Gas Engine Firing Problem

Although I'm a far cry from an expert with points and distributors, my neighbor has a problem with his 1966 Case 730 Gas Engine firing. He is getting fire, but not out the cap to the plugs. Here is what I have found so far:

New points and cap on the Prestolite Dist.
Rotor checks good.
Wire from Starter to + on coil
Wire from - on coil to distributor.
Points are gapped and firing correctly.

When the key switch is on, there is fire to the coil and back to the dist.
When the engine is turning over, there is no fire out of the cap, but with a test light on the - terminal from the coil, the test light is lighting every time it fires. This is with the test light grounded and touching the - on the coil. The test light is bright when it is firing. If you take the test light and then put in into the dist cap, there is no fire.

Any thoughts on this? Tried a second coil that I know is good, and the same thing. Thanks for any insight. Bob

As and adder to the above, maybe we need to take a temporary wire direct from the battery to the coil to make sure the trouble is not in the switch or wiring from the switch?

Also information to include:

Owner installed new switch and wired it the same as the old one
Owner installed new ceramic resistor.

The guy is super sharp with engines himself, but this one has us baffled, and we both know it is something simple, but just can't find it.

Pull the center wire off the distributor cap and hold it about a 1/4 inch from the block. Spin it over and you should get a good blue white spark that jump that gap. If you have that check for spark at each of the plug wires and it should be the same. If no spark at the plug wires but spark at the coil wire the cap is bad or the rotor is bad. Seen more then one new cap be bad and also seen rotors that where shorted to ground

(quoted from post at 14:03:36 01/12/18) As and adder to the above, maybe we need to take a temporary wire direct from the battery to the coil to make sure the trouble is not in the switch or wiring from the switch?

Also information to include:

Owner installed new switch and wired it the same as the old one
Owner installed new ceramic resistor.

The guy is super sharp with engines himself, but this one has us baffled, and we both know it is something simple, but just can't find it.

Click to expand...

Yes. Do the temporary jumper wire thing.

I heard no mention of the condensor did he replace it, is it hooked up, if it run before and he put a new one on maybe it is bad, with the china junk these days it is not uncommon for new stuff to be junk. If you have the old condensor put it back on and see what happens.

A question for me is. should the test light clamped to a ground, and touching the - on the coil show the points firing? The test light comes on and goes off each time the points open and close.

Not likely the problem but I had the pin that holds all the metal parts to the rotor come out. Without metal parts the rotor doesn't seem to feed spark to the plugs.

The fact that the test light was flashing when connected to the - coil terminal proves there is power to the coil, and the points are making and breaking the connection.

Do you know the coil is the correct resistance? Some coils need a resistor, some don't. You can look for any information printed on the can, research the part number, or check the ohms resistance across the + and - terminals.

With the coil out of circuit, a 12v no resistor needed coil will have around 3 ohms resistance.

A coil with around 1.5 ohms resistance needs a resistor.

For proper operation, the proper combination must be used. Too high resistance (a non resistor coil with a resistor) will give a weak spark. A resistor required coil used without a resistor will temporarily give a good spark, but will quickly burn the points and damage the coil.

Once that is established, check the spark quality at the coil tower. Should get about 3/4" or better spark to ground. If this spark is weak, try the old condenser, try a solid wire coil wire.

Then check for spark at the plugs Each wire should have 1/4" spark to ground.

If spark is going into the distributor cap, but not making it to the plugs, check the rotor. It could be grounded internally. With the cap removed, hold the coil wire near the rotor, arc the points. The rotor should not draw a spark. If it draws a spark, it is grounding through the distributor shaft.

Look the new cap over carefully. Be sure it is the same as the old one. Check the carbon post, be sure it is the same as the old one, check the position of the terminals that they are clocked the same as the old ones, check the height.

It's just a matter of process of elimination, you'll find it!

I just got a mans 135 running, had new condenser and all else. I went through one piece at a time and found that the condenser was the problem. I believe some of these new auto store parts just don't work out.

Did the engine run before the points & other parts were changed? Don't know about Presto Lite parts, have got 2 sets of CNH points with the rubbing block on backwards. Will keep things interesting. Compare your old points with the new set. Just a thought

Every time the points open the test light should come on and when they close it should go back out. The way a coil work is it build up a charge when the points are closed and when the point open it releases that charge so with a test light hooked up the light should go out when the points are closed because the coil is completing the circuit

On the Prestolite distributors we have had a problem with the rotors. They ground out to the shaft.

When the engine is turning over, there is no fire out of the cap, but with a test light on the - terminal from the coil, the test light is lighting every time it fires. This is with the test light grounded and touching the - on the coil. The test light is bright when it is firing. If you take the test light and then put in into the dist cap, there is no fire.

Click to expand...

Are you using the same unpowered test light to check for high-tension spark at the distributor cap and battery voltage at the low-tension coil terminals?

If so, be aware that a low voltage unpowered test light is NOT the tool for checking for spark on the high-tension side!