Cota 247 Carbie

[Tee-Why]

One things springs to mind is the vent lines, or overflow lines (new lines?). Are they clear and not restricted, and even though you have been riding around and tuned it to run since rekit, is the fuel filter and flow from tap ok, not blocked.
I had a bike some time back that suffered from the screen filter inside the tank for the fuel tap being coated in old fuel, and restricted the flow. Need to clean the whole tap too if you can pull it apart. (I saw one of those riveted types on the w/e that couldnt be dismantled!)

How is the air filter, not over oiled? This restricts air flow and would be making it run richer.
However the above would be noticeable over the whole rev range, but you may have tuned the bike over the partial restriction.

Another possibility, hows the fuel tank cap, breather line free and not restricted?

[keychange]

Yes I have checked fuel flow (would affect normal running), replaced the fuel and washed and very lightly oiled the filter and as I say it's running really very well except at warm idle. If it stalls when cold it is easy to restart but when it stalls from warm it doesn't want to fire - suggesting the plug is loaded but when I check it, it isn't (or doesn't appear to be) if I kick it over with plug removed there is no fuel vapour or wetness around the plug hole - which therefore suggests it is leaning off... an air leak which opens when warm would do that wouldn't it. I will check for that .. although if it's the manifold joint the crap alloy of Amal warps when tightened onto the manifold so it won't be easy to fix I will play around and test this line of thought.

If that doesn't work I will replace the float back to the old non-adjustable unit and then that only leaves the pilot jet which I have again removed and checked - I should compare the pilot jet size to make sure it is correct. It's amazing that a 2-stroke carbie that is pretty basic, how many things can affect the idle?

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I have tested for air leaks and no signs once again when it stalled I removed the plug and it was lean - I replaced the plug with brand new B6HS ( recommended) and restarted letting it run under low throttle for 30 seconds - released the throttle and she died. And again at every restart after that.

I came back to my desk e to update the post and as I wrote "as I see it if float level was low it wouldn't only affect the warm idle it would struggle from cold. If float level was high it would be a rich start which would be good while cold but then would be too rich when hot - but the spark plug is lean - as I wrote I thought it has to be Pilot Air (idle jet) screw . I have had this in and out a dozen times each time screw it in then 1.5 turns back then adjust in until splutter then out until it falters then back in 1/4 turn or until smooth and then lower revs and repeat. This time I upped the idle speed to around 1200 and turned the pilot screw in slowly a full turn and as I did so she started to splutter then be buggered as I kept turning she came good and ran smooth again. I lowered the revs to something like 800 and took her for a quick warm up run - then pulled up and she idled perfectly for 3 minutes so I lowered the revs to what I consider something around 500 ie: just comfortably above labouring and she idled happily for another 3 minutes. I think it's fixed. Weather permitting tomorrow I will go for a decent ride and take a spare plug just in case she fouls.

[keychange]

Happy to confirm that the idle problem is fixed it's nice and smooth and constant. The idle mix screw is less than one turn from full tight and yet after a 30 minute hard hilly run the plug still looks off-white - is it safe to keep turning the screw in ( it starts to splutter if I try) - I can only deduce that the new pilot jet installed is producing a much leaner mix than the previous jet - or in all my other adjustments the float level is too low, but performance is very good so unlikely I would have thought.

[David Lahey]

What plug are you running?

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David - I am back on the recommended NGK B6HS - previously B7HS

[David Lahey]

The idle air screw only affects the mixture when the throttle is closed so if you are going hill riding, don't expect that adjusting it will make the sparkplug look any different. If you are tooling around in tight trials type riding it would make a difference to the sparkplug appearance.
If you are going to flog it a bit on big hills, or along a road at speed, the 7 plug would be a safer choice, and keep the 6 for when you ride in a trial.
Modern fuels do not give the same visual messages on sparkplug noses that "super" petrol from the 1970s did, and if you use reference photos from a pre-unleaded fuel era source, you will be sadly disappointed trying to achieve the nice light brown colour we used to get back then. Plug nose colour is not the definitive tuning tool it used to be. If you can, stick to one fuel brand and type, and hope that they don't change the formulation too often, and you might get used to what you plug looks like in different riding conditions, and also when you make changes to ignition timing or fuel/air mixture.
I would set the fuel level to standard and leave it there, or you might have flooding problems when you start riding down big drop-offs and extreme steep downhills. AMAL Mk1 concentrics are notorious for going rich when the front of the bike is a lot lower than the rear.
An AMAL Mk1 concentric in good condition with standard settings will give a conservative, safe mixture provided there is no air leaking into the motor.

[keychange]

Thanks David

I wasn't really flogging on big hills just doing slow 2nd gear climbs and descents on fairly long around 45+% long paddock inclines ( decompressor works well on the descents) and then a 2 km mainly downhill run home in 5th at around 3500 rpm. The engine ran very sweetly all the way with very good response across the rev range.

You might not believe this but where I live we only have one garage within an hours drive so fuel brand and type is very limited :)

My point was that the recent idle stalling problem always showed up as lean and although I have screwed in the idle mix at least 3/4 turn which has fixed the stall problem the basic running still appears a bit lean. I take on what you say about modern fuel - however the plug prior to carbie refit was usually more sooty black (with mediocre performance) however I would rather foul spark plugs than burn holes in my piston so I would prefer a bit of oil to be showing. I'll try and get a decent 1 hour run and take some close ups of the plug. It' just that now that it's finally running well I don't want to damage it due to running too lean a mix - yes I'm paranoid but I do so want to have a serious ride one day soon.

[cedric van heerden]

Hi Andrew, where did you get your carbie kit?
Cedric

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Cedric

Go to http://www.amalcarb.co.uk/ - takes just over a week to here here.

Cheers

Andrew