Cleaning my BF20 carb (same as BF15)

[mf70]

After two days of trials on my "new to me" 2004 BF20, the next day the motor would not start. Spark was good. The plugs appeared wet, and my standard 2-stroke fix -- pull the plugs, blow them off, open throttle and pull it over to clear fuel -- did no good. No visible water in the oil.

I decided to clean the carburetor:

It needed it!

HOWEVER:

after two trips to the carb cleaning bench, it finally started last night on pull #2 with the FE system active, running until warm; however it would not run on idle at all without that. My assumption is that I have a clog in the idle pilot circuit. So...

1) Does anybody know the exact routing of the passages to the pilot screw? The output passage is easy to see - from the bottom of the needle cavity to the carb throat behind the throttle plate - but I don't see the supply drilling.

2) Can the carb body be dipped? Are there hidden O-rings still in the body that will be ruined? Is the grey color a paint that will be stripped or anodizing?

3) How much idle blowby normally comes out of the vent tube that extends to the intake silencer? I'm getting sharp puffs form the tube each time I turn the motor over.

 

[MattFL]

On the 50hp Honda, assuming that one might be similar; there are 2 holes on the carb bell. The bottom one goes to the pilot air intake. Shooting carb cleaner through there will pass it through the pilot passages where clogs commonly happen. Check if that carb has something similar, put carb cleaner and compressed air through it. That will likely help clean out the tiny passages. Just avoid getting cleaner on anything rubber or plastic.

 

[mf70]

The state of my knowledge:

As I understand it, the Honda carbs for the 15's on up have four separate fuel delivery circuits (numbers are from the Honda parts fiche images):

1) The FE (fuel enrichment) circuit, equivalent to the choke on conventional carbs: A piston #11 is lifted when the knob is pulled, opening a connection between the vacuum BEHIND the throttle plate and the intake silencer. A metered jet (one of the tall brass tubes on the body) is also uncovered, allowing fuel to be sucked up at the same time. When the throttle plate is opened, there is no vacuum, and no extra fuel is delivered.

2) The idle circuit: There is a passage parallel to the carb throat, reaching from the intake silencer, to the top of the inner jet tube #18, to a constellation of five tiny holes through to the carb throat, and finally to the pilot adjustment screw. Vacuum tapped from just behind the throttle plate and metered through the pilot adjustment screw sucks fuel up #18 jet tube. Idle throttle is set through the main throttle stop.

3) Main supply circuit: Fuel is supplied to the jet #13 and main nozzle #14 from the main jet #19.

4) Accelerator pump: additional fuel is actually pumped from the float chamber (through the second tall brass tube on the body) to a tube extending into the carb throat outside the throttle plate.

I think the idle circuit or the accelerator pump circuit is still clogged..

On other questions. the answer seems to be "don't use carb dip," but that's not confirmed.

 

[MattFL]

This must vary by model or year. My 1999 50hp does not have an accelerator pump, and it has a regular old choke.

 

[hondam]

The accelerator pumps were only used on engines with single carbs, the BF8-BF20.

You are right about the Carb dip, Honda does not recommend it. The recommended method is an ultrasonic cleaner with mild detergent. They claim that carb dip or even sea foam is too corrosive and can damage the sealant on the aluminum.