timing question
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New User
Joined: May 2007
Posts: 15
Likes: 0
From: San Jose
timing question
Picked up a '70 F250 390 4 speed and with a carter 4 barrel and edelbrock manafold yesterday. Driving it home the truck would not go over 50MPH as it would sputter and backfire and just run like junk.
we did check the timming at it was set at 6 - 10 ATDC, could be why it ran so bad.
Today I rebuilt the carb (carter afb) and added a Pertronix Ignitor Ignition.
The engine runs much better now with good throttle responce and revs with no problems the vucuum is bouncing between 19.5 - 20 in at 800 rpm's.
The strange part is the engine wants to run at 25 degrees BTDC at idle with the vac disconnected from the distributor (if I try to go much lower the engine will not run) I'm unable to check total advance as the vacuum module is bad and waiting for a replacement.
Any ideas on why it want's 25 degrees BTDC after the ignition upgrade and not the 6 BTDC the manuals call for? could I have put the distributor in wrong?
I just ran a check with a vacuum gauge, if I snap the throttle wide open and then close it drops to 0 at the hit of the throttle and then to 25 and back to 19.5 - 20 after I let off the throttle. that sounds how it should be.
The owner did say it had a burnt valve but I'm not sure about that now. will do a compress check next weekend
we did check the timming at it was set at 6 - 10 ATDC, could be why it ran so bad.
Today I rebuilt the carb (carter afb) and added a Pertronix Ignitor Ignition.
The engine runs much better now with good throttle responce and revs with no problems the vucuum is bouncing between 19.5 - 20 in at 800 rpm's.
The strange part is the engine wants to run at 25 degrees BTDC at idle with the vac disconnected from the distributor (if I try to go much lower the engine will not run) I'm unable to check total advance as the vacuum module is bad and waiting for a replacement.
Any ideas on why it want's 25 degrees BTDC after the ignition upgrade and not the 6 BTDC the manuals call for? could I have put the distributor in wrong?
I just ran a check with a vacuum gauge, if I snap the throttle wide open and then close it drops to 0 at the hit of the throttle and then to 25 and back to 19.5 - 20 after I let off the throttle. that sounds how it should be.
The owner did say it had a burnt valve but I'm not sure about that now. will do a compress check next weekend
Cargo Master
Joined: Jul 2006
Posts: 3,014
Likes: 91
From: Concord, CA
If it seems to be running so well, and in days past mechanics set the timing by ear, then maybe the timing for the engine is ok but the harmonic balancer is off. Kinda like 3 Mile Island where the gauge says the temp is ok but why is the coolant bubbling out all over the place...
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Check the damper. Pull number 1 spark and crank the motor in short burst with you finger over the plug hole (cold motor works best for this). Now rotate the motor by hand or with a wrench and using a dowel or a screw driver or dial indicator bring the piston to the top of the bore (actually it doesn't matter id you are on the compression stroke or not as long as the piston is at TDC) . Now look at timing marks and you should be at zero. If not within a couple a degrees, the damper has slipped and needs replacement.
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