1978 L48
Recently I installed a new MSD cap & rotor, EGR block off plate and aftermarket air cleaner (May not be related!). On cold starts the car bogs down with medium throttle applied. Issue persists if the car is in park and I blip the throttle. Runs fine at WOT but normal cruising stutters but seems to get better after I drive for a while.. which can take 10+ minutes of normal driving.
I've checked for vacuum leaks and I don't have any that I can find. Anyone have any ideas?

Thanks in advance,
Chris

 

Choke coming off too soon and/or engine not warming up enough with too cold a tstat.
Pretty normal in very cold weather.

 

The egr helps with cold driving conditions. Warms up the intake

 

The heat riser passage (which also supplies EGR only once you are at operating temp, cruise) warms the intake as Hugie said. Blocking it off completely wii require longer / richer choke operation, especially if you are still running the cast iron intake.

Many years ago I read an article that GM's testing determined that a 1/2" hole in the blockoff will provide enough heat to vapourize the fuel w/o hurting peak performance, so that is what I did w my aluminum intake. Works well.
Beware: if the block-off is a stainless steel shim, use the correct punch to make the hole (You probably don't have one. It uses a male punch and female die). Drilling SS is really tough and likely will pooch the shim, not cut a clean hole.

 

Cold air contains more oxygen than warm air and the carb can not temp compensate like digital fuel injection. So what's done instead is to keep the air at a constant (warmer) temp) and richen the mixture through the choke until it does.

The exhaust crossover and heatriser are one system that does that. The other is the preheat system in the air cleaner. Air arriving through the hot air stove on the exhaust is blended with outside air, thermostatically controlled, to provide air at a constant temp. If you have removed that it will contribute to your issue as well.

Steve g

 

Thanks guys, sounds like it's just somewhat normal then. I'll switch to an aluminum intake and see how/if it helps. Thanks! You guys are great!

 

I may be off base here but there is something that need to be said.
For those of you that have only driven injected cars in the past you need to understand that a carburated eng. needs a short amount of time to warm up. You should not just turn the key and go as you can with modern systems. Driving prior to warm up may cause the car to stumble and shifting into gear while on high idle may do harm to driveline parts.
I personally wait till my car reaches operating temp of 180 degrees before driving. But then I am retired and have all the time in the world and don't really concern myself with the expense of the fuel needed to warm my car up.
JMO!

 

If you can't get in and drive your car away as soon as the oil pressure shows up, your choke is not set properly. I worked in the field in the 70's and 80's in the cold of the Canadian prairies and I can tell you, people expected to be able to do that and adjusting chokes so that they could was a big part of our business in the winter.

With a properly set up choke you should be able to get in that car, kick the throttle to set the choke and start it. Kick it once to drop off the high step of the cam and away you go. If it stumbles and stalls quickly without warning and you find that you have to feather the pedal to keep it running your choke pull-off is set too wide. If it starts to load up,slowly losing rpm and eventually chugging until it stalls with black smoke accompanying it, it's not wide enough. This assumes that someone did not monkey with the choke rod to fast idle cam adjustments and that the hot air intake system (if equipped is functioning and the heatriser is working.

If you can start off fine but after 5 mins or so running you started to get the lean condition you need to increase choke tension.

This was my routine. Any complaint of cold driveability issues I had the car dropped off with the understanding I needed it overnight. When it came in hot, check the heat riser, check for cold (plugged) crossover, working hot air intake system and use a vacuum pump to check the vacuum pull-off for leakage. Correct whatever showed up then put it outside for the night. First thing the next morning I would go out, start it up, put it in gear and make it pull immedicately. Make any adjustment that was necessary then drive it for a few minutes to ensure no hesitation during warm up. It wasn't one of my most favourite jobs, working with bare fingers in -25 temps, but that's what it took to get them right. Many work fine set to specs, but many had to be fine tuned to achieve that.

As rudimentary as the technology was, it works. It just needs to be setup properly.

Steve g

 

I agree with Steve.

FWIW, I have full length headers, full duals, open element air cleaner and the afore-mentioned aluminum intake with the hole in the heat crossover. I set the choke duration / bimetallic so it is just slightly soft when it initially completely comes off. It runs great until then. Only reason I set the choke that way is so it is as lean as possible but still drives great cold. Being a stick, you just give it some gas and let the clutch out a bit slower than normal until it warms up a bit more (from a stop). Once rolling, she is good.
So I survive the lack of heat riser and heated intake air just fine.