weststar

K so Just bought an 81 rx7 with stock nikki carb and it has a few issues. since there is a ton of vacuum lines to and from this thing I was looking for a starting point to look from. So lets rule out some symptoms. There is definitely no loss of power, the engine runs smooth, idles well when cold, doesn't blow black, or any color for that matter save for a shot of blue at first start up. once warm it revs freely with no problems. Throttle response is better than my T2 13b.It will idle smoothly eventually but what its doing is really weird.You can rev the engine and it will do it in gear driving or in neutral.So rev it up and it revs normally then on decel it starts to chug. It sounds like a misfire almost but associated with the chug is a loud (when the breather top is off ) vacuum leak noise the engine will do this for a random amount of time (few seconds or until I get sick hearing it and jab the gas real quick) sometime jabbing the throttle will make it stop first try sometimes it takes a couple tries. Once the vacuum leak noise goes away the engine returns to a smooth idle.
Since I've been running a 13b turbo for the last few years with fuel injection this whole carbed rotary thing is new to me. Any help would be muchly appreciated.

OldMazdaProfessional

First off, know that all those vacuum lines at the base of the carb on the port side are attached to the insulator under the carb and they stay when you do a carb R&R.
Second- it's quite possible from what you are saying that you are experiencing normal operation or it may just need a slight TPS sensor adjustment.
The question is, how does it drive? Any power loss? Bucking or hesitation as you get on/off throttle?
Or is your concern just the noise at neutral throttle, because that may be 'normal'.

weststar

I found the issue, its sticking shutter valve (coasting valve) there is no power loss and only a little blue smoke at first start up after that it runs and drives great save for the shutter valve. When the shutter valve sticks open its has very little power and a rough Idle. It sort of makes the neutral throttle noise that my 13b T gets but it doesn't go away and very eccentuated. Then the shutter valve closes and it's fine until the shutter valve opens again at which point its will either do what its supposed to or stick open. Thinking of doing the rats nest delete and just putting an aftermarket cat on it, Even though we don't have Air care where I live

OldMazdaProfessional

Right, the diaphragm goes bad in those, pretty common problem. The shutter valve physically closes off the rear rotor from the carb venturi and bypasses a bit of air past the carb to lower vacuum in the manifold.
There's one on eBay right now. It's an easy part to change, just lift the carb up and remove 2 small bolts, slide the linkage off and slide the new valve on.
EZ fix.

Mazda Rx7 Rx-7 New 12A 12-A Shutter Valve With Gasket 1981 To 1985 | eBay

Kentetsu

Most of us just remove the shutter valve altogether (unless you have emissions requirements, which you probably do). You get to have the sunshine, but have to deal with the green *****. lol.



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