How a carbureted engine should start and run cold

[SirRobyn0]

I keep seeing some folks make comments on cold runablity that are incorrect. Comments to the effect that people should expect a carbureted vehicle to run poorly when cold, that a warm up period is required or that having to keep your foot on the gas to keep it from stalling is to be expected ect. I think most of us know that is not normal, even with todays gas. At the shop we consider 1 - 3 pumps of gas to be normal, the engine should then start and run without any other help or drama.

Part of the reason I'm posting this is so I (and anyone else that wants to) can reference this video for others later. It's been a rare stretch where I have not driven the truck for 5 days, so in the first video I'm going to fire it off and back it out of the parking spot (note 1 pump of gas only, no need to kick it down either). In the second video I pull out on to the farm road headed for the house, I try to keep the door open or window down so you can hear how the engine is running and watch the tach at the same time. It will cold start just this easy well below freezing.

Crank, start, and backing out:

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Driving on to the farm road:

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Having something to add to this or a question please comment.

 

[Matt69olds]

I have said this for years. When these cars and trucks were new, they were expected to start immediately in any weather, any environment, without fuss. Properly maintained, there is ZERO reason that can’t be mostly true today.

I say mostly because while it’s possible to properly adjust and set up the carb, todays fuel is far different than what was available 30 years ago.

 

[shortarms]

This is a stupid question, but how far are you pressing down on the throttle? Even in full screen, it's kind of hard to tell. Looks like maybe 1/4 or 1/3 throttle?

 

[Turbo4whl]

This is a stupid question, but how far are you pressing down on the throttle? Even in full screen, it's kind of hard to tell. Looks like maybe 1/4 or 1/3 throttle?

The throttle should be pressed all he way to the floor one time. This does two things, sets the choke closed and gives a full shot of fuel from the accelerator pump into the intake manifold.

As @Matt69olds stated, if every part of the fuel system is working correctly, the engine will start and be ready to drive.

Also, it should not matter what carb you are running, Quad, Eddy, Holley or whatever. Each needs to match the engine installed on and be set up and adjusted properly.

 

[shortarms]

The throttle should be pressed all he way to the floor one time. This does two things, sets the choke closed and gives a full shot of fuel from the accelerator pump into the intake manifold.

As @Matt69olds stated, if every part of the fuel system is working correctly, the engine will start and be ready to drive.

Also, it should not matter what carb you are running, Quad, Eddy, Holley or whatever. Each needs to match the engine installed on and be set up and adjusted properly.

That's why I was asking. My routine is to the floor. The last carbed truck I had was more than 10 years ago and I got to second guessing myself.

 

[SirRobyn0]

This is a stupid question, but how far are you pressing down on the throttle? Even in full screen, it's kind of hard to tell. Looks like maybe 1/4 or 1/3 throttle?

It's one full press to the floor. If it doesn't look like a full press in the video I'm not sure why it's a camera phone and maybe the lighting? But yes it's one full press to the floor and then crank with foot off the gas.

@Turbo4whl - Wayne, gives and a great explanation of why. And the results should be the same with any carb provided it is setup properly.

 

[shortarms]

If it doesn't look like a full press in the video I'm not sure why it's a camera phone and maybe the lighting?

yes, 100%.

 

[Juggernaut]

Wow, that is a well tuned truck. Wish I could get mine to start and idle like that. I like building engines, I'm just not great at tuning them

 

[SirRobyn0]

Wow, that is a well tuned truck. Wish I could get mine to start and idle like that. I like building engines, I'm just not great at tuning them

Thank you.

Patients and practice will go along way, what I mean is that being patient making small adjustments and retesting after sitting overnight, and knowing what to do to make those small corrections comes with experience more than anything.

What I do on a fresh carburetor is to set the choke blade first. On an electric choke, ideally it's about 50 - 60F outside and you'll want the choke to just close all the way. fire it up warm it up and set the mixture screws and warm idle. Most fast idle speeds are given to be set when warm ie. 1,200RPM so I'd set it to that.

Then let it cool overnight and fire it off and observe. If I feel the choke comes off to fast or slow rotate the choke coil a notch which ever way I think it needs to go. If the idle is to fast or slow turn the screw 1/8 of a turn in or out. It might take me a few mornings to get it dialed in how I want it, but I feel it is worth while for years of trouble free starting.

Hopefully that info will be useful for you. Feel free to ask questions if you want.

 

[Juggernaut]

My problems have been vast, and my engine tuning knowledge is low. This is the highest performance (mild) engine I have ever built. Trible knowledge for a 4WD truck is low RPM and higher torque, but I wanted something that would perform in the higher RPM range. My experience has been, with a 4.10 gear, 4 speed Muncie, and transfer case in low, I spend very little time below 3000 RPM. I found this with the mild 454 I used to have, it was kind of a dog in a mud hole, but now it is long gone. I now built a 350 with a solid bottom end, steel crank, Corvette rods, forged flat top pistons, all studded with King bearings. I installed Summit aluminum heads with 200cc runners, 2.02, 1.60 valves and 64cc chambers. Cam is a Howard's roller cam, 231 duration at 0.50, .508 intake .533 exhaust and Comp roller rockers. Edelbrock Performer RPM air gap manifold, 750 carb with electric choke, and MSD 6A ignition. First problem was timing the engine. I have 2 timing lights, with both the timing mark was jumping all over the place. Also it was in the WAY advanced area. Couldn't get it down below 8 degrees advance and the engine would stall. Maybe a tooth off? I ended up timing it with my vacuum gage, advance until vacuum fell off, then retarded till vacuum fell off, split the difference and set the distributor there. The distributor is pretty well centered, so I don't think it's a tooth off, and it runs GREAT. Getting about 13 inches of vacuum. When I rev the engine to 4000-5000 and let off, I get a backfire out the exhaust. Lower RPM is fine. To get it to start, it needs first pump, crank the engine, and second sometimes third pump while cranking. Takes a few minutes to idle on its own. High idle screw is all the way in, and the electric choke is set way rich. Idle screw is set for 800 RPM when it's warm. Best I can tell, it's set about 12 degrees advanced at idle, and about 38 degrees total advance. Any suggestions? Why is my timing mark jumping around, usually 4-6 degrees?

 

[Turbo4whl]

@Juggernaut Your air gap intake is no help with cold starts. The hard start, long crank, could be the fuel pump, is it new? If the check valve in the pump is weak, then the fuel will drain out of the line back towards the tank. The carb can also drain some.

GM added a decel valve to combat the off throttle backfire.

Trapped vacuum spark helps with cold engine start-up/ stay running.

The heat stove air cleaner is very helpful for cold weather starts, after all, you are in the snow belt!

 

[Juggernaut]

Everything is new except the MSD ignition and carb. Both came off this engine before the rebuild. It smells like it's running rich (gasoline odor), but the spark plugs look good, no sign it's burning rich. I've only been running it long enough to get to operating temp, so maybe the plugs aren't showing yet? Also, it's pretty fresh, only run it 4 times in the last 2 weeks for break in and tuning. This truck will be my hunting, fishing, trail riding toy, and will probably sit during the winter months.

 

[SirRobyn0]

@Juggernaut Generally speaking a timing mark that jumps around indicates a loose timing chain. I would be checking that first if there is a timing chain problem it'll never run right.

 

[Trucksareforwork]

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.

 

[potent rodent]

u say its running rich what carb ?