Robin Engine Nikki Carb mystery

[duane]

The PT422 starting surging, and application of some choke made it go away. This pointed to a carb cleaning. I took the carb off of the engine. To my surprise, there was a small hose connected to the bottom side. (See attached pix, I tried to circle the points of interest) It's connected to the idle screw adjustment. The idle screw was screwed all the way in, I mean seated. I proceeded to clean the carb and re-installed. The surging is gone, so the cleaning was successful, but why is the idle screw seated, and what does the hose connect too. It almost looks like it is connected to the crankcase, but I could not quite see because of the blower housing. The service manual and part manual make no reference to the hose connection. My experience had the idle air screw 1 1/2 turns out, but not on this one. It was provided this way from day one, and now seems to run fine seated.

It is a Nikki downdraft carb.

 

[westcliffe01]

Is it an idle air mixture adjustment ? And right now the screw is plugging it ?

The idle speed screw is on the left bottom of the carb picture unless I am mistaken.

 

[Gravy]

I've got a Robin, too. As far as I can tell, that screw and hose connection are a bowl drain for getting rid of water in the gas.

I took a jet reamer to the pilot jet on mine. It fixed the overly lean idle, but I went a bit too far. It's a bit rich at idle now. I really ought to get a new pilot jet and try again.

 

[duane]

Yes, I believe it is a idle-air screw adjustment. It's the screw with the spring on it. I have a kohler command vertical engine that uses a Nikki carb, that has an idle air adjustment which is open 1 1/2 turns for idle mixture control.

Gravy, the hose is connected back to the crankcase, so I'm not sure it is a bowl drain.

Hope someone can shed some light on it.

Duane

 

[Egon]

The hose is for crankcase venting.

 

[duane]

Egon,

Thanks, the crankcase vent is another line from the breather assembly to the air cleaner plate, this hose is about 5/8" diameter. This is a small hose, about 1/4" diameter. I've attached another picture. The screw is circled in red. It has a spring around the screw like an idle air adjustment, but has a hose that drops straight below.

I've attached a exerpt of the Robin parts manual for the carb, parts 32 and 33 are only listed as a screw and spring

 

[cqaigy2]

and the little hose is connected only to the carb with the other end hanging free? If so, i'd be surprise that it was idle control.

 

[Egon]

All I can do is Plead "A Seniors Moment" and let the more knowledgeable get on with things!

 

[Marks481]

I've got a Robin, too. As far as I can tell, that screw and hose connection are a bowl drain for getting rid of water in the gas.

Gravy wins the prize! Sort of. It is a carburator bowl drain.

Section 20 of the owner's manual (preparation for storage) tells you to drain the carbutator bowl, and attaches the following picture.

Since the second step of the procedure is to change to fresh engine oil, they make it drain into the crankcase so you don't dump gasoline on the ground.

 

[duane]

Gravy and Tim,

Thank you very much. I wonder why there is no idle screw adjustment. Also, those who are getting gas into your crankcase and believe it's you float jet, here is another source to check.

thanks again
Duane