burned valves
Thread Starter
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Senior User
Joined: Jan 2007
Posts: 184
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From: Ft. Riley, KS
burned valves
hi everyone,
I have a 78 351m that just wont run smooth rebuilt carb new plugs, wires, cap, rotor. now what a mech things is that the valves are bad. so how do i tell if the valves are "burned" and what does that mean? it has great compresion 125-120 on all 8 so well i think the valves are good. so what you think o and when i rev it up i get some black smoke not dark nor a lot but i can see it.
thanks jim
I have a 78 351m that just wont run smooth rebuilt carb new plugs, wires, cap, rotor. now what a mech things is that the valves are bad. so how do i tell if the valves are "burned" and what does that mean? it has great compresion 125-120 on all 8 so well i think the valves are good. so what you think o and when i rev it up i get some black smoke not dark nor a lot but i can see it.
thanks jim
Tuned
Joined: Jul 2003
Posts: 455
Likes: 3
From: Berkeley
redo the compression tests, squirting a little oil into each cylinder. if the compression remains the same, the blow-by (smoke) is coming from the valves. if the compression increases, the rings are worn. 120-125psi is not too bad and it's good that they are even, so the rough running may not be due to valves or rings...
make sure you've got all the plug wires on the right plugs. firing order is different on 351m/400 than 351w. also, don't run plug wires parallel to each other or you can get cross-fire. i think it's the #5 and 6 cylinders are particluarly susceptible, since they fire sequentially and their wires typically run right next to each other. (someone please correct me if i got that wrong...)
a worn distributor shaft might cause sloppy timing. so would a loose/worn timing chain. if you haven't already, replace it with a straight-up double-roller...
make sure you've got all the plug wires on the right plugs. firing order is different on 351m/400 than 351w. also, don't run plug wires parallel to each other or you can get cross-fire. i think it's the #5 and 6 cylinders are particluarly susceptible, since they fire sequentially and their wires typically run right next to each other. (someone please correct me if i got that wrong...)
a worn distributor shaft might cause sloppy timing. so would a loose/worn timing chain. if you haven't already, replace it with a straight-up double-roller...
Posting Guru
Joined: Dec 2002
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From: chicago burbs
i'm not buying the burnt valves theory, your compression test disproves it. a burnt valve would show as a very poor/dead cylinder on the comp test.
try this, you need a place where it gets extremely very dark, no light at all, so dark you can't see your hand in front of your face. lift the hood, shut off the lights and let your eyes adjust for a while, then look at all the plug wires, the coil, the dizzy cap, everywhere the spark goes. you just might see a light show that you aren't supposed to be seeing.
what's the possibility of bad fuel, water or moisture in the tank?
what's the possibilty of the EGR system being all clogged up?
try this, you need a place where it gets extremely very dark, no light at all, so dark you can't see your hand in front of your face. lift the hood, shut off the lights and let your eyes adjust for a while, then look at all the plug wires, the coil, the dizzy cap, everywhere the spark goes. you just might see a light show that you aren't supposed to be seeing.
what's the possibility of bad fuel, water or moisture in the tank?
what's the possibilty of the EGR system being all clogged up?
Thread Starter
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Senior User
Joined: Jan 2007
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From: Ft. Riley, KS
so i recheacked the timing and it is set at 12 btdc. and i didnt find any vacume leaks. if the timing chane did jump a tooth wouldn't the timing be off?
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Posting Guru
Joined: Dec 2002
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From: chicago burbs
what kind of coil is it, does it appear to be stock? there are different types of coils, some don't like to work with certain types of ignition systems. i suppose a coil could cheese out some before completely giving up...
yeah, if the chain skipped i'd expect to find the timing off considerably. is your recent dizzy cap a good quality one or a discount house special? i've seen new discount caps run worse than the old original.
yeah, if the chain skipped i'd expect to find the timing off considerably. is your recent dizzy cap a good quality one or a discount house special? i've seen new discount caps run worse than the old original.
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Senior User
Joined: Feb 2007
Posts: 188
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if teh valves are burned out does that mean that you have to replace the engine? Or can you just have the heads rebuilt? I was wonderin becuz my stepdad is buyin' another truck and the guy had straights on it, and caused the valves to burn out due to lack of backpressure. and we have to have the tranny rebuilt and didnt want to replace the motor to.
Posting Legend
Joined: Sep 2001
Posts: 30,255
Likes: 37
Staright pipes will not cause the valves to burn out.
Engine diagnosis:
http://www.users.bigpond.com/ergoff/vac1.htm
Engine diagnosis:
http://www.users.bigpond.com/ergoff/vac1.htm
Freshman User
Joined: Mar 2007
Posts: 34
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Carefull
Proceed with caution start with the least invasive first. The thing I have to ask is has the fuel been changed in the 78ford.. Been through this recently myself. Expensive tank! Skunktruck has this problem only occured on one tank of fuel? And 16-f100luvr check compression like packlet states and if the pressure dosnt come up pull the heads and get hardend seats installed and a good valve job. I have been racing a stock bottom end cleveland at 8000rpm for three years and built a very mild 350hp replacement to go cruiseing and one tank of fuel cost me alot of time and money! I have now an 87octane 10.5cp ratio cleveland that is somewhat finicy. Mixed gas I believe kero and gas cause a 200 mile engine to burn six valves. My build is on the threshhold of some detination and thermal efficiency issues and the kero cause the mixture to lean out without me knowing. 16-f100luvr be sure after makeing you repairs learn to read plugs and open headers are fun even on the steet!
Senior User
Joined: Feb 2007
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no, it's not on my ford, it's on my stepdads chevy! That could be part of the reason, lol. but were buying it for about 200 dollars, body is in great shape, it just needs the tranny rebuilt, and some valves are burnt out.