my two bobs worth, ok my carby was worse then whats happening
but there are little things such as the float being wrong (the thing inside)
as well my float chamber was not sitting right was angled a bit.

i dont suppose fixed or floating needles play a part?
could it be also that some cylinders are getting more fuel or air
then others check your spark plugs

Dashpot oil is just another way of making money for no real advantage. Have a look in any BMC workshop manual and see if you can find a car that has a requirement for dashpot oil from the manufacturer.
It is just oil. It is the correct grade of oil for some cars but it will not be the correct grade for every car.

ok, spent another 4 hours testing this afternoon.

i connected 2 fuel pressure guages, a vacuum guage and went for numerous drives with different settings so i could see the guages as i drove in varying loads/roads etc.

tested the fuel pump and it is putting out a constant 7psi.

tested after the fuel pressure regulator, set at 5psi, is outputting 2.5-3psi depending on load.

checked the earth strap, all good there.

tested spraying carby cleaner over carb while idling, very very small change but not really an amount i would expect to be causing this issue. maybe 20-50rpm change, no missing or massive rpm change. also tested all over the intake manifold for leaks, none found.

i dismantled the carb, dizzy and manifold, and ensured everything is 100% as it should be and setup correctly, lubricated etc etc.

best results after this 4 hour session, number 5 needle, red spring, 3 flats out from initial setting got the best idle mixture, and timing set to 10 degrees at idle and a max of 30 degrees at 4000rpm.

with the above, plugs are a nice even brown colour. exhaust tip is a nice colour also, no black smoke when accellerating etc etc. mixture seems close to where it should be i think.

with the above the car idles good, is good at low loads low rpm, and good at wide open throttle, but at that same spot 3500-3750 on the highway at light loads pulling up hill slightly it is still surging. not missing just a surging you can feel through your seat.

it does feel as though there is a starvation of something or a surging of something.

i also checked spark flutter at this rpm with my dialback timing light and the spark is solid all the way through the rev range. there is no spark fluctuation.

i noticed the piston in the carb has a lot of play in it, is that normal? could a worn piston cause this issue?

watching the guages, vacuum is fine, doesnt seem to flutter or vary at any given throttle amount, and fuel pressure is fine right through the range before and after the reg.

any further suggestions guys? any other tests i can do to try and solve the issue?

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'66 BRG Morris Mini Deluxe 60HP ATW

Autobarn. Penrtie $9.90 for 150ml. (Sorry thought it was 500)

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Cheers
Alec
1275LS - July 78 - XNFAD18Y/102918

No not normal to have a lot of play, yes could cause surging because it relies on the vacuum to raise and lower the piston and therefore the needle.

Also check the butterfly spindle is sealed properly. A leak here is common in worn SUs.

Finally, seeing as you are "feeling it through the seat", it could actually be a resonance in the exhaust system. I have often found that at 3-4000 rpm with certain exhausts that they resonate badly. A change in muffler or pipe length will usually fix this problem.

cheers
michael

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the world is a book, and those who do not travel read only a page
66 Mini Minor sponsored by http://www.lifeonthehedge.com.au/ The Dog Harness Specialists
It was a pleasure ausmini. I'll miss all you misfits and reprobates

Try and buy a little bottle of sewing machine oil, HANDY MAN OIL. It works wonders and has done so for many years.

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DID I MENTION THAT I AM THE BEST SUPPLIER FOR MINI ELECTRICAL PARTS AND REPAIRS?
"Let us remove your "SHORTS"

The timed drop test of the piston would show if it was worn.

Steve

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1964 998cc Cooper
1971 Mini K x 2
1972 Clubman GT

Yes, that is the purpose of that test.

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