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Ask Hank F. Your Own Question
Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 15267
Experience:  Certified on Onan and Generac generators
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We have a black max generator Honda gx 390 engine and you

Customer Question

we have a black max generator Honda gx 390 engine and you cant get to run, it only runs slow and sputters.
JA: What's the make and model of the generator? And can you guesstimate how old it is?
Customer: model is PM0496750 7 YEARS OLD HARDLY USED.
JA: Do you plan on doing the work yourself?
Customer: YES, CANT FIND ANYONE TO HELP
JA: Anything else we should know to help you best?
Customer: ADDED NEW FUEL DRAINED CARB. AND CHECKED THE PLUG.
Submitted: 1 month ago.
Category: Small Engine
Expert:  Hank F. replied 1 month ago.

Hello, my name is ***** ***** I am going to assist you with this.

There are a few things that can cause this:

1. Bad carburetor

2. Misadjusted/damaged governor linkage

3. Valve issue

Due to the fact that your unit has not been used for so long, at first glance I would suspect a bad carburetor - and it probably is.

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.
Today’s gasoline formulation goes bad in as little as 30 days.
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 all of a sudden, even while simply stopping to refill with gas.
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.

This can also happen even on a brand new engine or brand new carb. When the carb is built, it is run at the factory to tune it.
After tuning, they do not always get properly cleaned, or may not have enough/any preservative injected. Believe it or not, this is actually fairly common.

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.
If this is the case, you will need to change the oil and filter, if it has a filter.
Varnish can also cause the float needle to stick shut, and not allow any gas to flow from the carb bowl into the engine.

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. It takes months or years for this accumulation to build up – it just can not be removed in minutes.

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.

After sitting so long, I would also check the valves for proper adjustment, and make sure both are opening and closing.

Valves on this engine should be set to about .005"

Expert:  Hank F. replied 1 month ago.

Was this what you wanted to know?