Just trying to reassure myself I have not missed something.
1275, has been running very well but last time out developed a hesitation or misfire mainly at heavy throttle low speed which recovers if throttle is closed more. But also shows at low speed light throttle recvering as throttle is opened more.
I assumed it was stale fuel so put fresh 98 into tank which had perhaps 3 or 4 litres left. The problem improved somewhat but still remained slightly. Now seems to be back to previous poor running.
My next move is to totally drain the tank and fuel lines etc and start anew with fresh fuel.
Anyone have other suggestions than stale fuel?

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Cheers Dave.

SU damper oil level

Check the plugs aren't fouled up and the gaps are correct
Leads have good contact and resistance isn't too high
Points/rotor/cap all in good condition and points gap set correctly
Coil measures in spec

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Yeah sorry, should have said I have checked all the obvious usual suspects. My bad.
Oil level in damper normal
Plugs checked and show dark brown as usual.
Leads are 6 mnths old spiral steel. I will check resistance though
Cap and rotor fine.
New clean air filter fitted.

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Cheers Dave.

Anything to do with vacuum advance system if fitted, on the distributor? Leak in the tube or slight rupture of the diaphragm?
Maybe the carburettor piston not moving freely up and down in the dashpot? The oil in the dashpot doesn't lubricate the slide of the piston.

Dave,
You said in your first post, that your next move was to drain the fuel and replace with new fuel. Quite honestly that would be my first move, however replace with 95 octane plus an octane booster. It might cost a little more, but it is certainly a much better fuel than 98 octane. 95 with octane booster lasts for months rather than weeks.

GR gave me this advice a couple of years ago. He explained that 98 octane is too unstable. I just hope the new rules on fuels to be introduced in Australia, do not have a negative effect on 95 octane.

Good luck Regards Jon

I hate guessing but the ignition coil could be faulty, the KV demand is high under heavy throttle. Easier to change than draining the fuel.

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

Thanks guys, all good points.
No vac advance but will check piston.
I have not heard that about 95RON but the only options here in the sticks is 91 or 98. I will keep in mind but I am not convinced on octane boosters.
I had wondered about the coil amd your point is valid.
It is not too hard to drain fuel and as that is my gut feeling on the problem I will start there. If it is the fuel it brings other ongoing problems.
I will report back.

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Cheers Dave.

NGK plugs? I bet they've shat themselves

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did I tell you that I won a trophy?

Yeah I had that problem a few years back but I have had a good run so had forgotten the ngk plug issue. To be honest I am not sure wht is in there at the moment, I have many varieties here but I will check.
My money is still on the stale fuel though.

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Cheers Dave.

Well, its not stale fuel. Drained the tank, lines and float chamber and using fresh 98 from a high turnover servo the misfire/stumble/hesitation is still there and if anything is actually getting worse. After a run this morning I did a quick check on timing and mixture while it was at running temp and the mixture is showing quite a bit richer at idle than what I set it to. Timing is good.
So now to fault finding. Happy days.

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Cheers Dave.

running rich is a good sign that the ignition isn't doing what it should - plugs and coil

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did I tell you that I won a trophy?

Checked the plug leads?
I throw them away if over 5k ohm, or if resistance changes when bent about. Inductive core leads last much longer than the cheapo graphite impregnated string ones.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thanks guys, yep ignition was my thoughts too.
Leads are 12 months old spiral steel core jobbies but I will be checking all ignition parts asI work through it.

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Cheers Dave.

A bit cosy in the shed this arvo so not much done.
Coil measures ok and gives fat blue white spark.
Leads measure well within tolerance.
Replaced plugs.
No change so out of curiosity I adjusted the mixture. As expected it did not fix it so set back to original setting..
As I was packing up I put the timing light on all 4 leads and 1,2 and 3 were fine but No 4 shows an irregular misfire so I have something to chew on.

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Cheers Dave.