8n will start and idle fine but missfires at half throttle to full! PLEASE HELP

[The Fred]

You MUST first figure out if you have an ignition or a fueling problem.

Simplest way to check spark: Take a 12 volt test lamp and connect it to the condenser side of the coil and ground the other side. Start the engine and let it idle, watch the light and see how smooth the "pulsing" of the light is. As you load or rev the engine the light should still be smooth yet faster pulsing. If the ignition is your problem you will see the test lamp misfiring just like the engine. One can do the same with an inductive timing light to test the secondary (spark plug) side of the ignition.

Fred

 

[Utopia Texas]

Sounds to me like the centrifugal timing advance is not working properly. Also make sure the coil is wired properly. It will work wired either way but the spark will be much improved if the polarity is correct.

 

[rsallen]

It 'sounds' like ignition issues (yes, check the advance, missing springs or frozen weights). Have you tried the machine under a load?
What happens?
The only weird thing I've run into with carbs is that sometimes you'll run into one that'll will dribble off idle because fuel pressure is to high or because the float valve seat vibrates enough to allow too high of a float level (some are famous for it). The other thing I run into is the top and bottom halves of the carb don't really seal, ie. they are not really flat to each other allowing either internal vacuum leaks or an outside source of fuel other then the main jet.

 

[rsallen]

I just thought of some other off the wall things I've run into over the years.
First of all though I want to say double and triple check the carb and ignition systems.

Whoa, I just realized I don't even how the head or valve train is set up on an 8n. Or even if it's a 2 or 4 stroke engine, a flat head, F head valve set, overhead valves, push rods or overhead cam. lol
Let me know someone.

 

[8N Ren]

Check your 2 screws on the carb.
Check your sediment bowl screen for debris.
Turn your fuel on and take the drain out of your carb bowl to see if you have fuel flow for a min or two , not a dribble but a nice stream.

 

[8N Ren]

Is fuel flowing ok? Crank it open and remove the brass plug from the bottom of the carb and let it flow for a minute or so. You should have a decent amount with no dribbles (use a can or something to catch the gas).
2 carb screws need adjustment?
Float stuck? Give the carb bowl a wack with your hammer handle, NOT TOO HARD THO! you don't want to crack it.

 

[the old grind]

If the tractor has been sitting long enough the advance mechanism may be stuck (rusted) in one position. A light will then show timing at a prescribed rpm but perhaps not correct at others. If all is well, timing marks, under t-light, will shift as rpm changes.

A miss never seems to point specifically to fuel vs ign. Manifold/gasket leaks can cause lean & weak running that no adjustment can overcome. Whether new & computerized or 'old school', plug wires seem to be the last thing suspected/replaced. Water in fuel can give the symptoms OP has, but bowl drains let it out first, so doing that once is usually enough. Is fuel pump pressure ok?

 

[rsallen]

Okay,
Though not real research, I looked up 8n.
I assume this is an overhead valve engine.
Weird stuff........Do the basics first! ie., fuel & ignition homework!
Over the years I have run into one slightly bent push rod and a couple of weak or broken valve springs (all on different engines at different times of course (can it happen any other way?!)) but in each case the result was the same; compression good--wonderful idle---garbage from mid-range & up performance (missing).
What happens is at lower rpms the valve seats okay, at higher rpms the valve floats.

PS.
And if you DO get into the valve train, you may as well do a rough measurement on the rise of each cam lobe.

 

[mbryant51375]

Points @ 15 and plugs 25.

 

[Utopia Texas]

I have always set the point gap on my 8N's at 20-22 thousands.