84 Yamaha 4hp bogs down at low speed.

[abledog]

It is used on a sailboat generally getting in and out of the marina. I back out of the slip, slow to idle, shift to forward and increase speed to low and the engine bogs down and runs a few seconds with very little power and dies. However if I increase the speed very rapidly to medium or fast, 75% of the time, it runs fine until I reduce speed to slow. If it starts to bog down and I slow the speed to idle right away it will keep running. It idles fine. If it bogs down and I let it die it will start again on the first pull if I use the choke. When it is running normal in medium or high speed and I lower the speed to slow it bogs down most of the time. The problem gets worse as the summer season advances however the problem does not seem to be temperature related since I use it well into November.
The engine number is 4lk 6eol322182. In 1989 I had the same problem but with medium and high speed. I took it to a Yamaha dealer who told me they rebuilt the carb and tried everything with no success. They finally replaced the carb with a 6E002 and it worked fine untill 2001. Then I removed the internal gas tank and cleaned it and the gas filter, rebuilt the carb.and internal fuel pump, replaced the spark plug and adjusted the fuel/air mixture but nothing helped. Finally I increased the fuel/air mixture drastically by turning the mixture screw 2 full turns cc. This helped a great deal with only occasional problems until 2004. It does the same thing if the gas cap is unscrewed so it is not a clogged air passage. The cooling system is fine as it spits water normally. It does not have a glass sediment bowl or points.

If anyone has any ideas about how to cure this problem I would appreciate hearing them.

Thanks
Abledog

[boats.net]

The 02 carbs usually solved that problem. But it sounds like you still have a plugged pilot jet in carb. The hole is so small in pilot jet the best way to clean is remove from carb soak in Combustion Chamber Cleaner or Sea Foam, blow out with compressed air and hold up to light to see if can see light through passage hole. Check the Main nozzle for cracks or splits on the end.

[Clive]

I have just fixed a problem similar to yours my Yamaha 3.5AC woul donlt run with the choke out if I pushed in the choke the motor would die, so I removed the card striped it down got hold of carb service kit changed all the bits in the kit give the rest a good cleam runs like ne now,I cleam carb can fix aload of problems I wish we could get ehs ea foam in the UK it sounds very good for a fast fix,