Onan running on one cylinder-why

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namumac1

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My generator is only hitting on one cylinder. The compression is 100psi for both cylinders. I have replaced, spark plugs, plug wires ,and coil. Any ideas why the one cylinder isn't firing?
 

DonTom

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My generator is only hitting on one cylinder. The compression is 100psi for both cylinders. I have replaced, spark plugs, plug wires ,and coil. Any ideas why the one cylinder isn't firing?
Make and model number of the genny?

Did you check for spark?

-Don- Reno, NV
 

Henry J Fate

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Must confirm spark to bad cylinder and make sure it is in fact not firing at all. It's possible it is firing but poorly.

Compression looks good. Probably an electrical issue.

Replace both plugs with new. Replace air cleaner with new.

Swap parts around to see if they work on the other cylinder.

Don't trust new plugs, wires or electrical components out of the box. They can always be bad out of the box or damaged during installation

Good luck
 
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Utclmjmpr

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As the engine is running,,(on one cylinder),, try spraying a small amount of starter fluid into the air intake.. If this picks up the bad cylinder, then look for an air leak at the intake manifold on that side by spraying a small amount at that point while running..>>>Dan
 

namumac1

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But you don't know why, do you?

What if no gas is getting to the cylinder but you have a good spark?

Don't guess that you have no spark. Instead, try to prove it.

-Don- Auburn, CA
The plug that's not firing is all covered in soot. If the spark is weak what would cause it?
 

DonTom

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The plug that's not firing is all covered in soot. If the spark is weak what would cause it?
You didn't mention the make and model number of the genny so it is hard to guess. Is it a single ignition coil, or two?

Does it use the "waste spark" method? If so, you should be able to reverse the wires and see if the issue moves to the opposite cylinder.

-Don- Auburn, CA
 

Utclmjmpr

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Very simply,, if the plug is fouled it's not getting a flow of electricity from the coil for what ever reason.. Electrical flow and combustion is what keeps it clean..>>>Dan
 

DonTom

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The model is 4BGEFR26100J. The coil has 2 towers
Does it look like this?

That's the true transformer type and fires both plugs at the same time. Sparks twice as often as necessary on each side.

You can reverse the spark plug leads and it should make no difference. It uses a "waste" spark, which means every other spark is used. The spark that fires one side is the waste spark on the opposite side (both valves are open on the waste spark).

-Don- Reno, NV
 
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namumac1

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Does it look like this?

That's the true transformer type and fires both plugs at the same time. Sparks twice as often as necessary on each side.

You can reverse the spark plug leads and it should make no difference. It uses a "waste" spark, which means every other spark is used. The spark that fires one side is the waste spark on the opposite side (both valves are open on the waste spark).

-Don- Reno, NV
Yep that is exactly what it looks like. I have reversed the wires and it still misses.
 

Roy M

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Does it misfire on the same cylinder when you reverse the leads? If so it is not an ignition problem unless a plug has failed. A valve may be stuck.
 

Utclmjmpr

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The side that fires is on the power or compression stroke and the other side is on the exhaust stroke,, that's why only one coil is needed.. Harley Davidson has been using that method for 100 years.>>>Dan
 

DonTom

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Yep that is exactly what it looks like. I have reversed the wires and it still misses.
Misses on the same cylinder or opposite? What you want to do is see if you can move the problem to the opposite cylinder.

There should be NO difference on either spark plug wire from the transformer. They both happen at the same time and should be the same voltage.

Are you sure you do not have a bad wire to a spark plug? Check on an ohmmeter. I have heard of NEW wires being bad, two in the same new box of 8 for a car.

There should be NO difference on either spark plug wire from the transformer with your genny. The electronics before the coil provide both sparks the same way at the same time, just twice as many as really needed because of the waste spark.

Or a bad spark plug, especially if a Champion plug, new or not.

-Don- Ashland, OR
 

namumac1

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Sorry it took so long to get back but the trailer hasn't been here. I reversed the wires and the same cylinder had a good plug while the same one was all sooty. It seemed to make no difference.
 

Henry J Fate

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I would not qualify the test unless you put two new plugs into the cylinders.

If it made no difference (you switched the wires and the same cylinder did not fire and you installed two new plugs before the test) I would start looking at a valve problem with the bad cylinder.
 
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DonTom

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Reverse the spark plugs and see if you can move the problem.

I do realize you have already put in new spark plugs as mentioned in message number one.

Does that genny use the same carb for both cylinders or do each have their own?

Looks like everything else is ruled out.

-Don- Auburn, CA
 
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