Help troubleshooting a four stroke Honda GX engine

SchrodingersCat · 19-02-2023 7:28pm #1

Hi!,

I am having difficulty in starting a Honda GX250 four stroke engine on a Belle Cement mixer. Now, I know that this is a gardening forum, but I am hoping that there are more small-engine repair folks in this forum as these type of engines are so common on lawnmowers.

Ive put fresh petrol into it.
The spark plug is wet with petrol after a few pulls of the starting cord
I tried spraying "Ez Start" stuff into the carb but that didn’t make any difference.
The spark plug makes a spark when I tested that.
Ive dismantled the carb and cleaned it in a ultrasonic cleaner but it didn’t make any difference.
I tried spraying the same EZ start stuff directly into the cylinder, putting the spark plug back in, and trying to start it but that didn’t work.
Ive tried dropping a few drops of petrol directly into the cylinder, puutting the spark plug back in, and trying to start it but that didn’t work.
I removed the belt drive to the gearbox in case that was stopping it, no change.
I changed the oil as the old stuff had a funny tint. I put some car oil into it. Not sure if it suits, but sure it’s just to test it.
In the end, I was able to get it to start fairly consistently by putting a few squirts of car engine oil into the cylinder (see video: https://youtube.com/shorts/Fi4VJrkoNeo?feature=share

SchrodingersCat · 19-02-2023 7:35pm

Link to a video here where I could get the GXH50 engine temporally started using a couple of squirts of car engine oil into the cylinder:

https://youtu.be/B1cr8adg6As

Image of the previous oil that was swapped out:

artanevilla · 19-02-2023 7:40pm

Check the compression. Oil in the cylinder will temporarily increase the pressure in the cylinder (it's not compressible). You've ruled out fuel and spark already.

MIKEKC · 19-02-2023 8:04pm

https://www.boards.ie/discussion/2058284975/help-troubleshooting-a-four-stroke-honda-gx-engine

surely the plug shouldn't be wet.Cou!d it be getting too much petrol .Too much cnoke

The Continental Op · 19-02-2023 8:54pm

Did you clean up the needle valve attached to the carb float? I've had no end of issues with that range of engines after fuel has been left in it over winter.

The Continental Op · 19-02-2023 9:35pm

Its not an issue that affects Honda engines much but have you checked the fly wheel pin (well know issue on small B&S mower engines)?

If the engine suddenly stops there is a shear pin that connects the shaft to the fly wheel. If its anyway damaged then the timing will be out.

You can't normally judge the compression of those engines except perhaps with the oil trick because of the compression release thats built into the engine to make it easier to pull the engine over for starting.

I'd also add that sparkplug that works out of the engine may not work where you can't see it when its under pressure in the engine. I've wasted a lot of time in the past with a plug that sparked OK and seemed good. Always try a new plug!

Edit> One weird one I had on a similar cheap chinese Honda copy recently was the pull cord was too short. Seems the short cord would only pull it over one turn and I nearly pulled my arm off at the shoulder trying to start it. When increased the length of the cord so it pulled the engine over nearly two turns it started first time.

SchrodingersCat · 20-02-2023 10:30pm

Thanks for the responses guys

https://www.boards.ie/discussion/comment/120262516#Comment_120262516

Good tip. I checked the compression tonight. I was only getting 20 psi.

When I removed the meter from the spark plug hole, put a couple of squirts of motor oil into the cylinder, and check the pressure again I got over 90psi. Reading online, they reckon Honda engines should be at least 75 psi.

From a reading this low, they suggest that this is probably the piston rings that are gone.

https://www.boards.ie/discussion/comment/120262611#Comment_120262611

True, but I reckon that the plug was damp due to all the fuel going through without any successful ignition?

https://www.boards.ie/discussion/comment/120262823#Comment_120262823

Yep! I put the lot into the ultrasonic cleaner. I couldnt find any greasy deposits in it. I use carb cleaner everywhere in the carb too. TBH, the carb and engine seems to be in very clean nick even before cleaning them out.

https://www.boards.ie/discussion/comment/120263011#Comment_120263011

Cheers for the tips. I checked the flywheel and the pin, all were in grand condition. In fact, I struggled to get the flywheel off in the first place it was so tightly on. I actually put a small hole in the crankcase taking it off. Il have to jbweld it to seal it again. Good to know about the sparkplug, ive ordered a second one there to be sure. However, as the engine runs when there is oil squirted into the chamber, I suspect that the spark is fine. Thanks for the tip on the length of the pullcord. I am using a drill to start the engine though, as it was getting too tiring using the pull cord during all the troubleshooting.

Thanks for the information everybody. I will order replacement piston rings.

It looks like there is a bit of work installing them (see attached), but it will be a fun project.

GXH50_Assembly_Guide.pdf

The Continental Op · 20-02-2023 11:07pm

Think you'll also need a gasket kit as you have to split remove the rocker cover (hmmmm?? did you check the valve clearances look them up from memory they are around 8 thou for exhaust and 6 thou (inch) for intake?) and the main casing. Although last honda rocker cover I removed used high temp silicon (from a tube) and didn't us an actual gasket.

RE valve cleareses https://www.lsengineers.co.uk/media/advice_centre/Honda/GXH50_Adjustment_Information.pdf - no mention of gasket.

Certainly sounds like the engine is worn out. With cement dust flying around mixer engines are likely to suffer worse than your average mower engine.

Engine assembly https://www.lsengineers.co.uk/media/advice_centre/Honda/GXH50_Assembly_Guide.pdf - looks like no gasket set and its all silicon?

Full list of manuals and part numbers https://www.lsengineers.co.uk/advice-centre/honda-gxh50-parts-diagrams-and-manuals

Edit sorry didn't spot straight away you already had the assembly diagram.

SchrodingersCat · 20-03-2023 9:22am

Thanks for the information. The piston rings arrived so I replaced them. I bought a set of feeler gauges for adjusting the valves too.

To be honest, I suspect now that someone had overtightened the rockers on the valves instead of the piston rings being gone. There was no clearance in them. This was causing constant pressure on the valves meaning that they were alway open a smidge. This meant that the cylinder couldn’t get any compression. Adding the oil directly into the cylinder sealed up the valve just enough to give good compression temporarily.

Anyway, after putting everything back together, I got decent compression again.

I can report that she is purring like a kitten again! The engine is working great. I’ve learnt alot on small Honda engines with this project.

Thanks for all the help 😀

The Continental Op · 20-03-2023 9:38am

Don't forget to give it an oil change after a few hours use to get any metal out of the system from the new rings.

SchrodingersCat · 20-03-2023 10:27am

https://www.boards.ie/discussion/comment/120378412#Comment_120378412

That’s a good tip. Cheers, I will.

Help troubleshooting a four stroke Honda GX engine

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