# Honda GX390 strange problem



## legarem (Sep 9, 2017)

Hi
I repowered my snowblower with a near brand new Honda GX390 last fall

It start easily everytime, runs correctly from idle to full throttle (3600 RPM). When the motor has to deliver more power, the governor open the carburator butterfly and the motor bogs sputters but work. It is doing the same thing when it is at full throttle (3600 RPM) *and I open* the carburator butterfly manually with the governor rod. It doesn't rev more but sputters.

Here what I tried to get rid of this problem.

- Cleaned the carb
- Tried another carb
- Replaced the spark plug
- Adjusted the valves

With all these things done, the problem is still there. The only thing I suspect now is a faulty coil. Could it be the problem ?

Thanks


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## JLawrence08648 (Jan 15, 2017)

I go back to a carb problem, or a fuel delivery problem, restriction, fuel inlet float valve.


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## ELaw (Feb 4, 2015)

Did you allow the engine to fully warm up before testing? With newer engines that run leaner, you might have to let it warm up for 10 minutes or more before it'll run properly with the choke fully off.

Also have you checked for air leaks between the carb and the engine? Or possibly even a missing gasket?

Other than that, although the symptoms you describe 90% of the time are fuel problems, it's possible it's an ignition issue. Maybe see if you can find a used magneto (so you're not spending too much $ if it doesn't work) and toss it on there.


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## legarem (Sep 9, 2017)

There is no problem with fuel supply

When I unscrew the drain screw on the float bowl, gas is flowing freely when the fuel valve is open.


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## legarem (Sep 9, 2017)

ELaw said:


> Did you allow the engine to fully warm up before testing? With newer engines that run leaner, you might have to let it warm up for 10 minutes or more before it'll run properly with the choke fully off.
> 
> Also have you checked for air leaks between the carb and the engine? Or possibly even a missing gasket?
> .


It is always doing the same thing even after 1 hour of use. 

Yes, all the gaskets and heat insulator are in good condition and are not missing


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## orangputeh (Nov 24, 2016)

ELaw said:


> Did you allow the engine to fully warm up before testing? With newer engines that run leaner, you might have to let it warm up for 10 minutes or more before it'll run properly with the choke fully off.
> 
> Also have you checked for air leaks between the carb and the engine? Or possibly even a missing gasket?
> 
> Other than that, although the symptoms you describe 90% of the time are fuel problems, it's possible it's an ignition issue. Maybe see if you can find a used magneto (so you're not spending too much $ if it doesn't work) and toss it on there.


i have almost the same problem after i put on an aftermarket carb on my 1132. as long as i let it thoroughly warm up it runs fine. like 5-10 minutes. before then it will die or sputter when i try to move the throttle.

do these carbs run lean? is that the problem? how do you change that? would that be the pilot jet or the main jet?


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## micah68kj (Oct 8, 2011)

Possible air leak somewhere? Sounds like a lean condition.


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## RedOctobyr (Mar 2, 2014)

That's really weird. Swapping the carb should eliminate a bunch of potential (and maybe common) causes. But if you open the throttle plate manually and it still won't rev up, that (to me) sounds a bit tough to tie to an ignition problem. It's still running at the same RPM the whole time, so I'm trying to figure out how that could relate to the coil, as the sparks are still happening the same way. 

An inline spark tester might be able to show you if you are losing spark. That's easy to try, and cheap, vs something like replacing a coil. 

Is it possible the governor is connected wrong? Sorry to ask this, but when you move things manually, you're definitely only opening the throttle plate, and the choke plate is staying still, right? Just trying to think of easy things to check. 

When it sputters, does it smoke?


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## legarem (Sep 9, 2017)

I played with the main jet until I read rich spark plug condition with the same results.
There's no smoke when I try to open the carb butterfly with the governor rod
The choke isn't closed when I open the carb butterfly with the governor rod

Like you see, I tried everything,


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## RedOctobyr (Mar 2, 2014)

Any chance you could post a video? Maybe showing the throttle linkage area, while you open the throttle manually, so people can hear what it sounds like?


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## ELaw (Feb 4, 2015)

RedOctobyr said:


> It's still running at the same RPM the whole time, so I'm trying to figure out how that could relate to the coil, as the sparks are still happening the same way.


The sparks are happening the same way but not in the same conditions.

As you open the throttle, the pressure developed in the cylinder at the top of the compression stroke (when the spark fires) increases. The increased pressure requires more voltage to create an arc across the gap in the spark plug. So if the ignition system is marginal, it's possible the engine would run okay at light load but poorly when the throttle is opened.

Now that I think about it, another factor there is spark plug gap... the problem I mention above is worse when the plug gap is larger. So (to the OP), have you checked the plug gap? If you have and it's okay, you might try reducing it as a test. If the engine runs better, the ignition system might be weak.


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## legarem (Sep 9, 2017)

I'll try to do a small video next weekend

Only when I used a leaner main jet, there was some small back firing with flames with sputters when I opened manually the butterfly with the governor rod


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## RedOctobyr (Mar 2, 2014)

Good points, ELaw. And yeah, I'm a proponent of trying a smaller plug gap when the ignition system is in question. I'd rather have a smaller spark than no spark, and see if anything changes. 

The backfiring seems odd. Could the ignition timing somehow be too-late, perhaps?


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## legarem (Sep 9, 2017)

RedOctobyr said:


> Good points, ELaw. And yeah, I'm a proponent of trying a smaller plug gap when the ignition system is in question. I'd rather have a smaller spark than no spark, and see if anything changes.
> 
> The backfiring seems odd. Could the ignition timing somehow be too-late, perhaps?


As specified, the backfiring was there only with a leaner main jet. With normal or rich condition, there's no backfire, only sputtering.

Ignition timing is perhaps ok under 3600 rpm but ignition advance is perhaps wrong at higher RPM than 3600 RPM ?


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## tabora (Mar 1, 2017)

Over on the Honda forum, many of us are increasing the GX390 main jet to .042 or .043 to eliminate lean conditions...


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## tadawson (Jan 3, 2018)

legarem said:


> I played with the main jet until I read rich spark plug condition with the same results.
> There's no smoke when I try to open the carb butterfly with the governor rod
> The choke isn't closed when I open the carb butterfly with the governor rod
> 
> Like you see, I tried everything,


The *choke* (as you stated) should not be connected to the governor in any way, shape, or form! Only the throttle plate . . . Any chance the linkage is wrong? The choke should be wide open at all times once the engine is warmed up . . . 

- Tim


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## legarem (Sep 9, 2017)

tadawson said:


> The *choke* (as you stated) should not be connected to the governor in any way, shape, or form! Only the throttle plate . . . Any chance the linkage is wrong? The choke should be wide open at all times once the engine is warmed up . . .
> 
> - Tim


I know which one is the choke and which one is the throttle plate, You can't link the rod to the choke.


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## sock-feet (Dec 14, 2017)

Try this adjustable High speed jet. This will work on the GX engines too. The engine sounds like it is running lean. This will allow you to adjust at high speed and under load.

https://www.ebay.com/itm/HF-Predato...657008&hash=item4b378fa03c:g:QmYAAOSwF71Z144Y


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## legarem (Sep 9, 2017)

As stated, I tried a higher main jet number and had a rich condition. (dark spark plug reading ) The sputtering was still there but without backfire.


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## RedOctobyr (Mar 2, 2014)

It sounds like your options start to narrow, then, including (but not limited to, I'm sure):

- investigating weak spark
- checking for an air leak from the carb (somewhat unlikely, as you can make it rich)
- something wrong internally. A valves problem of some sort, incorrect timing, etc.


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## tadawson (Jan 3, 2018)

legarem said:


> I know which one is the choke and which one is the throttle plate, You can't link the rod to the choke.


Sorry . . . your prior wording indicated otherwise, just being certain . . .


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## legarem (Sep 9, 2017)

I suspect incorrect timing but only at high speed. If it was incorrect under 3600 RPM, the motor could be normally hard to start.. It starts easily and don't hesitate when I open the throttle.

About weak spark, with leaner jet, if the spark was bad, I supopose I could see dark coloration on spark plug if it doesn't burn fuel correctly ?

Is there an electronic advance in the electronic ignition of these motors ? If yes, is it incorporated in the coil ?

A leaking valve shouldn't make the motor smooth at idle ?

This is a tough problem to resolve.


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## orangputeh (Nov 24, 2016)

legarem said:


> I suspect incorrect timing but only at high speed. If it was incorrect under 3600 RPM, the motor could be normally hard to start.. It starts easily and don't hesitate when I open the throttle.
> 
> About weak spark, with leaner jet, if the spark was bad, I supopose I could see dark coloration on spark plug if it doesn't burn fuel correctly ?
> 
> ...


i'm not a mechanic but is there a way to check for vacuum leaks? what about an exhaust leak? would that cause problems? valve adjustment already brought up? proper heat range of plug?

I'm watching this thread closely because I have had the same problem and want to know what to check and remedy.


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## legarem (Sep 9, 2017)

Valve adjustment was done.

Spark plug is the standard recommended BPR6ES gapped correctly.


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## FlamingSpaghetti (Jan 8, 2018)

Just throwing it out there, but I did once come across an engine that started and ran perfectly and beautifully at low RPMs but once it reached the operating 3600 RPMs, it began to sputter and hesitate. The cam gear happened to be a couple of teeth off.


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## dsc440 (Feb 8, 2018)

legarem said:


> Valve adjustment was done.
> 
> Spark plug is the standard recommended BPR6ES gapped correctly.


You might check the exhaust for any obstructions ? / Governor possibly out of adjustment ?

Sent from my SM-N920V using Tapatalk


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## jonnied12 (Jan 14, 2017)

Did you check the flywheel key? It could be partially sheared.


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## drmerdp (Feb 9, 2014)

Partially choke the motor and see if it eliminates the studdering when going full throttle. If it stumbles worse, clearly fuel is
Not the issue. If it clears up then fuel is the issue.

Ignition coil can be the culprit but not very common. Use a in line spark tester and see if visually the spark is consistent or erratic.

Also there are ohm measurements for the coil to bench test it.


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## legarem (Sep 9, 2017)

Flywheel key was not checked but I remember when my lawnmower had a broken key, it was really hard to start

When I choked the motor, it didn't help


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## RedOctobyr (Mar 2, 2014)

If the flywheel rotated a lot, vs the crankshaft, it wouldn't run at all. Your timing would be way off. 

But shearing the key slightly, or making almost a full rotation on the crankshaft, could end up giving you just a slight timing change, maybe enough to just cause strange behavior. 

You might be able to see the flywheel/crankshaft key slots by looking down the middle of the flywheel, maybe with the flywheel nut removed. Perhaps you could at least get some info about it, without needing to actually remove the flywheel.


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## legarem (Sep 9, 2017)

Finally, I think I found the problem why this motor is sputtering when I open the throttle with the governor rod to rev it.

I found this here:

https://www.nrracing.com/product-p/stage1kit270-390.htm

Note: UT2s series Hondas (2012 & up) or later have a built in Rev Limiter in the coil and will require a new coil to exceed 4000 rpm.


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## RIT333 (Feb 6, 2014)

legarem said:


> Finally, I think I found the problem why this motor is sputtering when I open the throttle with the governor rod to rev it.
> 
> I found this here:
> 
> ...


How many RPMs are you planning ? Please don't point your chute towards my driveway. LOL


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## legarem (Sep 9, 2017)

3600 RPM is ok but I thought there was a problem with the motor because it was sputtering when I tried to open the throttle with the governor. This is the first motor I saw doing that. I had older GX340 and GX390 and they revved freely when I opened the butterfly with the governor. My suspicion about the coil was finally correct but this can be considered as normal.


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## RedOctobyr (Mar 2, 2014)

Interesting, so apparently coil-related, but not in the way we might have typically expected. That's good to be aware of, thanks. 

An inline spark-tester might have been able to help show the spark cutting out. Do you know what RPMs you can reach before it stumbles? Are you close to 4000? 

At least it sounds like swapping to a different model of coil might eliminate the rev-limiter. The Hondas sound more tolerant of higher-RPMs, but a feature like this might help avoid some blown engines (often other brands) when a governor linkage freezes at full-throttle, at least. This is annoying for what you're trying to do, but maybe not a bad feature overall, as a last-ditch protection measure.


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## York v45 (Dec 27, 2014)

tabora said:


> Over on the Honda forum.


Which forum? I have a gcv160 pressure washer that has been driving me nuts with the auto-throttle and jetting. It won't run with the air filter on it. I need some Honda experts.


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## legarem (Sep 9, 2017)

RedOctobyr said:


> Interesting, so apparently coil-related, but not in the way we might have typically expected. That's good to be aware of, thanks.
> 
> An inline spark-tester might have been able to help show the spark cutting out. Do you know what RPMs you can reach before it stumbles? Are you close to 4000?


I suspect the stumbeling is beginning near 4000 RPM. As soon as I open the butterfly with the governor rod when throttle is at 3600 RPM, stumbeling is beginning. Now as I know everything is normal, this is not disturbing me anymore. After checking everything on this motor, this was a situation I never encountered since I work on small engines.


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## RedOctobyr (Mar 2, 2014)

York v45 said:


> Which forum? I have a gcv160 pressure washer that has been driving me nuts with the auto-throttle and jetting. It won't run with the air filter on it. I need some Honda experts.


I think he means the Honda sub-forum. 
Honda Snowblowers - Snowblower Forum : Snow Blower Forums

Specifically the re-jetting thread. 
http://www.snowblowerforum.com/forum/honda-snowblowers/128193-re-jetting.html


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## RIT333 (Feb 6, 2014)

orangputeh said:


> i'm not a mechanic but is there a way to check for vacuum leaks? what about an exhaust leak? would that cause problems? valve adjustment already brought up? proper heat range of plug?
> 
> I'm watching this thread closely because I have had the same problem and want to know what to check and remedy.


If you spray carb cleaner, or starting fluid, or and unlit propane torch around various parts of the carb and see a change in running behavior, then you have found the leak.

But, I am not a mechanic either - just pretend to be one !


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