1986 250SX black smoke at idle

[shortline10]

Screwing it in leans / out richens .
If you get no differences while turning it , either that circuit is clogged or your choke plunger isn’t seating all the way in and that will definitely cause a rich condition .

[car46999]

Well I tried a lot of things tonight. Pulled the carb and checked/verified the following.

-Pilot screw is assembled correctly screw with o-ring near the slotted end. Spring on the intermediate diameter, washer then small o-ring.
-Pilot screw orifice is not clogged, compressed air is felt through the small hole in the main bore.
-Removed the slide assembly, verified that the needle is on the second clip down.
-Rubber plug inside the fuel bowl area is definitely installed and seals well.
-Measured the float level, with my shim I put in a few days ago it is at 18.5mm.
-I noticed that the choke plunger didn't slide in and out as easily as I would like. So although it is different I gave the Moose one I have a shot. I also installed the Moose spring as it was a few mm longer. Lubricated the choke plunger and seal with a little Superlube grease. Verified that the choke plunger seals by blowing air through the circuit and actuating the piston.
-Ran it with the choke cable disconnected, pressed down on the plunger to ensure it was fully closed.

-While messing with the choke I thought I found the smoking gun. It was not it, but I figured I would share this anyway since I had not seen anyone mention it as a possible choke open cause.
On the handle bar where the cable goes into the choke slider there is a metal clip that holds the cable. It has a few adjustment locations. On mine the metal clip had been bent in such a way it preloaded the choke beyond what Honda designed it to be. This could cause a choke open issue at all times.

This first picture shows the adjustment slots and my bent clip.


How it looks in the lowest preload setting.


But none of this fixed the problem. It is still smoking at idle (idle only). It is fuel, the plug is still dry and sooty.

I do not think it is a choke circuit issue because this would likely make it smoke at all RPMs. I confirmed this by activating the choke at a higher RPM and it started smoking. Same smoke color as the idle smoke.

I am stumped. I believe it has to be a circuit that is active at idle. If that is true it would not be the main, choke, pilot. I believe those are active across all RPM ranges. If I am correct, per the service manual that leaves the primary jet and slow jet.

Can anyone confirm if the slow or primary are idle only, or active at idle?

[shortline10]

Have you tried turning the pilot screw while it’s idling in and out until you get the best highest idle possible ? If you get no differences while turning it then their is an issue with either the pilot jet or choke circuits .

To answer your last question the pilot jet is for idle and just off idle only , the main jet kicks in at about half throttle , the needle kicks in just off idle to about half throttle .

[car46999]

I have turned the screw while idling, it seems to have little effect based on listening to the RPM change. I have started at 1.25 turns out, can run it all the way in slowly and the change is minimal. If I go back out past 1.25 turns there is a point where the RPM drops. But it does not stall.

I might pull the carb again and check the slow/pilot passages with compressed air while adjusting the screw.

Thanks for the replies and info.

[car46999]

New finding tonight, the throttle blade is bent. The last person who worked on this did a number on it. I know this could cause idle issues, but I am not sure if it would cause a rich idle?
It put up a fight getting it out, so I need to find a replacement.

I also removed the pilot(slow) and primary jet. I also removed the tube from the primary jet, and re-cleaned it.

I pulled the plug from the starter fuel circuit, and there is a brass tube in there. Does that one come out? How?

[ATC200X4716]

It sounds like you are getting fuel from another source in addition to the pilot jet because if you run the mixture screw in the fuel is turned off and the engine will die. The air running under the throttle plate causes a low pressure pitot effect pulling fuel up through that tiny little hole at the bottom of the venturi. It sounds like you are getting fuel up through the main circuit or choke, though not with corresponding additional air. Did you try lowering the float a little? I would definitely get the throttle plate straightened out. Everything about the plate, venturi, idle fuel port placement is precise such that at idle the pitot effect is right on pulling fuel up through that tiny hole.

[car46999]

ATC200X4716

I might have to try to flatten the plate out, I cannot find a new one. I think what you are suggesting makes sense, it might be throwing off the pressure that is present in the bore of the carb. Causing fuel to be added when it should not be.

I will see how well I can mend the plate and report back.

[car46999]

I am now pretty sure that the throttle plate is the issue. The bend was on the lower part of the carb main bore, and when the plate is closed it looks like it lands right between two small holes in the 6 o'clock position. So what I think was happening is with the bend the plate it never closed creating a low pressure over both holes all the time. This low pressure over both holes was drawing fuel from both circuits at a rate greater than it should have for that throttle plate postion.

I say I am pretty sure because while I was able to get the plate flattened out using a press and some large metal plate to help prevent over flattening, the throttle plate shape is not perfect. So it is not fitting well in the bore when closing in the bore.

Does anyone have a throttle plate they wish to sell? I could not locate a new one from Honda, and found none on ebay. Otherwise I might have to buy a parts carb from ebay.

Will the throttle plate interchange with any other non 250sx and 250es QA carbs?

Are the throttle plate the same on the QA04A 1985-1987 TRX250 (Fourtrax250)?

Thanks

[car46999]

Update on my progress: big thanks to FlyingW, he is sending me a throttle plate.

As soon as I get that and get it installed I will update the status.

[car46999]

Got the throttle plate in the other night, took it for a spin and let it idle that night. Brought it back out today during the day, and it appears that the throttle plate was causing the rich idle issue.

Thanks gain to everyone for your suggestions, and big thanks to FlyingW for sending me a new plate.