1986 250SX black smoke at idle

[car46999]

Looking for some suggestions on my black smoke at idle.

Bought a 250SX late last year and rebuilt the carb on it. Started with just a cleaning, then did all the jets.


This is the list of stuff I have done to the carb:

Tear down including removal of jets, ultrasonic cleaning for 8 hours+ (rotated during this period). Blow through all ports. Reassembled with original jets and new Honda float needle, Honda bowl gasket, Honda pilot screw oring.
Cleaned/polished float needle seat (was overflowing from overflow before doing this)
Tear down again, installed Shindy jet kit 03-028. (trying to fix black smoke at idle)
Moose Racing Choke Plunger Kit 46-1014 (I only used the seal, the plunger did not match the original) (trying to fix black smoke at idle) (choke moves freely)
Adjusted pilot screw. It makes a difference in running, but does not seem to affect the black smoke.


Ran it some more today, and it is still puffing black smoke at idle. It runs well otherwise, maybe slightly less power than I remember. But well.

If it sets idling long enough it will stall. It restarts without much trouble but will puff more black smoke.

I have been thinking maybe float, but wanted to get another opinion. I see many similar posts (86-87 250sx, 250es, 350x) with black smoke at idle, but no resolution.

Today I put a clear hose on the drain port of the bowl, here is the level indicated by the hose. Seems high?

https://i.ebayimg.com/00/s/MTYwMFgxMjAw/z/Ce4AAOSwg3Viw2F6/$_27.JPG?set_id=8800005007

[TrikeKid]

Float looks high to me judging off that line. Have you tried turning the gas off while it's idling to see if it clears up? That's what I would do to check for a float/needle issue.

[car46999]

Float looks high to me judging off that line. Have you tried turning the gas off while it's idling to see if it clears up? That's what I would do to check for a float/needle issue.

I did, and the behavior was not as I expected. I thought I might get a short period of good running and no smoke. Started idling really high, black smoke might have been better but RPM was really high.
I played with the petcock for a minute or so trying to find the sweet spot but didn't find one where the level went down.

[shortline10]

Make sure your not missing the rubber plug on the choke circuit .

[car46999]

Make sure your not missing the rubber plug on the choke circuit .

Rubber plug? Where is that located? The choke has a new seal on the piston/needle part.

[shortline10]

It’s part # 23 or you can buy the # 1 kit and it comes with all the rubber seals .
Basically it seals off the bottom of the choke circuit , just presses in the hole and the bowl keeps it in there .
With that open it could be flooding the motor .
https://www.partzilla.com/catalog/honda/atv/1986/atc250sx-a/carburetor

Here is the kit for sale on eBay , partzilla looks out of stock .
https://www.ebay.com/itm/184521385420?epid=21019477395&hash=item2af654e5cc&amdata=enc%3AAQAHAAAA8KNamkwK9qFtVAt8SomkbreoAJyvL KROd7%252FUpt1TF7lN7JW1%252BwJhxHzBlYMvEDBbmIb8sCc 7o%252BgIDJh5tun%252B042MRgD%252Fk32PrFFhFAQCTFKdh %252FMbHK6gesReYK%252BQimT04M5KjvWNTEPQs08aASreKFx VE873bJeGqTCbJbj5dlSd%252FYBXRcuhfNkPFtSfwgEdrg0t4 z6lgkQjktNlaENZbxmvGy1S8D8UWuCbhzOlW3JrILO63GLFJWG LuqktRA8%252B1p4wrqa8md0PywOzfIiBqU0WUPH8tPVBjTmXI jbPNERZdLwBPjD8dK8ttpmT%252BLsugg%253D%253D%7Campi d%3APL_CLK

[car46999]

Ah, I bought the #1 gasket kit, I am 99% sure I put that plug in. I checked the spare/old parts pile and it is not there, so pretty sure it made it in.

FWIW, I saw a post by Flyingw that said the plug didn't do anything. It was in his "ES-SX-350X Carb rebuild Tutoral".

http://www.3wheelerworld.com/showthread.php/161746-ES-SX-350X-Carb-rebuild-Tutoral

[deejaycee_2000]

8 hours + in an ultrasonic cleaner? That might have damaged your carb right there. Personally I would never have a carb in there for longer than 30 minutes.

[car46999]

It’s part # 23 or you can buy the # 1 kit and it comes with all the rubber seals .
Basically it seals off the bottom of the choke circuit , just presses in the hole and the bowl keeps it in there .
With that open it could be flooding the motor .
https://www.partzilla.com/catalog/honda/atv/1986/atc250sx-a/carburetor

Here is the kit for sale on eBay , partzilla looks out of stock .
https://www.ebay.com/itm/184521385420?epid=21019477395&hash=item2af654e5cc&amdata=enc%3AAQAHAAAA8KNamkwK9qFtVAt8SomkbreoAJyvL KROd7%252FUpt1TF7lN7JW1%252BwJhxHzBlYMvEDBbmIb8sCc 7o%252BgIDJh5tun%252B042MRgD%252Fk32PrFFhFAQCTFKdh %252FMbHK6gesReYK%252BQimT04M5KjvWNTEPQs08aASreKFx VE873bJeGqTCbJbj5dlSd%252FYBXRcuhfNkPFtSfwgEdrg0t4 z6lgkQjktNlaENZbxmvGy1S8D8UWuCbhzOlW3JrILO63GLFJWG LuqktRA8%252B1p4wrqa8md0PywOzfIiBqU0WUPH8tPVBjTmXI jbPNERZdLwBPjD8dK8ttpmT%252BLsugg%253D%253D%7Campi d%3APL_CLK
Ah, that plug. I bought the gasket kit #1, confident I put that in since it came in that kit. Double checked the spare parts/old parts and it is not with them.

[car46999]

Jumped back at this tonight and made the following observations/adjustments.

With the hose right up against the fuel bowl the level is just above the seam, 0.5 to 1mm above.
I gently rocked the bike and the level rose a mm or two above the seam. I assume the engine running would create a similar situation I created by rocking the bike.

At this point I took the bowl off and lowered the float level by adding a shim.
The level after this was just below the seam, and right at the seam after rocking the bike.

But, it is still smoking at idle.

[psujeepguy]

Probably as stretch, but could it be a weak spark creating an inefficient burn?

[ATC200X4716]

How does the plug look? black and dry? black and wet? tan?

[car46999]

Probably as stretch, but could it be a weak spark creating an inefficient burn?

Spark seems to be solid.

Here is a video

https://youtube.com/shorts/1lGRsYETrRw?feature=share

[car46999]

Plug looks to be dry and black. Sooty.
Plug 1
https://i.ebayimg.com/00/s/MTYwMFgxMjAw/z/Bu0AAOSwlrti2fyg/$_57.JPG?set_id=8800005007
Tried a new plug, this is after a short ride and some idle time.
https://i.ebayimg.com/00/s/MTYwMFgxMjAw/z/IXoAAOSwY~1i2fyi/$_57.JPG?set_id=8800005007

Also did two compression tests first cold, second hot. It is slightly low 160, minimum is 164.
https://i.ebayimg.com/00/s/MTYwMFgxMjAw/z/31wAAOSwyTpi2fyc/$_57.JPG?set_id=8800005007
https://i.ebayimg.com/00/s/MTYwMFgxMjAw/z/K8gAAOSwR3xi2f~F/$_57.JPG?set_id=8800005007

I read a few posts that said they adjusted the valves and it fixed their smoking. Anyone else adjust valves to get rid of black smoke at idle? I would add that the black smoke is only at idle. Off idle the continuous black smoke stops, with only a small puff when the throttle is at max.

[car46999]

Just recalled one more bit of information, while idling I attempted to adjust the pilot screw per the manual. I can run it all the way in until it seats (CW) and the motor just keeps running.

Just for clarification: screwing it in (CW) makes it richer, screwing it out (CCW) makes it lean?

[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.
https://i.ebayimg.com/00/s/MTYwMFgxMjAw/z/Pm4AAOSwXldi24X0/$_27.JPG?set_id=8800005007

How it looks in the lowest preload setting.
https://i.ebayimg.com/00/s/MTYwMFgxMjAw/z/KoAAAOSwz~Ji24X3/$_27.JPG?set_id=8800005007

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.