built big 390....tiny problem
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Joined: Apr 2010
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built big 390....tiny problem
so... here goes
im only 20, this was my first rebuild. My grandpa had a 390 cobra kit motor i bought from him, motor was tired so i decided to monsterize it. I put hyperutectic pistons in, bored it .030, mean high lift lopey cam, brand new heads, and a performer 390 aluminum intake. Everything in the motor is brand new.... just bought a new 750 edelbrock and stuck it on
I decided to put into a 72 f100 4x4 4spd
SO now i have the timing set at 10 advanced, took it to my mechanic, set the dwell and points, tested the coil...its good. Everything sounds as it should......
EXCEPT in 3rd and 4th gear cruise when i step on the throttle it makes a sound similar to a deisel from the engine compartment...... so basicaly when the engine has a load on it
I have tried asking a bunch of people whats wrong and nothing fixes it, can ayone help me?
Kaenan
im only 20, this was my first rebuild. My grandpa had a 390 cobra kit motor i bought from him, motor was tired so i decided to monsterize it. I put hyperutectic pistons in, bored it .030, mean high lift lopey cam, brand new heads, and a performer 390 aluminum intake. Everything in the motor is brand new.... just bought a new 750 edelbrock and stuck it on
I decided to put into a 72 f100 4x4 4spd
SO now i have the timing set at 10 advanced, took it to my mechanic, set the dwell and points, tested the coil...its good. Everything sounds as it should......
EXCEPT in 3rd and 4th gear cruise when i step on the throttle it makes a sound similar to a deisel from the engine compartment...... so basicaly when the engine has a load on it
I have tried asking a bunch of people whats wrong and nothing fixes it, can ayone help me?
Kaenan
Elder User
Joined: Dec 2007
Posts: 813
Likes: 6
From: Up Nort Minnesota
what kind of distributor setup?
Most here would probably agree it sounds like you have it sparking a little too soon. Too much advance in the timing.
But check out things like your advances. If it has a centrifugal advance (springs & weights) you might have too light of springs in it. As for vacuum advance You might want to check to see that the timing is falling back under engine load.
Turning your timing back is the quick & easy answer but you might want to find someone who understands the old art of reading & adjusting the timing curve through your rpm range.
I'd have to go out and pop the cap off one of my old point ignition FE's to reaquaint myself with the ford point distributors.
Someone here can probably be of more help to you.
Quick answer would be to bring your timing down about 5 degrees & see what that does for you.
But a properly functioning distributor can really make a huge difference in performance & economy.
"recurving" a dist is the art of making it do what's right for YOUR engine.
Good luck.
Most here would probably agree it sounds like you have it sparking a little too soon. Too much advance in the timing.
But check out things like your advances. If it has a centrifugal advance (springs & weights) you might have too light of springs in it. As for vacuum advance You might want to check to see that the timing is falling back under engine load.
Turning your timing back is the quick & easy answer but you might want to find someone who understands the old art of reading & adjusting the timing curve through your rpm range.
I'd have to go out and pop the cap off one of my old point ignition FE's to reaquaint myself with the ford point distributors.
Someone here can probably be of more help to you.
Quick answer would be to bring your timing down about 5 degrees & see what that does for you.
But a properly functioning distributor can really make a huge difference in performance & economy.
"recurving" a dist is the art of making it do what's right for YOUR engine.
Good luck.
Senior User
Joined: Feb 2009
Posts: 274
Likes: 0
From: Marquette, MI.
That sounds like spark knock, also known as pre ignition and that can cause engine damage. Are You running premium fuel? You might not have enough octane in Your fuel. What compression ratio is Your engine at? High compression requires higher octane to combat igniting from compression heat. Are Your brand new heads cast iron or aluminum? Aluminum heads transfer heat better meaning less chance for pre ignition from cylinder head temperature. Usually if You bump Your timing back a couple of degrees the spark knock will go away. It's been a long time since I timed a points distributor, but as I recall the original sticker on the valve cover called for 6 degrees of initial timing.
Elder User
Joined: Nov 1999
Posts: 796
Likes: 3
From: Anderson Northern Ca
Engine Ping
Kgreen,sounds like a timing issue.How does the engine run and perform at low speed and under load?Had the same issue with my new 390 when I rebuilt it a couple of years ago,turned out to be a timing issue.When I set the timing initially I tightened the distributor bolt and moved the distributor without realizing it and could not figure why it ran like a dog and made little power.After going thru all the same stuff as you did I finally went back and found out what I had done then re-timed it and now it runs great.A good upgrade for that engine would be a Pertronix Ignitor,I run it in all my old points trucks.
"bowties in the rearview mirror"
"bowties in the rearview mirror"
Elder User
Joined: Dec 2007
Posts: 813
Likes: 6
From: Up Nort Minnesota
Dude, I just noticed you said 750 edelbrock.
OK, on the front of the carb there are probably 3 ports for vacuum, a big one in the middle & a little one on each side if I remember right.
One of those little ports should be hooked to your vacuum advance. You might have the wrong one hooked up for your engine's timing needs with that cam.
try this.
Test the 2 little ports on the carb, one of them should pull vacuum at idle. Then when you snap the throttle open the vacuum should fade off. (this is the one you should try hooking to the vacuum advance) The other vacuum port should be dead at idle and it will pull air when you snap the throttle open. I don't think this would be the right one for your engine, just cap that one.
if you have that one hooked up then you are advancing your timing as you are loading the engine, I think you want the other one. The other one (the one that draws air at idle) will relax under engine load & let your timing fall back to the base setting of 10 degrees under load. Then at cruise it will come back in.
OK, on the front of the carb there are probably 3 ports for vacuum, a big one in the middle & a little one on each side if I remember right.
One of those little ports should be hooked to your vacuum advance. You might have the wrong one hooked up for your engine's timing needs with that cam.
try this.
Test the 2 little ports on the carb, one of them should pull vacuum at idle. Then when you snap the throttle open the vacuum should fade off. (this is the one you should try hooking to the vacuum advance) The other vacuum port should be dead at idle and it will pull air when you snap the throttle open. I don't think this would be the right one for your engine, just cap that one.
if you have that one hooked up then you are advancing your timing as you are loading the engine, I think you want the other one. The other one (the one that draws air at idle) will relax under engine load & let your timing fall back to the base setting of 10 degrees under load. Then at cruise it will come back in.
