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Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 14892
Experience:  Certified on Onan and Generac generators
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Briggs and Stratton 450 series 149CC lawnmower. Engine runs

Resolved Question:

Briggs and Stratton 450 series 149CC lawnmower.
Engine runs very poorly, almost stopping at times.
Have changed plug and air filter, and have cleaned all the small springs and linkages to carb - no improvement.
Engine is two years old and has had minimal use - small gardens.
Submitted: 5 years ago.
Category: Small Engine
Expert:  Hank F. replied 5 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.

This sounds like a carburetor issue - especially since it has minimal use.

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.

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.

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 5 years ago.

Is this likely after just a couple of years?

My previous lawn mower with the same engine lasted 6 years with just a service every year. I only changed it because the body rotted away!

It has been suggested to me that it may be the diaphragm? Apparently a common fault?

Expert:  Hank F. replied 5 years ago.
This is likely in 6 months on some engines.

Yes, it is very common for a carb to go bad on a small engine - this is the single most common issue we deal with.
Customer: replied 5 years ago.

Do you think I should buy a diphragm kit?

Are they easy to fit?

Expert:  Hank F. replied 5 years ago.
If your carb has a diaphragm, it should be part of the carb kit.

Yes, it is an easy fit - just make sure to pay attention to how all the parts come out, so they can all go back in the same way.
Customer: replied 5 years ago.

Does this engine have points and ignition timing which might need checking?

Forgot to mention that the spark plug was very black and sooty when I removed it.

Expert:  Hank F. replied 5 years ago.
There are no points.
If the engine is starting at all, the timing is good.

A black plug is another sign of a bad carburetor.
Customer: replied 5 years ago.

OK - thanks.

I'll try and find a carb kit and change the diaphragm and gaskets.

Thanks Hank.

Expert:  Hank F. replied 5 years ago.
No problem.

And don't forget - cleaning the carb means taking it apart and soaking it.
Simply changing the gaskets and diaphragm will not do a thing.
Customer: replied 5 years ago.
What type of solvent do I need to buy to soak it in - does it have a name?
Expert:  Hank F. replied 5 years ago.
It is called carburetor cleaner, and it comes in a 1 gallon can.
I prefer the NAPA brand, but Berryman's is good stuff, also.
Customer: replied 5 years ago.
OK Hank - not heard of Berryman's - are you in the UK?
Expert:  Hank F. replied 5 years ago.
No, I'm in the US
Sorry, I did not notice that you were in the UK.

You most likely won't have NAPA stores, either.

I am not sure what you would have over there.

What I would recommend is that you talk to an auto parts store or auto shop, and see what brand they would recommend.
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