How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Hank F. Your Own Question
Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 14888
Experience:  Certified on Onan and Generac generators
Type Your Small Engine Question Here...
Hank F. is online now
A new question is answered every 9 seconds

We have a Onan MCCK 6.5 kw Gasoline GenSet in our 1983 Boat.

Resolved Question:

We have a Onan MCCK 6.5 kw Gasoline GenSet in our 1983 Boat. Total hours 218.3. We put new spark plugs, points and oil change about 9 years ago. And, replaced the "Exhaust Elbow" about 6 years ago. The unit has always ran very well, this last week, we started the genset, it started right up and did not sound right. I checked the output exhaust and the water output was fine, but the water looked like it had "Air" in it, as it was almost "White". And, I could smell "Gas" when I went to the Bow of the boat. When I put the TV on, it lugged down a bit, but when I put the Stove on, it almost stopped the Genset. I checked the oil and Coolant, they both are Full; oil needs to be changed. The Fuel Filter has never been changed. Does this unit have a Oil Filter, I cannot find it? We only start and run the GenSet about every 3 to 4 months, when we anchor out. Thank you and we hope you can help us with this problem.
Submitted: 6 years ago.
Category: Small Engine
Expert:  Hank F. replied 6 years ago.

Thank you for choosing Just Answer for the solution to your problem.
My name is XXXXX XXXXX I am going to assist you with this.


Yes, this set should have an oil filter on it.

Since it has been so long since you have given it a tune up, I highly urge you to do so - you are long overdue.

But from what you are describing, it is your carburetor that needs the most attention.


As gas gets old, it turns to varnish and clogs up the passageways inside the carburetor, not allowing enough gas to get to the engine.

This condition is cumulative. Every time gas sits, the varnish builds up just a little more, like coats of paint, until eventually gas can not flow. It will not happen overnight, but the symptoms can show up over night.

The use of fuel additives, such as Sta-Bil or Sea Foam will not stop this process from happening. They will greatly slow it down, but the gas will still go bad.


When this happens, either the engine simply will not start, or it will not run without the choke on (this reduces the amount of air getting pulled into the engine, changing the fuel/air mixture), or it will run but surges.


Another issue that varnish in the carb can cause is that the varnish may not allow the float needle to seal properly against the seat, causing the flow of gas to not shut off when the bowl is full. The result will be gas overflowing the carb and running into the cylinder, and possibly out the air intake. If the gas gets into the cylinder, it will seep past the rings and down into the crankcase. This will be evidenced by your oil level being over-full and/or the oil smelling like gas.


The only 2 solutions are to either replace the carburetor or give it a good, thorough cleaning.

When removing the carb, make sure to take a good picture, or make a good drawing of where all springs and linkages are attached. This will make reassembly much easier.

Most people believe that cleaning a carb involves removing the bowl and wiping it out, then spraying some carb cleaner through it.

This is simply insufficient.

To properly clean the carb, you must remove it, disassemble it (making sure to remove all non-metal parts), and soak it in a commercial solvent for several hours. Soaking it overnight is even better.

Then clean all solvent off with a spray type carb cleaner, making sure to get lots of cleaner into every hole and passage there is. Pay special attention to the tiny holes in the bore of the carb, under the throttle plate for the carbs that have these holes. Use lots of cleaner. And make sure to wear safety goggles to avoid getting the over spray into your eyes. There will be over spray.

Dry the carb with low pressure compressed air.

When reassembling the carb, make sure to use a carb kit, when one is available for your carb.


Occasionally, even a good cleaning is not going to be sufficient, and you may end up having to replace the carb anyhow. Be prepared for this.


If for some odd reason this does not help, please let me know so I can assist you further.


Expert:  Hank F. replied 6 years ago.

Another thing I see quite frequently on older Onan sets is that one spark plug has gone bad, and it is only running on one cylinder. This would also cause this to happen.


Customer: replied 6 years ago.
Yes I agree with what you have said about the Carb., But Why does the GenSet run very Smooth when Not under a Load? Could it run Smooth if ( 1 ) of the ( 2 ) Spark Plugs were Bad? Your advise concerning the "Gas Smell" is Good and we will do a tune up nad Carb Clean, and Check the Spark Plugs.

But, my Question is Why does it Run or Boug Down when under a Load?
Expert:  Hank F. replied 6 years ago.

Yes, one of the spark plugs could be bad - see my above post.


If it is a bad plug, what is happening is that Onan engines are so well balanced that they can run on one cylinder and the average person would not ever notice. But once it gets put under a load, it then becomes severely off balance, and it shows readily.


If it is a carb issue, what happens is that it can flow enough gas to run well with no load on it, but putting a load on increases the demand for gas - the same as driving your car at 80 mph as opposed to 50 mph.

If there is a partial restriction in the carb, it can not flow enough gas to handle the load, and it will run rough, as the fuel mixture will be too lean.


Hank F. and 2 other Small Engine Specialists are ready to help you