I own a 1968 Mustang GT with a 390 and 4 speed. We used it for developing some or our products and the carb sat dry for about 4 years. When I tried to start it after that, there was 0 accelerator pump shot. Rather than rebuilding this 650, I just bought a cheapie 4 BBL and installed it. At least it had an accel pump shot and the car ran OK for moving it around, etc. Well then I started “driving” it and it seemed really low on power, but I wasn’t sure.
I installed one of our DataMite III loggers and just clipped the purple wire of an inductive pickup (DTM-IPU) to the coil wire. No other connections were made. I would measure the car’s acceleration rate with the built in accelerometer and just engine RPM.
To get a power curve in a vehicle, you place the trans in a gear where you won’t spin, but you won’t get going too fast. I chose 2nd gear. You lug the car down in that gear, like 2000 RPM or so. Then start recording data and go WOT (wide open throttle or full power), but not so hard as to break the tires loose. Our vehicle versions of the DataMite software have a special Test Type of “Measure Tq/HP from Accel” to produce dynamometer quality power curves. I was right, the car was a dog (no offense to dogs).
The easiest thing to add to diagnose the problem would be one of our DT3-AF1 A/F sensors. I had planned on doing A/F many years ago, so there was already a bung in the exhaust. I made a 2nd run now recording accel rate and A/F and the carb was way rich, with A/Fs in the 9.0 to 9.5 range.
Just as I was looking at the data, our good friend from Ford, Burgess Coleman just dropped by the shop to pick up a sensor for his car. We talked a bit and he said the accelerator pump diaphragms were notorious for drying up and getting really stiff. (I had previously thought some passages got clogged or some check balls got stuck and just didn’t pursue it.) He suggested to put in new diaphragms in the old carb and see how that carb ran. The old carb also had mechanical secondaries. (I did not pursue it, but it’s also possible the vacuum secondaries weren’t opening on the cheapie carb.)
Well, with $40 parts and some time, the old carb was back on the Mustang. I made a WOT accel in 2nd gear, now recording A/F and accel rate both. Well, this carb ran much leaner, in the 11.5 to 12.0 to 1 range, much better for good power. And power was up over 100 HP. The green power curve was with the cheapie carb and you can see is way down, over 100 HP, compared to the red graph. I backed off quite early on the first run with the cheapie carb (around 4800), because I could tell it wasn’t making power. Check out the power curves below from our Road Race/Circle Track DataMite software. The Drag Race DataMite software will do this type of analysis also.
Bottom line, if you are not measuring what is going on, you are just guessing. You need data to make informed decisions.