charlie1

85 GSE, before I bought car it looks like someone put a cherry bomb muffler on, everything else is stock. Is this muffler causing the back firing on down shifts and clutch in between shifts? Can I get rid of this by going back to a stock muffler?
Thanks
Steve

JoeyMazda

It could be your shut off valve. Sometimes it goes bad

88verttop

what does a shut off valve do and how does it cause back fires?

JoeyMazda

If the shut off valve stays open, it's going to cause backfiring. It is meant to prevent backfiring.

charlie1

Hate to ask this, but what shut off valve?

bstrange99

If I remember correctly, a stock nikki has some sort of idle down solenoid that's supposed to control the idle drop when you let off the accelerator. I'm sure you could find the info on it in the fsm.

DivinDriver

iTrader: (15)

Early stock Nikkis used a 'dashpot' to prevent the throttle plates from sutting too fast; looks like a vacuum diaphragm on the rear of the carb, but has no hoses connected to it. It's whole job was to slow the throttle closure; it really only comes into play from around 3500rpm on down. I don't know if they were still used on the 85's.

The shutter valve (probably Joey's 'shut off valve') appeared in later 1st gens, as part of the intake mainfold. It's a vacuum-controlled butterfly, like a second throttle plate, that is way down in the rear rotor runner of the intake manifold, and it shuts off air/fuel flow to the rear rotor under deceleration. They're pretty notorious, from what I've read around here, for sticking or developing leaks.

There's also the anti-afterburn valve (part of the air control valve) and the solenoid that controls it.