IDLE ISSUES - RX-7 Nikki Carb

 
 

Troubleshooting the Nikki

You might think that mixing gas & air is a pretty simple job. However, depending on the demands of the car at the time, it can become a little more complex. Problems that appear only at specific times can indicate a different problem with the carb.

There are other sections on this site depending on your issue - FLOODING, IDLE, CRUISING, ACCELERATING, and ACCELERATION AT HIGH RPM. Feel free to check the page corresponding to your issue.

Idle Issues

A few things happen at idle in a properly-functioning carb.

  • The throttle plates are *very* slightly open - just enough to let the minimum amount of air through to keep the engine at idle speeds (less than 1000rpm). Adjusted by the idle adjust screw on the lower right side of the carb when viewed from the drivers side.
  • The "main circuit" is not in use, meaning fuel doesn't come through the primary jets.
  • Fuel is pulled through the idle circuit at the bottom of the carb into the engine - the amount is adjusted by the "mixture screw" in the bottom center of the carb when viewed from the driver's side.

Before doing anything, check your float levels! Everything here assumes that you HAVE checked them and they're spot-on. There's a sight glass at the front of the carb (bumper-side), as well as the rear (firewall side). With the key on and engine off, they should both be at the half-way mark. You WILL need a mirror to check the rear, and probably a good flashlight as well.

If your RX-7 is idling HIGH:

CHECK FOR VACUUM LEAKS. This is the #1 cause of a high idle. A disconnected hose is usually pretty obvious, and it can usually be plugged with a bolt. If there's no *visible* source of a leak, you may have a cracked vacuum line. The easiest way to check for vacuum leaks is to get a spray-can of some sort of quick-start spray (you can use WD40 if necessary) and start spraying around the vacuum hoses/carb with the engine running. If you find a spot that when sprayed brings up the engine speed, you've probably found the source of your leak. Replace the cracked hose and go from there. A quick note - this is often best to do with the air cleaner on. Otherwise, some of the spray can be easily sucked into the top of the carb, bringing up the engine speed.You *can* do it with the air cleaner off, which gives you more room - just make sure to plug any vacuum hoses that went to the cleaner, and don't be tricked by spray that's pulled into the top of the carb.

If you didn't find any leaks, LOOK DOWN the carb and make sure the throttle plate isn't open. Don't confuse the throttle plate with the choke - the choke's the flap of metal at the top of the primary side and it MUST be open to see the throttle plate for the primaries. You may need a flashlight. Another indication that the throttle plate is open is if you see fuel coming out of the jets. Resist the urge to push a screwdriver down there to move the plate - I've seen one chipped, probably from exactly that (a chip missing will not only send a chunk of metal into your precious engine, but also cause excess air to leak causing a high idle).

One item to pay special attention to is the shutter valve: I've seen more bad than good up here in our brutal canadian climate. Incidently, it's not fun to get at. It's located on the firewall side of the intake manifold, just below the carb. Here are a couple pics to help identify/locate it:

Shutter valve (mazdatrix.com) Shutter valve location (mazdatrix.com)
First image is the valve itself. Second is the location as seen from the firewall side of the car when the carb is removed. Mazdatrix has an excellent writeup here.

It's purpose is to shut off the flow of air/fuel during deceleration, presumably both for emissions and to keep the car from backfiring. Replacing it's not a bad idea, although I simply removed mine, plugged the holes, and reinstalled it in a position that keeps the butterfly open.

Assuming you've checked for leaks everywhere you can (and are quite certain there are none), go through the following procedure. This effectively assumes that the throttle plates are stuck open a bit. If you're POSITIVE they're not, you're really looking at a vacuum issue, whether it's a line, leaking gasket, etc and should skip this part.

1) Reduce the idle speed with the idle adjusting screw - located towards the lower right of the carb when viewed from the driver's side (you will likely need a long screwdriver). It's pretty tough to see. Turning it counterclockwise (unscrewing it) lowers the idle. Clockwise raises the idle. What you're effectively doing is either closing or opening the throttle plate.

2)If you've adjusted it all the way out and the throttle plates aren't closing, something else is causing the plates to hang open. Check:
-Throttle cable (the one the gas pedal's hooked up to). Make sure it's not too tight keeping the throttle open. Check the other cables as well to make sure they're not keeping the throttle open (hot start assist, cruise control, etc)
-Deceleration dashpot (big circular thing at the back (firewall) side of the carb with a little plastic piece coming from the bottom. It reminds me of a UFO for some reason (or a spinning top). The plastic piece on the bottom is something of a plunger designed so that when you take your foot immediately off the throttle, it gracefully closes slowly rather than snapping shut instantly - probably to ease the RPM's down more slowly to prevent a stall. In any case, if this is too tight, it may need to be backed off. Easy enough to do - crack the nut loose at the top, and then spin the entire apparatus - it's threaded. Tighten the nut afterwards.
-Throttle opener. Not 100% sure but if I'm remembering right it's for the A/C. Basically receives vacuum when the A/C is on and raises the idle by lifting the "hook". Can be adjusted, or if you don't have A/C, disconnected and hose plugged.
Taken from carb manual

If you've checked all of those and the throttle plate still isn't closing... something else must be binding. Check around and see if anything's caught.



If you've ensured that the throttle plate's closing and you STILL have high idle, you've got a vacuum leak.

If your RX-7 WON'T IDLE below a certain rpm:

-You *may* have a weak engine. Do a compression check. PLEASE do a search on how to check compression on rotaries - some piston-engine testers are a bit long and may chip/destroy your apex seal. They also don't read correctly for rotaries. If you don't have a tester, you can do the "remove a plug and listen for a uniform sound" test. If you haven't done it on a vehicle before, again, read up first.
-You *may* have a weak ignition. Check all the plugs for spark. If the plugs aren't in great shape it may be worth replacing them anyway - it's very possible for a spark to look great when it's out of the engine, but when exposed to compression to not spark well or at all.

Finally, you MAY not be getting the proper air/fuel ratio.

1) If the engine will "kinda-run-but-stumbles-and-wants-to-die", make sure the engine is WARM (operating temperature), start the car, and get an assistant in the car to blip the throttle every so often to keep it alive so you have time to look down the carb. Make sure there isn't fuel pouring out the jets (except when the throttle's blipped by your assistant of course). If there's fuel pouring out, you've got a flooding issue.

2) If it's not flooding, adjust the idle mixture screw. Clockwise (in) leans the mixture, counterclockwise (out) richens it (adding more fuel). You might get lucky and tweaking this a little richer or leaner could solve the problem. If it's beyond that and you need to start from scratch... to adjust this, you typically start off adjusting the idle ADJUSTING screw high enough that the engine runs. Then turn in the mixture screw all the way (DONT overtighten, just snug), and then turn it out 2.5-3 turns. This *SHOULD* make it run a little rich at idle. Slowly lower the idle by turning the IDLE speed adjusting screw outwards until it starts to stumble, then turn the MIXTURE screw in slightly until it evens out. Keep repeating until at the desired RPM (about 750rpm I believe).

3) If the mixture screw's not doing a heck of a lot for you, something may be plugged. Time to take apart the carb.

 

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