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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / January 20071977 coupe deville 425 manifold vacuum behavior
Having a little trouble understanding manifold vacuum and what affects it. When I start my car the vacuum as measured from a port directly into the intake manifold is low, around 14 in Hg. As the car warms up and the choke opens the vacuum reading gradually improves to around 17 in Hg. I suspected a leak around the carb or the manifold to cylinder head. In fact found one loose bolt....wasn't it. Used map gas to try to find a leak but couldn't find one. I noticed that the timing was retarded about 2 degrees from spec of 18BTDC at 1400 RPM. Also, I believe the air fuel mixture is a little rich. All hoses where replaced and the vacuum measured with all vac ports on the carb closed in turn. My question for a good mechanic is will retarded timing and/or a rich fuel mixture have an effect on manifold vacuum? Thanks for your help. Sincerely, Tich. Yes of couse, but why do you think 17 ( is it a steady reading) is low on an old engine, what are the effects of this in respect to performance. > Having a little trouble understanding manifold vacuum and what affects > it. When I start my car the vacuum as measured from a port directly [quoted text clipped - 17 lines] > Sincerely, > Tich. Shep, Forgot to answer the performance question. I've had the car about a year and put on a thousand miles or so. It ran shitty from the get go but really crappy before I parked it about 3 months ago. The plugs were undergapped about 10 thousands and very carbon fouled. Found the air fuel mixture screws all "screwed up" if you'll pardon the expression. The right screw was backed out from lightly seated about 1 1/2 turns and bent. The right screw was about 4 1/2 turns. The curb idle was 125 RPM higher than it should have been also. I think after putting 126 bucks worth of parts on it it will run fine. Just waiting for a combination valve to repair the brakes. The only reason I bought this old Cadillac was for the 100 inch front seat wide enough for grandma's butt to take her to appointments :-) Previously, I was having to rent a car twice a week! Thanks for your post. Later, Tich Thanks for your response Shep. >From your reply I've learned that as a car ages one would expect the manifold vacuum not to be as robust as it once was. Must be valves get worn and don't seal as well etc. I just wonder now if a much lower cold engine manifold vacuum is normal. I'm envisoning that as the engine warms up the metal parts expand slightly improving the vacuum seal to the outside. Is this normal on an old engine. With a cold manifold vacuum of 14 in Hg seem low for a 425 cid with 100 thousand miles on it. Thanks again for your helpful response....I ain't no mechanic. Any hints are very welcome. Later, Tich > Thanks for your response Shep. > [quoted text clipped - 7 lines] > slightly improving the vacuum seal to the outside. Is this normal on > an old engine. That's probably not what's going on. A cold engine is much less efficient than a warm one. Its basically "loaded" by thicker oil and less efficient combustion (plus an over-rich fuel mix thanks to the choke being applied in a carbureted engine such as yours.) Take that added load off, and the vacuum increases. Perfectly normal. Agreed. >> Thanks for your response Shep. >> [quoted text clipped - 13 lines] > in a carbureted engine such as yours.) Take that added load off, and the > vacuum increases. Perfectly normal. |
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