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Sticking Carbs


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What happens:  Foot off throttle and the idle will stay up at anywhere between 1000 - 1500 rpm.

OK, so many in the past have suggested things like the chokes not closing completely or air leaks.  Not either of these.
It is purely a physical problem.  Clue as to why?  If a blip the throttle the idle most often does come down further to normal, i.e. 500 - 600 rpm.  In fact with the bonnet open I can even pull on the linkage to get the idle back down.

Why is the idle staying higher than it should?  I had rebuilt the carbs around 8 years ago.  Recently someone suggested using a lighter (10W40) oil in the dashpots.  Another recent alteration was to fit new butterfly spindle return springs, thinking the old ones were weakened - they were a bit.  I even added an extra return spring on the central linkage pedestal.  All has not helped.
I added a small rubber band folded double.  This did help but it only lasts 2 or 3 days before breaking.
The little extra spring did work at first for several years but the problem has gotten worse as of recent.

Something in the linkage or butterfly spindles area is obviously sticking or hindering free movement.  
Tips, pointers or suggestions are welcome.  Short of taking the carbs apart for a thorough going over and adding an even heavier spring (something I feel is only countering the symptoms) I have no idea where to go from here.


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Quoted from StagNL
Recently someone suggested using a lighter (10W40) oil in the dashpots.

Can't imagine why. The dashpots have almost nothing to do with idle speed, and from what you've said it's clearly sticky thorttle. The usual reason for suggesting thinner dashpot oil is a complete lack of understanding of how these CD carburettors work.

I had a similar problem on the Toledo at one point, but fitting a secondary return spring sorted it. The problem could be in the cable - which you can probably check by disconnecting it. Open the throttle at the carbs and let it close slowly. If there's no hang-up it suggests the spindles and linkage are OK and the cable is the problem.

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Each carb has a vent lever that acts on a vent piston. These can stick and cause problems such as you describe. Find the vent piston on your carbs and try freeing them up by moving them in and out whilst lubricating with a thin oil.

This picture shows the lever:

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make sure there is some free slack in the cable when the outer heats up it expands and tension holds the idle up .

if they have temperature compensators on the side make sure they are sealed with new o rings (2 on each) and the plastic plunger is adjusted to be closed at normal operating temperature,  ie turn the small nut in to close the valve, if they are open you will never adjust the idle /mix correctly


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Thank you gentlemen.  I will have a look at the suggestions.  Cable I have ruled out.  The cable operates the primary operation lever.  This is separate to the secondary and there is free movement between the two by design, i.e. the primary takes up slack before the secondary is activated.  
See clip:  https://www.youtube.com/watch?v=FOP61oVuOck

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  • 2 weeks later...

I hope I have found it. There seems to be more things happening at once. The LH vent valve that shuts off when the throttle is closed was not opening. This is opposite to suggestions that it might be sticking open. Secondly I noted the replacement butterfly spindle return springs were slightly wider than the originals. This resulted is the spring pulling a bit on the spindle to hinder its return. Markedly more on the RH carb. Strangely though, this probably was happening with the originals too. Anyway, the posted photo shows my solution - I bent the lever arms so the spring has a much straighter pull.
On the bench both carbs work as they should.  However on the road the problem was still present but there was an improvement.
I have resorted to using my extra spring again and this does work as it did before.  This is treating the symptoms though rather than solving the problem.  Oh well, as long as it works....

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