A puzzling problem

[PFord]

Sadly, some of the testing parts/tools I ordered were incorrect, so another order going in Monday. However I was able to test spark and it's absolutely solid across all plugs. So I think I can rule out ignition being a problem.

[Eugene]

I was able to test spark and it's absolutely solid across all plugs. So I think I can rule out ignition being a problem.

Not. Spark doesn't mean that you do not have an ignition problem. Diagnosed your occasional miss problem?

[Glen]

Hi,
To test the power of the spark the Battery Ignition unit is making, first lock the brake pedals together, then lock the brakes applied. If the brakes don't work right, be sure the Cub doesn't roll while doing the spark test.
Remove the ignition wire from the center post on the distributor cap, and hold the wire by the insulation, with the metal end about 1/4" from bare metal of the engine block.

Be sure the transmission is in neutral, turn the ignition switch to on, and run the starter.
The spark should jump the 1/4" gap, and be blue or white. A yellow spark is weak, and needs improving.
The engine may not run, or run right, with a yellow spark.
Do the test in a building, or not in bright light, so you can see the spark well.
The battery should be fully charged for the spark test. Charge the battery if necessary.
A low output charger is better, it may take several hours to charge it.

[Eugene]

You need to have the distributor correctly matched/timed to the engine. The distributor timed to the crankshaft. All distributor components in excellent condtion. Correct plug wires. Correct spark plugs. Plugs wires on correct plugs.

[PFord]

Ok, thanks everyone.

[Glen]

Hi,
Cub engines need the ignition timing set right, if it is off some, the engine may not run right, or have less power than it should.

The Cub service manual says to set the timing of the Battery Ignition unit using a timing light, like cars with a distributor in the past used.
Below is a page from the Cub service manual showing the timing marks a Cub the year of yours should have.
Mark number 1 should be aligned with the pointer when the engine is running at slow idle speed, 500 RPM.
Use only the pic and the words under it, they are talking about other things on the page.

http://farmallcub.com/rudi_cub/www.clea ... -01-43.jpg

As you slowly speed up the engine, the timing should advance until mark 2 is about in line with the pointer when the engine is at full speed, 2000 RPM for a 1975 Cub before serial number 248125.

[inairam]

These are called sintered metal filters. I have seen these in industrial filter applications. I have not seen them in small fuel filters.

Really?
https://www.finditparts.com/products/68 ... ters-sf898

I have seen them on vents for hydraulics and on regenerative blowers. i used them in a design for an air vacuum tempary sheet holder. I just have not seen them in fuel filters.

[PFord]

Hi,
Cub engines need the ignition timing set right, if it is off some, the engine may not run right, or have less power than it should.

The Cub service manual says to set the timing of the Battery Ignition unit using a timing light, like cars with a distributor in the past used.
Below is a page from the Cub service manual showing the timing marks a Cub the year of yours should have.
Mark number 1 should be aligned with the pointer when the engine is running at slow idle speed, 500 RPM.
Use only the pic and the words under it, they are talking about other things on the page.

http://farmallcub.com/rudi_cub/www.clea ... -01-43.jpg

As you slowly speed up the engine, the timing should advance until mark 2 is about in line with the pointer when the engine is at full speed, 2000 RPM for a 1975 Cub before serial number 248125.

Yes, thank you. I have a timing light, and my white marker pen just arrived from Amazon this morning so I can mark the timing marks more clearly. I found them last week. The problem is the the engine doesn't run long enough for me to look at the timing typically. It was dying in 3-4 seconds. So if I can get it to limp along a bit longer, I'll see if I can read the timing.

[Eugene]

I have a timing light. The problem is the the engine doesn't run long enough for me to look at the timing typically. It was dying in 3-4 seconds

Static time the engine, - - first.

[PFord]

I have a timing light. The problem is the the engine doesn't run long enough for me to look at the timing typically. It was dying in 3-4 seconds

Static time the engine, - - first.

Well, I know the piston is at TDC on the mark. I was able to rock the fan back and forth with the plugs out and watch the valves close as they should. I need to get the distributor cap off and see if the rotor is in the right place. Probably do that tomorrow.

[Eugene]

Well, I know the piston is at TDC on the mark. I was able to rock the fan back and forth with the plugs out and watch the valves close as they should. I need to get the distributor cap off and see if the rotor is in the right place. Probably do that tomorrow.

This site, look up how to static time an engine.

[PFord]

So it looks like it was a fuel delivery issue. I got the pony tank hooked up today. After a few cranks when the fuel delivery started, she ran just as sweet as you please.

So now, I'll do some basic maintenance, and things I had been planning to do, but I can do them more leisurely. And I'm going to pull the sediment bowl and fitting off the bottom of the tank to see where it might be gummed up. It's possible the sediment bowl screen was causing issues, and I've already changed that, but I'll just make sure to check everything while I've got the hood off and got easy access to everything.

[PFord]

Welp, spoke too soon! LOL!

I got some junk out of the way and had a good look at the carb. The top is warped near the fuel inlet. Undoubtedly letting in all kinds of unmetered air. I'll pull the carb off this week, get it in the ultrasonic, see if I can close the gap with gasket(s), and see how she runs. I may buy a new carb if I can't get this one straightened out. Even if this is not the ultimate issue, it sure isn't helping.

[Don McCombs]

I don’t think you’ll find closing the gap with extra gaskets to be a good solution. After you have cleaned it, if you want to send it to me, I’ll straighten it, no charge. Just return shipping. PM me, if interested.

[Bill Hudson]

I don’t think you’ll find closing the gap with extra gaskets to be a good solution. After you have cleaned it, if you want to send it to me, I’ll straighten it, no charge. Just return shipping. PM me, if interested.

Take Don up on his offer. It will be done right.

Bill