[metzman]

Hello everyone. I have a 1968 Chrysler Newport with a stock 383 2 barrel. It has 155000 miles on it with a replaced timing chain, distributer (points, cond, cap, rotor), new plugs, plug wires, and the carter BBD has been rebuilt. I've owned the car for 25 years. For the last two years or so its been running lousy. The compression is consistant around 115 lbs. The vacuum at idle (650 rpm) is steady at 18. The car runs fine when its NOT warmed up. Then in about 5-7 minutes it starts hesitating and missing. This is most noticeable with a little throttle with a load on it. At idle (at a stop light) it runs smooth. If I wait a few hours, it still hesitates and misses. There is a dead spot in the throttle where the missing is most prominent. When you let off on the throttle to just above idle it will smooth out. If you open the throttle more after the dead spot it will pick up on power but still run rough. If I run through the throttle in park, it is hard to notice any of these symptoms. It is mostly noticeable under load. If I wait until the next day and start out stone cold it will run fine for 5-7 minutes again. (non of the described symptoms occur for the first 5-7 minutes) The choke is opening properly and the fast idle cam works fine too. I've also swapped coils and the same condition persists. I'm not sure what else to check and could use some assistance on this one.

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[chuzz]

Sounds more like a carburetor problem to me, but I haven't fooled with a carb in 30 years. I'd suggest removing it and giving it a good cleaning and possilby a rebuild kit. It may be a bad accelerator pump or float sticking just a little bit. Does the car sit for extended periods of time, or do you drive it regularly? Maybe the jets need cleaing. IDK, I'm not really a mechanic. I just work on my own cars because I can't afford, nor do I trust, other people to do it for me.

 

[Bob Lincoln]

I agree, I'd rebuild the carburetor. It's probably running a little rich, which a cold engine tolerates well.

 

[dana44]

Good starting point, but in the meantime, check the heat riser in the exhaust manifold and make sure it is opening up properly, it may be stuck shut, half shut, or completely broken. Next thing you may want to do is replace a half quart of oil with a can of Restore Engine treatment, which should help your lower compression. I have used it several times and it hasn't failed me since. Another thing, given you have replaced the timing chain itself, you may also have lobes on the camshaft which have been known to wear down and not run smoothly when warm, seen that several times on big blocks over 125,000 miles.

Let us know what you find out.

 

[metzman]

Good starting point, but in the meantime, check the heat riser in the exhaust manifold and make sure it is opening up properly, it may be stuck shut, half shut, or completely broken.

The heat riser is not working properly. It is free, but has no thermostatic spring action to it. Its been like this for a long time. I just leave it in the open position.

Also, the choke vacuum diaphragm has a slight leak, it will not hold vacuum. But it does work if constant vacuum is applied. I will replace it when I rebuild the carb.

 

[dana44]

These two things should be addressed first, as in simply wire the exhaust butterfly open and the choke vacuum, diaphram open, see what happens.

 

[68RT]

Regarding the heat riser, what is the ambient temperature you are in. You might be possibly experiencing carburator icing. I don't really think that is it but it is a [possibility.

 

[metzman]

Update: The carburetor is in the middle of a rebuild. Also the intake manifold is coming off. Mice built a nice cozy home under manifold last winter. The cavities between the heads and the intake manifold surrounding the (riser crossover tunnel) were clogged up with droppings and seeds. I don't know what impact this would have on performance or if they could have fenagled their way under the valley cover. But I am interested in finding out what damage they've done to the insulation under the manifold.

 

[David Eidell]

A thorough inspection of the spark plug electrode and insulator may give you another clue. The engine should see an oscilloscope if the carburetor proves to not be the problem. I've seen everything including distributor bushings so worn out finger pressure on the rotor could open and close the breaker points .030" Good Hunting!

 

[dana44]

No damage is done when stuff gets into those vents. Get the shop vac out, stick a wire in there to clean it out while vacuuming, it is basically a space between the heater crossover and the rest of the head to keep the two center cylinders from overheating. The good thing is, you can pull the intake and clean it and not have to worry about water leaks. The new gasket, a steel valley pan, usually has a little tube of black RTV to seal around both sides of the port portion and the rails front and back. Put a little of the RTV in the threads so oil doesn't leak out of them.

 

[KOG]

Another possibility is that it's running lean when warm due to ethanol. It that's the case it could need to go up 1 or 2 sizes on the main jets. When it's cold the choke has it enough over rich to cancel the leanness of 10% ethanol.

 

[Bob Lincoln]

The crossover passage is to bring heated exhaust from the heat rise valve to the choke assembly, so it will open faster and fully.

 

[68RT]

The crossover passage is to bring heated exhaust from the heat rise valve to the choke assembly, so it will open faster and fully.

And to stop carburator icing.

 

[dana44]

And they also get carboned up and cause problems. An aluminum intake manifold works better than the stock cast iron manifolds if you take a piece of the old valley pan and cut a couple squares of metal to cover the heat crossover holes, and if you really want the excess heat, drill a quarter inch hole in each of the pieces of tin. Aluminum heats quicker so doesn't need the extra heat, some go so far as to not have the passage at all anyway.

 

[metzman]

Update: The carb is rebuilt and the intake has been cleaned out and retorqued to specs. Its running MUCH better now for a tired old motor. I don't have the cash to go through the rest of it (valves, cam, recondition block, ect.), but as long as it gets me to cruises without sputtering the whole way, I'm happy.

For the winter, when its not driven, I'm not sure what's better, keeping the tank empty and putting fresh gas in the spring, or keeping it full with a couple bottles of Stabil. Right now I'm leaning toward just putting in a few gallons and putting an old carb on it, so I can start it every few weeks. Then taking the rebuilt carb off, filling the bowl with gumout and keeping it in the house till I put it back on the road. What would you do?

 

[dana44]

Over the years of being in the Navy and being deployed (too many times to count) from 6-17 months at a time, I would simply fill the tank and leave it. I had one time I had to have a battery jumped to get the car to run just fine, but I never ever had a problem with the gas itself, whether the car sat in western Washington state through winter or Southern California. I usually had more hatred for the salty air or the dust drifts that would plug up the body drains and cause rust from the inside out.

Fill it and forget it. If it is in the garage, start it once a month, just disconnect the battery and leave it in place, 20 minutes of running (with the exhaust pointing outside, of course), the battery will remain fine.

 

[KOG]

That used to work prior to ethanol in gas. Doesn't work now.

 

[dana44]

2007 was my last six month deployment and it worked just fine, Ethanol aids better in absorbing any moisture, which is even better than just a full tank of gas in 1984. Heck, I've had gas in a tank that was four years old that still ran an engine fine. Pour a little bottle of Stabil into the tank if you don't think your gas is as good as mine.