How a carbureted engine should start and run cold

[SirRobyn0]

That fast idle looks slow to me. My '85 305 emissions decal (UHJ) says 1,800 RPM fast idle in P or N. It starts really well but roars a bit more than yours. Mine kicks down pretty quick, but definitely starts w/ higher RPMs.

Yours looked like it stayed below 1K RPM from cold start. Is that on purpose? Factory says higher I'm betting.

Right. But check that 1,800 RPM spec. That is to be set with the engine warmed up. So you warm the engine up set the fast idle cam on the highest step and set the speed to 1,800 RPM. When it's cold and you fire it off it's going to be lower than that. Like I was saying earlier I set it to spec and then tweak it to where it seems good. It fires up and cold idles at about 1,000RPM camera angle and the poor lighting may make it look lower but it's right about at 1K. At that speed I can toss it into gear right away and drive off. Over course it comes down a step when I move the truck but it's warmed up enough that it's fine. Also on cold frozen morning I can fire the truck up and let it idle to dethaw the windshield a little bit without concern of coming back to and the engine over speeding.

 

[Trucksareforwork]

Thanks SirRobyn, I learned something there. Explains my truck's "roar"

 

[fast 99]

This is a good subject.

Many carbed vehicles today are too rich. It could be motors will run "decent" rich; lean causes all sorts of drivability issues. Really don't think the vast majority understand what adjustment does what and given OEM carbs are all at least 40 years old many are far from stock. There are infinite adjustments, fast idle speed, choke pull-off. It takes the correct combination to have a good running engine.

 

[SirRobyn0]

This is a good subject.

Many carbed vehicles today are too rich. It could be motors will run "decent" rich; lean causes all sorts of drivability issues. Really don't think the vast majority understand what adjustment does what and given OEM carbs are all at least 40 years old many are far from stock. There are infinite adjustments, fast idle speed, choke pull-off. It takes the correct combination to have a good running engine.

I completely agree with your statement. I just had a 67 Satelite in the shop that another shop setup the aftermarket Holley Carburetor. Customers complaint was the exhaust smell. It was burn your eyes bad. So I went though and rejetted it, adjusted and set it up all properly. Another thing is I think guys still try to read plugs to know where they are at. As you know back in the day we looked for light tan to medium tan and being a good burn, but gas has changed and a normal running carb will result in almost a white plug with maybe a few spots of very light tan. Regardless gasoline has changed and and reading plugs just isn't as reliable as it use to be.

That and there are a LOT fewer of us that even know how to properly work on and tune a carburetor.

 

[wanderinthru]

This is interesting. Have no problem getting all of mine to start cold. I don't run a choke, it gets cold here and like the engine to spin a bit before it fires, so in the winter let it spin 3 to 5 seconds (depends on truck and starter and battery) let it get the oil moving then hit the throttle and they are running. Stay running even tho a rough slow idle but get smoothed out pretty quick. Warm weather just hit the starter and tap the gas and varoom. My problem is starting them hot, esp after being shut off for 10 minutes to a half hour or so. Have to treat all 3 of them like they are flooded, crank and hold the gas wide open? Any ideas on that @SirRobyn0
All 3 have Elderbrocks, 2 on alum intakes, other is on the cast GM and the hardest to start hot.

 

[SirRobyn0]

@wanderinthru Heat soak and fuel perculation are likely your hot start issues. Edelbrocks are terrible about that. A phenolic spacer, or even just the regular thick heat insulating gasket Edelbrock sells will help. That's what I did I put the regular heat insulating gasket available from Edelbrock on my truck and it solved 90% of my hot start problems. You live in a hotter climate so you might be better served with the actual phenolic spacer. The other thing is that there use to be additives blended into the gas to raise the boiling point which reduced vapor lock and perculation which were removed from most fuel blends sometime back. IIRC mid - 2000's. Because most squares have a fuel return line we generally do not have issues with vapor lock, but the edelbrock carb seems to have issues with perculation for at least some of us. I'm guessing the heat retention of the cast iron manifold is the reason that one is the hardest to start.

 

[Bextreme04]

This is interesting. Have no problem getting all of mine to start cold. I don't run a choke, it gets cold here and like the engine to spin a bit before it fires, so in the winter let it spin 3 to 5 seconds (depends on truck and starter and battery) let it get the oil moving then hit the throttle and they are running. Stay running even tho a rough slow idle but get smoothed out pretty quick. Warm weather just hit the starter and tap the gas and varoom. My problem is starting them hot, esp after being shut off for 10 minutes to a half hour or so. Have to treat all 3 of them like they are flooded, crank and hold the gas wide open? Any ideas on that @SirRobyn0
All 3 have Elderbrocks, 2 on alum intakes, other is on the cast GM and the hardest to start hot.

You are probably running rich enough all the time that it will start fine cold without the choke, but it causes hard starts when hot. You want to set everything up to run right at HOT and then properly set the choke up when COLD. Then it will start correctly and run right in all situations. I only need one pump 90% of the time. When below freezing i need a second pump for it to fire right up. High idle starts at around 900rpm when cold and rises up to about 1200-1300 once warmed up.

The biggest issue most people have is not cycling through the proper setup steps. If you don't set the idle mixture right or do this out of order, you will end up with issues. You need to set timing FIRST. That requires starting and fully warming the engine. Then go through and set base idle(also requires fully warmed engine). Then set idle air mixture screws for max idle(while adjusting base idle screw to maintain the proper idle RPM) and then lean the mix(by screwing in the mixture screw) until you take 50-100rpm off the max idle. Then you can set the throttle up onto the high idle step and rough set your high idle RPM(I prefer 1200-1400 RPM when HOT).

Then shut it down and leave it until it has fully cooled down for several hours. Best to wait and do the final tuning early the next morning when it is fully cold soaked. Go out and push the throttle fully for one press. Crank until it starts. If it starts, go out and adjust the high idle screw to the lower high idle value that you want(I prefer 900-1000rpm COLD). Let it warm on the choke until the RPM maxes out. Now readjust the high idle screw if necessary. You should only need to do fine adjustments now if the high idle is too high when HOT and nothing has been touched since startup. Once you have that set right, tap the accelerator once and make sure the choke pulls off and idle returns to the normal set base idle.

 

[SirRobyn0]

You are probably running rich enough all the time that it will start fine cold without the choke, but it causes hard starts when hot. You want to set everything up to run right at HOT and then properly set the choke up when COLD. Then it will start correctly and run right in all situations. I only need one pump 90% of the time. When below freezing i need a second pump for it to fire right up. High idle starts at around 900rpm when cold and rises up to about 1200-1300 once warmed up.

The biggest issue most people have is not cycling through the proper setup steps. If you don't set the idle mixture right or do this out of order, you will end up with issues. You need to set timing FIRST. That requires starting and fully warming the engine. Then go through and set base idle(also requires fully warmed engine). Then set idle air mixture screws for max idle(while adjusting base idle screw to maintain the proper idle RPM) and then lean the mix(by screwing in the mixture screw) until you take 50-100rpm off the max idle. Then you can set the throttle up onto the high idle step and rough set your high idle RPM(I prefer 1200-1400 RPM when HOT).

Then shut it down and leave it until it has fully cooled down for several hours. Best to wait and do the final tuning early the next morning when it is fully cold soaked. Go out and push the throttle fully for one press. Crank until it starts. If it starts, go out and adjust the high idle screw to the lower high idle value that you want(I prefer 900-1000rpm COLD). Let it warm on the choke until the RPM maxes out. Now readjust the high idle screw if necessary. You should only need to do fine adjustments now if the high idle is too high when HOT and nothing has been touched since startup. Once you have that set right, tap the accelerator once and make sure the choke pulls off and idle returns to the normal set base idle.

Thanks Eric I didn't even think to talk about the cold start or how that could affect hot start. Edelbrocks seem to be a little more prone to heat soak issues so running rich and the heat soak maybe compounding each other. as well.

Just like you and as you saw in my video 900 - 1000 cold is all that is needed and one can if wanted put it in gear right away and drive off just as smooth as can be.

 

[wanderinthru]

@wanderinthru Heat soak and fuel perculation are likely your hot start issues. Edelbrocks are terrible about that. A phenolic spacer, or even just the regular thick heat insulating gasket Edelbrock sells will help. That's what I did I put the regular heat insulating gasket available from Edelbrock on my truck and it solved 90% of my hot start problems. You live in a hotter climate so you might be better served with the actual phenolic spacer. The other thing is that there use to be additives blended into the gas to raise the boiling point which reduced vapor lock and perculation which were removed from most fuel blends sometime back. IIRC mid - 2000's. Because most squares have a fuel return line we generally do not have issues with vapor lock, but the edelbrock carb seems to have issues with perculation for at least some of us. I'm guessing the heat retention of the cast iron manifold is the reason that one is the hardest to start.

Yes Sir, that's what I was figuring. I don't have any Q jets that in't just plum wore out and past my patientce to rebuild. When i was really set on fixing this problem, found and joined this forum, I guess all the phenolic spacers were stuck in the Suez or waiting off Calif? Haven't looked of late but were unavaliable then.
If I'm understanding what I'm learning, the heat from the engine is a higher temp than the boiling point of todays e gas. It basically boils off/ evaporates out of the carb, hence the hard hot start. Correct? If that is correct then how come I can turn my hot truck off at the house one evening, next morning it fires as soon as it turns over? Shouldn't the fuel pump have to refill the carb?
Enjoying the conversation by the way.

 

[SirRobyn0]

Yes Sir, that's what I was figuring. I don't have any Q jets that in't just plum wore out and past my patientce to rebuild. When i was really set on fixing this problem, found and joined this forum, I guess all the phenolic spacers were stuck in the Suez or waiting off Calif? Haven't looked of late but were unavaliable then.
If I'm understanding what I'm learning, the heat from the engine is a higher temp than the boiling point of todays e gas. It basically boils off/ evaporates out of the carb, hence the hard hot start. Correct? If that is correct then how come I can turn my hot truck off at the house one evening, next morning it fires as soon as it turns over? Shouldn't the fuel pump have to refill the carb?
Enjoying the conversation by the way.

Also that boiling effect can push it into the intake in liquid or vapor form, which would explain it acting like it's flooded. But also what Eric said has merit to. If it's rich enough to start cold without any choke and run decent, it maybe rich enough to attribute to the hot start issue. I mean that would be easier enough to check, just lean the mixture screws and see if it starts better hot. I mean that's a real possibility but I'm betting heat soak is playing the biggest roll.

Also blocking the exhaust cross over in the manifold will make a huge difference in heat reaching the carb, but will most definitely make cold runability sucky.

I to am enjoying the conversation.

 

[wanderinthru]

You are probably running rich enough all the time that it will start fine cold without the choke, but it causes hard starts when hot. You want to set everything up to run right at HOT and then properly set the choke up when COLD. Then it will start correctly and run right in all situations. I only need one pump 90% of the time. When below freezing i need a second pump for it to fire right up. High idle starts at around 900rpm when cold and rises up to about 1200-1300 once warmed up.

The biggest issue most people have is not cycling through the proper setup steps. If you don't set the idle mixture right or do this out of order, you will end up with issues. You need to set timing FIRST. That requires starting and fully warming the engine. Then go through and set base idle(also requires fully warmed engine). Then set idle air mixture screws for max idle(while adjusting base idle screw to maintain the proper idle RPM) and then lean the mix(by screwing in the mixture screw) until you take 50-100rpm off the max idle. Then you can set the throttle up onto the high idle step and rough set your high idle RPM(I prefer 1200-1400 RPM when HOT).

Then shut it down and leave it until it has fully cooled down for several hours. Best to wait and do the final tuning early the next morning when it is fully cold soaked. Go out and push the throttle fully for one press. Crank until it starts. If it starts, go out and adjust the high idle screw to the lower high idle value that you want(I prefer 900-1000rpm COLD). Let it warm on the choke until the RPM maxes out. Now readjust the high idle screw if necessary. You should only need to do fine adjustments now if the high idle is too high when HOT and nothing has been touched since startup. Once you have that set right, tap the accelerator once and make sure the choke pulls off and idle returns to the normal set base idle.

Not to argue but, the 84 has a pretty big cam in it (Don't know bought it that way) and does idle a little rich, and is the easiest to start hot. It runs and pulls like a bat out of hell, plugs, smell or exhaust pipes don't show a rich condition in any of the 3. Sitting here thinking about it, the leanest running of the 3 is the hardest to start hot, the 75 with the cast iron intake. I'm asking questions here, giving the info I have the best I see it, wanting to learn and solve problems. Far as a choke goes, have thought of putting a manual on them, but frankly don't see where I need one as stated when it's cold I want it to crank a spell before it fires. And elec chokes have wires, I do not like wire unless 100 percent nesasary.
Appreciate it.

 

[Bextreme04]

Not to argue but, the 84 has a pretty big cam in it (Don't know bought it that way) and does idle a little rich, and is the easiest to start hot. It runs and pulls like a bat out of hell, plugs, smell or exhaust pipes don't show a rich condition in any of the 3. Sitting here thinking about it, the leanest running of the 3 is the hardest to start hot, the 75 with the cast iron intake. I'm asking questions here, giving the info I have the best I see it, wanting to learn and solve problems. Far as a choke goes, have thought of putting a manual on them, but frankly don't see where I need one as stated when it's cold I want it to crank a spell before it fires. And elec chokes have wires, I do not like wire unless 100 percent nesasary.
Appreciate it.

Hard to start hot is almost always from a too rich condition. Whether it is from fuel evaporation, jetted too rich, idle mixture too rich, leaking injectors/carb, etc... it almost always boils down to too much fuel or not enough air. That's why the slight crack of the throttle or pedal to the floor until it starts usually works for a no hot start condition, because you are allowing the engine to pump out the excess rich mixture. I bet if you had an AFR meter on all of those cars you would find that when they don't want to start hot they are super rich.

 

[SquareRoot]

Carb. lol

 

[wanderinthru]

Hard to start hot is almost always from a too rich condition. Whether it is from fuel evaporation, jetted too rich, idle mixture too rich, leaking injectors/carb, etc... it almost always boils down to too much fuel or not enough air. That's why the slight crack of the throttle or pedal to the floor until it starts usually works for a no hot start condition, because you are allowing the engine to pump out the excess rich mixture. I bet if you had an AFR meter on all of those cars you would find that when they don't want to start hot they are super rich.

Ok. Have thought about setting up one of those gauges just to see.

 

[SirRobyn0]

@wanderinthru @Bextreme04 An AFR meter takes all the guess work out of carburetor tuning or trying to figure out when or where aa engine is too rich or lean. You can definitely get by without it but there is a fair bit more of trial and error.