One big issue I still have is drivability at low rpms. If I open the throttle too quickly at say 1500-1800 rpms the engine will buck and backfire. The timing is fairly close to where it should be, and for now I have to assume the camshaft timing is correct, so Google says that my carb mixture is too lean. Now, I was actually thinking that I had the wrong power valve for my application which was making the mixture too rich when I cracked open the throttle. My Holley 4150 came stock with a 6.5 power valve which requires 13" of vacuum at idle to operate correctly. Since I'm only getting 8" I need a 3.5 or 4.5 power valve. But Google keeps telling me a backfire means lean not rich. So I pulled a spark plug to take a reading.
This plug looks very clean (except for the oil on the threads WTH). Too clean actually. It should be a light tan color and not this ghostly white. So she's definitely running lean. We had just driven the 'Stang back from a cruise-in so the engine had been running steadily at 2500 rpms for the last 30 minutes. So this plug is pointing to the main jets being too small. My Holley is stamped 4776-5 which means it came with 66 primary and 73 secondary jets. My old carb is stamped 4778-8 which should have 69 primary and 78 secondary jets as long as they weren't changed in the past. I think I'll remove the jets from the old carb and see what size they are.
The primary and secondary jets are located inside the front and rear fuel bowls, respectively. I removed the 4 retaining screws of the primary bowl on the old carb and then removed the jets. Instead of finding the stock 69 jets they were stamped 72! One of the side-effects of using a large carb on a small motor is that airflow is much slower than optimum, especially at idle and part throttle. The larger throttle plates don't have to open as much to supply sufficient air to the engine. This means that air moves slower through a large carb than it would through a smaller carb for any given volume, assuming that the smaller carb can handle the maximum flow required for the engine. Since carbs rely on airflow to "suck" fuel from the bowl, reduced airspeed means less fuel is being drawn in, making the engine run lean. So someone had previously increased the primary jet size to compensate for the slower airflow. Unfortunately that means I can't use these jets in my new carb. Jetting up from 66 to 72 is way too much. However, in a stroke of luck, I found a Holley jet kit on Amazon with same-day delivery! With this kit I'll have plenty of jets in all sizes to jet away to my heart's content.
While I'm waiting for the jets to arrive I reassembled the old carb and started removing the jets from the new carb. This is more difficult since the carb is still mounted on the engine but it's doable. I first siphoned as much fuel as I could from the bowl through the sight plug--the rest drains out when you remove the bottom screws.
I was able to remove the primary bowl with the fuel line still connected since it has a braided hose. Then I could just barely move the bowl out of the way and reach the jets with a screwdriver.
While I was removing the jets the metering block came loose so I just removed it from the carb.
This made it a lot easier to change the jets. I debated back and forth about how big of a jet to go to. Holley recommends going two steps at a time when changing jets, but I'm not sure 2 will be enough. So for now I swapped the 66's for 68's.
While I was draining the fuel from the carb I figured it was a good time to swap out that glass fuel filter before it breaks and starts a fire. I picked up a cheap chrome filter for now. One of the cheap hose clamps was stripped so I had to make a run for a replacement. I also wanted to replace the two short pieces of rubber fuel line but the local Autozone didn't have any on the shelf. Eventually I'd like to replace it with braided line like I have feeding the carb.
With the larger jets in place I started her up to go for a test drive. It might have been psychological but the throttle response seemed much better, allowing a smoother start from standstill. However, as soon as I hit 3rd gear, a quick stab at the gas pedal produced another backfire. So I continued around the block and parked it without letting the engine fully warm up. Since it was still backfiring I figured it was still "way" lean and not just "wee" lean. I didn't want to go all the way to 72 since that would mean I could have used the primary jets from the old carb and saved the $60 for the jet kit. So I swapped jets again, but this time I jumped up 3 sizes to a 71.
I didn't get to test drive the car for a few days until we went to a cruise-in at the local Rock 'n' Brew. She started right up and didn't backfire once on the trip there and back. The clutch still chatters a little, but not nearly as bad when I first drove the car. Right now the only drivability issue is when I try to crawl through a parking lot. While in 1st gear at basically idle speed the engine starts to buck. It's easily stopped by pushing in the clutch, increasing the revs a little, and letting the clutch back out.
I'll drive it a few more times before I do any more fine tuning. Ideally, to read the plugs you should drive at cruising speed for a while and shut down the engine and coast to a stop without letting the engine idle. Then you can pull a plug while on the side of the road. This will narrow down the reading to just the primary jets. If the primary jets seem good I'll then have to tackle the secondary jets.
What I'd really like to do (someday) is install a wideband oxygen sensor. This would require welding bungs in the header collectors for a pair of O2 sensors. Then I could install a gauge that would monitor the air/fuel ratio taking all the guesswork out of jet selection. Since I can't weld I would have to either take my car somewhere and let somebody else monkey with it or remove the headers and take them someplace. Maybe if and when I have to remove the transmission to fix the leak I'll try removing the headers since I'll have to remove the rest of the exhaust anyway.
