Benjamin4456

Background:
So I finally got around to doing the FD alternator swap (which I must say is a well worth mod), and as part of it I took the opportunity to remove the rest of my rats nest. Prior to the swap, my rats nest only had the relief solenoid left, which controls the port-air vacuum pot in the ACV (separate of the split-air system which was disabled previously). Basically, when at idle the whole system sends the air from the air pump to the port air injection port which puts fresh air into the exhaust. This is disabled automatically as soon as you reach above 1100 rpm or so. Previously I had attempted to repair my frozen ACV - once I got the valves moving I had to repair the diaphragms with some silicone in hopes they would still work at least partially. Ever since, they've operated about halfway, but I have always assumed that they have been the source of a vacuum leak. Just to note, I have kept the PCV system throughout all of this, including to the present. As the new alt was a dual pulley, it meant the air pump would no longer operate and thus I removed it along with the rest of the ACV system. In theory this would mean I have eliminated my last vacuum leak, but apparently I have created other issues.

Problem:
So essentially the car isn't idling nearly as smooth as it used to, even with tuning. It takes much longer to warm up with the choke out (it has to stay above 1.5k for a good five to ten minutes otherwise it dies at idle). Finally, when returning to idle, the rpm goes down nearly to the point where it would die, then it stutters, eventually returning to a decent idle. It is drive-able, but just barely and it's obvious something is wack.

So while writing this I came up with a possible explanation. The block off plate I'm using for the ACV is rather thin and somewhat flexible, and was also found in the car when I bought it. I also reused the gasket I had made for the ACV before, and when I held it up to the plate, there was one hole that was barely covered - the plate isn't shaped very well and is a little too small. I put it on anyway since I wanted to know if the alt worked but in hindsight, that has a good chance of being the issue; I probably created a vacuum leak. The car did require a richer mixture to idle, and that would also explain why without the choke it would die if it wasn't warm enough. Hmmmmm.... Ain't it funny when you solve your own issue in asking others for help. Well, I don't know that's the issue, but hey, it's a good chance.

Something else, the car backfires much less while engine braking. Perhaps another indicator? There is also a slight (rather quiet) ticking sound that is coming from the area where the ACV used to be. Maybe an exhaust leak too...

If y'all got any other ideas, throw 'em on out here. I'm open to all suggestions.

Edit: Dang, is there a way to fix thread titles? It should be complete, not compete.

Benjamin4456

I forgot to mention that the lowest I can get the car to idle at is ~900 rpm - another tell-tale sign of a vacuum leak. I won't have a chance to work on the car until perhaps late today or Monday, but regardless, throw on out here any advice y'all have.

Just noticed I didn't mention what car I've got. It's a 1980 RX-7 GS with a later model 12A that was running the matching emissions rack. Racing beat header to a custom exhaust. The carburetor is an '82 modded with an '83? (not sure about the year) t-body to add vacuum advance, plus the matching intake manifold and air box. No other major mods excluding the rats nest delete.

I should also reiterate that I have kept the PCV, which also means I still have the idle compensator valve.

Benjamin4456

Well..... I was right. Bonus points to the forum for solving the issue without any reply's . The ACV block off plate I was using was too small, and that combined with the existing gasket had created a vacuum leak. I ended up cutting my own dual layer plate from some aluminum sheet, while also making a new, blank gasket. Started it up and it idled at 450 no problem. I raised it up to about 750 rpm without electrical load and it's happy there. Smooth idle and the rpm drop on throttle release has disappeared. The car still backfires much less during engine braking, which I suppose make sense since the air pump isn't throwing fresh air into the hot exhaust anymore.

I hadn't mentioned it before, but when I first start up the car the tach bounces around (not with engine rpm) for a while. It does eventually settle out, but I've only noticed it since the alt swap. Any ideas on what might cause that? It's not an issue; just rather strange.

Thanks

Update: I decided to do some research into the tach issue and it seems like it's fairly common and has many possible causes. It's not really a problem, so I'll just consider this thread closed unless anyone has any comments or questions.

LongDuck

Nobody responded because they didn't know what engine, manifold, carb/EFI you were trying to troubleshoot.

Glad you got it worked out, and those block off plates sometimes don't.