76 Datsun B210 Carburetor issues

[JMAYHALL]

Has anybody had a carburetor that revs up the RPMs when shifting? At a stop light it purrs like a kitten, but when I accelerate and put in the clutch to shift it sounds like my foot is still on the gas pedal and the RPMs shoot up. This is my second carb and I've been to two mechanics that still can't figure it out. It also seems to lack power at the beginning of acceleration but once it gets to the high end of the RPMs it starts to pull power like it should, but you really have to stay on the gas pedal and hold that gear longer. I'm sure the cars next to me think I've never driven a manual trans before. Any ideas?

 

Thanks for your help,

Jeff

[distributorguy]

Huge vacuum leak.  Something's not sealing somewhere.    

[datzenmike]

No one has ever explained how a vacuum leak, which would lean out and destroy the idle mixture, makes the RPMs go up. Any time a hose has come loose on my L20B it won't or barely chugs at idle. It's usually the vacuum line to the air filter.

 

Find the BCDD, it looks like this...

 

 

If you have this style take the rubber cap off and turn the screw counter clockwise a couple of turns. Keep track so it can be put back if it doesn't help. The BCDD prevents the engine from returning suddenly to idle during high vacuum conditions like deceleration or between shifts. It's a carburetor within a carburetor that supplies gas and air to prolong the return to idle. It's triggered on and off by a vacuum signal. It may be set too sensitive.

 

I assume you've checked that the throttle cable is sliding smoothly and not hanging up when the engine torques to the side when accelerating?

 

There should be a slight increase in power when the secondary kicks in but if extremely pronounced maybe the primary is not providing enough gas? partly blocked jet causing it to be lean and power lacking?

 

 

[dat521gatherer]

Check your throttle return spring. Mine broke the other day while driving and made my rpms go crazy between shifts.