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:

1. 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.
2. Cleaned/polished float needle seat (was overflowing from overflow before doing this)
3. Tear down again, installed Shindy jet kit 03-028. (trying to fix black smoke at idle)
4. 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)
5. 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?

[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/ho...x-a/carburetor

Here is the kit for sale on eBay , partzilla looks out of stock .
https://www.ebay.com/itm/18452138542...ampid%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/showthr...ebuild-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/ho...x-a/carburetor

Here is the kit for sale on eBay , partzilla looks out of stock .
https://www.ebay.com/itm/18452138542...ampid%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

Tried a new plug, this is after a short ride and some idle time.


Also did two compression tests first cold, second hot. It is slightly low 160, minimum is 164.



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?