pikeapn

Hi Guys, I have been diagnosing a fast idle issue with my rx7 for the last week (since I bought it), and I am starting to believe that the only thing it can be is the carb. Granted, I don't know much, but that just seems to be the issue.

I have a completely stock 1981 RX-7 with a 12A, Stock carb, and 90,000 miles. It doesn't want to idle anywhere under 1000rpm. It will idle very nicely at 1500 to 1800rpms, and rough at 1200. (This has been with some small air-screw adjustments.) But unless I lightly choke it, it won't run anywhere lower than that. It was actually worse (idling at 2k) until I found a small vacuum leak in the rats nest (loose tube), but it hasn't gotten much better. I ended up disconnecting/capping every hose on the base of the carb except the vacuum advance line going to the distributor. But still not much better. I also checked that large vacuum line next to the exhaust manifold, but it was fine. Also, the fuel pump is rated at 2.5 to 4 psi, and was recently installed with a new filter. The guy I recently bought it from said that the car did sit all winter. And I suspect that it wasn't driven much before that either.

The spark plugs are all new. I drove it over 3 hours to get home and it ran great on the highway the whole trip (but that was with the idle set to 1800). The gauges and temp all seemed great also. When I turn over the engine, It seems like there are 3 even "whooshes", but I don't have a good ear for that kinda thing, and I don't have a compression tester on hand. The car runs absolutely fantastic above 2 grand, and it doesn't seem to be low on power at all. So I am really hoping that it isn't the engine seals going. It goes all the way up to red-line just fine. The car smokes a little when I do a cold start, but from what I have heard, that seems to be normal. Some of the screws holding the brackets to the carb are stripped a little, and a few were missing (but I found them in the glove box and put them back). So, someone did a little adjusting or something to the carb.

Like I said, I have put a full week into this thing, and I still haven't figured out what the problem is exactly. But I am also fairly new to fixing this kinda problem. Any advice?

Cookboy

iTrader: (10)

Buy a carb rebuild kit from rockauto.com and rebuild. All the info is here. You'll be glad you did.

RotaryEvolution

iTrader: (30)

the carb kit and overhaul itself will probably do nothing for your high idle issue but you may notice things that may be out of adjustment once you get the carb off like worn/sticky throttle butterflies/linkage, improperly set curb idle screw, broken idle speed screw, misadjusted throttle cable, etc, etc, etc.

the later model nikki idle screw is a bit obscured under the accelerator pump/choke bracket so often it is overlooked. the early ones were kind of tough to miss with the big flat head screw punching you in the face.

pikeapn

Thanks for the replies! While looking through the archives, I noticed a thread about a carb manual online. I think the link is broken because I can't get to it. Does anyone know if this manual is still online somewhere? RotaryEvolution, do all the carbs from 1981 have the side air adjustment screw? I have a Haynes manual for '79 thru '85 models, but the carb it shows has both screws in the middle of the carb base.

I suppose that I should order an intake gasket as well while I'm at it (in case that is the cause of a vacuum leak). What other precautions do I need to take into account for its replacement. I read somewhere that you can put freeze plugs into the intake water ports. Is this recommended? I found that carb stripping write-up in the archives and will probably end up doing away with all the emissions crap as well as the cat and air pump. I kind of like simplicity.

pikeapn

Ok, I found a very old compression tester laying around the shop. I did a quick and dirty compression test and all three pulses are exactly the same for both rotors. The gauge's valve stuck in the open position after the first rotor, so I can't get a reading for max compression on the second rotor. The one I did get was well over 100psi. (that was the rear rotor.) I am planning to get a good gauge one of these days.

Also, the engine doesn't seem to be getting enough fuel at low rpm's. I have plenty of fuel in the bowls (halfway on both sight glasses), but I usually have to put a little gas down the carb in order to start it. This only happens if I let it idle to death while trying to make it idle as low as possible. Isn't this an indication of plugged low speed jets or something? I got it idling at 1000rpms today, but it was really rough and didn't want to stay on.

Also, is this the right carb kit that I have attached as a screen shot? It was on RockAuto.com ($32.79), I just want to make sure before I order it. I am kind of confused if there were multiple carbs or not. And I can't find any numbers on my carb. (I suppose I could take it off and find something if I wasn't so lazy.)

RotaryEvolution

iTrader: (30)

so you can get it to idle lower but it just wants to stall out? if yes then yeah, it's usually due to clogged jets and air bleeds.

Cookboy

iTrader: (10)

That kit will work. Search 'Nikki rebuilding tips' in the 1st gen archive.

pikeapn

Thanks for all the info!