compression psi on 351m
Cargo Master
Joined: Apr 2001
Posts: 2,364
Likes: 3
From: Eustis FL
You can also adjust your mixture screws to get the best vaccuum possible. Set idle at the valve cover sticker's specs, and adjust left and right needles together, a quarter turn at a time, until you get the highest vac. You may have to adjust the idle a few times to keep it in spec.
Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
Yes, bad valve guides can cause vacuum leaks, but you would be smoking all the time, not just a puff of blue smoke.
If the plugs are brown, it is not lean. The timing is OK, and the accelerator pump works. So it is time to look elsewhere.
You should check the timing chain for slop. There is a way to do that, by cranking it one way by hand and setting the timing pointer on Zero. You observe the direction of the distributor rotor, then turn the crank in the reverse direction back until the timing pointer is again on Zero. Check the direction of the distributor rotor, it should point as before. I don't know how much tolerance is allowed, but if there is a significant difference, then the timing chain is stretched and your valve timing is retarded.
If the plugs are brown, it is not lean. The timing is OK, and the accelerator pump works. So it is time to look elsewhere.
You should check the timing chain for slop. There is a way to do that, by cranking it one way by hand and setting the timing pointer on Zero. You observe the direction of the distributor rotor, then turn the crank in the reverse direction back until the timing pointer is again on Zero. Check the direction of the distributor rotor, it should point as before. I don't know how much tolerance is allowed, but if there is a significant difference, then the timing chain is stretched and your valve timing is retarded.
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Junior User
Joined: Feb 2005
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Thanks Danlee I will do the check as you described when I get back from a trip to death valley in a few weeks. fortunately the truck isn't my daily driver it's more of a hobby than anything it's an early 77 f250 hiboy that I bought to restore. Not a frame off or anything like that because it was in really good shape body wise. No rust at all.
I had to replace the carpet, door panels and other cosmetic stuff and I put the steering conversion on to eliminte the worn out power assist.
I have not yet been able to justify replacing the engine since it runs sort of ok. The previous owner said he redid the lower end ,Im guessing he meant mains. And it has 20psi oil pressure at 800 rpms and 62 psi at 2000 warm. That and the fact that it still has 135 compression is why I can't justify a $3000.00 upgrade to a 400 ft lb 400hp screamer ( as much as Id like to)
Oh well maybe the timing chain will need to be replaced in which case I might just as well do a cam and oh the valve guides are worn so maybe some rebuit aussie heads and then oh what the hell Ive gone this far, might as well go ahead and put in a 400 crank.........you get the picture
I had to replace the carpet, door panels and other cosmetic stuff and I put the steering conversion on to eliminte the worn out power assist.
I have not yet been able to justify replacing the engine since it runs sort of ok. The previous owner said he redid the lower end ,Im guessing he meant mains. And it has 20psi oil pressure at 800 rpms and 62 psi at 2000 warm. That and the fact that it still has 135 compression is why I can't justify a $3000.00 upgrade to a 400 ft lb 400hp screamer ( as much as Id like to)
Oh well maybe the timing chain will need to be replaced in which case I might just as well do a cam and oh the valve guides are worn so maybe some rebuit aussie heads and then oh what the hell Ive gone this far, might as well go ahead and put in a 400 crank.........you get the picture
Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
Just check the timing chain. You can easily replace a stretched chain with a new timing set, but I wouldn't replace the cam without doing the whole job, unless you just want a RV type cam. A performance cam won't perform well with the stock compression of that motor.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Originally Posted by 200solomiles
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.
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Junior User
Joined: Feb 2005
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well bad weather forced me to delay leaving for warmer weather so I checked the slop in the chain. I did it a little differently than you described I had my son observe the rotor while i turned the crank
I could turn the crank about 10 degrees in opposite directions before the rotor began to move. That would seem excessive to me. It would translate to 5 degrees of late timing of the valves. Some degree of slop is probably calculated in but that seems pretty loose.. WHAT DO YOU THINK?
I could turn the crank about 10 degrees in opposite directions before the rotor began to move. That would seem excessive to me. It would translate to 5 degrees of late timing of the valves. Some degree of slop is probably calculated in but that seems pretty loose.. WHAT DO YOU THINK?
Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
Originally Posted by 200solomiles
well bad weather forced me to delay leaving for warmer weather so I checked the slop in the chain. I did it a little differently than you described I had my son observe the rotor while i turned the crank
I could turn the crank about 10 degrees in opposite directions before the rotor began to move. That would seem excessive to me. It would translate to 5 degrees of late timing of the valves. Some degree of slop is probably calculated in but that seems pretty loose.. WHAT DO YOU THINK?
I could turn the crank about 10 degrees in opposite directions before the rotor began to move. That would seem excessive to me. It would translate to 5 degrees of late timing of the valves. Some degree of slop is probably calculated in but that seems pretty loose.. WHAT DO YOU THINK?
Maybe someone else can weigh in with a similar experience.
I think a good double roller timing set set 'straight up' will help a lot.
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Junior User
Joined: Feb 2005
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What I find interesting is that my compression is still as high as it is. If you factor in the factory retard and the slop. I suppose someone could have replaced the factory timing set already.
My sons a chevy lover ( I'll carry the guilty of not having raised him right to my grave). I helped him restore a 71 camaro last winter. He put an adjustable timing gear arrangement on the engine and used the wrong spacer. Im not sure how many degrees it retarded things but the spacers looked like they would only change it by 3 or four degrees. It dropped the compression from 150 down to 100 the engine ran but didn't have any power and the vacuum dropped down to about 13 in.
My sons a chevy lover ( I'll carry the guilty of not having raised him right to my grave). I helped him restore a 71 camaro last winter. He put an adjustable timing gear arrangement on the engine and used the wrong spacer. Im not sure how many degrees it retarded things but the spacers looked like they would only change it by 3 or four degrees. It dropped the compression from 150 down to 100 the engine ran but didn't have any power and the vacuum dropped down to about 13 in.
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Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
Try to find some backup data for that procedure for checking the timing chain to see if it is a proper method and how much slop you will have.
I would hate to see you replace a timing chain for no reason, but it sounds like it is the problem in spite of the 135 psi compression reading.
I would hate to see you replace a timing chain for no reason, but it sounds like it is the problem in spite of the 135 psi compression reading.
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