Carburettor Woes (or something else?) - Help

[HoveActually]

Not a new problem but I'm really struggling to fix it - what am I missing?

My 1982 Sport (standard in every respect) has developed a horrible hesitation on opening the throttle, cold or hot. From ~3700rpm to the red line it pulls like a train and goes like a Sport should. Returning from a good run, both plugs are the correct colour, which suggests to me the needle position (middle groove) is OK. But between 2300 & ~3500rpm its B awful, pulling away from T junctions is fraught, unless I hold 4k rpm. Starting is quick (with electric starter) with just 1 cylinder needing to be choked. And both chokes seal OK.
The carbs are not unduly worn and are standard in every respect, jets, needles, slides, atomiser, etc. I've given them the ultrasonic treatment, checked and blown thro every passage way, renewed all rubber seals, washers etc and all rubber manifolds. Float height is correct. Standard air box has new filters.
Valve clearances are good (0.1mm) timing checks OK with strobe light and both cylinders correct at tick-over (1800rpm) and 6k rpm and the advance seems to progress as it should. I've changed idle jets, balancing the carbs with vacuum gauge, even going down to 35s which gave very erratic tickover. What is apparent is the idle mixture screw adjustment on both carbs is very insensitive and with 40 & 45 size idle jets, they have to be screwed fully home to get best tickover, and I've still got the hesitation. Is that the clue I've so far been unable to solve?

By the way I have a 1978 Strada which has perfect fueling and carb set-up is wonderfully straight forward - as per the text book.

[Daddy Dom]

Are both petrol taps flowing freely? Just a thought.
DD

[Bison]

Blimey, you've checked everything I would check!, my Sport had a very similar problem when I got it and it transpired the cam belt was on the wrong tooth, but I guess this problem suddenly appeared after your bike had been running well? In the flat spot have you tried flicking one of the chokes on?, see if that made a change?, could it be a plug or plug lead problem? (unlikely). Air leak?. Tank filler cap vent blocked? Do tell when you find it.
Good luck,
Alan.

[tobydmv]

Sounds like its too lean at idle. You might have an air leak. Try spraying some carb cleaner around the rubber boots and intakes and listen for rpm increasing. Mixture screws should be 1/2 - 1 & 1/2 out right?

[3potjohn]

This sounds so like a problem with a pilot jet pathway on other carbs.I have had a sort of waxy sh*t in them which really doesn't want to be cleaned off.Starts OK then hideous until throttle properly open then full power..Not good in traffic.

[EVguru]

My 1982 Sport (standard in every respect) has developed a horrible hesitation on opening the throttle, cold or hot. From ~3700rpm to the red line it pulls like a train and goes like a Sport should.

Developing a fault, particularly if it's on both cylinders suggests an ignition problem.

What is apparent is the idle mixture screw adjustment on both carbs is very insensitive and with 40 & 45 size idle jets, they have to be screwed fully home to get best tickover, and I've still got the hesitation. Is that the clue I've so far been unable to solve?

Possibly. You should be able to get each cylinder to cut out by leaning out the mixture and also to cut out (or at least missfire) by richening it up. Having to set the screws nearly all the way in suggests that you're unlikely to be running lean due to a blocked circuit or air leak. You can get some strange effects if the ignition timing at idle is wrong. Advancing the timing for a given idle stop setting will raise the rpm and also increase the idle vacuum (pulling in more fuel). Retarding the timing will lower the rpm and If the idle stops are screwed in to compensate you can end up coming off the idle circuit and onto progression. Progression mixture is controlled mainly by the idle jets and the slide cutaway. An over rich progression mixture often gives rise to a 'stumble' when pulling away. Modern fuel (which is better stuff in many ways) seems to cause this on Morinis because of the high proportion of 'aromatics'. The effect often gets worse the longer you spend idling as the excess fuel 'wets out' the manifold and port.

Have you checked the timing on both cylinders?

[HoveActually]

Thks to everyone for their input.
I've just spent the morning with multimeter, carb balancing and strobe light attached.

I think EVguru is right. I seem to have acquired an ignition/timing gremlin. I've rechecked wiring connectors/earths - but all are good.
Both cylinders seem to have very good sparks - engine fires/runs immediately (using electric starter) and will rev out provided you ignore the stumble!
However the more I've fiddled the more I'm becoming convinced at least 1 of the transducers is faulty. I've swopped them back & forth between cylinders and it seems that while both are providing healthy sparks, one of them now exhibits no or, a very erratic, advance of my paint dots/rotor marks when motor is revved. The other shows the sort of steady advance movement you'd expect/hope for.
The rear cylinder (carb) when fitted with the possibly good(?) transducer now seems to respond to the idle mixture screw as specified - hurray. However when the possibly good transducer is on the front cylinder that carb still won't play ball. God help me.
However another interesting fact has emerged - the (possibly) good transducer changes (increases timing advance) slightly depending on which cylinder its servicing. I think this is a known problem and I've read somewhere that you can bring the timing of the 2 cylinders into alignment by inserting a small resistor into 1 of the pick up leads. That is a refinement I might try once I've sorted my main problem.
So now I will try to get my hands on another good transducer - I just hope NLM have one that matches my pickup.

[hendre]

So now I will try to get my hands on another good transducer - I just hope NLM have one that matches my pickup.

I have a box of trnsductors that are checked by moriniservice.nl... many numbers floating around in there

[HoveActually]

So now I will try to get my hands on another good transducer - I just hope NLM have one that matches my pickup.

I have a box of trnsductors that are checked by moriniservice.nl... many numbers floating around in there

Hendre - thks for that. PM sent.

[HoveActually]

My Sport is now sporty again - hurray! However the fix was a long struggle & headache inducing
A new transducer has given the rear cylinder its proper advance characteristics. But that still didn't cure the carb adjustment problem.
So after re-reading many times, EVgurus comments and paras 3.4 & 3.5 of the Dellorto Carb Tuning Guide here
- http://www.ducatimeccanica.com/dellorto ... lorto.html - I decided to try lowering the fuel height in the bowls in case high fuel levels were a contributory factor to the rich running at low speed by messing up the idle / progression circuits.
Resetting the float heights to 11mm instead of 10 as per the book gave me my eureka moment. I had previously marked the float centres using a digital vernier so I could be sure of my measurements. The carbs now respond exactly as they should to the idle mixture screws which give best tick-over at ~1 turn open - and both are quite sensitive - 1/4 turn either way and she goes lumpy. Wayhay. Tick-over is now a very steady 1500rpm with the carb balance pipe fitted or with the brass stubs capped. I'm presently running her with the balance pipe fitted.
So the stumble / hesitancy on opening the throttle has completely gone (using 45 size idle jets) and she pulls cleanly in any gear (even 6th) from 2000rpm. But from 1/4 throttle to 3/4 throttle the engine sounded a little dull, i.e. a bit rich! The cure for me was to lower the needles one notch. Maybe they are slightly worn. Wow - acceleration is really crisp throughout the rev range and the motor sings like a bird - sorry I'm getting a bit carried away now, but this has been a long time coming. And finally plug chops both at WOT & at 1/2 throttle show the mixture is bang on.
The wife has now withdrawn her instructions that I sell it.
Anyway - thanks to everyone who offered advice and I hope that if anyone else has similar problems my cure may help.