Car runs really rough when cold
 

I just rebuilt a 351 and put it in my 76 Ghia. It has a comp cam extream energy cam, 4bbl manifold and a new edlbrk performer 750 carb/w electric choke. The car runs really rough when you start it, it sounds like its missing, I drive it for half a mile, still rough and gutless, turn it off wait 5 seconds start it up and it runs like a top and has alot of power. I thought maybe the carb, so I switched it with my 750 performer that was on my camaro, same problem. and the carb from the mustang worked great on my camaro, so its not the carb. The only vacuum lines are the vac advance, pcv valve , brakes, and a vacuum line going to the trans. Any Ideas on why its doing this ?

I have somewhat the same problem on my '66 coupe. The electric choke setting and the idle mixture on my 306 /Performer/Road Demon has to be just right for the engine to transition from cold to operating temp (180). even at optimum settings, I still have to baby it along until it's warmed up. Then, watch out.

Rev

Same problem here. On the other hand the torino now thats an engine practicly any carb I put on it just set the mixture and it will idle way down to 400 rpms with no problem. Always starts up on the first click cold or warm and with no choke. As for the exhaust rust has started to build on the inside of it because it burns so clean. Just baby it or go with a smaller carb.

Hmmmmmmmmmmmm, you got me on this one.

The funny thing is is that it will run rough until you shut it off and smooths out when you restart it. It runs so rough it wont idle at all, the whole car shakes. But when it runs good it idles at 1000 rpm and no vibrations. So I dont know, I dont think it is the choke, cause I tryed my edlbrock performer 750 with the manual choke and same prob.

Oui! That's a tricky one!

My advise to you is to take it to a shop with one of those engine analyzers. They'll be able to tell you if it's an ignition or a fuel problem. That'll cut your troubleshooting in 1/2!

I can't really think of any good ideas, so I'll just throw some at u.


Maybe your distrubitor cap is getting moisture in it at night and when u first start it up it has too much water in it.

Bad plug wires.

Bad coil, might not put out a full spark till it warmed up.

Sticking valves or lifters.

Bad head gasket-does your car put out any steam at any time, I mean a lot of it. Water could be getting into the cylinders.

Oil leaking into the cylinders.

Lifters pumpimg up too much or bleeding down too much after u shut it off and leave for the night.

Look for an air leak, could be leaking till it gets warmed up, making it run too lean for idle-after u shut it off and restart it, it might be resealing it's self.

Thats all I can think of for now.

Heres something eles I just discovered, when the car is running rough I can let it warm up, then put it in 1st gear and drive at 3000 rpm for about 15 seconds, it still is running rough the whole car shakes then all of a sudden it smoothes right out instantly and throws you back in the seat. I have checked the fuel pressure when it runs rough and then smooth and it stays at a constant 7 psi. As for the engine I just rebuilt it. It has 302 heads, comp cam XE 258 cam, 4bbl cast intake, performer 750 carb and the rest is all stock. Im going to block off all my vacumm lines and see if that makes a difference. As for some of your suggestions I just had the head off and there were no signs of antifreez or oil burning. I dont think it is the valves or lifters because of the way it smoothes out all of a sudden. Maybe Ill try a different cap and wires.

i took off all my vacuum, lines exept the vacuum advance and the only difference is that it ideled lower, at around 500 rpm, the car still shook and ran rough.

Hey Digger,
I'll throw in two things when you start the car cold have the air filter off and see where the choke is, which should be almost closed, and see where it is when warmed up.
Another thing is you might have a vacuum leak around the base of the carb or intake until the metal warms up and seals itself. Squirt some starting fluid around the intake manifold and see if the idle speeds up when its cold, this will find if there is a leak or not.

Sounds alot like an EGR valve problem to me, but since you don't seem to have one, we could probably rule that out. When you start the engine cold, do you by chance leave the ignition on for a second before starting, say to prime an electric pump? The reason I ask is because that will cause an electric choke to open too soon. Then, when you switch it off, and restart, the choke automatically closes . Just a thought. In any case, you need a helper to watch the choke while the problem is occurring, and see what's going on.

Take care,
-Chris

he said that he changed carbs. and it still happend. So I don't think it's the carb. It might be the spacer seal around it. To me, it sounds like it's missing real bad. I'd cheack all of my spark plug wires. I think It's a wiring prob.

I checked my choke and it is working properly. The electrical system is where I think the problem is, but dont know for sure. I have changes The electronic ignition boxes, pickup coil, coil, regulator and plugs. The cap and wires appear to be new but Im goint to change them anyway just to be sure.

Kaspar-
I know what you're saying, which is why I suggested the method with which the choke was being operated (signalled) as the cause. If, for example, he has an electric fuel pump, and turns on the key to build up pressure for a few seconds before starting, then the choke will start to open too soon. AND, since the cause would be the driver, the problem would remain with different carbs. Diagnostics are a tricky thing, especially online!

Digger-
You said the choke worked fine. Did the problem occur while you were verifying the choke? The reason I ask is because the only two items I'm aware of that effect idle qualityi n a non computer controlled engine, that change in 15 seconds, are the EGR and the choke. You don't have an EGR valve (correct?), which leaves the choke. If you wittnessed the choke operating properly while the problem was occuring, then we need to start from scratch.

Take care,
-Chris

The fuel pump is mechanical so the choke is not opening too soon, and it does the same thing even with my manual choke 750 performer. The choke is operating properly, you can see it slowly open. The car will run rough all the time until you shut it off and resart it, or leave it in first and drive it at 3000 rpm for 15-20 seconds, even this does not work sometimes. The car will run rough even if you let it get up to operating temp. It does not gradully get better, it runs rough or it runs perfect, more often rough.If any other info is required just ask.

Digger

I believe you, but that doesn't make any sense to me. What do you mean?

I still believe you.;)

If you can get your hands on a hand vacuum pump, remove the distributor cap and unplug the vac advance hose. Attach the pump hose to the vac advance port on the distributor and slowly apply vacuum. Does the assembly rotate smoothly? Release the vacuum. Does it return smoothly?

Are you sure your vacuum lines are correct (ported vs. mani)? Have you checked them with a gauge to verify the correct vacuum?

It's reaching a little, but you could have an obstruction in a vacuum line or port.

This won't be that tough. How do the plugs look? Does the rough running resemble a rich condition, a lean condition, or a timing issue?

It sounds to me like you've found a way to foul and then clear one of the spark plugs. What kind of ignition are you running?

Its alot more than a miss, its a total lack of power, almost like its running on a few cylinders. Oh ya its the stock ignition too

I think pntrbi is right. Mine does this to some degree and I think I'm over choked and plug fouled at first and then the plugs are cleared giving a correct air/fuel which delivers full power.

Rev

I printed it out to ask around, and also to ponder on it better.