The big Holley is probably at least 10 years old and has been sitting for most of that time so it's probably due for a rebuild. Even after spending the time and money to rebuild it I'm not confident it would ever perform well since it's so big. So I bit the bullet and bought a new carb. I wanted to stick with Holley, specifically a smaller double-pumper, because I like the looks and I could keep my fancy braided dual-feed fuel lines. I also wanted to keep my manual choke setup. The smallest 4150 double-pumper that Holley makes is a 600 cfm. I ordered 0-4776S which has a manual choke and silver finish.
The shiney new carb bolted right in since it's externally identical to the older carb. I need to replace that glass fuel filter soon--too much of a fire hazard. The engine fired right up since I had primed the primary bowl. I was able to set the idle down to 800 rpms whereas before with the older larger carb I couldn't get the idle below 1100. The engine sounded great and revved nicely but it would put out a bit of black smoke when I gave it a lot of gas so I think it's running rich. I took the car off the jack stands and backed it out of the garage for the first time in 6 weeks. Right off I noticed the clutch shudder was still there. As a drove around the neighborhood I also noticed the throttle response was actually too much. It was hard to accelerate smoothly. It reminded me of when I installed a twist throttle on my Honda ATV. I mistakenly hooked the cable to the two-stroke position instead of the four-stroke. This caused the throttle to open way too quickly. Now my Mustang felt the same way. Time to look at the linkage.
I had connected the throttle rod and return springs the same way they were on the old carb. The rod is attached to the carb so close to the pivot that there is only about 1.5" of travel with poor leverage near full throttle. So I swapped the attachment points for the rod with the return springs.
Now I'm getting about 2.5" to 3" of travel giving a much slower throttle opening with easier modulation. I still need to find a bushing for the end of the rod. A lot of other setups have the return springs attached to the bottom of the throttle linkage, pulling from the right. I might look into switching my setup that way since it looks cleaner. Although, right off it seems that having the rod and springs both pulling from the same side puts added tension on the throttle shaft causing addition wear.
