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GX390T2 QAPW
My gx390 on my personal pressure washer has been giving me a bit of a fit lately. i cleaned the carb several times (compressed air, soaking, and Carb spray), so i finally bought a New honda carb, and installed new intake gaskets. Thing idles beautifully now, however when i put it up to full throttle it surges. choke it almost half way and it smooths out, HOWEVER, i can manually move the governor arm through the RPM range and it will not surge. So i kinda think it's the governor making it surge, then again, i can choke it almost half way and it will smooth out. I've also checked for air leaks around the carb with WD-40, and there are no apparent leaks while running.
So i guess the Cheapest option first is i will adjust the governor, if that doesn't work then i may start on drilling out the main jet. I just wanted to be sure i am correct in my thinking that surging at high rpm will be a main jet issue and not in the idle circuit. as it idles perfectly.
any suggestions on a good micro bit set?

Surging at high speed no load is still idle circuit. You are getting enough fuel to run smooth at idle but not enough to supply high speed no load operation.

You say "it idles beautifully" and also you say "choke it and it smooths out". It sounds like it is running lean on the high speed circuit.

Hammermechanicman said:

You say "it idles beautifully" and also you say "choke it and it smooths out". It sounds like it is running lean on the high speed circuit.

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Yep, idles great with no choke. put it on full throttle or higher RPM than idle, starts surging. when i pull the trigger on the wand it barely smooths out.

I adjusted the governor this morning. No change.
I removed the plastic idle jet and sprayed it out as well as the passage in the carb. Now at full throttle it won't surge, but it surges at mid range RPM, and at full throttle under load (me spraying the pressure washer).
Still idles good.

Still sounds like it is lean on the high speed circuit. Might be time for the dreaded torch tip cleaner and ream out the high speed juet just slightly.

Assuming you purchased the correct carburetor, Honda P/N - 16100-Z5T-901. If you do have the correct carburetor and lets assume fresh fuel, I would look at two things to start with. 1. Insulator between carb and cylinder. Look for hairline crack from right-hand stud hole to center of insulator. The surging will tend to become worse as the engine warms. 2. Adjust the valves. 0.006" Intake and 0.008" Exhaust.

Honda Tech said:

Assuming you purchased the correct carburetor, Honda P/N - 16100-Z5T-901. If you do have the correct carburetor and lets assume fresh fuel, I would look at two things to start with. 1. Insulator between carb and cylinder. Look for hairline crack from right-hand stud hole to center of insulator. The surging will tend to become worse as the engine warms. 2. Adjust the valves. 0.006" Intake and 0.008" Exhaust.

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valve clearance and that insulator check out OK. i actually replaced that insulator after i got the pressure washer several months ago.
One thing, the exhaust really Stinks. almost burns your nose.... still idles GREAT, put it up to full throttle, surging starts... half choke, surging stops.

Were you able to confirm that you installed the correct carburetor? It is running to lean. You either have the incorrect carburetor and/or an air leak. What does you spark plug look like.

Honda Tech said:

Were you able to confirm that you installed the correct carburetor? It is running to lean. You either have the incorrect carburetor and/or an air leak. What does you spark plug look like.

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Spark plug only has about 15 minutes of runtime, definitely lean.

Removed the carb yesterday, I soaked it again in parts cleaner for 24 hrs, sprayed it out,ran my little wire brush through every hole i could find put it back on and she runs beautifully now. Not a single surge out of her for 30 minutes.