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My 1968 Mini 850 is running lumpy on load. Fine at tick over but struggles to reach speed and going up hills.
All four plugs are sparking but after removing and inspecting the plugs, I noticed the the plug from cylinder 1 is clean as new, there is only clean unburnt fuel on the plug. The other 3 are all sooty. Replaced the plugs and still the same thing happening, cylinder 1 spark plug is always clean. There seems to be nothing happening in the cylinder.
I have checked the gaps, done a compression test (125 psi at all 4 cylinders). Set valve clearance 0.30. News points, new condensor. Timing done, static and strobe.
The only fault that I have found was the vacuum diaphram isn't working, considering that the distributor is almost 50 years old, I will buy a new one. The coil seems to be the only thing never changed from new (1968).
I have no idea, please help.
Thanks. Rob

You need to determine if number one is sparking, easy enough when engine running just disconnect each lead one at a time and see if anything changes. I notice you haven't mentioned new leads on the lost of things tried, maybe give that a shot as well. The vacuum unit if leaking will cause an air leak so maybe try disconnecting it and plugging the end, also as it advances ignition it may cause running issues under load if not working.

As previous poster stated...you need to determine if # 1 is actually firing.
If not...possible causes.
Plug wire
Ends on the plug wire
Distributor cap has an internal crack...shorting inside the cap.

The vacuum advance problem will hurt your power as the distributor will not advance/retard the timing.
On a hill the timing need to retard otherwise you will have 'pinging' sounds etc. etc.
This is a different problem from the #1 plug issue.

If there's fuel and the plug is clean it sounds like there's no spark at that plug. Since the symptoms remain the same irrespective of new plugs, HT lead and/or distributor cap would be next on my list to check

The only fault that I have found was the vacuum diaphram isn't working

You can just buy the vacuum unit, no need to replace the whole distributor. Just make sure you get the correct vacuum unit, there are loads of different ones that look the same externally.

I have had dizzy vac advance unit not working on 25D unit, when I took it out there was no movement in the threaded rod coming from diaphragm.

A few gentle taps on end of rod freed it off, now works OK.

I think it must have had bit of moisture getting inside caused corrosion and the two parts stuck together. Other cause for not working would be split diaphragm, in which case you need to replace the vac unit.

There are 2 long and two short HT leads...

Identify the lead that is the same length as the No1 lead,
then remove and swap these two leads round.

If the problem moves from No1 cylinder to the "other" one,
the problem will be in the lead. If the problem is still in No1,
it could, as has been mentioned, be an internal short in the
distributor cap.

Are you sure that it's fuel on the plug and not water?

Ian

ianh1968 wrote:There are 2 long and two short HT leads...

Identify the lead that is the same length as the No1 lead,
then remove and swap these two leads round.

If the problem moves from No1 cylinder to the "other" one,
the problem will be in the lead. If the problem is still in No1,
it could, as has been mentioned, be an internal short in the
distributor cap.

Are you sure that it's fuel on the plug and not water?

Ian

Thanks Ian, I will try it tonight.
I am assuming that it is petrol. It smells of petrol, the cylinder and top of the piston is dry. Interesting though, I fitted a new head gasket last year after an overheating problem. I have done a compression test and all 4 cylinders are at 125-130.
If water is entering in the cylinder, would it still show the same compression as the other 3 cylinders?

carbon wrote:I have had dizzy vac advance unit not working on 25D unit, when I took it out there was no movement in the threaded rod coming from diaphragm.

A few gentle taps on end of rod freed it off, now works OK.

I think it must have had bit of moisture getting inside caused corrosion and the two parts stuck together. Other cause for not working would be split diaphragm, in which case you need to replace the vac unit.

Thanks Carbon, you are correct, it is the 25d. I will try what you mentioned. It seems to be the original factory fitted dizzy from almost 50 years ago, I am tempted to replace it. There is a new 45d available in a local auto parts store (nos).

Sorted, changed the leads, made no difference, changed the coil and the problem resolved.
Thanks.