Reviving a 1980 Porsche 924

I normally fail pretty hard at keeping build threads going, but I'm currently down following ACL surgery so maybe I'll stay on top of it this time. At any rate, it will hopefully serve as some reference for 924 and CIS stuff in the future. I've been down the rabbit hole, and unfortunately it seems like peak 924 was 10-20 years ago. Lots of dead links and sites lost to the rise and fall of geocities and the age of the DIY knowledge share website.

This all starts with my 2000 Tahoe Limited. Great truck, but I also have a GMT400 K1500 and my GMT800 Suburban 2500, so the Tahoe went on the block...and sat. For months. Though I'm ultimately trying to thin out my fleet, I eventually started looking for trade opportunities, and I found a Facebook ad for an old 924 "runs, been sitting for years. Needs maintenance. Will trade." I'm an E30 guy at heart, but I do love 924s and still have the 944 project that I got from DrBoost that I continue to chip way at. So, long story short, I loaded up the Tahoe, dragged it to Iowa, and came back with a 924.

Does this count as downsizing? I'd like to think so.

In the process of the trade, I learned quite a bit of the back story of the car, and suddenly a trade-and-flip started to look like a car I want to save. I'm such a sucker.

The original owner bought the car in Illinois in 1981 as a leftover 1980 model. they drove it up until April of 1991 when it went to the second owner in Davenport, Iowa. This owner drove it basically as a sunday car to the golf course until unfortunately dementia started to take hold. He lost the ability to drive, and the car was tucked away, forgotten, in his garage. It sat there for 16 years.

A family member eventually took some interest in the car, working to get it running again. They did brake lines, replaced/cleaned/repaired much of the fuel system, and replaced all the fluids to get it running. They drove it all of about an hour and a half before, as they put it, "turns out Porsche doesn't automatically mean fast". Interest quickly waned, along with family impracticality of a two seater.

So, completing the trade, I'm now the third owner of this car, which has what appears to be a true 40,000 ish miles on it. The mechanical condition of the car suggests that it probably didn't roll the 5 digit odometer, but I suppose it is possible. If the Pirelli P6 tires aren't original, they are certainly period-correct tires with the 1980s version of the tread pattern before this tire design was revamped.

The car is straight, but filthy. Layers of dust and dirt, plenty of scratches where it looks like boxes were stacked on the body, and a strange amount of crazing on the clear coat. It's a very nice example of a model that's usually pretty ragged these days, but not so nice that I won't want to drive it.

The interior is very clean, and while the dash is cracked, it isn't nearly as bad as most of these are. Nearly everything works, with the exception of the fuel gauge and a few burned out lights. It does run and drive, though it does need a fair bit of deferred maintenance which is what a lot of this thread will be about.

My goals are simply to bring it back to its former glory and make it a good driver's car. No engine swaps, probably no forced induction. I have other cars that meet those needs.

It fits right in next to my E30s - that 325es is also a bit of a rolling restoration at the moment.

So, what did I end up with? Besides the background story, the car itself is somewhat interesting. The build sheet is missing from the trunk area, but from what I can tell, it has these options:

• 4 spoke sport leather steering wheel
• Indiana Red Metallic paint, which was a $$ add option
• Dealer-supplied Lloyd floor mats (came with a dealer stamped receipt)
• 15" sport alloy wheels
• factory fog lights, which I have not seen many of on 924s
• Rear mud guards
• I think these were all standard by 1980 but maybe optional before this? manual sunroof, air conditioning, power driver and passenger mirror, power windows, 5 speed transmission

Things I have confirmed it does not have:

• The vaunted M471 package, which I think is what later became the M030 pack for the 944.
• Limited slip differential
• Rear disc brakes
• Sport seats
• Sport option suspension

What that means is the suspension is 160 lb/in front springs, 21mm front anti roll bar, 22mm rear torsion bars, no rear anti roll bar.

There are several versions of the 924 EA831 engine. Mine is the "best" US market engine, with a VC stamped block. That means I get a rollicking 115 horsepower due to the higher 9.0:1 compression ratio. Earlier NA cars are 8.0 or 8.5 depending on the year, making 95 or 110 hp. Euro cars are 9.3:1 and make 125 hp due to dramatically more ignition advance and no catalyst. It's not clear to me if the euro cars also have slightly larger valves, the internet hasn't been clear on this.

I did drive it around some after getting it home just to start making a list of needs. Dog approves. Interestingly, the car has been sitting so long that everything is stiff and just feels reluctant. After puttering around for awhile (remember, 40+ year old tires...) it has actually loosened up quite a bit. The engine, shifter, brakes, and even the steering all continue to get better with use.

It seems like the engine is what people disliked the most about these cars. This one runs, frankly, shockingly well. It's got an odd high idle when warm, but it is smooth, torquey, and generally has great manners. I've not had it over 3500 rpm because as far as I can tell, the timing belt is original.

You'll see a lot of stuff about how the EA831 is a VW bus engine. That isn't quite right, but it's close. Apparently the engine design came in through Audi, where it was used in the 100 sedan. VW already had a similar size engine, the EA827 I believe, but they did also put the EA831 to use in the LT truck/van. It may have been a 2.3 liter engine in that application. This engine also made it into postal jeeps, as well as a few AMC models as the 121c.i. 4 cylinder.

The EA831 is non-interference and the NA 924 version uses a unique Porsche cylinder head with a Heron design - the head is flat, and the power cylinder geometry is in the piston. The 931 (924 Turbo) used different pistons and a different head that had integrated chamber geometry. The biggest issue  with these cars appears to be the head design. They don't flow well, and they have very poor knock resistance. Few workarounds exist, and the poor head flow means that bolt-on modifications largely make no difference without first addressing the head.

What am I going to do with mine? I'm going to keep the CIS (I actually kind of like it the more I learn about it) and plan to get it running well. The usual tune up stuff, plus some ignition stuff to convert to European settings. Then we will see where we're at. The wildest thing you can expect to see in this thread engine-wise would probably be a lightweight flywheel, some basic head and manifold porting, and a cam, but that would all be way off in the future. I also own a 318is - my target is basically to match the performance of that car when it was stock. I think that should be very achievable, and it may even be able to do that after just getting it running well. The rest is just making stuff work well, like replacing suspension bits and whatnot.

So, there's the introduction. And of course, what introduction would be complete without a headlights-popped picture?

Lots of exploratory work since we have the thanksgiving holiday. Started with the basics, checked y'all kinda of stuff and made a list. It's long, but a lot of it is fortunately "adjust x thing" instead of "replace x thing".

Starting with the rear brakes. These drums are not self adjusting, and the shoes must be individually adjusted. Pedal travel seems more normal, but now that the shoes are engaging my guess is there will be a second round of adjustment once the shoes and drums are re-bedded in after sitting. Shoe thickness looks great, and bonus is the parking brake works again. Though the pawl doesn't seem to catch consistently. 

The fuel gauge also doesn't work. I checked the wiring, and grounding the gauge and low fuel light made the dash do its thing, so the sender is the issue. Tore it down to find the low fuel wire broken and the float had split from the jumper that changes the resistance. The ground strap is also somehow failed, so basically everything needed some attention. Not sure if it will last, but figured I would try it before buying a $200+ replacement. 

I did also buy new tires, some Fallen Sinceras in the stock 205/60r15 size. Most tires that fit these wheels are not performance oriented, but this strikes me as a car that might be more playful with a bit less grip, like a BRZ/FRS. 

The original tires end in date code 1 and are 3 digit. 12/79 build date, and the car was first sold in 1981 so I have to guess they are 1991 tires and were in fact replaced once. I think tires went to 4 digit codes in 2000, and the car was parked in 2003 anyway 

The P6 was a really good looking tire.  You could spot them from a distance, although they were usually on German cars. 
 

I'm enjoying this already!

So much 80s! When can I have a spin in it? 

@Woody I wish I could have found something that looked similar, that tread pattern really does just look right on the 924, and my e30s for that matter. It wasn't worth $320 per tire to me to get a vintage tread pattern though...

@Chappers this is jumping ahead in the thread a bit but I did something to the 80s alarm system and now I can't get it to start. I guess that will keep me from succumbing to the temptation of driving it around before it's registered. 

Lots of time spent researching parts ...some stuff showed up and about half of it didn't fit. I'm not entirely sure if it is a parts interchange problem, or if it's an issue of my car having been serviced with other parts over the years that aren't quite the same. But since everything appears original, I suspect the former. 

First off is the tie rods. The outers have a lot of play and look like water got in them. The inners aren't bad, but figured I would do it all at once. Unfortunately, the parts that showed up are a much different length than original, so now we wait another week to see if the second guess is more successful. 

I also bought new hatch struts. The connections are wrong for the light and defrost somehow, and I found out about the hatch delamination issue and the Jeep Wrangler alternative after the fact. So another round of struts, on the way. 

Checked a bunch of stuff that could have contributed to the high idle, but I won't know if I fixed it until I resolve the alarm immobilizer issue. This car is a 1980, which sources conflict as to if it should have a vacuum limiting valve. Mine had one. I hate hanging revs, and some of the lines were badly cracked and NLA, so that's all deleted now. 

I tested the AAV, it works like it should. 

The 1980+ cars like mine have an idle and WOT switch on the throttle. I'm not entirely sure what they do, but the idle switch wasn't plugged in, so that's fixed now. 

Someone had unhooked the vacuum retard on the distributor, and their plug for the vacuum line (an undersize nail) was leaking. That is all cleaned up now. 

Throttle got an adjustment. It was only opening the primary throttle, just cracking the secondary. I actually had to modify the pedal stops under the dash to get both plates to WOT, so that should make the car feel much better. 

Clutch cable is adjusted now, the lower nut had completely unthreaded itself and the cable was just sitting in its hanger. That should cure the balky shifts and reverse grind because it was dragging a bit before. 

I had a set of H4 lights in my parts stash from a 951 I parted out. Swapped those over so I can see at night. In the process, I learned that the 951/S2 actually have completely different headlights and nose panels. I had no idea. 

Otherwise, lots of little stuff with few pictures. New air filter, plugs, wires, cap and rotor, that kind of thing. I'm moving pretty slow as I'm only about 4 weeks post op on ACL surgery, so it is somewhat demoralizing seeing how little I get done each day.  But progress is still progress!

Great thread. Following!

The CIS hates vacuum leaks, pulling the injectors and new orings is not a bad idea either.  Once you try to start it if there are problems just hot wire the cold start injector to 12V.  The warm up regulator can also be tweeked to adjust fuel mixture, there is probably an instruction somewhere on the web on how to do it.

In reply to porschenut :

Yeah, once I get the engine fired up again, vacuum leak check is high on my list. It actually cold starts great so I think the 5th injector is likely okay, it just idles at ~900 rpm cold and then by the time it is up to temperature it's idling at 1500 rpm. The throttle stop still has the original paint mark on it, so I'm guessing I've got a leak plus maybe something going on with the WUR. This car, having the O2 sensor, has a frequency valve that adjusts mixture via control pressure. From what little I can tell online, it appears that the idle and WOT switches on the throttle are used to set an enriched control pressure, and in between them the valve cycles based on O2 sensor input. It's possible that having the idle switch connected will help, too.

The online documentation for the early 924 is remarkably sparse coming from the BMW E30 world. Nearly everything I read is in conflict with some other source and this entire car seems to be a combo of parts that shouldn't exist, so at this point I'm just measuring as much as I can directly to figure out what's going on.

This is the manifold vacuum limiter that my car shouldn't have:

This is WOT when I got the car - not enough to trigger the WOT switch, let alone allow any airflow. Both plates open fully now. Throttles on these are 36/46mm staged plates. The Audi 5000 and early Golf/Cabrio throttles are an "upgrade" with a 53mm secondary plate, but the internet suggests that in stock form this engine can't use the extra airflow anyway.

Early cars up to maybe 1979 had a restrictor in the primary throttle that was ~25mm. Seems odd to me, maybe they needed extra vacuum for something as those cars had EGR, vacuum amplifiers, etc.

These cars use rubber mounted alternators, and the mounts on mine are completely gone. The bushings are tiny, and while cheap, I don't like the design and Porsche themselves eventually went away from this setup. Bushings are 19mm OD and M8 (8.4mm) ID. I've got some steel inserts on the way that will be welded into the stock mount pivots. As it sits, the fan on the alternator is actually hitting the mount, and the water pump pulley can touch the upper pivot bolt. Not good. Going to clean it up a bit while I'm in there so I can figure out where the oil is coming from, and it is getting a water pump and timing belt while I'm in there just to be safe.

It still seems like a pretty low mile car from everything I've seen so far, but it isn't a car that was kept terribly clean under the hood or anything like that. More of a "typical" owner who enjoyed driving an interesting old car as a cruiser than an "enthusiast" owner attending to all the details. Of course, every seal is dried out and there's now a puddle of oil under the driver's side strut from leaking...all the things that happen when a vehicle sits.

gearheadE30 said:

hey that looks familiar...

I don't have anything to add at this point, but it's already interesting to see how much different the early 924 is compared to the 924S I have. I mostly just assumed it was the drivetrain only, but when I saw the drum brakes back there, that was a surprise. In any case, good luck!

(btw, can you just swap in 944/924S trailing arms and disc brakes?)

In reply to irish44j (Forum Supporter) :

I have definitely done some lurking in your build thread and immediately thought of you when I brought this thing home  it is very interesting to compare the two cars. Even though they're fairly close in age and the 924 made it to 1988 and the basic design existed to 1991 in the 944, it's very obvious that my 924 is really a 1970s car vs. the relative solidity/electronic/materials advances of the 1980s E30. 

The early 924 is very different. The interior and basic body panels are about all it shares - the front subframe is steel on my car, the engine hangs from mounts at the rear instead of the later car's conventional compression mounts, front brakes are solid discs, rears are drums, etc. A lot of stuff on the 924S is rubber isolated that isn't on the 924 - the steering rack is a good example of that. Your car would have mounting bushings like a 944, mine is rigidly mounted to that front cross member. My car is also I believe the first year of the rubber isolated rear torsion tube, the earlier cars were solid mounted to the body. Not dramatic changes, but enough that, for example, it's basically impossible to put a 944/924S engine into an early 924. Or, at least, it's not any easier than going nuts with an 07K swap or something. 

Yes, I can swap in early rear 944 or 924S parts, since I have a rubber isolated torsion tube car. This would get me 23.5mm torsion bars (I think mine are 22), a rear anti roll bar, 5 lug hubs, etc. I actually have a rear 1984 setup on the floor next to the car, but I don't have any of the front parts to do the swap. The biggest challenge there is that the 944 and 924S have conventional front/rear brake split. The 924 is a cross-linked brake split, and the caliper and master cylinder sizing is completely different. So a suspension swap also means doing a brake system swap.

I'm not totally opposed to that, but I have to know I want to keep the car for a while if I'm going to go to the trouble. My fleet is still too big - along with all the bikes, I have the 924, 944, 318is, 325es, K1500, and Suburban. Both Porsches and both E30s are projects, and it doesn't really make a lot of sense to have two of each. But I like them all, so for now that's a future Jordan problem. 

A lot of the brake and suspension is off the VW super beetle parts bins.  The setup works fine on the street, on the track it needs much bigger brakes.  When doing upgrades be careful of some stuff.  For example most headers are too close to the starter so heat soak kills them.  I tried heat shields and wraps but ended up louvering the hood on that side.  Extreme but it was a track car so it didn't matter.

gearheadE30 said:

In reply to irish44j (Forum Supporter) :

I have definitely done some lurking in your build thread and immediately thought of you when I brought this thing home  it is very interesting to compare the two cars. Even though they're fairly close in age and the 924 made it to 1988 and the basic design existed to 1991 in the 944, it's very obvious that my 924 is really a 1970s car vs. the relative solidity/electronic/materials advances of the 1980s E30. 

The early 924 is very different. The interior and basic body panels are about all it shares - the front subframe is steel on my car, the engine hangs from mounts at the rear instead of the later car's conventional compression mounts, front brakes are solid discs, rears are drums, etc. A lot of stuff on the 924S is rubber isolated that isn't on the 924 - the steering rack is a good example of that. Your car would have mounting bushings like a 944, mine is rigidly mounted to that front cross member. My car is also I believe the first year of the rubber isolated rear torsion tube, the earlier cars were solid mounted to the body. Not dramatic changes, but enough that, for example, it's basically impossible to put a 944/924S engine into an early 924. Or, at least, it's not any easier than going nuts with an 07K swap or something. 

Yes, I can swap in early rear 944 or 924S parts, since I have a rubber isolated torsion tube car. This would get me 23.5mm torsion bars (I think mine are 22), a rear anti roll bar, 5 lug hubs, etc. I actually have a rear 1984 setup on the floor next to the car, but I don't have any of the front parts to do the swap. The biggest challenge there is that the 944 and 924S have conventional front/rear brake split. The 924 is a cross-linked brake split, and the caliper and master cylinder sizing is completely different. So a suspension swap also means doing a brake system swap.

I'm not totally opposed to that, but I have to know I want to keep the car for a while if I'm going to go to the trouble. My fleet is still too big - along with all the bikes, I have the 924, 944, 318is, 325es, K1500, and Suburban. Both Porsches and both E30s are projects, and it doesn't really make a lot of sense to have two of each. But I like them all, so for now that's a future Jordan problem. 

interesting stuff. Having a 924S I never really looked at 924 information of course, just focused on early 944 stuff for my application. Pretty neat how many upgrades they made over the change from the 924 to the 944, for sure.

Even with the lower power, I imagine this one will be a blast to drive - and surely more raw than the 924S (as you note, like a 70s car). I wouldn't even sweat the lack of LSD - I've *never* really missed it on mine, and that's with more power, I still can almost never break the rear end free lol. 

Also reminds me of my old 1984 e30 318i project, which of course had solid discs up front, drums in the back, tiny shocks, an ancient engine and basically very little of it translated over to a late 80s e30. 

Always love background cars - neighbors Impala? Mid-80's?

Dudes driving around a 40 year old car for a daily driver?

@irish44j - I'm not too worried about the lack of LSD, especially since there's no rear anti roll bar. I did plenty of experimenting with no rear bar autocrossing E30s, sure I had an LSD but it was so worn out it wasn't really doing much anyway.

Since this car isn't destined for track use, my plans are to keep the 4 lug setup and stock brakes. I do have some other suspension upgrades lying around from parted out cars that might go on it if I like it enough to keep it longer term. The engine was pretty unenthusiastic in my driving so far, but I think getting it running right will fix or at least dramatically help with that. Raw is the right term though, if the engine wakes up a bit, I really enjoy the rest of it. 

@Datsun240ZGuy - that is a Caprice Classic LS, 1990! The last year they made them. I had a 1989 wagon for a while, it was pretty cool having matching Caprices in the neighborhood. My neighbor bought it new, he mostly drives the Buick sitting in the driveway behind it on the rare occasions that he does drive. He's in his upper '80s and doesn't drive much, and it's been months since I have seen the Caprice move. It's pretty badly rusted underneath, to the point where I'm a bit concerned about the integrity of the rear control arms. That rear anti roll bar you see hanging down is like that because it has rusted right off of the control arm on the driver's side.  

Mostly still waiting on parts, but I did remove the rest of the alarm system and it runs again! Lots of wires removed under the hood. Lots of loose hardware under the dash too, so tighten things and replaced some missing hardware that will hopefully make it all a bit less rattly.

Also swapped in the Jeep hatch struts and did some wiring mods to make the 924's odd hatch light/defrost grounding work. These are lower pressure than the stock struts which, at least in theory, will help prevent the hatch from delaminating.

While I was in there, I adjusted the hatch pins so it doesn't rattle and installed some Sylvania Zevo LEDs in the brake lights. I've had good luck with them being both brighter and more noticeable than incandescents in other cars, and this one is no exception. This car has the euro rear light arrangement, not sure if that was normal on 924s or not. The middle light is the taillight, outermost light is the brake, and the inner big lights are rear fogs which aren't connected to anything. 

Oddly, these cars have a pressure switch for the lights instead of a contact switch like modern cars. You can actually drag the brakes without the lights coming on.

Have you ever thought of wiring the inside fog lights to be additional brake lights?

I'm in the "can't be too visible" camp especially with something as old, fragile and special as this!

Gear:

These are really great little cars for how they were engineered.  I've put my early 1977 model through alot in my 11 years of ownership.  When it failed me, it was due to something that I overlooked.  Like on a 6 hour road trip, packed full with tools, suitcases for a weekend at Watkins Glen, the nuts holding the exhaust fell apart, or when lapping at the Glen the rubber exhaust hanger dissintigrated on me, on track.  I have not really found the drum brakes to be much of a problem, yes they are a Pain to adjust, but it is doable in a half hour or so, and have not made me want to swap the suspension out for the 944 version. 

Although not the most powerful cars, they handle incredibly well and can keep up with the flow of traffic.  Their design is nicely understated and they are becoming rare to see as not many people really took care of them.  You are finding this out as you go through yours.

If I may a great source of information is Clarks Garage. 

Enjoying your thread.  Keep it up.

jfryjfry said:

Have you ever thought of wiring the inside fog lights to be additional brake lights?

I'm in the "can't be too visible" camp especially with something as old, fragile and special as this!

That's common in the BMW E28 world.

I'm a big fan of LED bulbs in the back, with the old wiring, old grounds, old switches, old bulb contacts  you can be down a volt or two by the time it hits an incandescent filament which is a significant reduction in light. The LED drivers though are designed to run on a range of voltage and will not affect the light output (just slightly higher, but still much lower than incandescent bulb current draw). If you do add bulbs where the rear fogs are the stock wiring shouldn't have any trouble with the extra LED's either.

@jfryjfry - yeah, that is definitely a possibility. The LEDs are there for exactly that reason, and more lit area is never a bad thing. Especially since there is no high mount brake light on these. There are no bulb sockets in the housings in that position, but they are a snap in piece so should not be too hard to find more - looks like some people sell them on Ebay. 

@Mike924 - Glad you're enjoying it! For now all that stuff is staying stock, just making it all work like it is supposed to before considering upgrades. I don't see this ever being my primary track car since some of the other vehicles in my fleet are better suited for that, and I don't mind adjusting the drum brakes, that was pretty easy to do. You are right, it's rare to see a "nice" one. Honestly, it's even rare to see a beat up one anymore. 

Clark's Garage is great, mostly 944 and later stuff but there are good bits in there. It seems like several of the workshop sections are broken, which is a bummer. 

Got new tie rods on and ran some errands in the 924 last night, longest drive yet! The gearing is sooo much longer than my 318is. 3000 rpm at 65-70. It actually shifts very nicely now that the clutch is adjusted, and removing the manifold vacuum limiter was also a massive improvement. Much less rev hang.

Outer tie rods are easy to get for these (I went with Delphi TA770) but the inners are goofy and generally either listed as NLA or show an incorrect part for over $100 each for at least my 924. After too much time looking through catalogs, I found the Moog EV271 (also available as Mevotech MEV271) for the ~1989 VW Jetta with manual steering. They are the correct length and bolt on. The boot rubber donut thing is in a different place, but it still works. There are two minor issues that I'm still keeping an eye on:

• The counterbored hole in the inner end of the inner tie rod is deeper. The originals appeared to bottom out against the end of the rack, and then have the jam nut tighten on the end. These have a deeper pocket inside that doesn't quite reach the end of the rack before running out of space on the threads for the jam nut.
• The 924 had some little rubber steering stops added to the tie rods externally in 1979 or so (the circle piece at the far right of the below pic). These don't fit the new inners, but the end stops still seem to be in the right place internally so not sure this is an issue. And the earliest 924s didn't have this anyway.

Top two inners came off a 1983 944 with manual steering. This is what is listed where I found "924" inners for sale, they are too short. Clean with no boot is from the Jetta, bottom is original and you can see the innermost section that is machined round on the original is where the steering stop bumper sits.

Now for the bad, or at least the not great.

• Major warm/hot restart problem. I need to fix this before I burn the starter up, and I'm not sure what's causing it.
• I broke the odometer. Stupid me tried to reset it while moving. I know better.
• Bearing grinding noises. I am still waiting on a water pump to show up, I know the bearing is loud in that, hopefully that and the new timing belt tensioner take care of it. Not surprising for a car that sat for so long
• Since it runs again, I did a vacuum leak check. All good, except for the rubber seals around the injectors, so I'll need to replace those. Hopefully they're available.
• And this one, which I could use some brainstorming help on. The car intermittently runs fine, and then falls on its face. I don't think it's an ignition issue with how simple these cars are. It's not totally breaking up and cutting out, so it's either ignition timing moving around, or, more likely, I have some kind of fuel delivery problem and the mixture is changing as I drive. I have checked that the CIS flapper door thing moves freely, all good there. The fuel pump was replaced recently and the car was converted to single pump. I need to check pressure and whatnot, but being new to CIS, if anyone knows of common areas I can start with that might make mixture wander enough that the car hesitates badly and makes no power, especially at low revs, that would really help me out. It will occasionally go back to "normal" if I let it rev out a bit.

You might want to look for a inexpensive air/fuel ratio gauge, the cheap ones with a wide band sensor seem to work ok.  Of course a ignition issue can also affect the fuel ratio out put at the gauge, so the ignition needs to work properly.