Hey there all, got a good one for ya.  Got a freshly rebuilt 350 with a edelbrock 600cfm carb brand new and jetted for my alt. of 9000ft.  Here is the deal my truck runs great when its cold, but skips when I get about say 3500 rpm.  Thats just the beginning, as the motor warms up to operating temp. it starts to miss more ofter, and then after running for a while longer it seems like it running on 5 or 6 cyl.  What could be wrong?  I replace the carb thinking it was a flooding issue, that didn't help.  When I check the plugs they are wet and black so I thought oil fouling, but if I let the truck sit for 2 or so hours it starts back up fine and runs like the cold situation again.  My thoughts are timing or maybe distributor?  Any help would be great.  

I could use more info about your truck. What year, Does it have smog equipment?

When the truck is cold, presumeably the choke is on and it is running richer. this could cover up a small vaccum leak but not a really big one.

at idle there is not much load on the distributor shaft so bad bushings aren't always noticed. When the shaft deflects at higher rpm the distance between the reluctor and pickup coil or rubbing block and points cam begin to open up on one side, causing a bad and unpredictable miss. when this happens the spark becomes weaker and weaker. Sometimes the richer mixter will still light but after the choke releases that obviously changes.

By all means check this out. I have seen distributor shaft bushings go bad in a short time but I don't always find the cause. A new HEI distributor is not that expensive from PAW or summitracing or JEGS. Maybe 200 bucks.

When I check the plugs they are wet and black so I thought oil fouling, but if I let the truck sit for 2 or so hours it starts back up fine and runs like the cold situation again.

Smell your plugs, if you smell gas, then set your jetting back to the way the carb came out of the box, usually the carbs are shipped on the rich side and should work out of the box. If they smell like oil, then the rings have not broke in yet. Put a load on the motor to break in the rings.

Well here is the problem, the motor has about 4 thousand miles on it, when you say put a load on the motor, what do you mean.  Could the rings not being broke in foul the plugs after the motor runs for a while.  I would think it would be oppsite, ie the piston is cold at start and therefore small then as it warms it expands?  I could be totally wrong, I will try a new dist. and see if that works.  

when you say put a load on the motor, what do you mean. Could the rings not being broke in foul the plugs after the motor runs for a while.

Rings work from pressure from the combustion, you want to find a hill or a little hard acceleration to get the rings to go against the cylinder walls to start to seat and break in. If the rings do not break in they will glaze from the excess oil and never break in and always use oil.

Here are the instruction from GM:

I had a 82 malibu with a tired 229 chevy v6.I did cam ,lifters chain, rngs and bearings.I used perfect circle rings.The ring instructions said just what u posted.I could not believe this because it countradicted what new car owners manuals said.Well I did the break in in the P.C suggested and  after that I ran a compression check and had 165 psi on the first rotation.The plugs were always white and it ran beatifull untill I traded it for another vehicle with a bum motor and started over.  

9000 feet is a lot of altitude, this requires everything be pretty much spot on.

If the engine runs good when cold but not when warm it would make one think that the warm mixture was too lean and the mixture was closer to correct when the choke was on. However, the fouled sparkplugs lead off in a couple different directions from that assumption.

One could indicate that the choke is not coming fully open. This would result in an overly rich mixture when the engine warms up, which would certainly lead to fuel fouled plugs. A 600 cfm Edlebrock could be a Q-jet or a Carter, which is it? This could also be indicative of an excessivly rich idle or main metering combined with insufficient choke. Overly rich main metering could be an issue with jet and/or metering rod sizes, or with the return spring under the vacuum piston(s).

The other situation could be ignition, with the fairly low absolute cylinder pressures at your altitude, you need a high energy spark to insure consistent firing. This really requires an HEI or other high energy electronic ignition to generate sufficent voltage. Then the next problem is getting that voltage to the sparkplugs. A high quality rotor and cap that's clean and corrosion free. A cap with additional ribbing on the inside is better than a smooth surface in that it requires any loose voltage to find a longer path to any adjacent terminal or ground thus making it more diffuciult to loose voltage to places other that where it's intended. The secondary wires and boots (to the spark plugs) need to be of high quality and condition to contain the voltage. As atmospheric pressure is reduced, it becomes eaisier for voltage to leak through the insulation or take advantage of any defect. You may have to play with combinations of resistor wire and non resistor plugs, or resistor plugs and non resistor wires, or non resistor wire and plugs, or resistor plugs and wire in order to hit a unique combination that works better at your altitude than what the OEM recommends. A higher voltage coil will increase the problem of keeping high voltage inside the system till it gets to the plug. For sparkplugs, you may need to experiment a little. Possibly a heat range hotter than what the factory recommends will help keep the tip clean if the engine is simplying responding to low charge density and thus low cylinder heating. A little tighter spark gap may also help if getting voltage on the plug is a problem.

Spark advance is another issue with a relationship to altitute. Higher elevations like 9000 feet can require more or at least sufficient spark advance again to fire the thinner mixture densities which take more time to burn. Again the vacuum (if present) and the centrifugal advance systems need to be functional and have the proper curves. If you don't have a vacuum advance, I'd put one on as this really helps mid RPM/power settings to run smoother,  produce more power and get better fuel economy.

A way to check for ignition voltage leaks is of course to listen for the popping sound of arcs and of course running it in the dark and looking for "lightning" from the ignition wiring.

I'd start simple and take the aircleaner off and fire it up; observing the choke to see that it's closed when cold and opens as the engine warms. Then do the "lightning" check of the ignition as neither of these tests cost anything but 15-20 minutes of your time and a little gasoline.

Of course all this assumes that the cam is timed properly and the fuel pump is delivering proper pressure, little things like that.

Bogie

Thanks for all your help, come to find out that it was Ignition, bought a MSD Hei distributor and fixed all my problems with one fell swoop.  Just a couple more questions, Timing right now I am at about 12deg inital, when I go more I don't get any pinning or knocking just the truck runs different, not to say any better or worse, is there any number I should be looking for, and how come I can't get the motor to ping?  Not that I want that but just wondering why.  

Thanks again for fixing a problem that has been troubling me for months.  

Thanks for posting the solution, now we don't have wonder what ever happen.

Your altitude, as bogie said, reduces charge density and flame propagation within the cylinder. You might say gas being equal, altitude increases the octane of a fuel. This is why here in Southern California there are three choices in octane 91, 87, and 89 ( which is a blend of the two ) where as in Denver or Vail they sell 85 and 87. By reducing the octane of gas sold at higher elevations the factory tune at sea level will at least run up there.

In the old days cars sold at higher elevations were calibrated differently from the factory. This is made less necessary by smarter cars with smarter computers but on carberated cars it's still worth calibrating to your local area's altitude. Leaner jets and a shade more ignition timimg are possible where the air is thinner. In San Bernadino it would ping like hell.

As for total advance you could probably get away with 38 or forty degrees, Initial plus mechanical ( assuming WOT and no vaccum adv. ).During cruise when vaccum advance kicks in that number could be greater. Let PING be your guide but I wouldn't try more than 14-16 degrees intial advance, it's usually not necessary with a good working distributor.

Good luck

ACAJOE