I’ll throw in my 2 cents. Experience with big blocks over the years, especially if bored, tend to have this hot start slow spin. Some say the rings and pistons are too hot and are ready to expand once stopped??? I have solved it several ways. 1st I try retarding the timing until the slow spin is almost gone but still have sufficient vacuum for the brakes. 2nd I have converted the battery and starter cables over to 2 gauge and put longer bolts through the starter/bellhousing and putting nuts on the back for a tight connection. 3rd a high torque starter.
As for the rpm increase could you being running hot enough that the vacuum switch in the thermostat housing is switching the vacuum to the distributor which will increase the speed.
The '69 carburetors have the same issue and the same lever. If the return spring is too weak they have inconsistent idle speed because the throttle doesn’t always shut.
I tinkered a bit with it today. I checked the float levels since it’s easy to do. The front one was what I would call right. Just a little trickle right at the bottom of the site plug hole. The back one I thought was way to high fuel pouring out. I readjusted it, but the exhaust smelled so clean I think I went a little too lean so I’ll raise that back up a bit. None of this changed anything with the idle issue or really the current driveability.
When it’s cold or warming up it idles and seems to take fuel like it should. Since we’ve gotten into warmer weather here it’s seemed to develop a hesitation/misfire at low throttle say 1000 to 2000 rpm range when starting off but then seems to clear up. I can also feel random misses at highway speed normal driving rpm just kinda seems to randomly and quickly cut out for a split second.
Again only when it’s warm. None of this is under full throttle just normal driving. I did stick my foot in it one time a while back and it did great plenty of power didn’t feel or hear anything wrong. Haven’t heard any pinging or spark knock at all.
What typically causes this kind of behavior?
Unless the rear float bowl is empty, it will not affect you idle mixture. If empty or stuck, it will. If the missing was there with the high float level in the rear, that may have been the reason. You have lots of suggestions in this thread, and it would be great if you answered each one so we can help you.
Rob
I have heard from many car friends from across the spectrum - from Early V8s to hotrods to muscle cars - any car with a carburetor - complaining about the modern gasoline/alcohol fuel that they have foist upon us. The symptoms you describe - hesitation and misfire are ubiquitous. After you’ve tried everything else try spending a couple hundred dollars on a full tank of racing fuel that doesn’t contain alcohol and see what happens.
In my experience todays gas works fine with our old engines. I have not experienced any problems with cars I work on when properly tuned. Now does ethanol gas cause problems when storing your car? Absolutely! Ethanol attracts water and causes corrosion in fuel system components. But it burns very slow and acts like old 99 octane gas with less power. Yes, our old engines would make more power on the gas blends of yesterday, but they run just fine on todays gas.
Here is a post about todays gas and the is very revealing about how a high performance engines runs on various blends of gas and how much advance the engine can manage.
https://cccforum.discoursehosting.net/t/ignition-resistor-wire/11855/17
Wish I could get you car in my shop. I have a List 3795 Holley carb off a 1968 Cougar GT-390 coming my way. I will wet test it on a known GT-390 engine that has no issues. I could do the same for your car, but again you may have other issues not related to the carb.
Rob
What type of ignition are you using? The old style points can act up. Going with an electric drop in conversion like petronix might settle it down.
If the distributor needs to be rebuilt or replaced, do it
The cleanest way to find vacuum leaks is with a propane torch unlit. Just listen for the idle to increase.
The other option that I ended up doing was switching carbs entirely.
I went with a 500 cfm Edelbrock AVS2 carb. A larger cfm but otherwise identical one can be had in 650 cfm.
Yes yes yes, I know it’s not original but it’s a good looking carb and it’s a zillion times more street friendly. The previous carb was a Holley that I ended up sending free of charge to another forum member.
These carbs run right out of the box with only minor tweaking required & my gas mileage jumped up from about 9 miles to 13 miles per gallon. My warm up times are essentially half of what I had to do with the previous one.
You might or might not have to add a fuel pressure regulator. I just have the regular old plain Jane fuel pump and no complaints.
Top end horse power is very slightly less but the low end torque is a little bit more. I also added the smaller diameter Edelbrock air filter, makes adjusting much easier.
The main adjustment you need to do is setting the accelerator pump to the longest stroke possible to minimize hesitation.
The only other little gotcha is that you have to block off the hole in the manifold where the hot air choke tube used to go to about a small exhaust leak.
Either JB weld or a tap and die set are your friends.
Noticed today a little puddle of fuel in the front most valley on the drivers side intake manifold. Looks to be leaking from the accelerator pump on the front of the carb. Not sure if it’s the diaphragm or the gasket, but the off idle hesitation it’s doing and possible lean out could be due to this right? Either way the timing is too high and I’m dialing that back to 6 and going from there. What are the best carburetor gaskets to get for one of these? It’s all factory stuff.
Is it okay to turn a carburetor upside down to fix the pump diaphragm or does it have to stay upright?
If there is fuel still in the bowls it will drain out the vent tubes. So have a large pan handy to catch the fuel, then you should be good to go.
Noticed today a little puddle of fuel in the front most valley on the drivers side intake manifold. Looks to be leaking from the accelerator pump on the front of the carb. Not sure if it’s the diaphragm or the gasket, but the off idle hesitation it’s doing and possible lean out could be due to this right? Either way the timing is too high and I’m dialing that back to 6 and going from there. What are the best carburetor gaskets to get for one of these? It’s all factory stuff.
I’ve got a Holley rebuild kit. Are the base gaskets in that sufficient or do I need something else? I think I’m going to spring for the green GFLT accelerator pump diaphragm that’s for alcohol laced fuels this go round. What are the torque specs for the carb to intake bolts?
I had the exact same problem with my 428 car it needed a rebuild and the idle mixture screws were to fat making it run on and idle high same as yours.
Car has the thick carb base gasket. Anyone know torque specs for carb to manifold nuts?
I’m super confused now. I put the carb back on. Replaced the pump diaphragm with a GLFT ethanol resistant one. Cranked. Felt like it had a dead cylinder but smoothed out. Checked timing. First time I checked it I used the mark someone had put before. Today I crawled under and marked 0,6,8, and 10. Hooked up my dial back timing light and initial timing is around 30. The vacuum advance is pointing straight forward. Something is for sure off here cause there’s no way it would run at 30. Any ideas? Turn right to retard left to advance right?
Did you disconnect & plug the vacuum advance?
Yes and it made no difference. Now it has a rough idle/shake.
I went around the distributor disconnecting each plug wire to see if it made a difference.
Wires for cylinders 4 and 7 made little if any difference. There is spark at the cap all cylinders.
Those look to be the hardest plugs to get to. I need to get to those plugs to change. What’s the best way? Don’t see much if any room to do it.
#4 is the rear one on the passenger side. Just a regular ratchet and a spark plug socket. It’s one of the easiest ones to get to.
#7 is tough, as is #8. I normally remove the master cylinder to change those. Alternative is to reach beneath the master cylinder but the older I get the less nimble I have become.
I wonder if the timing chain has slipped. Sounds like it is getting worse, not better.
I pulled the passenger valve cover. Everything looks good. Cranked it over and all rockers had the same amount of movement. My concern was a wiped cam lobe.
What all needs to come off to get to the plugs? The emissions equipment is in the way but I can’t seem to get that unbolted. Also can’t seem to get the plug wires off the plugs. I at least want to get to number 4 and check it.
If the timing chain jumped is there any way to compensate for that? Since I have the passenger valve cover off I’ll check and see if TDC is truly where it’s supposed to be.