Darren Groves Posted May 14, 2020 Report Share Posted May 14, 2020 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 Link to comment Share on other sites More sharing options...
Pete Lewis Posted May 14, 2020 Report Share Posted May 14, 2020 Daren not wishing to seem "£$$%^& but is the disc the right way round , ??? pete Link to comment Share on other sites More sharing options...
Darren Groves Posted May 14, 2020 Author Report Share Posted May 14, 2020 1 minute ago, Pete Lewis said: Daren not wishing to seem "£$$%^& but is the disc the right way round , ??? pete Haha....yeah, it's definitely in the right way. I think if you try them the wrong way, the disc is a long way off closing. Darren Link to comment Share on other sites More sharing options...
Pete Lewis Posted May 14, 2020 Report Share Posted May 14, 2020 didnt think you had that wrong , well the only other idea is the choke fast idle lever or one of the nasty bits of interwoven links is holding it open some how let us know when you solve it Ha ! pete Link to comment Share on other sites More sharing options...
Darren Groves Posted May 14, 2020 Author Report Share Posted May 14, 2020 Backed off the idle and fast idle screws to ensure no interference....will get it on the bench on Sunday to see what's happening.... Cheers Link to comment Share on other sites More sharing options...
NonMember Posted May 14, 2020 Report Share Posted May 14, 2020 It may just be the photo but it looks to me that the disc isn't quite centred on the shaft - sitting a fraction high - which would possibly cause the problem. Link to comment Share on other sites More sharing options...
dave.vitesse Posted May 15, 2020 Report Share Posted May 15, 2020 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 1 Link to comment Share on other sites More sharing options...
Anglefire Posted May 15, 2020 Report Share Posted May 15, 2020 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 1 Link to comment Share on other sites More sharing options...
Darren Groves Posted May 15, 2020 Author Report Share Posted May 15, 2020 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 1 Link to comment Share on other sites More sharing options...
Darren Groves Posted May 15, 2020 Author Report Share Posted May 15, 2020 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 1 Link to comment Share on other sites More sharing options...
daverclasper Posted May 15, 2020 Report Share Posted May 15, 2020 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?. Link to comment Share on other sites More sharing options...
Darren Groves Posted May 21, 2020 Author Report Share Posted May 21, 2020 Problem sorted....finally got the butterfly to sit correctly, idles nice now....👍 1 Link to comment Share on other sites More sharing options...
daverclasper Posted May 22, 2020 Report Share Posted May 22, 2020 Hi Darren. Well done. Any particular tips on how you did it?. I couldn't get one quite right, that I had managed ok before. Dave Link to comment Share on other sites More sharing options...
yorkshire_spam Posted May 22, 2020 Report Share Posted May 22, 2020 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! Link to comment Share on other sites More sharing options...
JohnD Posted May 22, 2020 Report Share Posted May 22, 2020 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. Link to comment Share on other sites More sharing options...
daverclasper Posted May 23, 2020 Report Share Posted May 23, 2020 12 hours ago, JohnD said: In the abscence (I hope) of a handy Rizla packet Cheers folks. Always got plenty of the thin blue Rizla's, so I'll remember that. Link to comment Share on other sites More sharing options...
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