to having the wrong fuel mixture as it was running rich with the incorrect carb - 1 1/2 SU.

We recently refurbished our 28/36 Webber and had it set to the correct jets using the sacred vizzard book, it did ran a lot better but we had the problem of it being underpowered. This however was due to the incorrect timing but we have since rectified it.

All we are really left with now is excessive exhaust smoke, it’s faint greyish/blue. I know blue means oil but we still think this could be down to carb adjustment. We also considered the valve stems being worn but as the MOT is on Tuesday we want to try rule out serious problems.

We did also consider maybe the was a lot of carbon build up in the exhaust due to the previous rich mixture we had on the old carb and maybe it just needs a good run down the road to clear the system out, could be wishful thinking.

We checked the recently bought spark plugs and we found that 3 and 4 are looking normal, brownish colour but 1 and 2 are blackened with soot not oil, which does indicate a rich mixture on those two alone, for some reason plug number 2 hole was letting exhaust smoke but could down to the valves being opened thus letting gas trapped in the exhaust manifold to escape.

We did do the valve clearances not long ago so ruled that out too. We did a compression test twice and found all cylinders are showing good high compression which tends rule out worn valve guides and rings thus leaving only rich mixture as a probable cause which is reflected in the colour of the spark plugs. With the 1 1/2 SU, all four plugs were sooty. With the 28/36, 1 and 2 remain sooty.

Not sure on the best way forward so any help would be fantastic :]

Thanks

[Icey]

The 1.5” carb on a 1275 wasn’t strictly wrong, some models with the A+ came from the factory with a HIF38 which is the same. I’d take a slightly under size SU over a Webber any day, but I accept that’s personal preference.

If it was running rich (enough to emit black smoke) for a while you could have damaged the bores which would lead oil burning.

A typical compression test will only really tell you that the valves are sealing and the gasket isn’t blown. It won’t tell you anything about the condition of the bores. Poor ring sealing leads to poor oil control which artificially increases apparent compression - the test doesn’t care what’s sealing the pistons, oil or rings. Leak down tests may give a better idea what’s going on, as would taking the head off and inspecting/measuring (but even that wouldn’t tell you if the rings were toast).

To sort the fuelling out you need to either get someone like AC Dodd round (but he won’t touch webbers) and/or get it to a rolling road with an operator who can tune carbs (a rare thing these days). Just reading plugs won’t get you all the answers.

Oil burning can of course be other things like valve guides/seals, tell-tale for that is smoke when starting and after periods of engine braking.

[Steve220]

Black soot can also be from burning oil. I would do a compression and leak down test first. If its showing bad readings, I'd be looking at the cylinder head or maybe piston rings.