Pete Lewis Posted May 16, 2020 Report Share Posted May 16, 2020 two things stop a piston dropping with a nice clunk onto the bridge a stiff thicker rubber diaphragm the needle is fouling the jet as Johny says needs re centering undo big nut 1/2 turn give it a light tap and re nip recheck the drop . to detcect whats going on with piston lifting you only need to lift it a couple of mm and then theres a hint of change a small speed up rich a small drop weak no change ok the hiss on the rear is probably correct the lack of hiss on the front as said needs the throttles balancing the air flow , so need to be set pretty much both open the same and needs the rod clamp loosening and set both idle screws turned in 1.5 turns from just touching the casting boss. then re nip the rod clamp if in doubt bring it over and i can twiddle it at 6ft away Ha ! Pete Link to comment Share on other sites More sharing options...
NonMember Posted May 16, 2020 Report Share Posted May 16, 2020 As others have hinted, that sounds to me like the front throttle plate is fully shut and the engine is running entirely on the rear carb. Adjust as Pete says but beware of slippy linkages. Both my Vitesse and GT6 have, at various times, suffered from a loose linkage resulting in the front carb not opening. On the GT6 it just made it feel gutless - nice and smooth still but no power - whereas the Vitesse managed also to overheat (but the difference is probably down to having slipped while driving in the Cairngorms whereas the GT6 did it in East Anglia) Link to comment Share on other sites More sharing options...
Robin Posted May 16, 2020 Author Report Share Posted May 16, 2020 Hi Johny thanks for the reply. With the engine off the piston on both carbs rises and falls correctly but when I start the engin the piston On the rear rises slightly on tick over (about 700 rpm) but falls when switched off. When accelerating, , the piston on the rear carb rises before the front one. The other thing is that no matter how high I raise the piston on the front carb it makes no difference the the engine note so I’m not sure if this is a problem and if it is how to fix it. Any ideas? Link to comment Share on other sites More sharing options...
Pete Lewis Posted May 16, 2020 Report Share Posted May 16, 2020 it could still be the front carb is low on fuel when you lift it ....how far ??? as suggested do the two throttle plate actually open ..is the front one staying closed ??? Pete Link to comment Share on other sites More sharing options...
daverclasper Posted May 16, 2020 Report Share Posted May 16, 2020 On 05/05/2020 at 09:37, Pete Lewis said: tip the carb upside down and it should be 17/18mm gap between the float and the casting ( the float will be at an angle , measure the shortest gap) Hi Pete. I think I have always measured the the longest distance between the bottom of float and gasket face, which on reflection, is not correct. Guess a removal of carbs to check this is in order?. Cheers, Dave Link to comment Share on other sites More sharing options...
Robin Posted May 16, 2020 Author Report Share Posted May 16, 2020 1 hour ago, NonMember said: As others have hinted, that sounds to me like the front throttle plate is fully shut and the engine is running entirely on the rear carb. Adjust as Pete says but beware of slippy linkages. Both my Vitesse and GT6 have, at various times, suffered from a loose linkage resulting in the front carb not opening. On the GT6 it just made it feel gutless - nice and smooth still but no power - whereas the Vitesse managed also to overheat (but the difference is probably down to having slipped while driving in the Cairngorms whereas the GT6 did it in East Anglia) Will try and check everything as Pete suggests but the performance seems ok - not down on power. Can’t work out why raising the piston on the front carb has no effect on the engine revs . Link to comment Share on other sites More sharing options...
Robin Posted May 16, 2020 Author Report Share Posted May 16, 2020 48 minutes ago, Pete Lewis said: it could still be the front carb is low on fuel when you lift it ....how far ??? as suggested do the two throttle plate actually open ..is the front one staying closed ??? Pete Hi Pete i can lift the piston on the front as high as it goes but no change of engine note. Also if I lift the Piston and then rev the engine I can see the throttle plate opening so I assume the linkage is working ok. Overall performance seems fine. Link to comment Share on other sites More sharing options...
Pete Lewis Posted May 16, 2020 Report Share Posted May 16, 2020 thats awfully wrong, can you get her over to sunny luton ???? is there any sign of fuel at the jet orifice ??? in the throat Pete Link to comment Share on other sites More sharing options...
Iain T Posted May 16, 2020 Report Share Posted May 16, 2020 Peter's right something very wrong, it does sound as though there is no fuel to the carb. What about checking the fuel line to the carb, is anything getting through? Iain Link to comment Share on other sites More sharing options...
johny Posted May 16, 2020 Report Share Posted May 16, 2020 51 minutes ago, daverclasper said: Hi Pete. I think I have always measured the the longest distance between the bottom of float and gasket face, which on reflection, is not correct. Guess a removal of carbs to check this is in order?. Cheers, Dave No Dave you've got it right, I think we're just getting mixed up here. It's measured from the lowest point of the float when in its raised position (i.e. as if the float chamber was full of fuel) to the bowl joint face. Link to comment Share on other sites More sharing options...
NonMember Posted May 16, 2020 Report Share Posted May 16, 2020 1 hour ago, Robin said: Will try and check everything as Pete suggests but the performance seems ok - not down on power. Can’t work out why raising the piston on the front carb has no effect on the engine revs . I assume you're raising the piston at idle, not when you're checking the performance 😁 If the linkage is only a bit off, so that the front throttle opens less than the rear, you may not notice the power being low, but you could be getting almost no air through the front carb at idle, which would explain why lifting the piston has no effect. It would also explain the rear piston rising faster than the front when you rev it. (When my GT6 was "gutless" the linkage had come completely loose and the front throttle stayed at its idle position even at wide-open pedal.) Link to comment Share on other sites More sharing options...
Pete Lewis Posted May 16, 2020 Report Share Posted May 16, 2020 open the front idle stop to increase idle revs does this have any change to your lifting not reacting ??? pete Link to comment Share on other sites More sharing options...
Robin Posted May 17, 2020 Author Report Share Posted May 17, 2020 14 hours ago, NonMember said: I assume you're raising the piston at idle, not when you're checking the performance 😁 If the linkage is only a bit off, so that the front throttle opens less than the rear, you may not notice the power being low, but you could be getting almost no air through the front carb at idle, which would explain why lifting the piston has no effect. It would also explain the rear piston rising faster than the front when you rev it. (When my GT6 was "gutless" the linkage had come completely loose and the front throttle stayed at its idle position even at wide-open pedal.) Yep- that pretty much sums it up - So I guess I need to check the linkage. Is it just a question of loosening the linkage bolt? Link to comment Share on other sites More sharing options...
Robin Posted May 17, 2020 Author Report Share Posted May 17, 2020 15 hours ago, Pete Lewis said: thats awfully wrong, can you get her over to sunny luton ???? is there any sign of fuel at the jet orifice ??? in the throat Pete Hi Pete i’m going to have another play today but if I get no luck I could get over to you on Friday or Saturday morning if you’re ok with that - we can keep our distance! Robin Link to comment Share on other sites More sharing options...
Pete Lewis Posted May 17, 2020 Report Share Posted May 17, 2020 yes thats a plan friday might be best stick your finger in is there any fuel around the top of the jet ??? pete Link to comment Share on other sites More sharing options...
Robin Posted May 17, 2020 Author Report Share Posted May 17, 2020 24 minutes ago, Pete Lewis said: yes thats a plan friday might be best stick your finger in is there any fuel around the top of the jet ??? pete Hi Pete ok - tried increasing the idle screw on the front carb to increase the revs and this time when I lift the piston the engine does respond but at the same time the rear carb starts to leak Petrol through the brass tube. When I decrease the idle screw on the front carb, raising the piston has no effect but the rear carb no longer leaks. think Friday would be good! Is 10am ok? Link to comment Share on other sites More sharing options...
NonMember Posted May 17, 2020 Report Share Posted May 17, 2020 7 hours ago, Robin said: ok - tried increasing the idle screw on the front carb to increase the revs and this time when I lift the piston the engine does respond but at the same time the rear carb starts to leak Petrol through the brass tube. When I decrease the idle screw on the front carb, raising the piston has no effect but the rear carb no longer leaks. Weird!! I can't really see how getting some throttle opening on the front would cause a float problem on the rear. Unless... you actually have a fault on the rear float valve but when the whole engine is idling on a single throttle it draws just enough air (and hence fuel) to prevent it overflowing. If so, the correct approach is to fix both problems - clean the rubber sliver out of the rear float valve and adjust the front idle stop to get equal air through both carbs. Link to comment Share on other sites More sharing options...
Pete Lewis Posted May 17, 2020 Report Share Posted May 17, 2020 10am is fine assuming the weather is sunny Pete Link to comment Share on other sites More sharing options...
Robin Posted May 18, 2020 Author Report Share Posted May 18, 2020 15 hours ago, NonMember said: Weird!! I can't really see how getting some throttle opening on the front would cause a float problem on the rear. Unless... you actually have a fault on the rear float valve but when the whole engine is idling on a single throttle it draws just enough air (and hence fuel) to prevent it overflowing. If so, the correct approach is to fix both problems - clean the rubber sliver out of the rear float valve and adjust the front idle stop to get equal air through both carbs. Agree - Weird!! Link to comment Share on other sites More sharing options...
Pete Lewis Posted May 22, 2020 Report Share Posted May 22, 2020 well I leave Robin to confirm she's running ok but apart from some wear ?? allowing rear balance idle being a bit high and not responding to idle screws the main problem was front diaphragm dimples out of place putting the air piston 90deg out and set both jets to 2mm down and all tickety boo all done from 6ft away magic hopefully he is on his way home with a few more horses Pete 1 Link to comment Share on other sites More sharing options...
Robin Posted May 22, 2020 Author Report Share Posted May 22, 2020 3 hours ago, Pete Lewis said: well I leave Robin to confirm she's running ok but apart from some wear ?? allowing rear balance idle being a bit high and not responding to idle screws the main problem was front diaphragm dimples out of place putting the air piston 90deg out and set both jets to 2mm down and all tickety boo all done from 6ft away magic hopefully he is on his way home with a few more horses Pete Yep - much better! Thanks Pete Link to comment Share on other sites More sharing options...
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