compression psi on 351m
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compression psi on 351m
My manual does not say what would be an adequate psi for my 76 351 m. Only that it should be even on all cylinder and 80 % of the original psi. Im guessing that 150 might be the psi of a fresh from the factory engine and that since mine is 135 on all cylinders it is probably ok. Do any of you know what is normal for this engine from the factory.
I am researching this because I am trying to find out why my vacuum is so low. it is barely in the yellow zone on my guage which if I recall is about 15 inches and steady The truck runs just sort of 'fair' and I have to use quite a bit of leg pressure to brake or at least I think so. I have searched for leaks and can't find any
I have read about these engines having factory retarded timing sprockets and late valve timing is one of the problems listed for low vacuum. Any ideas?
I am researching this because I am trying to find out why my vacuum is so low. it is barely in the yellow zone on my guage which if I recall is about 15 inches and steady The truck runs just sort of 'fair' and I have to use quite a bit of leg pressure to brake or at least I think so. I have searched for leaks and can't find any
I have read about these engines having factory retarded timing sprockets and late valve timing is one of the problems listed for low vacuum. Any ideas?
Cargo Master
Joined: May 2003
Posts: 2,561
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From: Earth
I would look for a vacuum leak in your vacuum lines. Little cracks in those hoses are not uncommon. Typically your engine will run a little rough if you have a vacuum leak, but that is where I would start. Your compression numbers seem fair to me.
Postmaster
Joined: Aug 2001
Posts: 4,270
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From: Delaware
Steady vacuum is good. I would spray carb cleaner around the carb base and the throttle shaft in the carb as well as the vacuum lines, while the motor is idling.
If the idle speed pick up then you will know where the leak is.
If the idle speed pick up then you will know where the leak is.
Freshman User
Joined: Nov 2007
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From: Benton, Ill.
My 400 had a vacuum leak that I couldn't find. I finally pulled the intake and found the gasket was leaking on inside. It had been a problem for a long time and was a gradually worsening problem. Since then I've also replaced the booster and most hoses and have around 20in at idle after warmup. You need to get it up to operating temp to check. Anybody who has messed with these much knows they are more cold natured than a witches heart.
Cargo Master
Joined: Apr 2001
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From: Eustis FL
The 351M in my '77 Cougar ran with 90 lbs compression average. Ran nice and smooth, but not much power left in her. It had so many miles on it, I had to retire it for the '74 Montego I found with 400 engine. Great choice!
15 inches of vacuum is acceptable by any means, but check your brake booster for leaks.
15 inches of vacuum is acceptable by any means, but check your brake booster for leaks.
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I actually took all the vac lines off the engine and capped the outlets just to isolate the problem. with the lines disconnected and plugged I still had only 15. and spraying carb cleaner around manifold and carb had no effect.
77hoss may be right, it may be where it can't be detected or maybe 15 is all i should hope for, but still, with 135 in compression I wouldn't think it would be that low when I really stomp on it it stumbles then backfires then slowly revs up while making a sucking sound and yes the accelerator pump works.
the more I think about it the more I think it might be one port in the head since the engine doesn't seem to have even power as it accelerates but wouldn't that cause the vacuum guage to fluctuate? Maybe not.
77hoss may be right, it may be where it can't be detected or maybe 15 is all i should hope for, but still, with 135 in compression I wouldn't think it would be that low when I really stomp on it it stumbles then backfires then slowly revs up while making a sucking sound and yes the accelerator pump works.
the more I think about it the more I think it might be one port in the head since the engine doesn't seem to have even power as it accelerates but wouldn't that cause the vacuum guage to fluctuate? Maybe not.
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Cargo Master
Joined: Apr 2001
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From: Eustis FL
Sounds to me like a carburetion problem. What carb are you using? If it is a factory type, you might want to try a rebuild kit, or if you have the money and were thinking of upgrading someday to a 4V intake and carb, now would be a good time. Definitely can't lose on some quality Edelbrock parts.
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Cargo Master
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From: Earth
Originally Posted by 200solomiles
...I really stomp on it it stumbles then backfires then slowly revs up while making a sucking sound and yes the accelerator pump works.
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I 'll pull the plugs after the holidays and see if one shows signs of running lean. The carburetor is a carter afb 4 bbl 625 cfm sitting on top of an edelbrok streetmaster 400 aluminum manifold. They were on it when I bought it and I have not been able to find out much about that intake but I have rebuilt the carb. because the accelerator pump wasn't working
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The timing was good I did advance it a little just to see if it would make any difference but it didn't. The plugs look good but they only have about a 1000 mi on them . I replaced them when I got the truck. The old plug were ok also as I recall but I don't know how many miles they had on them.
a thought occured to me over the holidays. couldn"t worn valve guides cause low vacuum? I know the guides in this engine are worn since I get a puff of blue smoke when I accelerate after decelerating. my manual doesn't list worn guides as a possibility but it could be ...right?
a thought occured to me over the holidays. couldn"t worn valve guides cause low vacuum? I know the guides in this engine are worn since I get a puff of blue smoke when I accelerate after decelerating. my manual doesn't list worn guides as a possibility but it could be ...right?
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Did you check the timing exactly with a light? As little as 3 degrees off can cause backfiring. When you're trying to solve car trouble, you have to check things one by one to eliminate the problem - instead of continuously generating a list of "could-be's" without actually checking any of them. It also sounds like your accelerator pump isn't dialed in. With the air cleaner removed and the truck shut off, look down the throat of the carb and pull back the throttle - you should see two nice even streams of gas shoot at the primaries. It definitely sounds like you have a lean condition.
Last edited by fmc400; Dec 27, 2007 at 02:48 PM.
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fmc 400 what makes you think I have a lean condition? My problem is that I have low vacuum ,which can cause a lean condition I admit, but a lean condition would not neccessarily cause low vacum.
yes I used a light. And as I said the plugs look good though they don't have enough miles on them to show alot they were a light brown and pretty even not carboned up or burned.
The accelerator pumps works. as I said in a previous post I rebuilt the carb because of the accelerator pump.
I am eliminating things one by one. Right now I am back to having a leak of some kind to explain my vacuum loss. which is why I posed the question 'would worn valve quides cause low vacuum' since I am pretty sure the guides are worn.
yes I used a light. And as I said the plugs look good though they don't have enough miles on them to show alot they were a light brown and pretty even not carboned up or burned.
The accelerator pumps works. as I said in a previous post I rebuilt the carb because of the accelerator pump.
I am eliminating things one by one. Right now I am back to having a leak of some kind to explain my vacuum loss. which is why I posed the question 'would worn valve quides cause low vacuum' since I am pretty sure the guides are worn.