1973 351C (Mustang Mach I)
* Replaced 2V heads with 4V (yeah, I know)
* Replaced 2V manifold with 4V
* Replaced 2V carb with Speed Deamon 650CFM, Electric Choke, Ford
* Replaced cam with CompCams Magnum 270H grind Hydraulic flat tappet cam,
* Installed Hooker headers.
* Installed MSD-6AL and MSD Pro-billet disttributor
Car starts but won't stay running until warm. Run-on severly, sometimes
backfires when turning off. Gets 5mpg-8mpg (should be 12-18).
Already installed resistor as recommended by MSD if you have run-on
Where should I look?
(email is snipped-for-privacy@Me.org where "Me" is emike)
Contrary to what you think, you are getting too much air....... this is THE
major cause of after-run.
Start by making sure that the choke opens fully when warm... from the sound
of it, you'll also need to adjust either the choke idle speed screw, the
choke pull off or both. With the motor warm, place the fast idle screw on
the second step of the fast idle cam (it's on the choke side of the carb)
and adjust the idle speed to about 1600 rpm - using the fast idle screw,
natch. With the motor cold, the choke pull-off should open the choke by
about 3/16ths of an inch (give or take depending on your climate).
If you are still having problems with after-run, after getting this stuff
right you may have to remove the carb and look from the bottom side of the
throttle plates. With the choke fully OPEN, look to see if the bottom of the
transfer slot is visible (this is a rectangular slot just above the small,
round idle hole). Another clue is how much control you have over the idle
quality with the mixture screws.... Your 270 cam shouldn't be lopey enough
to require drilling the throttle plate but I will say that, after 35 years
of doing this stuff, I still haven't seen everything there is to see. If
(and ONLY if) everything else is right, you can start with a 1/64th drill
bit and drill a hole about 1/8th back from the very front edge of each
throttle plate. Again, with that 270 grind, I can't see you going much more
than 1/64th, but if you feel the need to increase the size of the hole to
regain idle quality or bring the idle speed down (this set up should idle
comfortably around 750 in gear), work your way up in 1/64th increments. If
you botch it, the holes can be soldered shut....
This modification is not for the faint of heart, those that possess ten
thumbs nor is it recommended for those that have said "that's good enough"
at any stage in base engine prep.
where "Me" is emike)
The condition Jim was describing and the modification to correct
it (drilling the holes) occurs when the base throttle opening is
so great that too much of the idle transfer slot is exposed below
the throttle plates. Drilling the air bleed holes allows that the
throttle plates be adjusted further closed restoring the proper
relationship at idle position of the throttle plates to the
transfer slots. The maximum that the idle transfer slot that
should be exposed below the throttle plate is .045 inches.
This is all well documented by Holley.
With the bad gas mileage, coupled with the run on and idling problem, it's
gotta be the carb. Do you have the old carb (hoping it was a 4V)? If so,
put it back on and see what happens.
Run on (dieseling) occurs because too much gas (and air) is being dumped
into the carb throat at shutdown. Ford carbs have an electric anti-
dieseling dashpot that you use to set the idle speed. When you cut off the
key, the dashpot closes and the butterfly valves close completely. Does
your carb have this device? If not, try slowing the idle down, and see if
the run on continues.
However, that would have nothing to do with getting bad gas mileage. Carb
way too rich, timing problems, a large vacuum leak, just a few things that
could cause it.
I' been living with one of these boys since '72. First, the
4V heads poor air flow qualities at low RPM can cause you a
lot of grief when cold. You can minimize the problem by
making sure everything else is right. Your description
would lead me to thing you have a vacuum leak somewhere.
One problem I have had on several occasions is the intake
gasket - they can be a bitch to seal especially if one or
both of the heads has been milled and the intake has not.
If you are using only the pan gasket as many do, it can be
hard to seal. For best power when warm, you need the pan
gasket to keep the hot oil off the bottom of the intake
heating the mixture. Unfortunately, this is also
detrimental to a street driven car. I have up on the pan
gasket and installed the Felpro permatorque gaskets without
the pan. I only used a bead of silicone at each end of the
intake to seal the valley. The very narrow seal surface
around the 4V ports do not help to seal.
You mentioned the timing chain. I hope you installed a new
timing set when you installed the cam and made sure the cam
bearings were in good shape. The Clevelands and their
cousins in the family seem to have a problem keeping them
and the crank properly oiled resulting in fairly rapid
bearing wear. Valve guide wear can be another source of
wear resulting in problems in the 4V heads.
Another area of problems is valve settings. I know the 351C
does not have "adjustable" valves but they are adjustable
during engine setup. The lifter preload bust be set
correctly. If not, the lifters may tend to act "pumped"
when the engine is cold but be fine when the oil warms
enough to allow the lifters to bleed a bit more quickly
which will result in better idle. This can be aggravated if
you not only installed a high volume oil pump but, decided
to give it a boost with a spring shim. A stock 351C pump
will go well past 100 psi on cold start and you stock guage
will not show excessive oil pressure because it has a pin in
the guage to limit it on the high scale. If you have not
done so, go back and check each and every valve for proper
All that said, if I were in your shoes, I would drag out the
vacuum guage to get an idea of how the vacuum is reading as
the engine warms how much, needle fluctuations, how does it
read as the rpms ar increased, etc. thet may give us some
idea of what is happening. Check your ignition timing. The
351C like a lot of base advance. The spec was 16 deg BTDC
with the hose off and plugged. Make sure you have the
advance hose attached to the carb ported vacuum. It is the
one that sucks more when you open the throttle. If you have
a dual diaphram distributor, you should at least convert it
to a single diaphram unit and have it recurved. The folks
at CompCams can give you a wealth of info on the correct
curve to go with your setup. Run a proper compression check
to be sure it has good compression. I don't know what
pistons you have or exactly which heads but, if you have
aslightly dished piston to go with a large chamber head, you
may have enough static compression to accommodate your 270
cam. A 270 cam will probably need at least a 9.0:1
comprerssion ratio to run decently because the cam bleeds
off quite a bit of pressure at low RPM. That is why a good
close observation of the vacuum readings and compression may
Once you have done all your backtraking over what has been
done to determine exactly what your parts combination is and
that all the extyernal diagnostic work has been done, I
would head for the intake gasket and get that straight.
While you have the intake off for that, pop, the rockers and
make damn sure the preload is correct.
Only after making sure everything else is right. you can
think about drilling the throttle blades as Jim described.
If you screw that up, you either have to remove the blades
and solder the holes or replace the carb if you go too far
or if you find that was not the problem.
For a group that lives 351C try here for more help
I think you will find more than you ever wanted to know one
way or another
On Sat, 06 Mar 2004 23:22:46 GMT, "Whole Lotta Tom"
Thanks, I'll update my link.
I hope Mike, the OP, has a bit of patience. He is going to
need it. I think he would have been better off in the
beginning, to stay with his 2V heads with a little cleanup
and use the Edelbrock 4V manifold with that 270 cam and it
would have probably fallen right in place with good street
driveability and some punch in the mid to top end. The 2V
351C's were much more pleasant to drive - they just didn't
have the the mid to upper end of the 4V. The 4V is doggy on
the bottom end if it has enough cam to keep it moving on the
top end mostly because of the heads. I learned that the
hard way. If I could order mine all over again knowing what
I know now, the Torino would be a 429 upgraded to a 460 by
now. Only after I bought it did I learn that heavy cars
need torque more than HP.
Check your timing and make sure it's not advanced too far ahead of
what it should be. Also, does it run hotter than normal? The run on is
usually caused by excessive combustion chamber temps, which can either
be caused by problems with the cooling system, or the timing advanced
too far ahead. When you replaced the heads, did you install the head
gaskets properly? If you don't get them in the right way, it will
block one of the cooling ports and cause the higher temps.
Before you drill the throttle plates be sure that they are closing evenly
and completely. With the carb off hold it up to a light and look through the
throttle bores with the throttle fully closed. You'll probably see a bit of
light. The main thing is that it be the same amount on both primary bores,
and the same amount on both secondary bores. This isn't normally a problem,
but it has been known to happen, especially with a rebuilt carb. Also be
sure the secondaries aren't too far open to begin with. They need to be open
slightly, but too much initial opening will give you too much air.
Every day is a good day- it's just that some are better than others.
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