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Just trying to confirm which components use which vacuum source as I am replacing a carb on a stock engine.The engine still has EGR, EFE, PCV and charcoal canister as well as hot and cold air filter housing and AIR ( I know no vacuum requirement for 'Smog'). As far as I can work out, the only thing that likes ported vacuum is EGR. My understanding is that ported vacuum is zero at idle. Whats the situation with vacuum advance on a standard fit distributor? I thought it only requires vacuum advance as the revs climb above 1500, i.e ported vacuum, yet all the diagrams I see it comes from manifold vacuum!!! Here are the ports on the new Rochester carb I will be fitting with what I think are the correct ports labelled for each item. ( Canister also gets a small ported vacuum source teed of the EGR supply) The one I'm not sure about is the dissy one, it appears to be 'above' the throttle butterflies but maybe runs inside the carb to ported pressure or is manifold vacuum correct???
and where does the S connector on the underside of the air cleaner connect to?
From: At my Bar drinking and wrenching in Lafayette Colorado
On that carb, the only "ported" nipple is the one you have labeled "EGR". It sticks out of the throttle plate on the forward, passenger side corner at an angle. All other ports, except the big bowl vent ("Canister") and the rear fresh air vent (incorrectly labeled "air cleaner") are manifold vacuum, including the one on the forward, driver's side labeled "Distrib." (On the earlier 4MV Q-Jets, that nipple is usually ported vacuum. On the M4M carbs, it is manifold vacuum.) Your labels are all correct, except the fresh air vent in the rear of the airhorn: That nipple is for the hot air choke air inlet tube that goes down to the intake manifold exhaust crossover.
On later cars with the 2-port vapor canister (no provision for carb bowl vent line), the "S" tube connects to the bowl vent nipple to pull bowl vapors up into the air cleaner for ingestion into the engine (you have the nipple labeled "canister", which may not be correct for your year car).
Good luck with that commercially rebuilt carb - most of those have significant problems and issues that are not easily rectified.
Lars, Thanks for that. I'm still referring to this bag of bits and the jigsaw seems to be slowly falling into place! Yes..I think this car probably was a 1980 305California car, with a 5.0 L engine and auto, the carburettor and ignition timing working under the Computer Command Control system, if I'm correct? This is presumably why it has the 'ECM' carb and the other Californian emission requirements under the hood. The vacuum arrangement fits the schematic you posted ,,apart from no vac to smog pump, thats looks like an electric plug on the Div valve..What I know is the car apparently was salvaged and given a new VIN by the Californian DMV in the mid 80's.and then it's spent its whole life in California until it was imported to UK in 2019. At some stage while in the US a replacement engine was fitted, although they seem to have kept everything else the same (carb, dissy, emissions), engine details of which I think i have decoded,....: Block 10066036...350.....4 bolt main...TargetMaster/Goodwrench crate engine, 2-piece rear seal, "Hecho en Mexico" Made in Mexico. 1M0615 9VP Not 100% sure on the suffix but the M = GM of Mexico, 06 = 06mnth mo of year, 15 = 15th day of month. 9= could be year 9? VP is the suffix added to all of the goodwrench/targetmaster 260 HP engines? Now, as you say I'm having real fun trying to sort this car! I've got it, however, and am trying to make a go of it as it was running like a sack of spanners! I'm very appreciative for all the help both on the UK Corvette forum and this, thanks for all your help guys......
The carb I have sourced here in the UK is a 17085226 ; 1566 HLY. I'm sure that means something to you but was told by the 'expert' I bought it from it was suitable for this engine! The carb looks like a straight swap ( I have the old ECM one off now). My only thoughts now are, is the distributor suitable and with most of the electrics detached from the old carb, am I running into ever more problems?
Ported or mainifold vacuum?
