Need a little help with a problem 351M
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Joined: Sep 2004
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From: Abilene, Texas
Need a little help with a problem 351M
My grandson bought a 77 F100 with a 351M engine, 2WD, auto tranny, dead stock, but it won't idle properly. We have rebuilt the carb (2150 Motocraft 2 BBL) and changed the plugs but I can't get the idle mixtures down where they should be. Runs much better now, but still won't idle.
I suspect that it has a vacuum leak in the intake manifold or a hose. I am going back down to where he lives in a couple weeks to trouble shoot it some more.
With the idle down where it should be, the truck dies at stops or slow-downs, but with it high enoug to not die, the truck will go about 20MPH, foot off the gas.
When we changed the plugs, it had been runing on the choke for just a few blocks so I wasn't surprised to see the plugs pretty well sooted up, but the back plugs (#4 & #8) had no soot on them. I know that should tell me something, just can't remember what...
I suspect that it has a vacuum leak in the intake manifold or a hose. I am going back down to where he lives in a couple weeks to trouble shoot it some more.
With the idle down where it should be, the truck dies at stops or slow-downs, but with it high enoug to not die, the truck will go about 20MPH, foot off the gas.
When we changed the plugs, it had been runing on the choke for just a few blocks so I wasn't surprised to see the plugs pretty well sooted up, but the back plugs (#4 & #8) had no soot on them. I know that should tell me something, just can't remember what...
Last edited by 01PSDRVPuller; Feb 11, 2008 at 09:05 PM.
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From: Eldorado Ca. USA
When you go back, bring with you one of them small propane cylinders, with a torch attachment. Use gas from the unlit torch to find and identify vacuum leaks. The vapors will change how the engine is running temporarily. Carb cleaner will work as well, but its more flammable. It will also might cause hoses that are cracked to swell and temporarily seal. Check for broken/cracked/rotten hoses and fittings, and check especially around the EGR valve. If the EGR valve has a leak (valves from this era were notorious for being junk) it will cause a leak. If the EGR valve is open at idle, it will also make the motor run poorly, and very rich.
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From: Spanaway, Washington
With the idle down where it should be, the truck dies at stops or slow-downs, but with it high enoug to not die, the truck will go about 20MPH, foot off the gas.
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Joined: Jan 2001
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From: Central NYS
Originally Posted by 01PSDRVPuller
We have rebuilt the carb (2150 Motocraft 2 BBL) and changed the plugs but I can't get the idle mixtures down where they should be.
Also, a sticky fast idle linkage will give an inconsistant idle speed, as will an improperly adjusted choke fast idle adjustment screw.
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From: Abilene, Texas
Good answers guys! Let me respond to some of them. First, I know that black plugs indicate too rich a mixtue. As I had stated, we started it and only drove about 8 blocks (to the parts store to buy new plugs, then back home) so I knew the choke had not come off and the mixture would be rich. my question was why the two rear plugs weren't black too.
I had just rebuilt the carb on the day before so I am sure that the float was not leaking and the enrichment valve was brand new. I'm not sure about the float level because the kit instructions called for the float (with the needle valve closed) should be 3/8 inch below the carb rim. That was a lot (maybe 3/8 - 1/2 inch higher than it was when I tore it down. But, it wouldn't idle then either!
The throttle dash pot doesn't look quite like that one, but close. I don't think it works.
The emission systems on this engine have been mickey moused with BBs and capped off hoses, I don't know if I can get it working to factory specs.
I had just rebuilt the carb on the day before so I am sure that the float was not leaking and the enrichment valve was brand new. I'm not sure about the float level because the kit instructions called for the float (with the needle valve closed) should be 3/8 inch below the carb rim. That was a lot (maybe 3/8 - 1/2 inch higher than it was when I tore it down. But, it wouldn't idle then either!
The throttle dash pot doesn't look quite like that one, but close. I don't think it works.
The emission systems on this engine have been mickey moused with BBs and capped off hoses, I don't know if I can get it working to factory specs.
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Postmaster
Joined: Aug 2000
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From: Eldorado Ca. USA
Originally Posted by 01PSDRVPuller
The emission systems on this engine have been mickey moused with BBs and capped off hoses, I don't know if I can get it working to factory specs.
<~~~~~not an expert.
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From: Abilene, Texas
Update on this post:
we got the truck idleing properly and running much better, now all we need is a throttle dashpot (which I cannot find anywhere, on or off line. The vacuum dashpot is discontinued and i can't find a selonoid to convert it to electric. Ford swithced to the selonoid in '78, but all those are discontinued also)
ALSO, IT HAS A "RAPPING NOISE" at around 100 to about 2100 rpm. It is quiet below and above that range. I have isolated it to the #1 cylinder rocker arm area, but have not yet pulled the rocker arm cover to look at the pushrod and rocker arm bearings.
So...Anyone have any ideas on the noise and/or the dashpot?
I figure we can just wire into any wire that is hot with the key on and hook up the selonoid to that wire.
we got the truck idleing properly and running much better, now all we need is a throttle dashpot (which I cannot find anywhere, on or off line. The vacuum dashpot is discontinued and i can't find a selonoid to convert it to electric. Ford swithced to the selonoid in '78, but all those are discontinued also)
ALSO, IT HAS A "RAPPING NOISE" at around 100 to about 2100 rpm. It is quiet below and above that range. I have isolated it to the #1 cylinder rocker arm area, but have not yet pulled the rocker arm cover to look at the pushrod and rocker arm bearings.
So...Anyone have any ideas on the noise and/or the dashpot?
I figure we can just wire into any wire that is hot with the key on and hook up the selonoid to that wire.
Last edited by 01PSDRVPuller; Feb 25, 2008 at 09:40 PM. Reason: Correct spelling
Postmaster
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From: Eldorado Ca. USA
All of my trucks that have dashpots idle just fine, they are mis-adjusted, missing or not hooked up on rigs with and without AC. My AC rigs dont have AC that works, so if your AC doesnt work, I wouldn't worry about fixing it.
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From: Spanaway, Washington
This is the dashpot I finally purchased (ebay search ford dashpot or dash pot). Had to be creative as the bracket on the carb is for the electric version and the screw-in hole is larger. Works though! http://cgi.ebay.com/ebaymotors/1965-73-DASH-POT-MUSTANG-2-4-BARREL-NEW-TORINO-FORD_W0QQcmdZViewItemQQ_trksidZp1638Q2em118Q2el124 7QQcategoryZ33550QQihZ006QQitemZ160209049269QQrdZ1 QQsspagenameZWD1V
The electric one you see in the picture (in my previous post) came with the truck and obviously I don't need it. Can't guarantee that it works, but you are more than welcome to it, no charge. Just pm or email me your info.
The electric one you see in the picture (in my previous post) came with the truck and obviously I don't need it. Can't guarantee that it works, but you are more than welcome to it, no charge. Just pm or email me your info.
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From: Abilene, Texas
Thanks to you both! I'll check out the ebay listing. The truck has a vacuum dashpot now, but it is missing th spring and bumper.
Anybody got any ideas on the noise under the rocker arm cover?
Anybody got any ideas on the noise under the rocker arm cover?
Postmaster
Joined: Aug 2000
Posts: 3,312
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From: Eldorado Ca. USA
Originally Posted by 01PSDRVPuller
Thanks to you both! I'll check out the ebay listing. The truck has a vacuum dashpot now, but it is missing th spring and bumper.
Anybody got any ideas on the noise under the rocker arm cover?
Anybody got any ideas on the noise under the rocker arm cover?
