I've got an 85 GS with stock nikki 4bl and recently I've had problems with the secondaries not opening most of the time and when they do they also stick open after the throttle is released. Is there a vacum line collapsing perhaps or is it all internal vacum problems? I've worked on a nikki 2bl on my B2000 and it was all internal vacum but I never had a problem like this. Any help would be appreciated.

 

could be the diaphram and/or spring itself..

 

My guess is the linkage is sticky. Have a buddy hold the throttle pedal to the floor, hose down both sides of the carb where the throttle shafts are with PB Blaster, and work the linkage manually.

 

I'm working on this exact same problem right now.

I've noticed something.

When opening the 2ndary venturis with my fingers (manuall, car off) they only open a wee bit. It takes a lot of force (me pushing hard w/ a screwdriver on the levers on the front) to get it to open any further.

Could this be my problem? is it supposed to open freely?

 

Please respond my car is in the driveway w/ all the carb stuff sitting about...

 

Venturis? I think you mean the throttle butterflies. But yeah, the throttle butterflies should open quite freely. It sounds like the linkage is binding somehow.

Just so you know, if you look down the carby the venturis or chokes are the hour-glass shaped restrictions pressed into the body of the carb.

 

Not unless the primary throttle blades are open.

There are two sets of linkage to the secondaries. One linkage is on the firewall side of the carb, and it prevents the secondaries from opening until the primary is about 50% open, and then it progressively allows more secondary opening, until the primary is at 100% which allows 100% secondary opening.

This is why when you work the secondaries by hand, you MUST have the primaries open fully.

The other linkage is on the radiator side of the carb, connected to the diaphragm assembly, and this is what actually opens the carb. A passage goes to one of the primary venturis. The more air flows through the primaries, the more vacuum is created in the passage, and this vacuum (not manifold vacuum) is what opens the secondaries.

 

Doh! Peejay's right. The primaries need to be open before the linkage will allow the secondaries to be opened.

 

Ok so i open the primaries, the 2ndaries do open.

what more could be wroing?!?! ack i dont like this.

I've wd'40d' a bunch more stuff on the outside, lemme go try it out.
peace

Geoff (thanks for the help guys)

 

They open? So what exactly is the problem still?

 

Thanks, Peejay, I'll give the PB a shot when I have some time off tomorrow and see where that gets me.

 

Ok, all the info have been really helpfull, thanks.
Any idea what could be wrong, when going full throttle it wont go past 5500rpm, like hitting a wall, still runing stock jetting, with big streetport.
had a IDA on, no problems..

 

Fuel filter possibly?

 

I'm a bit new to the 12A motors, but on the older 13Bs, there is a little plug with a screw slot in the side of the carb that goes into a vacuum passage that feeds the secondary diaphragm. the plug has a hole drilled through it sideways that the air must pass through. Sometimes, these plugs are screwed in in such a way that the hole through the plug doesn't quite line up with the vacuum passage on either side. Usually, the hole is oriented in a straight line with the screw slot, but not always. For this reason, it is a good idea to remove the plug and confirm the orientation of the hole drilled through it. Sometimes they will not line up when reinstalled, necessitating the use of some sort of sealant so that when you reinstall the plug, it won't move. If the hole is not lined up with the vacuum passages on either side, the secondary diaphrgm won't get as much vacuum as it needs to open fully. Anybody know if this same thing occurs with 12A rotaries? -WG

 

Well, the linkage is working fine and still no improvement. I guess I'll have to say goodbye Nikki, hello Mikuni.

 

Dunno if this has been said or not, but on the pass side of the car (carb) you will see a circular metal piece with 4 phillips screws, that is the diaphram and the spring inside. you might wanna chek it to be sure that the diaphram doesnt have a crack/hole etc in it

Lubing the linkage is a good idea, but be sure to hold the throttle open when you try to manually adjust the secondary linkage, it wont move smoothly unless the throttle is like 3/4 or so to the floor.

 

Or you could've done a search and found my post in the archives.

 

A diaphragm problem wouldn't be causeing the secondaries to hang OPEN sometimes.

That could only be caused by sticky/bent linkage, and/or if the small return springs on the secondary throttle shaft are broken.