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Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 15278
Experience:  Certified on Onan and Generac generators
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I have a Honda GX620 engine that powers a thermalarc

Customer Question

I have a Honda GX620 engine that powers a thermalarc predator welder/generator. This thing gives us fits all the time. I usually start it several times a year when its just setting. I started it probably 3 months ago. It starts right up as long as its choked. As long as the choke is out partially or all the way it runs normal. It does have an auto/run switch. I have it in the run switch but when in auto mode it throttles up as necessary when load is applied.
Again it runs fine in either position as long as choke is out. What it does is surges. High/normal rpm to low/normal rpm. I am guessing its probably a piece of dust in the carb as that is usually what gives it fits. But usually when that happens it wont start. The only other thing that stands out to me is that its in the back of our pickup so it isn't quite level and I know its very sensitive to being level. I really kinda suck at small engines. I am very mechanically inclined and can take just about anything apart and put back together very successfully. My real issue is I need to weld some fence for our dairy for animal/human safety and cant wait weeks while a shop gets around to fixing it. anyway you could get me through this?
Submitted: 2 months ago.
Category: Small Engine
Expert:  Hank F. replied 2 months ago.

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

What you are describing is a bad carburetor.

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.

Customer: replied 2 months ago.
I will try cleaning it. I don't think I have the commercial cleaner. What I have is from John Deere and is what all their mechanics use and recommend. However these are much larger parts. I am not sure I can get a kit in a reasonable timeframe but will try. What do those kits usually have in them? It may be mid day tomorrow before I know if this helps. Possibly sunday.
Customer: replied 2 months ago.
I may need some further clarification as I roll along as some of the things you have explained I'm not completely familiar with. In the past carburetors I have dealt with just had gaskets that lost the seal or prime to the bowl.
Expert:  Hank F. replied 2 months ago.

Not all carbs even have kits available. Sometimes you just have to replace the carb.

Even of the carbs that there is a kit available for, they differ in what is in them.

Gaskets, float needle, needle seat, jets - any combination or all of it may be in a kit.

Customer: replied 2 months ago.
before I ripped everything apart I leveled the unit out and started it. Within probably 2 minutes it started to run normal. Choke is pushed in and its back in auto mode running nicely. I know that this particular setup is very very sensitive to the slightest incline. Such as the one that the pickup bed has due to the overload springs. At this point I find it very difficult justifying taking the carb apart. Also I forgot to add that we don't leave gas in any of our small engines. So it has new fuel put in on each use.
Expert:  Hank F. replied 2 months ago.

I don't know how far out of level you had it, but it would have to be very off level for it to run bad like that.

Customer: replied 2 months ago.
Not much out of level. Running really awesome now though. I know the thermalarc manual warns of this. I sure wouldn't think it would matter that little bit.
Expert:  Hank F. replied 2 months ago.

Running off level has more to do with oil than anything. If it is too far off level, the oil flows to one side of the crankcase, where the pump can't pick it up.

Customer: replied 2 months ago.
Sure. Makes sense.
Expert:  Hank F. replied 2 months ago.

The same warnings are in RV owner manuals.