Morris Minor Owners Club

Well - there should be a little raised part on the bridge of the carb - that's designed to stop the piston going right down so no air can get in. That hasn't been 'cleaned' off has it ?
Other idea - assemble the carb damper etc - but put NO oil in there - to see if that helps ?

i havent noticed a little raised part, but ill have a good look tomorrow, and ill try the no oil in the carb.
will report back on the findings as well as picture of the innards of the carb

it really does sound like the piston's binding and not rising properly.

You've centred the needle, cleaned everything off.

A bit of a crazy thought, is the needle/jet the right combination? i.e. could the needle be getting jammed in the jet

is the is the piston nice and clean, free from marks, scratches, gouges

Are the needle and the piston straight? i.e. not bent?

Andrew

It really is a strange one ! Other possibilty I suppose - is the spring too strong - has it been modified in any way ?

its just a normal spring that came with the carb.

well i got to look at the carb today, i recentred the jet (just to be sure) and gave it another clean and emptied the dash pot and went for a little drive, all was well and it worked.
so for some strange reason my carb piston goes flat against the jet, yet it still starts and it runs without any oil in the dash pot.

so the life of me, i cannot work out why this is the only su carburettor in existance that does this.

The only thing I can think of is that the spring is way too strong or way too long.....?

its the original spring to the carb and the whole thing is about 3 years old

It should start and run with no oil in the dashpot.

If you open the throttle very slowly (car stationary) the revs will rise, but if you snap it open, it'll pop and bang. The oil is there to slow the rate of rise of the piston and provide a momentary enrichment (like an accelerator pump on a weber) without it, the piston rises too quickly and the mixture would go too lean for acceleration (If you knew that, apologies I'll shut up )

If it accelerates with no oil in the dashpot, maybe it's way too rich?

how far below the bridge is the jet? You may be able to see from the inlet, or take the dashpot off and have a look. IIRC 2.5mm is about right as a starting point.

If you look down the inlet tract when you open the throttle and this strange running happens, can you see the piston rise?

Is the damper or damper tube bent?

Andrew

Andrew

how does a daper tube become bent?. i replaced the whole damper with another one, but both where not bent.
the piston does rise when the throttle is opened and closes again.
not sure how i can measure the distance between the bridge and the jet, its abotu 2.5 turns down on the mixture adjustment.
i did give the car a good run (about 5 miles) on an A road today, as well as in and around Grantham, and except reving slightly higher, it was fine.

Well - just continue with it and be glad!

thats the plan

how does a daper tube become bent?.

No idea, just a suggestion, I was running out of them! stranger things have happened.

not sure how i can measure the distance between the bridge and the jet, its abotu 2.5 turns down on the mixture adjustment.

2.5 turns sounds good, I use a vernier calliper to check the distance, mainly as a starting point, and usually with twins to make sure they're both the same before starting to tune.

i did give the car a good run (about 5 miles) on an A road today, as well as in and around Grantham, and except reving slightly higher, it was fine.

Fantastic. Sounds like you may have cracked it somehow. Keep driving it!

Andrew

My 'racing' Mini went best of all (when doing a Sprint in the 'single carb' class) with NO damper fitted to the carb. It had a horrible flat spot and would have been hopeless on the road - but on the track it was possible to anticipate - and floor the throttle early - it won it's class!

Re-reading this, 3 thoughts come to me:
You say the timing is fine- did you check with a bulb, or have you actually checked that the points gap hasn't closed down? Can still appear to be fine statically when trying with a bulb even when there is hardly any gap!

The piston going hard down worries me- there should be a little round plastic bung thing inset in the underside of the piston which protrudes a small amount so as to keep a small gap under the piston when it is down.

Is the vacuum advance diaphragm holed? Combined with a lack of the little round plastic thingy in the piston, that could be bleeding air into the carb and mucking things up.

i checked the points a while ago, gave then a good clean and checked with a set of feeler guages. i cannot remember what it actually was as i have just woken up.
as for the platic bung, it is noted for its abscence, as it has been since i bought the carb new a few years ago.
finally, the vacuum advance is fine, i know this due to having accidently upset the timming whilst trying to find the cause of the bad running, which has given it a backfire, but turning the wee knob on the bottom of it will fix that (though which way will be a case of trial and error)

Well - the missing 'bung' is the answer - I pointed to this a while back - that there should be a raised section to stop the piston closing completely. I didn't know it could be plastic ?

i am actually quite confused, the bottom of the float is flat, just the needle sticking out of it. the bore of the carb is flat, with the brass jet in the middle.
their isnt any provision for a plastic bung, and the carb is nice and smooth and hasnt had any raised ridges to stop the carb going flat to the bottom of the bore. its just a standard carb bought from a reputable morris minor spares place.

If it's still running - don't worry about - enjoy!

now thats the problem, its running like a bag of cats atm. it backfires, stall, power comes and goes and its a total pig to start. i am going to check everything thoroughly again tomorrow, from static/ignition timing, right through to filters, all electrical connections and cables. if that dosent locate the problem, ill stand and scratch my head for a while and use the scooter instead for a while