Here's the 'patient'....1984 RX-7 GS, completely stock. Carburetor rebuilt by the Rotary Shop in Fort Worth on Vickery Blvd 4 years ago. ACV replaced last year so the car would pass emissions for inspection in May 2006, which it did. New NGK BR8EQ-14's just went in today and it had no effect on the problem which is thus.

Coming to stop at a red light or stop sign or what have you...the engine will idle smooth at 1000 rpm (new plugs added 150 rpm to idle which I haven't worked on yet, but the problem is the same, so the engine speed doesn't seem to be a factor, but I could be wrong...) for about a minute and then suddenly will start to stumble like it has a SBC with a big cam under the hood. The rpm's drop with the stumble and they jump up and down from 700-950 rpm's. It never ever will die while doing this. You give the engine a little gas to get the revs to 1500 rpm and the idle goes smooth again and stays at 1000 or so rpm once again for yet another one minute period. It will do this for as long as you want to play the throttle.

Thinking vacuum leak, I got some of my favorite all-purpose spray, Gumout's Carb Cleaner, and misted some spray at the air cleaner. The idle smoothed and jumped immediately and then went back to the stumble as soon as I stopped....I then proceeded to hose down the carburetor 360 degrees and all over the rat's nest and the stumble kept on going like I wasn't doing a thing.

I'm now at the point of wanting to try the classic "tear-down-the-intake-manifold-and-carb-and-reinstall-it" magic cure which works 90% of the time on these old cars. Since this is my daily driver until I get Furious back on the road, I can't exercise that option just yet. Before I go down that drastic route, I'm hoping someone out there has had this problem and can give me some simpler solution involving solenoids or check valves I don't know about...

Thanks to any and all who can help in this dilemma...

 

Shutter valve? Idle mixture? Vacuum advance connected to constant (rather than ported) vacuum source?

I dunno, that's all I got right now and I'm late for a meeting.

 

this sounds just like mine. I have 85 gs with racing beat exhaust. when the idel is below 1000 rpms it sounds like a big block with racing cam. rev it up past 1500 and it runs smooth. i also change plugs fuel filter and check for vacume leaks. maybe carb rebuild is next?

 

First, get your idle down to 750 and check your timing. At 1000 rpm, the vacuum advance is kicking in, changing the timing. Since the idle smooths out spraying carb cleaner in the air cleaner, check the vacuum hoses that connect to it and the bimetal valve at the base of the air cleaner that goes to a vacuum source.

Also check the float levels of both fuel bowls and the filter screens in the banjo bolts where the fuel lines connect to the carb. You might run some Seafoam through the fuel tank too.

Other than that, the only suggestion I can give is to break out the FSM and follow the troubleshooting section step by step.

 

I never thought of the vacuum advance messing with idle...I'll begin the tune-up this weekend and follow the idle set-up instructions I saved from Sterling back in the day. Thanks for the tip.

As an asides, it has now moved up to 1500 rpm with the be-bop happening in the 1100 rpm range...looks like the Sea Foam is still getting junk out of the engine and/or carb.

Amazing what a fresh set of plugs will do for a rotary...that's an increase of about 550 rpm from before...

 

1500 is a large vaccuum leak. you should be able to locate that leak by spraying each hose with carb cleaner until you find the one that gives the response.

 

If your float level is just a tad to wide, it will give the same problems. I ran into this when I had just rebuilt a carb. If you replace the needle and seats, not all carb kits specs are the same. The floats may need to be adjusted one way or the other to get them right.

 

Been there, done that....from my initial post above as written:

"Thinking vacuum leak, I got some of my favorite all-purpose spray, Gumout's Carb Cleaner, and misted some spray at the air cleaner. The idle smoothed and jumped immediately and then went back to the stumble as soon as I stopped....I then proceeded to hose down the carburetor 360 degrees and all over the rat's nest and the stumble kept on going like I wasn't doing a thing."...

bad83, I think you may be onto something since the carb was rebuilt in FTW but actually installed by a shop here in South Arlington...since they got the vacuum and OMP lines wrong, I'll bet they didn't set that up right either...it's never really been as smooth since the day I dropped it off there 4 years ago. I personally haven't had the car except for the past year and a half when I had to play repo man to a buddy that couldn't pay for it after we had agreed on a price. Get everything in writing, even with very good friends. It made the whole ordeal a lot less stressful...

 

I missed that 360* remark, sorry.

If after messing with the floats and idle adjustments it still doesn't idle lower, another thing to look out for is the throttle plates. The first time I rebuilt my carb I got **** and unscrewed the throttle plates to scrub. When I put it back together, the idle went up about 400 rpm because I reinstalled one of the plates a little crooked so that it no longer closed completely. Took me a very long time to figure that one out. If they're reinstalled upside down it'll have the same effect because they're beveled at the closing edges.

 

Thread from the dead returns to complete the story....although the carb was only 4 years on a rebuild at that point, it WAS the problem. Sam at AnB Auto Center popped the top, did some quick cleaning and that was the end of the stumble and die routine.

The carb looked absolutely fresh from the outside. Never dismiss the obvious.

 

Good to know. Thanks for posting up, even if you did have to shake some dust off of it.





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Now, that's what I call followup!

 

Just giving back to the community!! It's been a long while since I've posted here in the 1stgen tech section.

 

I had the same problem with mine. Got new br8 plugs and reset the timing after changing the vacuum advance to the 2nd port. Reset the idle and played with the mixture a bit. It idles rock solid at 900rpm now and even starts easier. I'm running a sterling with 2gdfis.