datachicane
01-11-15, 07:29 AM
On the way back from Portland a couple of months ago, I noticed my daily driver Mustang was running very, very poorly when I stopped at an exit. This is the highly modified '65 convertible I'd posted pics of earlier. I bumped the idle up enough to keep it from dying and babied it home. I went through all of the usual stuff- spark and fuel were fine, I installed block-offs on the two secondary carbs and swapped carbs around to verify that I hadn't sucked any crap into a jet, swapped out the PCV, etc. The cam is too lopey to do much good checking vacuum, but as poorly as it was running it would have to be one heckuva vacuum leak, which I couldn't find in any case. The temp stayed rock solid, which also seemed to eliminate a vacuum leak of that size.
I'd spend a day or so on it, get buried with work for a few weeks, sneak in another hour or two, repeat until last weekend. I decided the only logical next step was to pull the head and check for camshaft failure, and I knew given my workload that getting a day free to work on it wasn't going to happen anytime soon. I swallowed my ego and took it to a shop. I built this engine (and the one before it), and it's the first time this car's been to a mechanic in 25+ years and 200k miles. That hurt.
Most shops won't touch a car like this. Luckily there's an old-time mechanic still around that I used to go to in high school, before I figured out what I was doing. It took him nearly two days to find the problem, and it was my fine engineering to blame. These engines were designed for durability, not performance, and one of the performance shortcomings comes from the #3 and #4 exhaust ports being siamesed together into a giant outlet, probably over 3x the size of the other ports. Aftermarket port dividers that separate these and knock the ports back down to size used to be popular back in the day, and I've run a few in various other cars with the same engine. The mfgrs recommend that they either be high-temp epoxied or tack welded in place, and I've had both fail, usually grinding a hole in your expensive headers in the process. When I built this head for the first time back in the early '90s I wanted to make certain that never happened with this one, so I drilled and tapped the head and bolted the divider in, nice sanitary countersunk install. Problem solved. The intake manifold and cylinder head is also integral on this engine, and you can just see the end of the bolts looking into the intake from the primary carb mount.
Fast forward to a couple of months ago. The high-temp threadlock on one of those bolts had failed, and vibration had loosened it until it dropped out and exited through the exhaust system. This left a neat, efficient port between the exhaust and intake manifolds, so vacuum was radically pulsing in time with the exhaust pulses, while leaving the average right where it should be (thus the constant temps).
Yeah, nice design.
At least it's back on the road. Glad I'm not working for SpaceX.
I'd spend a day or so on it, get buried with work for a few weeks, sneak in another hour or two, repeat until last weekend. I decided the only logical next step was to pull the head and check for camshaft failure, and I knew given my workload that getting a day free to work on it wasn't going to happen anytime soon. I swallowed my ego and took it to a shop. I built this engine (and the one before it), and it's the first time this car's been to a mechanic in 25+ years and 200k miles. That hurt.
Most shops won't touch a car like this. Luckily there's an old-time mechanic still around that I used to go to in high school, before I figured out what I was doing. It took him nearly two days to find the problem, and it was my fine engineering to blame. These engines were designed for durability, not performance, and one of the performance shortcomings comes from the #3 and #4 exhaust ports being siamesed together into a giant outlet, probably over 3x the size of the other ports. Aftermarket port dividers that separate these and knock the ports back down to size used to be popular back in the day, and I've run a few in various other cars with the same engine. The mfgrs recommend that they either be high-temp epoxied or tack welded in place, and I've had both fail, usually grinding a hole in your expensive headers in the process. When I built this head for the first time back in the early '90s I wanted to make certain that never happened with this one, so I drilled and tapped the head and bolted the divider in, nice sanitary countersunk install. Problem solved. The intake manifold and cylinder head is also integral on this engine, and you can just see the end of the bolts looking into the intake from the primary carb mount.
Fast forward to a couple of months ago. The high-temp threadlock on one of those bolts had failed, and vibration had loosened it until it dropped out and exited through the exhaust system. This left a neat, efficient port between the exhaust and intake manifolds, so vacuum was radically pulsing in time with the exhaust pulses, while leaving the average right where it should be (thus the constant temps).
Yeah, nice design.
At least it's back on the road. Glad I'm not working for SpaceX.