Runs rough when hot

[Enrico8n47]

Hi, everyone, I've had a Ford 8n, has run well for years, I've accumulated about 150 hours on the present coil, points, plugs, condenser. I cleaned the points, set the ignition timing and tested, it was running great. After an hour of mowing, it lost power and was backfiring through the exhaust system. After it cooled down, I tested each cylinder, it was getting a strong spark, and it fired right up and ran without problem. The coil was bench tested and is within spec. I'm thinking either 1) the timing somehow changed, or 2) the coil may be breaking down from the heat of operation. I want to get some other opinions. Please feel free to opine or suggest additional tests. Thanks!

 

[5030]

Check the compression?

 

[EdGooding]

Are you still 6-volt?

 

[John0829]

Typical failing coil symptoms. While it is acting up, DO NOT wait for it to cool down, immediately pull a plug wire and check the spark I am sure you will see it is yellow and not blue like it should be.

 

[Enrico8n47]

Typical failing coil symptoms. While it is acting up, DO NOT wait for it to cool down, immediately pull a plug wire and check the spark I am sure you will see it is yellow and not blue like it should be.

John, thanks for your input, I figured that was the logical symptom.

 

[wolc123]

It might be a compression issue and you may need a valve job.

 

[Thomas]

Just shot...radiator fins clean?

 

[ultrarunner]

It sounds like a common old Ford problem with a bad condenser the cause.

Especially the backfire and lack of power.

If it was just lack of power or stopping it could very well be fuel related...

I have an old 1930's coil tester and so far only one bad coil have I found.

There were a number of imported condensers that failed miserably... especially after 45-60 minutes...

One guy ended up changing out several new ones.

I had a brand-new Ford Scrip condenser and said try this one... that was over 20 years ago and still running on the one I gave him.

 

[Enrico8n47]

UPDATE: The coil was replaced, all 4 plugs were checked for color and gap, all normal and not worn out. Still harder to start, backfires and runs rough when warm. I just pulled the distributor to check timing and dwell, they are as close as I can get to spec using Tim Ponn's Grand Valley Tractor 8n front distributor adjusting tool (great product!). Next I'm going to try new points & condenser, since I heard that condensers have been problematic in recent years. Points and condenser are relatively inexpensive and simple to change. I prefer to try less expensive remedies first, and really don't want to do a valve job on it. Air flow and fuel flow are good, so it should be a spark issue.

 

[DarkBlack]

Have you checked your valve clearance?

 

[Enrico8n47]

Have you checked your valve clearance?

No, I don't know how to do that. I'm checking Youtube for a video, it appears a special tool is needed to lift the spring, only then insert can you insert a feeler gauge. Have you ever done this job? Any tips?

 

[MTGreen]

Look through this thread -explains valve adjustment: John Deere 855/856 hard starting

Also YouTube

 

[MTGreen]

No, I don't know how to do that. I'm checking Youtube for a video, it appears a special tool is needed to lift the spring, only then insert can you insert a feeler gauge. Have you ever done this job? Any tips?

You shouldn't be using any kind of tool to compress the valve springs unless you are taking out the valves. What you're doing is adjusting the clearance between the top of your valve and your rocker arm. You do this when the springs are fully extended/valve fully closed

 

[drizler]

Typical failing coil symptoms. While it is acting up, DO NOT wait for it to cool down, immediately pull a plug wire and check the spark I am sure you will see it is yellow and not blue like it should be.

I always checked mine by substituting with a known good one that was always around the shop. Coils can be weird in their failure modes so swapping out always worked best for me.

 

[Enrico8n47]

You shouldn't be using any kind of tool to compress the valve springs unless you are taking out the valves. What you're doing is adjusting the clearance between the top of your valve and your rocker arm. You do this when the springs are fully extended/valve fully closed

The 8n has a flat head, no rocker arms. I'm guessing the clearance is between the lifter and the valve stem.

 

[Enrico8n47]

I always checked mine by substituting with a known good one that was always around the shop. Coils can be weird in their failure modes so swapping out always worked best for me.

I did that.

 

[Enrico8n47]

I'm also going to try some Marvel Mystery Oil to see if it can help free any stickiness in the valvetrain.

 

[Enrico8n47]

Have you checked your valve clearance?

Not yet, I've just seen a youtube on how to do that and will check it as well.

 

[DarkBlack]

Not yet, I've just seen a youtube on how to do that and will check it as well.

If checking your valve clearance, is more involved than you want to deal with, you could do, as an alternative troubleshooting check, a compression test, while it’s hot. You’d have to pull the plugs out quickly after stopping it, as to get your readings while it’s still hot. A cylinder with inadequate clearance will read low compression

 

[MTGreen]

If checking your valve clearance, is more involved than you want to deal with, you could do, as an alternative troubleshooting check, a compression test, while it’s hot. You’d have to pull the plugs out quickly after stopping it, as to get your readings while it’s still hot. A cylinder with inadequate clearance will read low compression

True only if no hardened valve seats & clearance gets tighter. With hardened valve seats clearance opens up and compression is not affected