OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters Four Stroke Trimmers & Brushcutters GX25 Honda

A bloke dropped of his Honda GX25 brush cutter because it won't start. Around Christmas I fitted a new carby to it and it worked fine.When I changed the carby I also put new fuel lines and filter on it. It primes fine. Now the only way I can get it to fire up for a second is with starter fluid and wide open throttle. It is obviously a carby issue, just not sure what to look for, anybody have any ideas?

Hi NormK,

Have a very close look at the insulater block between the carby and the cylinder block as it could be cracked.

Hi Norm and Bruce,

Yes do all the easy checks first like Bruce mentioned before dismantling the carby.

I've had a few like this , just to save time I always disconnect the intake fuel line from the carby and put a spare fuel line on with primer and dip the end of the fuel line in a cup of fuel with no fuel filter , then run the snipper for a few minutes as sometimes the extra flow fixes the problem and also tells you
if the filter is gummed up or carby gummed up.

Even though you put a new filter on it around Christmas, a few things could have happened to it since then:

Microscopic Fuel Flakes: If the old fuel sat in the tank and started to break down, it can create a fine, almost invisible gel or powdery scale. When you put fresh fuel in, it may have washed all those loosened contaminants straight into your new filter, instantly plugging it up.

Another problem I always see is people leaving the fuel tank or petrol tin on cold concrete which attracts moisture.

Water Contamination: If water got into the tank, some fuel filters (especially high-quality felt or paper ones) will absorb the water, swell up, and completely block fuel from passing through, even though they look spotless to the eye.

As we know it's still priming but can be restricted flow.

Another thing I've done before is remove the 2 lines from the carby and blow some compressed air down them , but the air gun I use is a lot bigger than the small fuel line connectors so it's not getting a full pressure blow out.It can just loosen sticky valves or partial blocked jet etc.

If it still won't run during the cup test:

That confirms the blockage is inside the carburetor itself. Since it primes, we know the primer pump is pulling fuel through the carb, but the engine can't draw fuel out of the carb on its own.

If the cup test fails, check these two specific things inside the carby:

The Main Jet is Blocked: The primer circuit bypasses the main jet to fill the bulb. If the main jet is blocked by varnish, the primer will feel perfect, but the engine's intake vacuum can't suck any fuel when it's cranking.

When you pump the primer, you are manually pulling fuel around the engine's main feeding zone just to prime the pump chamber and fill the bulb. Once the engine starts, it stops using the primer circuit entirely. Instead, the engine’s internal vacuum has to suck fuel through the metering needle and the main jet.

If that main jet or the internal nozzle is blocked by a tiny bit of varnish, the primer will feel absolutely perfect, but the engine will starve the second it tries to run on its own (which is why it only fires when you force-feed it starter fluid).

When someone bring a snipper back a few months later with carby trouble we know it generally shouldn't be too bad as it had a new
carby last time.

Most times the cup trick and air gun blowing some fuel and air through usually clears a carby that's not that old.

I do the easy tests first and sometimes check the mixture screw in the middle of the black plastic throttle link and if that doesn't
work you will need to dismantle the carby ,sometimes there is a little mesh screen in the carby that can also block up with old fuel
that will need cleaning out.

The needle
The Quick Fix: Gently turn it all the way in until it bottoms out (don't force it, or you'll snap the needle tip), then back it out about 1.5 to 2 full turns. That is usually the factory baseline. If it suddenly starts from there, you know it was just out of adjustment.

The Little mesh screen
it sits right at the entrance of the carburetor's fuel room, it acts like a bouncer. When old fuel sits, it turns into a gummy film right on that screen, acting like plastic wrap. Fuel can still pass through when you force it with the primer bulb, but the engine's weak natural vacuum can't suck fuel through it.


Make sure jets are clean as you have done these a million times by now. Just summarizing what you already know.

One last thing I forgot to mention, I always try another primer before a tear down.

Even though a primer bulb looks and feels like it’s working when you pump it with your thumb, it can absolutely fail to hold pressure or vacuum when the engine is running.

Here is exactly why that happens:
The Secret Check Valve inside the Primer

Inside or directly beneath that little plastic primer base, there is a tiny, delicate rubber check valve (often a little duckbill valve or a tiny orange rubber umbrella).

When you pump it: Your thumb creates a massive, forceful suction that manually forces the valve open to pull fuel up.

When the engine runs: The engine relies on a tiny, microscopic vacuum pulse to draw fuel. If that little rubber check valve has softened, warped, or gotten sticky from ethanol, it won't seal properly.

The Result: Instead of pulling fuel from the main jet, the engine sucks air backward through the primer return line, leaning the engine out instantly. It will only fire on starter fluid because it's starving for fuel the second it cranks.


So the fuel line in a cup of fuel trick I will use a good known fuel primer on that line. You probably also have had a lot of primers only
half work too.

Cheers
Max.