Honda GX120 with Predator Carb having issues

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MikeH1021

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I recently came into ownership of a GX120 Pressure washer for free and have done some mods to it. When I first got it, It had a pressure washer carb that would either run at idle, or at full throttle, with no other options in the powerband, so I swapped the carb out for a stock predator carb I had lying around, which worked well enough to use for a while. Next I removed the governor and upgraded the valve springs to 22lbs and added a high flow straightpipe exhaust. Anyways, I'm having issues where the engine RPM's are higher at 3/4 throttle than they are at full throttle and I think it's because it's running either lean / rich but I have no clue. Should I try to re-jet the carb or just buy a stock carb for a gx120 / mikuni?

edit: also forgot to add the engine turns off when set to any choke other than full choke; weird.
 
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mckutzy

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Hello and welcome....
Wow your running some serious springs there and I'm guessing not upgraded the con rod yet...
Equally speaking I reckon they're too much for the the cam to handle...
With that being said... Yes you'll need to re-jet... Best way is to do a plug test first...

Is say do a little at a time... Put back the stock springs(they also help to float the valves to prevent inadvertent con-rod dissection), you'll also have a better baseline for dialling the carb in...
 
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MikeH1021

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Thanks so much dudes

Alright. I'll swap back to the stock springs for now and do some testing. When I did remove my spark plug earlier it was very black with soot so I guess it's running rich. So if it is running rich, should I buy a new, smaller main jet (that's the right size) to put on the predator carb or just buy a stock 120 carb to jet myself? I hope I can re-use the predator carb.
 
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mckutzy

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Go to a lawnmower/tool rental shop, and talk to one of the mechanics there... Theyll mostlikely have a junk box of jets and other assorted carb items. See what they want for some...
Ask if they have any for a stock GX120, if not they can be modified, and redrilled to a smaller orifice size...
Look up on jet re-sizing and drilling...
 
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mckutzy

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As I understand.. Therein lays the aforementioned modification that takes place... Aparently you solder the hole shut. Then drill the new hole.

I won some good marks back in the day in science class, explaining how to drill a square hole....
Wasnt able to actually demonstrate it, but was able to explain an show the theory.
 
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TT540

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Why give up on the stock carb? I say you clean that and get your choke/idle issue fixed. get the carb working through out its different modes of operation. Then go into modifying it.

When you get to reading plugs, it's important to check them right after a full/long pull. As in, just cut the engine and coast after running at top speed. For you, you can run at 75% throttle for 15 seconds; then check that out. Then, run at 100% for 15 second and check again. the duration might need to change.
 
firemanjim

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212 carb is way too big.... Your engine ran at idle or wide open because it was intended for an industrial application ,ie power washer. All you needed to do was remove governor to fix that issue, or place the governor spring rod in one of the other holes in the arm.
Put the stock springs back in it, before you put a new vent hole in the block.
You're not gonna pump out much HP with that engine. Be happy with it and save money for bigger , proper engine. Pulling the governor, adding a free flow filter, and put your stock carb back on.
 
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