dave88sw Posted August 8, 2018 Share Posted August 8, 2018 I'm just finishing up putting an su carb on my series 3 2.25. It hadn't occured to me until I went to connect it up but there's no vacuum feed for the distributor on the SU carb. Would it be ok to connect it up with a T piece into the brake servo feed? Thanks Dave Quote Link to comment Share on other sites More sharing options...
lo-fi Posted August 8, 2018 Share Posted August 8, 2018 There really no vacuum port on the SU? How strange. Ideally you want as close as possible to the throttle plate - I'm not sure if the vacuum will drop as readily in the servo line - but I doubt it'll worry a 2.25. You could always put a vac connection in the carb? Drill through somewhere close to the throttle plate (manifold, not filter side), put a piece of brass tube in secured with epoxy if you're not confident of getting a nice press fit. Quote Link to comment Share on other sites More sharing options...
dave88sw Posted August 8, 2018 Author Share Posted August 8, 2018 No, the carb came off a Rover V8, the vac advance i think was connected to the other carb (or manifold, can't honestly remember but there's no port on this carb). I had considered drilling the carb but i don't really want to have to remove it again if i can avoid it. As it's not a performance engine do you think it'll hurt it to connect it straight to the manifold? Cheers Dave Quote Link to comment Share on other sites More sharing options...
TSD Posted August 8, 2018 Share Posted August 8, 2018 Doesn't the vac take-off for the dizzy need to be upstream of the throttle plate, otherwise the advance would be maximum at idle, instead of building rapidly as the throttle is opened? With the drilling just above the throttle plate, the initial throttle opening movement moves the vac take off from atmos side to manifold side of the plate. Quote Link to comment Share on other sites More sharing options...
dave88sw Posted August 9, 2018 Author Share Posted August 9, 2018 I've been googling all night and you find lots of different opinions. It seems that the advice varies as to which side of the throttle plate the vac port should be and whether it should have "ported" or "manifold" vacuum. If anyone is interested, i found this article, which would appear to be trustworthy and explains the whole situation very well: http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm Paragraph 6 of the article is where he discusses manifold or ported vacuum if you want to jump straight to that. Having read through that, i think i'll try it with the vacuum line connected up straight to the manifold and report back on how it runs. I'm just desperate to hear this thing run, it's been 7 years since i had a proper drive in it!! Thanks for the help Dave Quote Link to comment Share on other sites More sharing options...
cackshifter Posted August 9, 2018 Share Posted August 9, 2018 It should be fine without vac advance, its a part throttle economy thing, so run it up without, sort it later. After all at full throttle there is no vacuum. Quote Link to comment Share on other sites More sharing options...
ThreeSheds Posted August 10, 2018 Share Posted August 10, 2018 7 hours ago, cackshifter said: After all at full throttle there is no vacuum So what sucks the petrol in? Quote Link to comment Share on other sites More sharing options...
lo-fi Posted August 10, 2018 Share Posted August 10, 2018 Vacuum caused locally at the jet by venturi effect Cackshifter is right, it'll run without it. For anyone Interested, the excellent article Dave posted above goes into why its desirable to have, though. Quote Link to comment Share on other sites More sharing options...
ThreeSheds Posted August 10, 2018 Share Posted August 10, 2018 34 minutes ago, lo-fi said: Vacuum caused locally OK - you have me there... But Bernoulli would argue that the gas moving through the manifold would be at a lower pressure than ambient just by the fact that it is moving... so I would suggest that there will still be 'vacuum' (ie reduced pressure compared to ambient) in the manifold below the (even wide open) carburettor... Sorry, but I am a programmer - I am paid to be pedantic! Quote Link to comment Share on other sites More sharing options...
lo-fi Posted August 10, 2018 Share Posted August 10, 2018 Hehe. Yep, no argument there I think the idea having the port right next to the throttle plate is that it's furthest away from the pistons causing local under-pressure on the inlet strokes. Atmospheric pressure being higher causes flow, but at that point the static pressure will be very near atmospheric as there's little restriction upstream of that point save the throttle plate, who's position we essentially want to measure. This same might not quite be the case with the connection to the brake servo in the manifold, but I'm sure the 2.25 won't care much. 1 Quote Link to comment Share on other sites More sharing options...
cackshifter Posted August 10, 2018 Share Posted August 10, 2018 (lol)I have no disageement with the theory of ThreeSheds' post but the vac advance capsules intentionally don't work with the sort of vacuum levels at full throttle he describes, and normally have a fair bit of preload so that you need a certain amount of vacuum before anything starts to happen. Their aim was to improve part throttle economy while waiting for electronic ignition to be invented, because more advance is needed if the cylinder isn't full, but isn't needed if it is. It used to be the tweak to plug or remove the vacuum advance for performance tuning to a) minimise the risk of detonation from over advance and b) give a more secure mounting for the points, as it was thought unlikely part throttle economy was a consideration on, say, a rally car, and most tuners with rolling roads couldn't adjust or change them.Quite often the diaphragm failed and no-one noticed for ages, so it shouldn't stop it from running (checked by sucking on it and seeing if the baseplate moved). Sorry, but I am a programmer - I am paid to be pedantic! Quote Link to comment Share on other sites More sharing options...
ThreeSheds Posted August 11, 2018 Share Posted August 11, 2018 On 8/10/2018 at 1:25 PM, cackshifter said: Sorry, but I am a programmer - I am paid to be pedantic! Quote Link to comment Share on other sites More sharing options...
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