Honda GX620 Struggles under load

[RMR]

I've got a long time pressure washer retail/repair customer with a machine that is not able to produce the needed rpm to get the pump up to pressure. It is a Honda GX620 QAF1 GCAD 2041087 . It had a new carburetor installed, before I took possession of it. It has good compression at 170lbs/160lbs. and passed a leakdown. The RPMs are set to 3640-3720 (it doesn't like to sit still). Runs like a champ without a load, but as soon as the load is applied, it drops to about 2900 (Needs 3400 at the pump), and starts to hunt and backfire a little through the carb. and it is running rich. What am I missing here? Defective carburetor? He brought me the original carb today, that I will be running through the ultra sonic tomorrow to reinstall....

[RoyM]

Did you check valve clearances? Ground the spark plugs in turn, the rpm's should drop off equally. If nothing happens on one it is not producing power. If the engine quits the other one is the culprit.

[KE4AVB]

What pump are you running and do you have the correct tip installed? The tip itself needs to rate for the maximum gpm of the pump or one size larger. Here one of my customers is running Cat 66DX40GI1 using the 390. If I install the 4.0 gpm tip it overloads the engine. If switch to 4.5 gpm tip then it run at full rpms and at nearly full 4000 psi.

Suggest backing off the pressure regulator to 50% and work from back to the pumps max rated pressure. You do need a pressure gauge for this. That way you can tell where the overloading starts at. Also the 620 did have two different carburetors (depends on serial) and the wrong one may have been installed. Plus there is three different main jet sizes that can be used. If it is over rich you may need to downsize the main jet.

[RMR]

Did you check valve clearances? Ground the spark plugs in turn, the rpm's should drop off equally. If nothing happens on one it is not producing power. If the engine quits the other one is the culprit.

Valves are set, However I did not ground the plugs, looking for a weak cylinder. I will though...

[RMR]

What pump are you running and do you have the correct tip installed? The tip itself needs to rate for the maximum gpm of the pump or one size larger. Here one of my customers is running Cat 66DX40GI1 using the 390. If I install the 4.0 gpm tip it overloads the engine. If switch to 4.5 gpm tip then it run at full rpms and at nearly full 4000 psi.

Suggest backing off the pressure regulator to 50% and work from back to the pumps max rated pressure. You do need a pressure gauge for this. That way you can tell where the overloading starts at. Also the 620 did have two different carburetors (depends on serial) and the wrong one may have been installed. Plus there is three different main jet sizes that can be used. If it is over rich you may need to downsize the main jet.

That is his department, and he's been doing this for 30yrs. We've been working together on this, so I'll say the pump side of all this is right.

[KE4AVB]

That is his department, and he's been doing this for 30yrs. We've been working together on this, so I'll say the pump side of all this is right.

Then I can't be of any help for if the pump is something like 4.5 gpm @ 5000 it would be too be big for that 18 bhp engine.

I'll just step out of this thread and ignore any further posts.

[Merkava_4]

Also the 620 did have two different carburetors (depends on serial) and the wrong one may have been installed. Plus there is three different main jet sizes that can be used. If it is over rich you may need to downsize the main jet.

That's the answer right there.

[RMR]

Not sure how I missed that above. I will clean the OE carb and reinstall to see if that's it. Thanks.