I have a 66 with a 347 and I'm running a Mighty DEMON 650. I am experiancing a heavy load hesitation in second and third from anywhere around 4500 to redline (6K). First gear is fine. I am running a holley blue through a regulator at around 7.5 to 8PSI. The car is timed right, mixture seems fine, floats are on the money and the only thing I have not addressed is the power valve. Could this be my problem?
Thanks in advance for the assistance. Been busy and haven't posted for a long time.
Here is an explination on how a power valve works.
When an engine is idling the diaphragm ‘sees’ a high manifold vacuum. The diaphragm responds by moving in the direction of the vacuum, which overcomes the force exerted by the spring and pulls the needle into the seat, thus closing the valve. As the throttle is opened and manifold vacuum falls, at a given point the force exerted by the spring overcomes the declining manifold vacuum and pushes the needle away from the seat, opening the valve. Fuel from the fuel bowl flows through the valve and through two small holes located beneath the valve- the power valve restrictions- then flows into the main fuel wells, joining the fuel supplied by the main jets. With a power valve you can use smaller main jets, which provides good low RPM performance and drivability, yet when the throttle is opened and manifold vacuum falls, the power valve supplies the extra fuel needed for acceleration and high RPM running.
Your carb may be too small. Sounds like maybe powervalve closeout. You need a vacuum gauge to read in you car. When you have a carb that is too small, the vacuum created can actually start to close the powervalve. You should have less than one inch of vacuum at WOT. As a test and a test only, put in a 10.5 if you don't have a guage. This is not a fix. The 10.5 has a stronger spring and will close as easily. You may need another carb. My friend made 455 HP dynoed with a 302 and the same carb. But because of the vacuum he was drawing, he would need a bigger carb if he made one more HP upgrade. Good luck!!
If the hesitation goes away then the fix would be a larger carb? I was always under the impression that if a carb was a little small you would get better throttle response and not have any negative effects other than a little less HP.
Please enlighten me because I am about to put it in the shop for some professional tuning.
If it was me I would play with jet size in the secondary side and see what the does. if no change I would look at another culpret other than carb. I had a car do about the same thing it was a week spark. wireing in car was old and wasnt supplying enuff current to coil to fire at upper rpms. put msd in ran great. before I thought about another ignition setup I would pull a plug and see what spark looks like it should be blue.
oh I forgot if you have petronix in you dist I will put money on it being bad. I tried one went bad within a month same problem. my friend tried one and the same thing happened to him he put stock stuff back in runs great.
A 6.5 ? I can almost promise that is too small. It will waot too long before it gives your motor the extra fuel it needs. The closer the pv is to 0, the longer it take to open the pv...sometimes long enought to starve a motor of fuel. If the 10.5 cuers it,yes,you may need another carb. BUT BUT BUT... If you are running a 6.5 and it need a 9.5 it's will need a vacuum guage. Have someone sit in the car seat while you floor it to redline. If your car draws more thatn 1-1.5 in of hg. you need a new carb. Yes you do have to run it all the was on the car and yes it looks dumb as hell. What are you going to do ya know ?