vacuum guage readings
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vacuum guage readings
I put a carb spacer on my 79 f 150 400 and i also invested in a vacuum gauge. I didnt check the gauge before i put the spacer on but i put it on after and checked the manifold vacuum and i think i may have a problem. It reads about 14-16. It "vibrates" basically between 14-16. i rev it up and it has a steady sweep than it goes back to idle like i think it should and vibrates from 14-16 again once it is back to idle. I checked out http://www.users.bigpond.com/ergoff/vac2.htm it has illustrations that helps pinpoint problems and it looks like the one that it has for defective ignition, but it says slowly. The gauge moves fast. The gauge does look like the illustration but i would not call that illustration slow. So anyone have any idea what my problem may be or is this normal? Do i need to pull my plugs and check them? They are only have been in for a 1 1/2 yr. The motor was rebuilt a 1 1/2 yrs ago. Thanks for the help.
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It has a comp cams 268h cam. around 15k on a rebuild. i reved it up but i didnt hold it at a steady rpm but i reved it up and it sweeped up steadily and dropped steadily. I will try it tonight or tomorrow and see what i find out.
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So you think i am running to lean? I am pretty sure that the screws are set right. I messed with them a little when i had the vacuum gauge hooked up to the manifold vacuum port on the carb and i turned the screws in and out and they were set to the highest vacuum. That is how they should be set correct? maybe i will back them out a little and try that. Well i am going to adjust the timing using the vacuum gauge and then i will go from there i guess. and suggestions or opinions are greatly appreciated.
Laughing Gas
Joined: Feb 2003
Posts: 1,234
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From: In the woods of Oregon
strange, I only pull 12.5 vacume with my comp cam 268h. My needle stays steady at about 600rpm idle. Vacume will change a little from timing and idle speed adjustment including idle screws. My screws are about 3.5 turns out. Probaly the difference is in the gauges.
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Posting Legend
Joined: Sep 2001
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12.5 seems way low. A good engine with an OEM cam will pull around 21". If the cam timing is retarded or a slightly longer duration cam is used it will pull less or around 18". It would take a very radical cam to only pull 12.5".
3.5 turns seems like a lot also. Normally I will start a carb up with 1.5 - 2 turns then tune from there.
3.5 turns seems like a lot also. Normally I will start a carb up with 1.5 - 2 turns then tune from there.
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ok i pulled all my plugs and the electrodes were white. So this means it is pry to lean and maybe the timing is messed up correct? I put new plugs and wires on adjusted the timing a bit and richened the carb a bit so we will see what that does. One of my spark plugs gap's was way too close. I didnt put them in the mechanic who rebuilt the motor did. So i was wondering is it just an error on his part or did something happen to cause it to close the gap? I think i will play with it some more tomorrow if i do not work but we will see. The vacuum gauge still has the "gitters" but i richened it and it made it not so bad so i think it is ok now. If i rev the motor and hold it the vacuum gauge steadies and it does not have the "gitters" anymore also. Also the motor does use up a little oil. It doesnt use up a whole lot but it does use a bit. Dont know if it has any relevance to this subject but figured i would throw it in.
Last edited by Josh79; Mar 28, 2005 at 04:34 PM.
Laughing Gas
Joined: Feb 2003
Posts: 1,234
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From: In the woods of Oregon
need to invest in a new vac gauge then, I thought 3.5 turns was alot also but thats what gives me the highest vac. I will double check again tomorrow. Could worn pushrods and rockers cause this? My engine runs great so I'm not realy worried, but if I can tune it better, well heck yeah.Doesn't speed up the idle if I turn them in either. But like I said she runs good, but your post makes me think she can run better.Thanx.
Post Fiend
Joined: Jul 2003
Posts: 11,945
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From: Spokane, WA
Damn, I'd like to see an engine that pulls 21". I get 17" out of mine and it's pulling some big vacuum with a stock cam. The really hairy cams that I've seen still used on the street are pulling under 10".
Postmaster
Joined: Aug 2001
Posts: 4,270
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From: Delaware
Yeah, my motor is lucky to see 10" of vacuum at idle, but I am running a 281 degree hydraulic roller cam with 69 degrees overlap. I had to change the springs in my Edelbrock carb to keep it in cruise mode at low RPM.
With a 268 degree cam you could be OK at 14" of vacuum.
With a 268 degree cam you could be OK at 14" of vacuum.
Mountain Pass
Joined: Jun 2000
Posts: 242
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From: earth
I'd spray the carb with cleaner (if it looks gummy/varnished/dirty), check the cap/rotor and if it's still fluctuating, check/adjust the engine on a smog machine.
Don't forget altitude...higher elevation causes lower readings. I think my truck increased ~1" every 1500' drop in elevation. At 4500/5000 feet, my truck pulls ~16" with the edelbrock 2172.
Chris
Don't forget altitude...higher elevation causes lower readings. I think my truck increased ~1" every 1500' drop in elevation. At 4500/5000 feet, my truck pulls ~16" with the edelbrock 2172.
Chris