OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Honda Engines GXV160 not running well, too smokey

Hello Gents,

Alas this is a bit of a long running saga - continued from here (https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/118994/1.html) but as that was 2yrs+ ago I thought I'd start a new thread.

In the past, it wasn't running great but it was usable - but last time it was blowing so much dark smoke I found myself feeling sick after 15mins usage & had to stop. Tried to tinker with carb settings but to no avail. Hence here I am hoping for guidance.

I will try to describe issue & whats been done/tried to assist those who assist me:

ISSUE:
Engine blows noticeable dark smoke when revving up (ie you hit a thick patch of grass, the revs slow, then when it gets back to standard RPM it'll puff smoke).

SETUP OF MOWER
Currently has quality full synth 10W-40 oil in it (freshly changed in to see if would help, nope!). Has a sparkplug thats one step 'hotter' than standard one (as recommended on here). Regularly has Redline S-10 fuel cleaner put through. Air filter clean. Everything else is standard, does have a Masport factor snorkel specifically for the GXV160 - BUT I do not run the extra high mounted filter in it - I only have the standard low fitted Honda filter, the high intake of the filter's intake has very coarse fly screen in it, but nothing else - so air flow would not be restricted vs a single low filter. Fuel tap is always used so fuel not left in the carb, fuel stabiliser is always used in all fuel.

THINGS TRIED
Multiple changes of oil to higher viscosity, hotter sparkplug, new airfilter, trying to adjust the two screws on the carb (though I was kind of adjusting, viewing results and then adjusting to get no smoke - but could not change correctly). Tried to adjust the governor (followed Youtube tut but not sure if it was done right).

QUESTIONS
1) Could the valve clearances be the issue? I have had the mower 7yrs or so now, lots of use and I have never adjusted or checked them - I have asked here if I should do or if it is hassle for no good reason, never got any replies.

2) I suspect whatever the issue is - I've further stuffed things up by changing other settings TRYING to fix. i.e the carb is probably incorrectly adjusted, the governor is likely incorrectly adjusted etc. Is there an order I should try and use to get each of these bits correct as they all seem to affect each other so I'm unsure which one to do first?

I will note this - the carb fitted (which I am certain is the OEM carb, has two screws to adjust it. A larger plastic one towards the top of it (not sure the proper setting for that one or what it's for) & a smaller metal one thats on the front, lower down on the carb. VERY ODDLY this screw is only adjustable ~1/4 of a turn either way, as the plastic cover under the air filter box seems deliberately placed to block this being altered any more.

In contrast the replacement GXV160 carb I bought has that screw (which I think is called the pilot screw) being fully adjustable.

Thanks in advance - sorry for the long winded info but didnt want to miss things that might be needed.

Nick

Hi Nick have you tried running it with the air filter housing removed?

Hi Norm,

Yes I did at one point way back - but I am happy to do so again to dispel the thought that the housing itself is a factor - as mentioned it's a factory fitted one that Masport used to put on their GXV160 fitted mowers and whereas Masport intended you to run two airfilters in it - the honda one down low and a Victa long cylinder one up high - I don't run the Victa one - instead just having some coarse mesh to block large debris.

I will remove the housing, run and rev and report back.

Cheers. :-)

Hi Norm,

Ok ran with the airfilter housing removed. Did it make any difference? Ummm perhaps a tad less smoke - there is still very noticeable noise when the engine is at full revs, it's not a happy full throttle - to me it sounds quite possibly like incomplete combustion of fuel. Hard to explain.

Of course I had to mess with both the adjustment screws, well just to see if either of those helped. The 'pilot screw' which is as I said only turnable around 1/4 of a turn total, seemed to do nothing at all - as I suppose you'd expect from 1/4 of a turn range. Seeing as I cannot have ever changed that I don't see that could be the issue.

The larger plastic screw.....well that has a lot of range on it - clearly it alters the idle speed. I've got the workshop handbook pdf on the GXV and used a cheap tachometer to set the proper speeds a while back. At one end of adjustment it has a very low idle, which when you add throttle results in a further dip of the RPM and a lot of smoke before getting to full RPM. When set the other way its a higher RPM, less smoke to full RPM.

I am sure this screw is set wrongly......and likely the governor is set wrong as well - as of recent times it's had a very slow and bad respnose to hitting heavier grass etc - which means you have to slow, cop a load of smoke etc.

On the air filter housing - is a very fine line between having it sealed against dust ingress - and making it unhappy with the air flow. I will note I still have the foam prefilter on the airfilter, all are in very clean condition.

I did seal up the housing's edges with some grease etc to try and keep a dustproof seal on it, the idea being all airflow came down the high mount inlet point.

Is there a way I can reduce the fuel mix richness? When I go from idle to stop, it will often do a loud explosive backfire.

Even though it wasn't very old, replaced the air filter with a brand new one - took the prefilter off. Seemed to eliminate the smoke to a large degree, I think the remainder could be sorted by just adjusting the governor etc.

Hi Nick, the governor is obviously set correctly because if it is wrong the motor will run flat out

Hi Nick,

I'd adjust the valve clearance first ,turn the motor so one valve is fully open then adjust the other valve,then turn the motor
so the one you just adjusted is fully open and adjust the clearance on the other valve.

manual with clearances here https://www.honda-engines-eu.com/files/files/owners-manual-gxv160u-english-32z4p602.pdf

All the mowers I've ever had used one filter, they never ran two filters always had just the filter at the carby end or just a filter at the snorkel end even if it was physically possible to put two filters in.

Valve clearance for this mower should be checked every 200 hours, for me every time I cut the grass it takes an hour,
so would be around 7 years.

When you turn off an engine, the spark to the spark plugs stops immediately. However, the engine can continue to rotate for a few more revolutions due to inertia. During this brief period, fuel and air may still be drawn into the cylinders. Since there's no spark, this unburned fuel mixture gets pushed into the hot exhaust system. The residual heat in the exhaust (especially the muffler) can then ignite this unburned fuel, causing the loud "bang" or "afterfire."

Common causes of backfiring/afterfiring when turning off an engine:

Rich fuel mixture: If the engine was running too rich (too much fuel, not enough air), there's more unburned fuel to ignite in the exhaust.
Incorrect ignition timing (especially retarded timing): If the spark is happening too late in the combustion cycle, combustion might not be complete when the exhaust valve opens, pushing unburned fuel into the exhaust.
Worn or faulty ignition components: Bad spark plugs, wires,
Engine not allowed to idle down: Turning off an engine at high RPMs can pull more unburned fuel into the exhaust. Allowing it to idle for a short period before shutdown can help.

Exhaust leaks: These can draw in oxygen, which can combine with unburnt fuel and ignite.

How too little valve clearance relates (or doesn't directly):

Too little valve clearance means the valve doesn't fully seat or closes too late. Specifically, if an exhaust valve has too little clearance, it might not fully close. This could potentially allow some hot exhaust gases to linger or unburned fuel to escape prematurely into the exhaust system while the engine is running, potentially contributing to a backfire.

However, the primary cause of backfiring when turned off is the unburned fuel in the exhaust igniting due to residual heat, which is often a result of the factors mentioned above (rich mixture, timing, etc.) rather than just tight valves.

Sometimes on motors that have a running problem it pays to clean the exhaust out if there is a partial blockage.

The dark smoke is usually over fueling , eg float setting wrong or needs a new needle, Inspect the float needle and seat. Ensure they are sealing properly and not worn, allowing fuel to continuously flow.

Sticking Choke: The choke might not be fully opening, restricting airflow.
Clogged or Incorrect Main Jet/Pilot Jet: While less common for too rich (usually causes lean conditions), an aftermarket or incorrect jet, or even a partially obstructed one, could cause problems .I've had partially blocked carbies before that were causing over fueling.

Cheers
Max.

Max, thank you - will check out in daylight tomorrow - much appreciated.

Folks, running a lot better - just used for an hour doing the front yard - well PART of it (joys of acreage!).

Haven't done the valve adjustments yet but will do within the next few weeks, just need to complete a cycle of mowing so I can have a tad of time to breath as the kikuyu slows down.....finally!

Much thanks to all.

Just on the valve adjustment process.

When it comes to getting the valve head cover off, when it's not been removed previously - so seems most often to be using a gasket of applied silicone etc - whats the best approach for getting this off, which minimises destroying the valve cover by bending it?

Paint scraper? Heat Gun? Razor blade?

And then following on from that - do folks prefer to have a replacement gasket on hand (i.e the exact pattern in proper gasket material)? Apply some high temp silicone? Or just screw back on and watch for leaks?

Thanks in advance.

Hi Nick,

I find if you run the motor first so it's warm you should be able hit the cover off easily with a bit of wood and hammer
or big screw driver and hammer.

If it has a gasket that rips and you clean the surfaces with petrol or thinners but leave the old gasket in ,silastic
high temp will seal the gasket back together.

I have high temp silastic but if I didn't I'd just cut a new gasket from gasket paper if the old one was damaged.

Gasket Paper 1.5mm Thick S-207 150mm x 150mm Sheet is about $10. with free post or you may get silastic cheaper.

If you don't have wad punches it's probably easier to get a VALVE COVER GASKET FOR a HONDA GXV160 for $10 with free post
when not using silastic.

A couple youtube clips to remove the cover but I haven't had a look at them.

If the cover gasket doesn't get damaged when removing the cover I'd just put it back on.




Cheers
Max.

max, much thanks, cheers