Hi guys, just a quickie. I built my 312 last year and shes done about 500 miles. I built it stock and fitted an Isky E4 cam with an adjustable timing set. When installing the cam I dialled it in but I can't remember what figures it came out at. Anyway she runs fine but needs a lot of juice at idle just to run smooth, i had to wind the mixture screws well out to get it to idle at all. I suspected a vacuum leak, i found one and fixed it. I swapped out the Holley carb for a new Edelbrock performer (500cfm) but still have the same issue, needs a lot of mixture to idle. I've just decided to check the cam timing. Checking the cam card it states inlet opens at 22degrees at 17 thou lift, mines reading 26. At 50 thou lift it should be zero, at this lift I'm reading 2to 3 degrees on the crankshaft. Im not sure if this would cause my weak idle situation, or should I be looking at some other issue? I did a smoke test on the inlet and found no leaks. Thanks.

What is your initial ignition timing?

The cam card figures are usually lobe lift, not valve lift.  Make sure you are getting the right lift measurement.


Lawrenceville, GA

Just a random thought. If you're running a PCV valve, try blocking it off and set the idle. There's been some talk that some valve/engine combinations need to be restricted.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Running ignition timing at 10 degrees initial, it's ok up to 12 but tends to ping at any more than that. The car runs great at speed and acceleration is good, just have this rough...Ish stumbly idle that can only be bettered by richening the mixture to the point where the exhaust stings your eyes. Spark plugs show it's running weak. It all points to as vacuum leak but I have not been able to find one.

I'm not running a pcv valve on this engine. Perhaps I'm trying to make it idle too low with this cam. I'll do a vacvuum gauge reading once its running again, the radiator is currently removed because it sprang a leak....again!!!

Does this problem get better at 12 degrees?
What is your idle speed?


Lawrenceville, GA

It does improve it slightly but only by the fact it speeds it up, I then slow it down with idle screw but it still runs lean . I don't have a tachometer on it at the moment, I'm guessing it doesn't like any less than 800rpm or it stumbles when I select drive. It would idle quite low and smooth before I rebuilt the engine with idle mixture screws out about 1.5 turns, I almost have to screw them nearly all the way out now to get it to idle at at all.

And that timing is with the vacuum advance disconnected and the hose plugged, correct?

In order to trouble shoot effectively, you need to have all of the info possible, including RPM.  Otherwise, it is more of a guessing game.



Lawrenceville, GA

I put an E-4 in my 312 along time ago in a standard shift car and the cam profile produces a rough idle. While 260 deg. is not an extremely radical cam the extra lift and duration along with a tighter lobe separation angle creates more overlap that results in lower vacuum and a rougher idle. If your degree wheel readings are accurate it looks like your cam might be a few degrees advanced, which I wouldn't call a gross error. When installing my cam there were three of us there checking each other on accuracy and it is right on and it idles rough, so I don't think you would gain the idle your looking for by retarding the cam a couple deg. even though that is a step in the right direction. The engine likes 10 deg advance on ignition and it pings if I don't use higher octane gas than what is sold at the pump. The engine idles at 750 rpm and surprises people how hard the engine pulls at high rpm in high gear. If a smooth idle and take off coupled to a Fordomatic is what you like you might be happier with a stock '57 245 hp cam.

Thanks guys. Yeah it's difficult to get any data as it's not running at the moment. I guess the question i was asking is would retarding the cam give me a better idle? The E4 was the only cam I could get hold of easily here in the UK.