mtbtimr Posted July 7, 2016 Share Posted July 7, 2016 So we recently picked up some twin carbs off Samdash's 1300... They have AUC870 and AUC871 on each of the body castings. Our current single carb setup uses a single 1 3/4 su carb. The issue: Currently... We have the timing cover (AKA clutch cover AKA fly wheel housing) venting straight to the atmosphere through the v. We have our rocker cover also venting straight to the atmosphere. The vacuum advance from the dizzy plugs into a little nipple on the front of the carb. There is no other vacuum port on the carb. All is well. With the new twin carbs... There is a vacuum port on each carb, currently joined with a Y piece (presumably for the flywheel and rocker cover ventilation). There is no vacuum advance nipple on either carb. So should we somehow plug the vacuum advance from the dizzy into Y piece and leave the flywheel and rocker cover venting to atmosphere? A bit of reading suggests the carbs are post PCV design or something... Any one understand??? Quote Link to comment Share on other sites More sharing options...
Ghostchips Posted July 7, 2016 Share Posted July 7, 2016 Yes i would plumb the vacuum advance in as you said and leave the atmosphere vents as they are. Sounds about right. 1 Quote Link to comment Share on other sites More sharing options...
mo999 Posted July 7, 2016 Share Posted July 7, 2016 Also take a look at Andrews one as it has the correct venting setup from the rocker cover to the inlet manifold and correct vac advance 1 Quote Link to comment Share on other sites More sharing options...
Muncie Posted July 7, 2016 Share Posted July 7, 2016 If your going to run an airbox your breathers would go into that. But you have individual filters right? If so tee them buggers together and vent them at the ground or if your keen tap the inlet manifold and fit a pcv valve. Having your crankcase under vacuum does help keep the oil in a British engine. Quote Link to comment Share on other sites More sharing options...
Mr Vapour Posted July 7, 2016 Share Posted July 7, 2016 Having your crankcase under vacuum does help keep the oil in a British engine. Is that because the vacuum in teh motor stops the oil falling out?? 3 Quote Link to comment Share on other sites More sharing options...
tortron Posted July 7, 2016 Share Posted July 7, 2016 because pressure slips past the rings and blows the oil out the subpar seals its the #1 reason to supercharge your A series tbh 1 Quote Link to comment Share on other sites More sharing options...
Borgweiser Posted July 8, 2016 Share Posted July 8, 2016 There should be a vacuum port in the right hand carburettor, it may just be lower than the previous one and a bolt on metal type. Remove the crankcase breather and cut open the top, remove the insides and weld back together, always blocked and full of tightly wound caked gauze. Then either run to atmosphere for max stink- or plumb up to carburettors via a T piece as well as the rocker cover. Insufficient breathing leads to leaks as stated due to high crankcase pressure/gearbox in sump dramas. Make sure vacuum advance actually works also, number one fault with poor performing and over heating A series, dead advance unit 1 Quote Link to comment Share on other sites More sharing options...
Borgweiser Posted July 8, 2016 Share Posted July 8, 2016 Those vacuum ports are only in operation after the piston rises, so no good for ignition timing Quote Link to comment Share on other sites More sharing options...
Muncie Posted July 8, 2016 Share Posted July 8, 2016 I call it the bong principle 1 Quote Link to comment Share on other sites More sharing options...
mtbtimr Posted July 8, 2016 Author Share Posted July 8, 2016 Turns out the carbs are off a late model cooper S which didn't come with vacuum advance. The ports with the Y connector are for the flywheel housing breather. http://www.theminiforum.co.uk/forums/topic/318572-twin-su-carbs-dont-have-a-vacuum-advance-nipple/#entry3401538 1 Quote Link to comment Share on other sites More sharing options...
Nominal Posted July 8, 2016 Share Posted July 8, 2016 It's better to have the vacuum advance connected, particularly for fuel economy. You could add a port to the intake manifold. Quote Link to comment Share on other sites More sharing options...
mtbtimr Posted July 14, 2016 Author Share Posted July 14, 2016 "Not necessarily a good idea as the Vacuum is not controlled. In the HS type Carbs (and most others), the Vacuum take off for Vacuum Advance lines up with the Butterfly such that at Idle, there is next to no Vacuum for the Advance. This reduces the likelyhood of overheating while waiting at Traffic Lights and the like." Overheating vs bad fuel economy.. seems like its best to go for the lesser of two evils...? 1 Quote Link to comment Share on other sites More sharing options...
xsspeed Posted July 14, 2016 Share Posted July 14, 2016 yeah cook the engine and replace with K24...I'll show myself out 1 Quote Link to comment Share on other sites More sharing options...
Frosty Posted July 19, 2016 Share Posted July 19, 2016 One of the carbs will have a tiny nipple between the butterfly and the manifold, that is where the vacuum advance line goes. The two larger nipples are for the vents, you can use ether the rocker cover or the clutch housing one you don't need both hooked up, block the other one off. The A series will leak every where without one or the other connected. If the little nipple isn't in one of the carbs drill a small hole in the manifold and fit one there, it needs to be between the throttle butterflies and the head or the advance won't work properly. Quote Link to comment Share on other sites More sharing options...
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