OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Honda Engines Honda gxv390 running issues

Gday,

I have an old Honda Gxv390 motor mounted on a victa pro12 frame. The motor was originally found sitting in a farm shed with no plug in it. I replaced the plug and completely cleaned out the carby best I could. After a few pulls I managed to start it but it would run for a little while with the revs lowering and raising again before the motor would die completely. This happened twice and then I tried to start it for the third time and it didn't want to start. There was smoke coming out of the carby (I assume from the crank case) while trying to pull the rip cord after it ran. The motor is pretty much stock except I removed the muffler and it was exiting straight out of the head (only way the motor would fit without custom exhaust).

I would appreciate it if you guys would be able to tell me what may be happening with this thing. Thanks.

Hi ProXu1Victa

Do you have a welder? I suggest making the custom exhaust before trying to run the engine again. Maybe even try using some water pipe. Running the engine without the muffler for extended periods is a bad idea. There won't be enough back pressure to stop the flames from burning up the valves. Did you see flames coming out of the exhaust port?

The valves, especially the exhaust valve, may have lost its ability to seal off the exhaust port during intake, compression and combustion. This means that most of the fuel/air mixture may be lost during these strokes, leading to poor power and starting. You may need to take off the cylinder head and check the valves to see if the valve faces are still seating well against their respective seats and the valves have not disintegrated. I am not completely sure but, I believe the valve heads have been heat treated more than the stems. If the valve head is heated from exhaust flames, then that will soften the valve material and make it prone to wear a lot lot sooner.

Also, if there is smoke in the crankcase (called blow-by), then your rings are shot, or the cylinder walls are scratched to buggery. However, some smoke coming out of the carby may be due to the intake valve not closing properly during the exhaust stroke. Either way you will need to take off the cylinder head to take a closer look. All that may be needed is a good clean (to remove carbon deposits), some valve lapping and adjusting the valve clearances (according to Honda'sspecs) - best case scenario. Not sure how handy you are or if you have the nessesary tools or time to complete such a task. So, good luck!

Regards,

xhall

Thank you heaps for the in depth response, should be helpful.

I was planning on making an exhaust, would you still recommend a muffler or would it be fine if I create a little back pressure with some piping? I knew that running it without one was not the best idea, but it didn't run for too long (30-40 seconds or so) so I doubt much damage was done to the valves in that time. I will still check them though. I do not recall seeing any flames at all.

My first thought of the blow by is that the rings are shot, So I will look at that. I was thinking it may have fuel issues, as I was running an old filter, a somewhat dodgy fuel tank and I don't really trust the carby as I rebuilt it without using a kit. So the cutting out could be that?

If you don't mind the noise then you don't have to put the muffler back on, but your neighbours might feel differently. I also doubt that much/if any damage was done to the valves after running it for such a short time. You will need to ask someone else about carb problems for this engine, my experience is limited to a handful of side valve Briggs engines, with pulsa jet carbs and a few Walbro and Zama barrel carbs. Carb issues (such as blocked passages from dirt, or stiff/hard diaphragm pump), old fuel, clogged fuel filter, and dirty/rusty fuel tank or damaged gaskets can definitely cause hard starting/ rough idling. Rough idling can also be caused by an overly lean fuel mixture.

Thanks for the reply,

I don't mind the noise, it's a farm vehicle anyway. I have a piece of exhaust I will try to fit on it. When I rebuilt the carby, I just wound the jet all the way in, so I imagine it's probably running a little lean. Would there be any other causes for crankcase smoke? Or should I be preparing for an engine rebuild.

Going by your first post there is a good chance it will need a rebuild. The fact is was found with the plug missing is that prior to it being parked up there was an issue and that was why the plug was removed, always a bit of a warning sign

Well I'm that case, I either rebuild this or the seized Briggs 12 oppose the mower came with.
Which one would you recommend rebuilding? Seems like there is more parts available for the Briggs them for the Honda.

I'm not sure what Honda parts are available but if I could I would build the Honda any time. The Honda may only need a bore hone and a set of rings. Problem with the Briggs you never know when they are going to put a leg out of bed. Then you have to decide what it is going to cost or you get a new 13hp Chinese motor for about $400