Classic/Antique Car Repair: bad running 413 RB engine, Engine wont idle after storage

Question
Hi Dick,

I would like to ask you a question about the engine of my 1960 Imperial, the 413RB. I put this car in storage last winter (October 2008) like I always do, but when I tried to start the engine two weeks I won't fire up. As a matter of fact, it was very very hard to crank. The starter was only turning the engine at a very low speed. First I thought the starter was at fault,but when I tried to crank the engine by hand I knew I was in trouble. I have taken the plugs out to see if it turns easier then, but no. I then shot in a few blasts of PB Blaster and some Marvel Mystery oil. Let is soak in for the night and found out the next morning that the engine is turning over much easier by hand, and turning twice as fast on the starter as before. So there was definitely rust on the cylinderwalls, don't ask me how this is possible. Never had this problem before during the 4 years that I own the car now.

After cleaning the plugs several times, I was able to start the engine for the first time this season. But it runs far from good...First it only started on 2 or 3 cylinders. After cleaning the plugs again the engine is now running on 7 or 8 cylinders, still have a miss every now and then.

I checked the valves but they all are opening and closing like they should (took the valve covers off to check that). So I am pretty sure that there aren't stuck valves.

I did a lot of work on the engine last winter, replaced the lifters, pushrods, rocker arms and shafts, installed new plugs, new sparkplug wires (Magnacore KV85) but didn't try to start the engine then, stupid me!

Although the engine has come back to life, it only runs good at half trottle or full throttle. As soon as I release the throttle the engine stops. When going to full throttle the engine is spitting petrol back through the carburetor (Edelbrock). As far as I know this is an indication of a lean mixture? Could it be that the engine is starving for fuel when I release the throttle, because the carburetor has gummed up ? The engine is normally running on LPG, but is running now on petrol because I wasn't able to start it on petrol.

I also noticed, while I was checking the ignition, that I get a yellow spark from the coil. I always thought that the spark should be blue or white?

Do you have any thoughts why the engine won't run at low speeds (anything lower then half throttle)?
Could there be any damage done to my pistons/rings due to the fact that there was obviously surface rust on the cylinder walls?

Many thanks,

Rob van der Es
1960 Imperial
The Netherlands

Answer
Hi, Rob;
I would do a compression test on the engine first - that always tells us what the condition of the cylinder walls is now, and also if any of the valves are sometimes sticking open.

Spitting back through the carburetor can be due to a very lean mixture (we'll check on that in also - stay tuned), but it can also be due to an intake valve failing to close all the way.  If a sparkplug fires in a cylinder with a leaking intake valve, there will be a backfire through the carburetor. You would not be able to see this problem visually by watching the rocker arms, because the intake valve will appear to be totally closed - it only has to be open a few thousandths of an inch to cause the backfire.   

To check on the very lean mixture question, do the following.  Have some one hold the throttle steady, (so the engine runs at a steady speed), and with this condition established, slowly slide an obstruction (in your case I would recommend a piece of sheet metal) over the top of the air intake (air cleaner removed), paying very close attention to the engine RPM.  If at ANY point as you obstruct the air inflow, the engine speeds up, you definitely have a lean mixture condition.  This could be caused by a gasket leak around the intake runners or the carburetor base, or a problem with your brake booster or other vacuum operated item in the car.

If the result of the compression check is good (all 8 cylinders high and even within 10%), and obstructing the air flow slows the engine, we'll need to investigate the carburetor.  The Edlebrock is a very reliable carburetor, but it is possible that the backfires have damaged something.  

I am guessing the carburetor is OK, and the problem is still internal to the engine or a very serious vacuum leak.

I'll wait to hear what you find out with the compression and lean mixture test - then we'll plan the next step.

Dick