Honda Gx390 Will not Rev up over Idle.

F

fradan13hp

Member
My first post, been reading a while, but I have a problem that I need help with.

I have a Honda gx390 engine. I got it on a leaf blower I bought, and I have been struggling on this one. I have repaired plenty of these engines, but this one has me stumped.

It will start, and idle. as soon as Throttle is given, it shudders and shakes and wont rev any higher than idle.

I have replaced the carb ( tried that after I cleaned original)
New plug.
I lashed the valves that were out of adjustment to 0.06 and 0.08 inch
Replaced coil.....would not run with new coil, removed it and reinstalled factory coil-start immediately
I changed the blown head gasket (kept digging and finding things wrong).
After changing the head gasket, same thing-no rpms, but now with ALOT of kickback on pullcord when starting. Rechecked valves-spot on adj.

Today I pulled the back cover off.....measured 1.25 lift on the lobes of cam and some wear/pitting on exh lifter. That could be from the decompressor touching it?
The decompressor looks like its working, but maybe worn.

I dont see any broken pieces on case. the governor rod is there, and the gear looks fine.

I was thinking the cam was worn, so thats why I opened it up. It seems like its off time from how it shudders so much when giving throttle. The cam stuck in cover when removing it, so I couldnt see if it jumped a tooth.

thanks for the help
 
Last edited:
I should have pulled the flywheel first, but I took the rear cover off now. I guess I could slap things together and pull that off......
 

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You had the head off ? The valves were sealed good .
Compresion or leak down test . The decompression tab looks a little wore but that wont affect operation .
Spray some either in will its running with throttle .
Hook a timming light up .
Mechanically it looks fine .
 
flattop1 said:
You had the head off ? The valves were sealed good .
Compresion or leak down test . The decompression tab looks a little wore but that wont affect operation .
Spray some either in will its running with throttle .
Hook a timming light up .
Mechanically it looks fine .
Click to expand...

I did have head off. Assume valve had reasonable seal. Not bent..no marks.
Saw head gasket blown...went to work replacing that.
I dont know how to time these, but i can time a 700hp small block ford,lol

Ive heard these things wear cams at times....maybe the ramps worn?

But i still think its something basic. Starts 1 pull, idles great, pull throttle and SHUDDER AND SHAKE BAD. Wont go over idle speed.

Im looking to fix this as a work engine...ie leaf blower 3600rpm. Not a race piece
 
Ok....question..
Is this coil backwards?
I Assumed it was correct because thats how i got it

Im thinking..........it was on backwards and i swapped coils same way...
 

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Timing you need to put your own refernce mark on it or mount up a degree wheel on output shaft .. Bring it up to tdc on compression stroke with magnet under the coil . Make a paint dot on flywheel near coil post . Light it up and see if it moves forward or back .
There are some differnt coils for the 390's . Otherwise it looks right .
 
flattop1 said:
Timing you need to put your own refernce mark on it or mount up a degree wheel on output shaft .. Bring it up to tdc on compression stroke with magnet under the coil . Make a paint dot on flywheel near coil post . Light it up and see if it moves forward or back .
There are some differnt coils for the 390's . Otherwise it looks right .
Click to expand...


Thats how it was
I mounted coil this way now.
Which is correct?
 

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