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What is normal V-12 Intake Vacuum?


40ZephSedan

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My 40 sedan has had problems stalling at stop signs after warm. Since I had a intake crack at the port for wiper vac, I put on a new gasket and a good almost NOS intake manifold, and two new head gaskets. But did not solve the problem. (but it did confirm you need to re-torque both gaskets- bolt do loosen-up). Found another leak at oil breather vent tube into bottom of air cleaner- extended and sealed that. It still stalls when I stop at the stop signs. Then starts right back up. Do it again and the same happens. Takes the fun out of driving the car! I am thinking I should check the intake vacuum with a gage, curious what vacuum pressure I should be seeing if I run a check?

(Jeff - the Buckeye says to say hi to the Buckeye!) Paul

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Guest B1rdman

about 17 to 20

i would think the V-12 should read about the same as other engines.

i would get the gage because a bad leak i think would show you what you are looking for at idle, before you leave the house.

just a opinion, ever one has one, kinda like a brain, i have at least 1/2 of one left.

gene

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Since there is no connection between the braking and the carburetion I suspect that the carburetor idle stop may be a little out of kilter? might check to see if the arm is contacting the idle speed screw fully. Sometimes the throttle arm gets a little loose and when releasing the throttle quickly it may move past the idle stop screw. The idle vacuum should be 18 to 22"Hg at sea level if the engine is in good condition. The higher the altitude the lower the vacuum.

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Hi 40 zeph !! Get a can of "carb cleaner, yellow aerosol ..cheap stuff..spray around intake , carb base..wiper hose fitting.. vac brake line down the ft of engine.the engine will speed up when it finds the vac leak..really cool trick..be careful..but, that how the pros do it...

.What kind of gasket / spacer set up are u using?? easy place for a problem.. Are you certain all cyls fire..?? pull 1 wire at a time enough to see if engine sounds diff..1 weak / dead cyl will do the same thing..stuck valve..etc with 12 cyl, u can have a dead one, and hardly know it......

oh, and that vacuum gauge is a wonderous tool..it should / must read steady..bouncing reading is telling..

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  • 1 month later...

Back from short jaunt, and tried whole separate distributor/coil/brushes/points/condenser assembly to rule it out totally for my stalling problem. Thought maybe centrifugal timing weights were sticking, until engine stalled, then fell back. But 2nd dist made it run worse, backfire, no power, trouble idling worse. (probably on 6 cylinders!) So next swapped 1 condenser and solved that problem on the second dist assembly-also runs fine, but still problems stalling at stop signs once warm. Idle screw is centered on cam OK. Vacuum test shows it varies from 17 to 17.5" reving and idling. Not great, but maybe not the problem. But I will try spray next around carb base to see it speeds up. (tried already around intake manifold and didn't show any problems there). Good idea on maybe one bad cylinder causing minor vac problem but major stalling problem.

Probably should do spark and compression on all 12 cyls, then maybe swap the carb. Thx to all for ideas and help! Will get it one of these times! P

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Just because they dont seem to have been mentioned - have you tried the idle mixture adjustment? What about the ignition timing?

17" is just a little on the low side for idle vacuum but I dont know the age or condition of your engine

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After I rebuilt my 48 V12 it was running 18"HG at idle with the factory intake and carb, after 500 miles it was still at 18"HG. After I switched it to the Edelbrock 2x2 with factory carbs the vacuum went up to 20"HG. I talked to a few people that have been doing this for a long time and right around 18"HG is normal.

The only reason I can think of for higher vacuum with multiple carbs would be that the engine doesnt have to pull from so far away.

Would be interested to see what reading someone with a 3x2 setup is getting, if its like my 440 six pack challenger it should read what a stock engine reads.

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