Fast idle DCH340 carburetor suspecting BCDD issue

[aussietom]

Hello everyone,

 

I have been searching the forum and the web for diagnosing a fast idle.

The final part to be removed from the main carburetor (Nikki DCH340) body is this component (image). It sits behind the BCDD and it looks like the BCDD plunger should push on it (and subsequently move). My manual and rebuild kit don't identify it nor display it in the exploded view. Does this component come out? I have soaked it in petrol and given a few gentle taps but no success.

 

Background:

My 1979 Stanza has been sitting for about 10 years and intermittently started and ran. I'm planning to get it back on the road and it has developed a fast idle. Long story short, I have stripped the carby to be ultrasonic cleaned and put a rebuild kit through it.

 

 

This is is the view from underneath it, where it is protruding into a bore.

 

 

 

Edited September 1, 2023 by aussietom
Changed BCDD from BDCC

[datzenmike]

 

There were several methods of resetting the BCDD after use. It's the round thing with six screws and rubber cap in center covering an adjustment screw. Just turn counter-clockwise to reduce BCDD sensitivity.

 

Looks like yours controlled by a vacuum line or solenoid? I think the solenoid is controlled by a switch in the speedometer when it drops below 10KPH.

[aussietom]

Hi datzenmike, thanks for the reply.

 

Yep mine is controlled via a vacuum line. There is no solenoid like others I have seen.

I think my BCDD is fine though, the plunger moves in and out smoothly.

 

Its the brass component (first image in my first post) in the body that appears to be seized (unless it is meant to be fixed firm). Are you aware if this part moves?

[datzenmike]

The BCDD is a 'carb within a carb' in that it supplies gas and air to the engine under strong deceleration conditions to relieve a severe over lean mixture where the fuel does not burn properly causing elevated hydro carbon emissions. When the intake vacuum lowers, it is supposed to turn off. There should be a sensitivity adjustment somewhere that turns it on and off?

[aussietom]

Yep I can see the sensitivity adjustment. The plunger coming out the side of the BCDD though must 'close/open a port' or push on another plunger/needle and seat arrangement to supply the extra mixture. At the moment I can't see what the BCDD is actuating. Do you know if the brass item in the first image can be removed from the carby body?

[datzenmike]

Ive only worked on the one I posted a picture of. I've seen them deleted and a cover plate put on. Deleting it might not disable it. It may be stuck on or set too sensitive that it's too easily triggered.

 

When working it's hardly noticed because it happens when slowing down from  high speeds or high revs in a lower gear. 

[aussietom]

Thanks for the help datzenmike. For anyone interested, I believe the brass surface is locked into position and does not move under normal operation. I've decided I will delete the bcdd (replace with blanking plate), this should eliminate another variable by disabling its operation.

[datzenmike]

Just make sure that it's blocked off internally and not simply covered over and on all the time.