Why? Cold runs rough
 

I know we all have experienced it, I know most all cars are this way. What I don't know for sure is why does a car run rough until it is warmed up?

It's possible there is more than one reason, but because of a choke being the answer in most cases I would assume it has to do with air/fuel mixture. But I'm trying to get to the root cause and effect.

Can anyone help explain this?

Thank you
Joe.

No different from a smokey wood stove in the morning. The carb isn't fully atomizing the fuel, the pressure drop in the venturies causes cooling of the mixture (refrigeration effect), the whole engine is cold and suffers from poor / incomplete ignition/ combustion

cold run
 

X2

Joe -

What 440 said, but also the metal parts haven't "resized" itself to to operating temperature and the oil is still "thick". We've all been some what spoiled by "modern cars" that pretty much allow you to turn the key and, except for the most extreme conditions, start driving.

With the Charger, I don't start moving it until the mechanical temp gauge starts moving.

Archer

Your car will start when it's cold?!?

That is one thing I hate about the Doba is that it absolutely hates cold starts especially when it is cold outside.

440roadrunner and archer got it dead on!

Thanks guys.

Here is the 2nd part of the question.

Giving that heat is good to help atomize the fuel, at what point is it better to have a cool intake? For example blocking off the exhaust passage through the intake and aluminum intakes to dissipate heat.

Thank you
Joe.

Joe -

Like everything else, fuel temp optimization/atomization works on a bell shaped curve. I'm going to guess that optimal s somewhere in the 160 - 180 range, but I have nothing but a little experience to base that on. Too low and you get a "wetter" fuel charge and too hot, it will vaporize prematurely.

The standard way of "fixing" that isn't with fuel temp with with the AF mix, commonly called the choke. Since your running a double pumper, the odds are good that with a couple of full pumps, you'll have enough fuel to get the engine going -without using the choke (I removed mine), then I think it's a matter of time until the engine reaches (expands to) operating temp.

If you have a manual electric fan or a thermostatically regulated one, it should allow the engine to heat up quickly enough.

Archer

the heat riser may not be opening fast enough !

Thank you guys.

I'm really not having trouble getting it started. I turn her over with the starter until she starts to build oil pressure then I pump it a couple of times and then feather it until she stays running. It has a manual choke but no cable to it.

More over I'm trying to understand the relation ship between heat - fuel and air. Picturing a bell curve in my head helps to understand it.

The engine is running better now than it ever has, much better throttle response, Much better. Cold or just getting warm she stumbles with the slightest fast acceleration. When completely warmed up, very little stumble at all. But still completely dies when going from idle to WOT.

You know what it acts like, it acts like when it hits zero vacuum the gas is shut completely off, like it runs out of gas. Now I know it's not the accelerator pumps, that thing shoots a ton of gas down the throat when you pump it, it almost seems like when it hits zero vacuum there is nothing to carry the gas down through the intake.

She idles at about 700 rpm with 18" of vacuum. Drive your foot to the floor and she drops to 0" and you could not shut it off faster if you flipped a switch.

I'm loosing my mind over this thing.

Joe -

Rickyr could address this better, but you still running the Holley DP? If so what size Power Valve are you using?

(Still amazed you're getting 18 mm of vac at idle on a "worked" engine.)

Archer

Even more amazing it doesn't burn a drop of oil!
That's what keeps me going with it, it really seems to be a good engine.

Yes the Holley 650 DP is still on there, although I have been toying with the idea of putting the 600 Carter AFB back on. The 600 AFB is what Edelbrock recommends for the Performer Intake.

The power valve according to Ricky is sized real close as it has about 13" vacuum idling in gear... 1/2 of 13 being 6.5
Carb comes stock with a 6.5 power valve.

I think I will right a book about this when I finally get it figured out :)

It's all fun.... to a point, I really am looking forward to the fix.

Joe, here's my .02 worth, put that AFB back on and see if it's any better. I think with that DP holley it's just too much carb, 0 vacuum at WOT is too low.Did you try adjusting the timing to a little more advanced?

john

barfly
 

Does the cam card tell the max RPM it is good for?
Cubic inch X max RPM divided by 3456 = CFM of carb you need
If the Holley DP is the right size, it might be a bit rough when cold
but way better performance when up to temp
any time with any carb when you go to wide open throttle the vacuum should go to 0 at first. and then pick up some as you hold it there
but if it stays at 0 then that would be a problem.
I hope this helps

that's what happens at WOT. you lose all vacuum. have you ever been in a car with the heater on and stomped it to the floor and the heater quits? blowing hot. that's what it does. a diesel has no vacuum. why cause the plenum is wide open all the time. same if you open the butterflies there should be no vacuum at WOT>

In a stock engine at WOT, vacuum will drop to "near 0" around .03-.04, which is enough air flow through the carb to start fuel flow from the main nozzles. With a larger carb with bigger venturies,even with accelerator pump action the vacuum will not be sufficient to start the main circuit causing it to stumble or stall. Sorrry,this is what happens when you take physical science in school,I tend to generalize things without going into details.

let the flaming begin,oops! sorry,I forgot this isn't moparts!

I went out to change the carbs out this evening after work but alas, it's still winter and I have no garage. :rolleyes:

It does drop to zero, not near zero, but zero. And it does make sense to me that if nothing is pulling the gas down it more than likely just squirts into the manifold and lays there. I half a$$ed thought that the duel plane intake would suck it down better and I think it does, just not that much better.

If changing the carbs doesn't fix it this time, I'm just going to put a board under the gas peddle so I can MASH it down as hard as I want and not open her up all the way :D

Joe.

Well this thread started out with Why does cold run rough and ended up with my same old trouble of the engine dying when I go from idle to WOT.

Guess what fellers!

NO MORE HESITATION!
NO MORE STALLING OUT!
NO BACKFIRE!
AFTER A FULL YEAR OF MESSING WITH THIS, IT"S FINALLY OVER

(covering my butt with both hands) ;)

I put the Carter 600cfm AFB back on yesterday evening, warmed her up and took her for a spin. From a dead stop, no brakes, just a big tromp to the floor and off and rolling I went. Now granted it was no neck snapper or anything like that, but no hesitation and no stall!!

The only thing I can say is it boiled down to that the Edelbrock Torker 340 single plane intake was the trouble from the git go.

My first big fix was to try a different carb, a Holley 650 cfm DP. In order to get that to run at all I had to pile on (2) 1" carb spacers. Ran "okay" but still the same old trouble.

My next big fix was the Edelbrock Performer Dual Plane intake. Ran a lot better even after removing the carb spacers, but still the same ol' same ol' stall and hesitation.

And as I said above the next big thing was to remove the Holley and replace it with the original Carter.

I keep say next 'BIG' thing because I tried new jets in both carbs, new springs in the Carter, several different carb spacers, timing, new dizzy and on and on.... those all go into the next "small" thing column.

I want to thank everyone who chimed in to help me with this in several threads over the last year. I can' begin to tell you how good it feels to have found the answer.

Saturday my son gets his first lesson on changing out rotors and brake pads on the front of the truck. And we will be reinstalling the hood scoop he has refinished. Next we will be refinishing the bumpers and then there's that neutral safety switch and shifter adjustment and then......... :)

Thanks again everyone!
Joe.

congrats man!

Joe,I bet you are so happy!!

john

That 340 torque r is a good intake , maybe a little more high RPM HP than the dual plane. I would bet the carb is not set up properly. I remember having an 850 DB pumper on a 383 worked fine.. How big is the cam? If you have the idle screw turned in too far it could be past the transistion slot, what ever it is called in the carb That could cause your problem too..

I would not say the 340 Torker is a bad intake, only that it did not run well on my motor. As a matter of fact it's for sale if anyone wants it.
$50 you pay shipping from zip code 49646 :)

Joe.

Way to go Joe!

I came upon this thread a bit late it seems but i wanted to share a bit of info anyway in regards to warming up.

The optimal operating temperature is between 180 and 210 degrees this allows for best fuel economy, power, and oil lubrication, i think where people get the lower numbers is the opening of thermostats but many of those open at lower temperatures because if you throw cool coolant into a hot engine block you can damage it, so it cant warm up all the way with thermostat closed. (info comes from my college textbook, working on a technician degree)

my vehicle is notorious for the cold starts i go through, my step dad refers to my '70 fury as "the cold blooded bitch" and as much of a bitch as it can sometimes be to warm her up, after a year of tweaking mine its running smooth now as well. Iv narrowed my problems down to my edel brock 4bbl carb+matching manifold just being too much air to too little fuel(318 engine).

does you car have headers? if so, you eliminated the heat crossover in the manifold. that makes the car take longer to heat up