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High Idle


Darren Groves
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Hi All,

Started my Spitfire MK4 for the first time, full engine rebuild etc.....

Started fine but can't get the idle down, the front carb is pulling in too much air even though idle screw fully wound out. Started to investigate and it was pretty obvious what the issue was (see pic). In the fully closed closed position the throttle disc isn't a great fit, so swapped with the throttle disc with the other carb exactly the same...so regardless of which throttle disc I use the problem stays with the front carb. The carbs were fully rebuilt by me last year, so the discs, spindles etc. were all replaced with new items from Burlen.

I rebuilt a fair few HS2's in the past, but never had a disc not fit well....any tips of tricks? Replacement Carb Body?...there is one on eBay at the moment.

Cheers
Darren

 

DSC_0185.JPG

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I agree with Rob the disc may not be centred correctly. The disc should be centred with the throttle fully closed, throttle screws and choke cam dis-engaged. Undo the two screws holding the disc to the spindle and centre the disc Then move the spindle in and out to ensure the disc is in the centre of the spindle slot.

Dave

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Are you sure it is this that is the problem - given that some air has to get in the engine at idle.........

I had a massive problem with my spit idling too high and it turned out to be a leak on the manifold to head flange. Can easily prove it by running the engine and dropping a bit of oil on around the mating face - which should drop the idle as it seals it - or a bit of spray brake cleaner which may raise it - though can also seal for a bit. Petrol is good - if you are brave ;)

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3 hours ago, dave.vitesse said:

I agree with Rob the disc may not be centred correctly. The disc should be centred with the throttle fully closed, throttle screws and choke cam dis-engaged. Undo the two screws holding the disc to the spindle and centre the disc Then move the spindle in and out to ensure the disc is in the centre of the spindle slot.

Dave

Hi Dave, 

Have done many of these over the years, this is how I fit them and I retried again yesterday, but it just wouldn't sit right. 

I ran out of time last night, so will have another bash on Sunday when I have a day off.

Cheers 

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2 hours ago, Anglefire said:

Are you sure it is this that is the problem - given that some air has to get in the engine at idle.........

I had a massive problem with my spit idling too high and it turned out to be a leak on the manifold to head flange. Can easily prove it by running the engine and dropping a bit of oil on around the mating face - which should drop the idle as it seals it - or a bit of spray brake cleaner which may raise it - though can also seal for a bit. Petrol is good - if you are brave ;)

Pretty sure, doesn't take much of a gap around the throttle disc to bring up the revs. I did spray easy start around the mating faces with no change in revs. 

Cheers 

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They normally supply those butterfly's in service kits, though I've never understood how they can wear around the higher and lower edges (maybe the sides could a bit), as they are normally sitting away from the carb body slightly, to allow for idle, I would have thought?. 

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I tend to fit them with the screws slightly loose, then snap them closed from fully open (let the return spring do it's thing) THEN tighten the screws if I can't see any gaps.

Sometimes a repeat fully open, snap shut is needed to "shake" them into place. If the disc is tight in the spindle a "jiggle" of the spindle might be needed.

Edit note added: Above procedure with the idle control screw backed well off!

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The test used to be, would the butterfly grip a cigarette paper , all around.

In the abscence (I hope) of a handy Rizla packet, I use my finest feeler gauge, 0.02mm. less than 1 thou.  It can take a while, and even a tiny gap can make the difference.

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