Join Date: Mar 2002
Location: Austin, TX, USA
Re: bad miss, stumble on acceleration? please help?
Don't feel bad. Many of us have suffered the exact same symptoms. There are two basic things that contribute - timing and carb setup.
I see this is your first post - welcome aboard. Search the archives for topics like "vacuum advance", "carb(erator) too big", "stumble","bog", and "3456" (factor in a square bore carb sizing formula). You should find lots of basic info to get you started. There are lots of little details.
Here are my thoughts which are likely to coincide with one another:
* The cam may be a bit too radical for the torque convertor on your car.
* The 650 carb is a perfect fit for a 350 street motor. Make sure it's at factory baseline specs for starters. Consult the Holley web site for jet sizes and other baseline calibration details.
* If the carb is a mechanical secondary type, and you have an automatic transmission, you'll need to refrain from mashing the gas to avoid the bog - unless you have a high stall convertor.
* If you have a vacuum secondary carb (best for auto-trans), perhaps the secondaries are coming open too soon.
* Vacuum leaks are a prime culpret. Anything that allows air to get past the butterflies without affecting fuel metering will cause a lean out that often creates a back-fire throught the carb. My Holley's front accellerator pump lever was hitting the number 1 runner on my Edelbrock Performer RPM intake creating a huge leak at the base gasket.
* Base timing isn't set correctly. Base timing should be between 10* and 15* BTDC in most cases. Engine / cam specs vary the optimum setting widely. Cut and try is needed to find the optimum.
* For all but the most radical motors, vacuum advance is a huge benefit. It will improve idle speed, raise idle vacuum, and make for better street manners. If you have anything other than a stock cam, an adjustable VA unit is a good idea. It allows you to set the vacuum level for full advance as well as limiting the total amount of VA advance.
Be patient, don't tweak too many things at once. You're fine tuning your setup and it takes a few carefully thought-out passes to get it right. Use your plugs to help you determine if the fuel mixture is right, but beware reading a cold plug that been in an idling motor will always be black. Get the plugs good and hot by driving the car if possible, and read them immediately after.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI