Starts easily when cold with a couple of pedal pump primes, and runs excellent.
However, after it has warmed up, if I turn it off, it is harder to start again. I find that I have to crank for longer with the gas pedal held about a quarter of the way down. Eventually it catches and, again, runs well.
Except... after it is fully warmed up, when I let off the gas (between shifts or to stop), it is often very slow to return to an idle. When it is cold or just warming up and I release the gas, the RPMs drop immediately. The hotter it gets, the more it starts to drop the RPMs very slowly when letting off the gas. Eventually it does get back to a proper idle, but can take 10-15 seconds (guessing, not actually timed). So, when I come to a light, push in the clutch, let off the gas and brake, it often races at starting around 2,000 RPM and gradually drops to normal.
Oddly, sometimes it just drops to an idle like it should (most of the time not).
Any ideas? Perhaps a bad choke? It is a Holley 4bbl that is only about 18 months old.
Given the symptoms it sounds like it might be your distributor. Put a timing light on it and see if the timing is jumping around at or around idle.
The hard starting is a common issue for most people. Not always the same issue for everyone but usually a heat induced issue. Some of it has to do with new formulated gas that boils at a much lower temp then it use to. Thus boiling gas in your carb. If its fully warm and you try and start by putting the pedal halfway down and leave it there while cranking it should start in few seconds.
If this is the issue. blocking off the heat crossover in the heads, phenolic spacer above the carb, and insulating the fuel line can help a lot.
If the issue is just the starter, fatter battery cables, insulated battery cable/starter or mini-starter can be a quick and easy fix.
Could be your Idle mixture, sometimes that will cause a slow return to idle. But if it sometimes drops right to idle and sometimes slow to return it may be the throttle arm/shaft sticking or the throttle cable sticking. Give them both a squirt of lube and try it out. Do you have two throttle return springs? Good Idea to put one pulling fore and one aft to help reduce wearing the throttle shaft bore.
Thanks! I sort of assumed that the two things were related, which us why I mentioned them both here. But, after reading your replies, perhaps it is two different items. I'll try both for sure.
rohrt - it definitely sounds like the formulated gas problem you mentioned in regards to the hot starting as it cranks just fine, but requires the gas pedal down as you described (but only when fully warmed up).
Bluebird - I do have two return springs, so I'll try lubricating everything first.
Just a thought, peddle down to help start is what helps when a an engine is flooded. Fuel pressure can remain after shut off and sometimes leak past an old or improperly adjusted needle and seat. Might be something to check. Again, just a thought.
Kind of sounds like two different problems to me. For the hot hard start. Rohrt is right. The gas today can have a very low boiling temperature. I have a phenolic spacer, carb heat shield and lowered float on mine. If the float is set just a little high a hot engine can expand the gas enough to push it out of the boosters creating a flooded condition. To start, the throttle must be opened to allow more air into the engine. Lower the float level to a little below the sight glass or sight hole and see what happens. Don't lower too much, will mess with the transition. Holley carbs don't like a lot of inlet fuel pressure. Double check fuel pump pressure. The idle return problem sounds like a sticky linkage. Perhaps a misbehaving choke or choke linkage? Can you replicate the problem with the car sitting in the driveway? Years ago my dad bought a new '69 Belair. The drivers side motor mount separated and would hang the throttle under odd conditions - like my mom driving a bunch of us kids around. Would never hang with dad driving. Finally hung for the service writer after mom bitched at him. He nearly drove dad's new car through the dealership wall. Us kids had a ball that day.
I would agree with bigchief and also add that increasing the size of the pos and neg cables will also help as most the cable were barely sufficient back in the days to handle what they were for and will help with starting!