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Discussion Starter #1
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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I go back to a carb problem, or a fuel delivery problem, restriction, fuel inlet float valve.
 

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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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Discussion Starter #4
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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Discussion Starter #5
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?
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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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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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Possible air leak somewhere? Sounds like a lean condition.
 

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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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Discussion Starter #9
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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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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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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Discussion Starter #12
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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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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Discussion Starter #14
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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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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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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Discussion Starter #17
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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Discussion Starter #19
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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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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