Quickish question on Honda GX25 carb

Hi,

Been struggling to get my Honda GX25-based tiller back to working. At this point, I'm suspecting one of two things:

(1) The carburetor doesn't visibly dispense gas. Is that how the GX25 carb works? I'd have thought that when I primed the carb, that the bulb would fill (which it does), and that gas would eventually drip out of the needle and into the main carb chamber, waiting to be sucked into the engine. Is that how this type of carb works?

I'm on my third carb, but the 2nd and 3rd are knockoffs from amazon. The second had a really flimsy plastic sheet in place of the spring that came with the OEM carb. I haven't torn down the third carb yet. All that to say that https://www.lawnmowerforum.com/showthread.php/57125-Incorrect-parts-for-GX-25 resonates with me.

(2) I've certainly had the tiller stall out on me a couple times recently (before it stopped starting) due to being unable to turn the tines (hit a rock, too much bermuda grass roots caught up in the tines, etc). After reading this forum a bit last night, I'm now concerned that my timing belt is off by a notch or two. Does that sound right?


As background, I've taken apart a lot of the engine -- pretty much everything that didn't involve taking out the piston or messing with the timing belt. Both before and after the teardown, I was and am getting about 90 PSI of compression (no idea at what RPM), as measured by one of those cheaper compression kits. I used some liquid gasket to seal the engine. I get a pretty decent spark from the plug and have now tried at least a couple different spark plugs.

Thanks!

:welcome:

Got to the Walbro page https://www.walbro.com/parts-services/.
Down the bottom right side you will see a link to manuals and videos.
Download the WY series ( Rotary valve ) and read it.

A primer bulb is actually not a primer bulb it is an air purge pump which does nothing but remove air from the carb so the metering chamber is full of fuel
It does not flood the carb.
The quick test to see if a carb is working is to slide it off, turn it sideways then blow air ( low pressure ) through it.
You should see the vapourised fuel coming out the engine side.
If the peepers are not a good as they used to be blow onto a piece of newspaper then feel & smell it .

It is very rare for the timing belt to skip a tooth as it is very short and very direct unlike the serpentine one on your car.
IF the belt is out of time then it has stripped a few teeth.

The clutch will slip or the key in the flywheel will usually break before the belt strips a tooth.
The internal parts that generally give you grief is the rockers / followers
One arm of the triangle breaks or the pad wears through.

The GX series is also very sensitive to fuel tank pressure or lack there of.
By far the most common reason for GX 21 / 22 /25 / 30 / 35 running poorly is a tank that leaks pressure , a blocked fuel or leaking fuel cap or the supply line going soft & pinching off.
I replace a lot more fuel lines & tank grommets than I do carbs

Search out Joe Pace - two stroke diagnosis on u-tube for details about how to pressure test a tank & carb properly.
Everything he says up to pressure testing the crank case seals applies to both 2 & 4 stroke hand held engines equally.

What exactly is your engine doing or not doing as the case may be ?

Wow, your response is gold! Thanks very much!

I didn't even know that a pressurized gas tank was an issue. I'll have to learn more about that.

Direct link to the rotary valve carburetor section of the aforementioned video https://youtu.be/cDaOcNrS6BM?t=808 (for those onlooking later)

Now watching https://www.youtube.com/watch?v=V9vTjBeEgQE , which is the fuel tank pressurization video. For $5, I might just go ahead and replace the gas cap. That pressure device looks kind of pricey, given I'm not super likely to re-use it. OTOH, you suggested the fuel lines & tank grommets, so maybe I'll swap those out first, since I've got those parts already. I actually tried pulling out the grommet to replace it before I even knew that it was called a grommet! Couldn't pull it out with a pair of pliers, though -- will search around for videos on that.


The symptom is that the engine doesn't start at all. This problem occurs even when I spray starting fluid in there... which implies that maybe it's not a fuel delivery issue. Running out of ideas, though.

Video on grommet removal: https://www.youtube.com/watch?v=hxg9vnVIQHY

They make it look so easy... makes me wonder if the grommet is indeed the issue, as maybe it's gotten too brittle and no longer sealed.

You can not test a small carb without a pressure & vacuum gauge of some sort.
HF & Similar places sell cheap pressure pumps & vacuum pumps.
There is a plastic MityVac that is fairly cheap.
When I took over the repair run the previous owner had a simple bulb pump like a doctor uses to take blood pressure
So one end sucked & the other blew.

Now getting back to your problem.
Get yourself a nice new spark plug.
Spray a tiny amount of starter fluid down the plug hole and pull the rope.Try it a couple of times.
If the engine does not fire then the ignition timing is out.

The most common reason for this is the flywheel key being broken.
The simple check is the magnets should be under the coil or just leaving the coil when the piston is at TDC.

A pressure failure usually will allow the engine to start but after a while it bogs down or stops
The engine should start regardless of the pressure integrity unless the carb has an internal pressure leak.
Internal pressure leaks only happen after 10 thumbs Tony has pulled it apart & reassembled it wrong or decided the old gaskets are good enough to go back in.

When fitting grommets or passing tubes through grommets, silicon spray is the stuff to use not WD 40 which actually makes then sticky.

The rubber fuel line on the GX series has a finite life & goes soft.
When that happens it pinched off where it goes under the clip.
When you fit the new line, one should have thick walls and one should have thin walls
The thick wall tube is the SUPPLY line and has the filter on it.
The thin is the return.

People see colours on the videos and think it is the colour which determines which tube goes where.

Bert - I respect your experience, but fuel tank pressure or lack thereof is not nearly as important as you suggest.

The carburetor on a two-cycle (or mini 4 stroke) engine does not pressurize the fuel tank, and it does not require a pressurized fuel tank to operate properly.

The pressure buildup inside a fuel tank is solely a result of fuel expansion. There are only two ways that pressure can affect the carburetor, and neither of them are beneficial:

1. Excessive pressure can cause the needle valve to pop off and flood the engine with fuel

2. If the check valves in the primer system are weak or faulty, pressure from the fuel tank can force fuel backwards through the primer circuit and flood the engine.

Now, that said, the fuel tank must have an operational vent allowing air in to replace the fuel pumped out. If the vent becomes clogged, the carburetor will starve for fuel. (This was described very well in the video.) Under no circumstances will the engine run properly if the fuel tank is under a vacuum.

If you believe a pressurized fuel tank is necessary for proper operation, please explain why...

Clamp you trimmer in a vice start it then when hot remove the fuel cap & see what happens
The tiny capacity of the fuel pump is not sufficient to keep up with the engine and in particular pull the fuel up from the bottom of a tank turned sideways.
It requires the tank to be pressurized.
If you are old enough you would remember the old outboards with the pressure pumps you had to boost every now & then to keep the fuel supply up to the engine.

Since discovering Joe and taking the time to start with the fuel tank testing the number of carb pull downs has dropped about 60%.

The fuel tank and the carb are a semi sealed pressurized system.
I am fairly sure the Walbro service video tells you this , the service manuals do.
IF the pressure inside the tank gets too high then the metering needle pops off , the metering chamber overfills, the pressure inside the carb increases and the engine stalls out rich.
The opposite happens if the tank pressure is 0
The cap allows air to be drawn into the tank and either it or an external vent limit that to 7 to 10 psi depending upon the carb & engine.

Pressure in a cube carb is the same as float height in a bowl carb

A Walbro service manual:

https://www.google.com/url?sa=t&sou...FjAQegQIBBAB&usg=AOvVaw3_f4gcn81wYcu8XXdb8Sew

This manual goes over pressure testing the carburetor, but doesn't even mention pressure testing the fuel tank.

Which service manual are you referring to?

Here is Zama's theory of operation. Again, no mention of the fuel tank. http://www.zamacorp.com/carburetorsystem-as249.html

OK, Tom we will have to agree to dissagree on this one.
I had a quick look around for where it came from and apart from Joe Pace came up with nothing.
It occurred to me that a tank would not be a carb problem but an engine problem so it may have been in one of the two stroke engine books but no, not in the Honda book & not in the Tecumseh book.
However replacing leaking fuel tank grommets & fuel caps has fixed a lot of hand helds that come in with " occasional running problems " to the extent that how it is the first thing I do on a hand held service.
Since working this way the carb rebuilds have gone down by around 2/3 .
The bulk of the hand helds I work on are Honda GX series , better than 1/2 after that stihl ,then a mix of Shindiawa, kawakasi , with very occasional Huskies & cheap trash.
Probably due to what was stocked by the locals stores before the big boxes starting selling unfixables like Ryobi & house branded Chinese trash.