Distributor cap shorting out

rtstrack · #1 (**permalink**) 06-21-2008, 05:02 PM

I have a tri powered 428 with an oval air cleaner. The air cleaner is sandwiched between two metal plates. The bottom plate sits directly on top of, and touches the metal carbs. The bottom plate also extends out over the top of the distributor cap. The bottom plate touches the middle distributor boot, the one that connects the distributor to the coil. What I noticed is that when the metal plate and air cleaner are set in place, the engine won't fire up. It turns over and gets gas, but wont fire. I take OFF the plate and it fires up immediately. I started the engine without the plate on, and tried setting the plate back on top of the carbs. Two things happened, the engine shut off immediately, and I got a major shock. So apparently their is an electric current that is being conducted by the boot and the metal plate. I guess I need to first try replacing the boot, but is this normal wear for a boot? Or is there a bigger problem and even after I replace the boot I'll still have the same problem? Should I try putting some type of resistant material in between the two? What in the heck is going on?

Ryan

Ronbo · #2 (**permalink**) 06-21-2008, 05:18 PM

Check clearances in the cap, rotor.

What's happening is the air cleaner is providing a "better" path for the spark current, which means you have some wide gaps in the distributor.

Your coil wire may have an internal break as well.

Your rotor may also be out of index as well, but this would be less likely.

New cap and rotor will likely take care of this.

rtstrack · #3 (**permalink**) 06-21-2008, 05:35 PM

Ok I should have mentioned this up front. I'm a mechanical novice, so I need detailed explanations on EVERYTHING.

"Check clearances in the cap, rotor." What am I looking for? What should the clearance be?

"Your coil wire may have an internal break as well." Is this within the coil? Could I actually see it if there was a break? Should I just replace the coil to be safe?

"New cap and rotor will likely take care of this." Ok the cap is straight forward and i can replace that with ease, but what would be wrong with the cap? I think its the boot on the wire that is allowing the electricity to flow through.

How do i replace a rotor without messing things up worse?

Thanks for all your help

PANAVIA · #4 (**permalink**) 06-21-2008, 06:19 PM

ok, so the basics.

What Ronbo mentions sounds plausible.

so here is the semi detailed explanation.

Aluminum is a conductor (wire) and will transport electricity from one place to another. with that said, there are two or threee things that come to mind with your issue.

1. Mechanical (force) interference: the air cleaner oval base of aluminum is putting pressure upon the the distributor cap causing some sort of misalignment or aggravating some sort of damaged part inside. This could be the distributor cap, its rotor inside that directs the spark to the various spark plug wires, or the base distributor base itself.

With the air cleaner installed, there should be clearance between the top of the distributor wires , and the bottom of the air cleaner base.

2. Electrical issue: the Ignition coil (small redbull can attached by 1 large wire to the center of the 8+1 wire distributor cap.) If this is having an issue where the rubber isolation of the coil to distributor wire is unable to keep the electrical current inside the wire and transport it to the distributor cap , it may arch (like a little lightening bolt) to the base of your air cleaner. (look for little black marks on the underside of the air cleaner base.

What I would do.

1. Check and see of the distributor cap and wires show signs of damage, age or wear. If so --replace them.

1B. place a piece of cardboard 5"x5" on top of the distributor cap and install the air cleaner base, middle and lid. if the cardboard compresses or dents at all, STOP. you have a interference issue and need *not* to have the air cleaner base PRESSING DOWN on the distributor cap. if is does not press down on the cardboard , you can try to fire up the engine and see if the it will fire up,* if it will * then you have an electrical (spark plug wires and coil wire issue) problem.

2. figure out how to get some space between the base of the air cleaner, and the top of the distributor. this can be accomplished by a spacer between the the carburetor top and air cleaner base (say 1/4") and double check hood clearance so that the hood closes and does not contact the TOP of the air cleaner oval. another option would be to add/insert a second carburetor gasket underneath the carb jacking it up 1/8".

2. Verify that your distributor is not damaged, or its cap cracked.

3. Figure out if the rotor (inside the distributor cap) is rotating freely and is not damaged .

Send me a PM if you get stuck.

--Steve

Ronbo · #5 (**permalink**) 06-21-2008, 06:38 PM

Your wire between the coil and distributor may be broken down internally. (common with carbon core wires)

Cap and rotor normally come as a set. I would check the center contact carbon ball in the cap to make sure it didn't fall out. (again common problem) Your rotor will have a spring loaded contact to the center cap pin, this should contact the cap pin and can be checked by placing a small piece of clay on the contact then install / remove the cap and see if the clay is completely penetrated.

You've got a gap somewhere in the path to the plugs that's large enough to make the jump to the air cleaner an easier route...

Rick Parker · #6 (**permalink**) 06-21-2008, 06:43 PM

You have a direct short from the center wire (from the coil) to ground (engine block, carb body etc. The base of the air filter should NOT sit on the distributor as you have indicated. The boot covering the wire may be worn to the point that it is unable to provide insulation to prevent the secondary spark jumping to ground. If you are able to measre the distance from the to of the installed air cleaner assembly to the closest point of contact with the hood. You may put balls of clay on the corners of the air cleaner when it is cold, then close the hood gently and then measure the compressed thickness of the clay. Then purchase a spacer or thick heat insulator type gasket that is close to the thickness of the clay and install it under the carb to effectively raise it above the distributor by the thickness of said item, also replace boot on coil terminal.

Ron61 · #7 (**permalink**) 06-22-2008, 01:32 AM

As Rick said above, the spacers on the carbs at the base of the air cleaner are the easiest cure. Mine never touched, but since I was running such a high voltage an only had a very little clearance I used the spacers and never had any problems.

Ron

Rick Parker · #8 (**permalink**) 06-22-2008, 12:02 PM

You may also be able to find some thicker gasket that will fit on the carb openings where the air filter base attaches. Anything to gain some distance between the filter base and distributor. If you are using an MSD distributor you may be able to find a cap that is shorter. I investigated this on one occasion.