390 missing

[390owner]

I have a 390 in my 79 bronco. It has a miss when the engine is cold in a hard pull with low rpms. Once the engine is warm it wont miss at all. When cold and missing it sounds like it is hitting on about 5 or 6 cylinders. I have replaced everything I know electrical including coil, pick up coil, plugs, plug wires, ignition switch, wiring harness from dist, to control modular, and the control modular, I have messed with the carb( edlebrock 600), I checked the compression, it was from 135-150. I also added new ground wires from battery to frame and block to frame and a few more. The coil is getting 11.5 volts on the positive side and 6.5 on the negitive while idling. The thing I cant get is it only does it while the engine is cold once it is warm the only thing it does is I can fill a little cutting out on a long hill pulling hard. The other thing is when it is missing when cold and I shift to 3rd and step on the gas it runs just like it should it only happens in a hard pull almost lugging. The engine has about 100000 miles on a professional rebuild. It is a great engine. Also the vacuum is 16 while idling. Here are what I know about the engine, it is bored 60, has an edlebrock 600, headman headers, stock ignition and a rv cam, not sure on the lift of the cam. Any ideas?

[machoneman]

The lifters may/are bleeding off when it sits. Pull the covers cold, rotate each cylinder with the crank pulley bolt to the cam's heel and push down on the p-rod side by hand to see if the lifters have lots of play. Also, check that the lifters/cam aren't worn. Best done hot with a dial indicator or steel rule with each at max lift to measure off the retainers.  JMO. 

[ScotiaFE]

The engine has about 100000 miles on a professional rebuild. It is a great engine. Also the vacuum is 16 while idling. Here are what I know about the engine, it is bored 60, has an edlebrock 600, headman headers, stock ignition and a rv cam, not sure on the lift of the cam. Any ideas?

Well it could be an intake leak. Maybe snug up the intake bolts.
What do the plugs look like?
It might be getting close to needing a refresh.
I would go back to the guy who built it 100000 miles ago and ask him to build another one. Great Job!

[jayb]

It could easily just be a carburetor problem.  Too much choke when cold, making the engine very rich, for example, or maybe it is just really rich all the time, and once it warms up the heat helps vaporize the liquid fuel in the intake tract.  If you happen to have another carb, or can borrow one, swap it on to see if the problem disappears.  Or maybe just rebuild the one you have.

[fekbmax]

I had a similar problem with my bus. Had a 600 edlelbrock. It missed terribly when cold and ran better after warmed Up but still would miss under a heavy load. I adjusted the carb, overhauled it , then after no luck took it offand tossed it on the bench. Stuck a cheap re-man 600 vac Holley on and it ran perfect. . Could have been a combination of the carb/choke and maybe a vacuum problem with it to idk but once I changed the carb my troubles were over.

[390owner]

Plugs look great after 10000 miles I did try to tighten the intake, The electric choke it tied open it has been for years. This problem started in the last year or so. I did think about changing the carb I have another 390 in my ton truck with a 600 also. I will also try the lifters hadent thought of that

[390owner]

Ok I changed the carb with no luck. On the way to work this morning it did the same thing it always does when cold. The thing about it is I can make it do this missing or if I drive it real easy it wont do it only when it is pulling hard with low rpms.. Also I did a compression check on the engine a while back after it had sat all night and the compression was fine. If it were a lifter problem shouldent the compression check have showed something?

[shady]

sure sounds like ign. maybe go through that again. plug wires ain't worth a shit these days, that's the first thing I would try.

[jayb]

I think a lifter problem would give you a lower valve lift, but the valves would still close fully.  Since a compression test is done at cranking speeds, I doubt that you would see a lifter issue on a compression check, even if the intake valve was just barely opening. 

Since its not the carb, I agree with Shady that you should be looking at ignition next...

[cjshaker]

So nothing has changed even after all the replacement parts you put on it? It still acts exactly the same? If so, that pretty much rules out the externals, although I might try a set of Autolite 45's and use the wires from your 1 ton just to rule them out. Also, have you confirmed your advance is working properly? Little issues are  magnified when an engine is cold.

If that doesn't make any progress, I'd start to lean towards a vacuum leak. Things can seal up when they heat up. Might try looking at the carb base gaskets, spacer gaskets if used, rubber vacuum lines etc.

But if you say you can also feel it cutting out on long hard pulls even when the engine is warm, it may be a valve issue. Again, problems are magnified when it's cold. When you do the cold compression test, look for very even pulses. A hit and miss pulse can be a sign of a sticking valve, and that won't show up in the numbers OR a leakdown test.

[Joe-JDC]

Check your vacuum advance diaphram is working properly.  Pull the hose at idle and see if the speed changes.  Increase the rpms and see if the stumble is there, put the hose back on at higher rpm and see if it increases.  If the gasoline mileage is terrible, that is my suspicion.  Joe-JDC

[390owner]

Ok I just changed the plugs and wires like 100 miles ago. I have never seen a plug wire cause one to miss when cold and then not hot. The vacuum advance is also new less then 100 miles ago. It does increase rpms when I plug it in to a full vacuum port, I just replaced the carb gaskets and sprayed starter fluid around the base of the carb with not rpm changes. I have replaced all the vacuum lines. So if the valve is not opening all the way when cold could cause this problem? And like I said I hauled a load on my 16 foot trailer today and on a couple of long hills I could feel it cutting out very slightly nothing like when it was cold this morning. It seems like it is running on 5 or 6 cylinders then all of a sudden runs fine I can NOT make it miss like this when warm. I have nearly killed the engine lugging in 4th down the highway.

[Joe-JDC]

Have you checked your fuel tank for water/condensation collecting in the tank or fuel filters?  Have you run a can of fuel injector cleaner or something similar to remove any water in the fuel?  I dumped two bottles of the injector cleaner in my vehicle when it was skipping like that, and added only a few gallons of fuel, and the skip went away immediately.

If you had a bad valve, the skip would not go away at any rpm.  Joe-JDC

[cjshaker]

I wasn't saying a bad valve, but rather one that may be sticking slightly. A hit and miss deal. I've seen it before. All the valve has to do is not quite SEAT all the way and it will produce a light miss. This can also be seen if you hook up a vacuum gauge and watch it while the miss is happening. A valve that is hanging slightly will affect the vacuum reading to a noticeable degree with a quick, short "jolt" to the needle. A hard miss is VERY noticeable, when 1 or 2 cylinders completely misfire. It's especially evident when you're pulling something. You can also have a light miss, where the cylinder just isn't producing full power. That's less noticeable, but you can still feel that something "isn't right".

And I know you said you changed the coil, plugs and wires, but even new parts can be faulty. Happens all the time. Ask just about anybody here if it's happened to them and the answer will be yes. That's why I suggested trying the wires, and possibly the coil, off of your 1 ton, so you didn't have to spend any money. Just because you have never seen something happen before doesn't mean that it can't happen. I'd also be curious as to what type of plugs you're using?

You also mentioned that the choke is tied open. It could also be a lean miss when the engine is cold. As the engine heats up it becomes more efficient, so the leanness may be going away or the engine can just "deal with it" better.

There are lots of things that can cause the issue you describe, you just have to be methodical and check them all. Sometimes double check them with a known good part.

[390owner]

Ok as far as replacing wires and plugs and the coil I have done that twice. This is my second set of wires and plugs and coil. As far as the carb being lean I richen it up until it burnt your eyes at the tailpipe. I have also replaced the pickup coil  three times. The only thing I have not changed  is the dist, cap. I also just put the valve cover back on and all the rockers are tight. I turned the engine and checked then turned the engine and checked the others all tight. I am wondering about the valve you were talking about. I am using autolite plugs and accel wires. Thanks for all the ideas I am at a loss on what to do