351C problem. Help!

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
kickdown. * Replaced cam with CompCams Magnum 270H grind Hydraulic flat tappet cam, * Installed Hooker headers. * Installed MSD-6AL and MSD Pro-billet disttributor
Problem:
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 problems.
HELP!
Where should I look?
Too rich? Fuel pump? Timeing Chain? Manifold leaks?
Thanks!
emike (email is snipped-for-privacy@Me.org where "Me" is emike)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
HTH
Jim Warman snipped-for-privacy@telusplanet.net
where "Me" is emike)

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

If there is too much air, how in the world does drilling a hole in the throttle plate help anything? That only deliveres MORE air (and mixture.)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Ok, I get it now. I'm no Holley man. Yes, the run on is always caused by too much air/fuel mixture in the manifold at shut down. Sounds like this could cause it.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
donutbandit wrote:

He just finished telling us that he swapped the old 2V carb, intake, and heads for 4V stuff you dimwit!
Merc Thundersnake#16
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

And this is all you have to add? Asshole.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
A dumb shit wrote:

Why thankyou for your kind words!
Merc Thundersnake#16 69 Mustang sportsroof
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 06 Mar 2004 11:22:07 EST, "E. Mike Durbin"

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 lifter preload.
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 help.
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
http://www.network54.com/Hide/Forum/119419
I think you will find more than you ever wanted to know one way or another
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
lugnut wrote:

For a new, easier way to reach that same message forum, just go to:
http://www.351cleveland.net
--
Tom
TS3
http://www.geocities.com/styleline58 /
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"E. Mike Durbin" wrote in message:

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.
Pintobro
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.