Guy's,



I’m having some problems with a rough engine stumble when I initially depress the throttle. Consequently with the stumble I have a hard time pulling out without stalling unless i get the RPM's up.



The engine ran great prior to the installation of a Schneider 274F cam; specs are 274 duration with 230 at .050. 480 lift and 110 lobe separation. Power bands from 2500 to 6000 RPM, I realize this is a lot of cam for a basically stock 312, though it is hooked to a 4 speed with 3:89 gears.



Also the carb is a Holley 465 CFM 4 barrell. I have checked the accelerator pump adjustment and it is spot on. Timing is set at 16 degrees initial advance.



Any help at all would be appreciated.

You have answered your own question.  The problem didn't appear until you installed the cam, and you admit it is a lot of cam for a stock 312.  What distributor are you using?  If it is a '56 or earlier, it is marginal at best.  You can gain a little more low end torque by advancing the cam a little.

John - "The Hoosier Hurricane"

It's a 57 distributor, I figured the cam was the root of the problem but was hoping it was something else that could easily be fixed.

Hi Guys,  I wouldn't weigh in here except that I had the same problem with my '57 312, and I have the exact same cam that you do.  Hoosier John has forgotten more about carbs and Y-blocks than I'll ever know.  My engine is not too far from stock with a Blue Thunder aluminum intake and Hedman headers.  I've got two Holley carbs, the 465 like yours and a 600 cfm.  With both of them I had a serious stumble when taking off from a stop, and an occasional backfire through the carb.  After the backfires I replaced power valves at times and played with the idle mixture also.  The distributor is a 1957 with a Pertronix Ignitor in it.  I changed over to a 600 cfm Carter AFB that I got through eBay, model number 9635.  I've had to play with the accelerator pump adjustment and richen up the idle mixture, but the stumble is gone and it's easy to drive on the street.  I think I'm running about 12 degrees initial before TDC at idle.  I'm running a 4-speed with the 3.10 rear end.  Hope this helps.  Good luck, Nick Brann - K.C., MO

This could be as simple as richening up the idle mixture or putting some additional advance into the intial ignition timing.  New camshaft = new tuneup parameters.   Here are also some other items to look at.

 

Was the camshaft degreed in when it was installed?  If not degreed in, then you’re running blind in that area without doing some kind of check for that.  John’s suggestion on advancing the camshaft may be the obvious fix and especially if the camshaft is sitting in a retarded position now.  Has both the vacuum and mechanical advance portions of the distributor been checked for proper operation?

 

Did you perform a vacuum check before changing out the camshaft?  If so, then do another vacuum check now and compare the readings looking specifically for a drop in vacuum at the same rpm with the new camshaft.  On the flip side, this could be as simple as a vacuum leak that occurred during reassembly.

 

A compression check on all the cylinders before and after the camshaft change will also give an idea on how much the dynamic compression ratio was lowered as a result of installing a camshaft that has a later intake valve closing event.

 

Besides fattening up the idle mixture, recheck the float levels in both the front and rear carb bowls and loosen up the valve lash.  A low float level in either end of the carb will contribute to some hesitation issues.

Lorena, Texas (South of Waco)