2004 Honda 20 horse four stroke stalls in idle

I purchased the boat / motor used, and was told by seller that the motor stalled when running in forward at idle speed. The motor starts fine, warms up and idles down fine, runs fine at all speeds - until running at idle. It will run for a few minutes at idle then just shut off. The motor starts right back up again and runs fine at any speed, until idling again, then it'll start right back up, and take off... when it shuts off it does not stutter or shake as if it's running out of fuel or dying a slow painful death. It just stops as if I shut it off intentionally.

I have removed the carb and cleaned, but it looked fine. I checked the bystarter for continuity and it was ok. I was not able to measure it's movement however. I have had the boat a year now, so I know it's not the fuel, especially since it did it to the previous owner. It's not a major issue, but sure is a pain... Any ideas are greatly appreciated. Thanks much - kyounger

My bet is the carb needs to come off again - for a more focused cleaning. This time, have a really close look at the bottom of the brass tube that runs from the bottom to the top of the carb. Hidden INSIDE that tube is a tiny jet, not much bigger than a whisker hair. If fuel can't get through that, the motor is not going to idle correctly when warmed. Acid test, the one to make absolutely sure that jet is clean, is to be able to see light when sighting through that tube's length. No light at the end of the tunnel, it's not clean. Make sure your wearing your glasses for this one..... or go find somebody with better eyes. Do not force anything through that jet or you'll be buying a new one.....

The second place is a totally hidden jet in the passage between the float bowl and the bottom of the bystarter area. Dirt in that area will cause low speed headaches as well. Carb cleaner and forced air are about the best bets here.

Speaking of brass tubes, Alan gives you good advice about the "jet set" tube. I would add that it goes up through the carb throat and is sealed by a small oring. Make sure you have a new oring on hand as it is important. Use a pocket screwdriver to gently pry, with a twisting motion, the tube from the top of the carb throat after you have removed the emulsion tube. See items 16, 17 and 8 in the link below.

https://www.boats.net/catalog/honda...l-range/20hp/bf20d4-shsa-2004/carburetor-auto

That tiny tube can also be cracked so look it over very carefully and replace it if so.

Another brass tube is the accelerator pump spray tube. It protrudes into the incoming air stream at the carb throat entrance. It has a very tiny spray orifice at the tip that clogs. If that nozzle clogs, it will cause a variety of running issues. If you remove and disassemble the carb for cleaning, it only makes sense to at least check that it is clear. You do that by locating the passage in the main body that feeds the tube and spraying cleaner into it.

With the throttle linkage held wide open by using that little screwdriver again as a prop rod, spray the cleaner into the passage that feeds the accelerator pump spray tube. The fluid should exit the tiny nozzle and stream out past the open throttle plate 6 feet or more. YES! 6 to 8 feet is the stream that you need to see to ensure that the spray nozzle is fully open. If not, you will want to use the spray straw that comes with the cleaner can and carefully insert it through the rear of the carb throat and "backflush" that little nozzle. You may need to do this several times to clear a clogged nozzle.

While it doesn't seem logical that an accelerator pump circuit can cause hard starting, stalling and low power, this particular carb will do one or all of this if that nozzle is even partially plugged.

Also, while you're at it, carefully remove and drain all liquid from the fuel chamber. Item 2 in the link below:

https://www.boats.net/catalog/honda/outboard-by-hp-serial-range/20hp/bf20d4-shsa-2004/fuel-pump-2

The fuel chamber acts as a water catch and needs to be drained every so often to prevent problems.

Another reason the engine may stop while idling in gear is simply a low idle setting. These outboards idle best around 900rpm in neutral. Some people find this too high for trolling and will lower it. It could be as simple as that.

Good luck.

Thanks so much for the great information. I'll carefully start looking into these areas. Again thanks so much! Kyounger

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