A plug reading on all eight cylinders showed a light tan color, perfectly normal. It's a good fuel mixture and doesn't burn oil.
Here's one reason Doug's 289 didn't perform to potential: a throttle not opening fully. Jon adjusted the rod and fixed the problem.
Our troubleshooting efforts turned up this vacuum hose between the oil-filler cap and the carburetor-base PCV port. Do you see the restrictor someone fabricated at the carburetor-base end? This is a vacuum leak serious enough to alter air/fuel ratio and cause poor performance.
While we were tossing the improperly installed PCV valve hose, we decided to check carburetor-base and spacer gaskets. Both were deteriorated, causing vacuum leaks and poor performance.
Dyno Test: '65 289
Owner: Doug Holland
289-2V, C4 automatic
When Doug Holland arrived at Jon's shop, his Mustang was plagued with a definite misfire and rough idle. Doug's original 289-2V had received only one valve job in its 42-year history. We were convinced the misfire was ignition related or rooted in weak compression. Testing would prove us wrong on both theories. Jon decided to do the baseline test exactly the way the car came in.
Pull 1: Baseline
Peak horsepower: 98.27 at 3,500 rpm
Peak torque: 167.59 lb-ft at 2,650 rpm
Air/fuel ratio: 12.8:1 at 70-mph cruise; at wide-open throttle, 12.0:1
We began troubleshooting with a check of the timing and ignition system integrity. The Autolite single-point distributor had seen better days. However, it produced a healthy spark on all eight cylinders. One of the first moves to help this distributor should be installing a PerTronix Ignitor II and curving the advance units, which makes a significant contribution to performance and reliability. We weren't prepared to do that in the time available, so we continued with the same ignition system.
Jon wondered about compression considering the soft misfire he traced to the No. 2 cylinder. The compression check was remarkable with all eight cylinders coming in at 105-110 psi each. A cylinder leakdown test yielded a healthy 289. So why the misfire?
Pull 2
Peak horsepower: 87.65 at 3,250 rpm
Peak torque: 157.69 lb-ft at 2,600 rpm
Air/fuel ratio: 14.6:1 at 70-mph cruise; at wide-open throttle, 13.0:1
When Jon observed air/fuel mixture, he concluded that horsepower could be improved by reducing jet size from 52 to 48. Ironically, horsepower fell off a pinch to 87.65, but torque increased to 157.69. This tends to break all the rules because, theoretically, horsepower should have increased. There are other factors going on here, including a marginal ignition system. We still had the misfire.
Pull 3
Peak horsepower: 83.92 at 2,900 rpm
Peak torque: 151.87 lb-ft at 2,900 rpm
Air/fuel ratio: 14.6:1 at 70-mph cruise
When Jon decided to try another dyno flog using the same 48 jets and ignition timing, something caught his attention: the throttle linkage. It did not open the throttle fully with the accelerator pedal floored. Jon adjusted the throttle rod to allow the throttle to open all the way.
Pull 4
Peak horsepower: 136.7 at 3,500 rpm
Peak torque: 205.10 lb-ft at 3,500 rpm
Air/fuel ratio: 14.6:1 at 70-mph cruise
After Jon took care of the maladjusted throttle rod, faulty carburetor base gaskets, and improper PCV valve hose, positive changes began with the right air/fuel ratio and throttle positioning. These were simple fixes costing less than $5. Jon also swapped in larger 49 jets to richen the mixture and adjusted the idle mixture to help improve idle quality.
Despite Jon's best efforts, he was unable to get rid of the misfiring plaguing Doug's Mustang throughout our testing. Althought Jon checked virtually everything, compression, cylinder leakdown, spark quality, and timing, carburetor gaskets, and vacuum hoses, the misfire remained throughout our testing.
During each pull, we couldn't help but notice black smoke coming from the exhaust at wide-open throttle and deceleration. We sprayed carburetor cleaner around the intake ports and didn't observe a change in engine operation, which would indicate a vacuum leak on top. We concluded there was a vacuum leak at the No. 2 cylinder's intake port underneath, causing the affected cylinder to misfire for no apparent reason. We suggested Doug replace the intake manifold gasket, which should correct the vacuum leak and get rid of the misfire. We also advised Doug to invest in a rebuilt Autolite distributor, retrofitting it with a PerTronix Ignitor II to improve idle quality, cold starting, and drivability.