Specs: rebuilt factory carb (17082204 E4ME), SBC 350, iron vortec heads, "baby hot cam" (215/223 LSA 108 473/473), flat top pistons, custom prom (L69 timing tables stepped down for vortec heads and target rich/lean set to .6 from .41), and custom MAP sensor voltage regulator to boost voltage signal

Car runs great except at idle... In park it is less noticeable but in gear it's honestly annoying. Basically, I'm getting a weird surge that shakes the vehicle for a few seconds then smooths out. No trouble codes, and dwell is varying between 30-35. I don't expect this thing to idle like a stock engine, but this feels almost like a misfire. Plugs are just a hair lean and introducing/removing air from the carb does not seem to improve things.

The only two things that come to mind is the knock sensor interpreting things wrong at idle or the volume of air/fuel in the idle circuit is simply insufficient for my specs.

 

Probably is. As we all always used to say, about 80% of all carb problems are found in the ignition.

Not likely.

It's plenty. The stock carb in my 83 L69 car was almost adequate in that area for a 400 with a 230°/230° solid lifter cam, 64cc double-hump heads (same chamber size as yours), and the stock L69 chip.

Never heard of any such. I'd be interested in learning more about this.

Sounds EXTREMELY suspicious. I'd start by doing away with this. I'd also suggest putting the stock L69 chip back in it and see what happens.

 

As a reminder, a "factory rebuilt" carburetor is never a bolt-on-and-forget item. Float level specifications vary by engine and application. Base idle mixture settings should be accomplished on an engine running at normal operating temperature before the idle mixture solenoid adjustment is even considered. Throttle position sensor adjustment must be set to the correct range after initial adjustment, and would likely be adjusted again AFTER final mixture and curb idle settings. Instructions indicating that and detailing procedures should have been provided, or references made, withing the instructions sent with the "rebuilt" carburetor.

 

@sofakingdom I bought all of the stuff required to burn proms a few years ago. The ZZ4/L69 bins are available as well as the 8079 ECU source to use in tuner rt. I can screen shot my tables when I get off work. I upped the rich/lean flag threshold to see if that would allow me to run things richer at idle in closed loop.

The map regulator I made is simple - cams that produces lower vacuum at idle will trick the computer into thinking there's a greater load on the engine then there really is. A 12v regulator circuit allows me to "boost" the reference signal going into the MAP sensor and raise the output (going to the ECU) to the proper "factory" range. A switch allows me to use the factory voltage.

What's the best way to determine if its ignition related? I can pull the plug wires one by one, but it doesn't feel like your typical misfire.

@Vader I rebuilt the carb myself. Float was set to factory specs, idle mixture screws are 3.5 turns out, TPS at .5, dwell varies between 30-35 and is adjustable. Car idles around 650-700.

 

Good news. You're way ahead of the game if you already set up the carb upon rebuild. We see too many cases where a carb in a box get s delivered bolted on, and expected to run properly as received.

If you can perform a power balance test by pulling ignition wires one at a time and monitoring the RPM drop, it could be revealing.

 

As promised... my current timing tables and other useful L69 prom info. I pulled all of my spark plugs yesterday - let me know what you think of the color. They have a little over 2,000 miles on them and all look like the two pictured below. Plug #8 was slightly whiter.

I've also included my MAP sensor regulator I built.


Custom L69 Prom

#1 cylinder plug

#7 cylinder plug

My map signal “regulator”

Wiring schematics

 

I forgot to mention... I cranked the engine last night and put the timing light on it. Not sure if this is normal but I'm getting about ± 5 degrees of timing movement with the engine warmed up and idling.

 

That seems unreasonable. As I recall, when the EST is active the adder to base timing at idle is pretty stable. That makes me wonder about the EST and/or knock sensor.

 

I thought so too. BTW if my mark is correct I'm getting 27 degrees of timing in park at 850 rpms. It's retarding and adding 5 degrees and then momentarily at the target timing which is effecting the idle rpm. I guess the question is what component is causing it? I'm assuming that even the carb fueling could cause fluctuations and effect the rpms which would show up in the timing.

 

Pretty cool software. Looks like Windows 98 or later. Not at all what I'd expect for disassembling a MC680x from the late 70s such as what was in the L69 from decades before what you show on your screen. I had no idea anyone had ever brought that 70s microcontroller into the 21st century like that. Which is not say that it "can't be done", or "hasn't been done", or any such; only, that I've never seen it. Back when I was working with that sort of thing (the 70s and 80s), we had a monochrome screen, with nothing but text, and the numbers were usually hexadecimal (that being what was stored in the chip), with an address at the far left and a table of 16 hex values per line. And of course the MC680x was very much a one-time-only programmable device, which means, it's not the same as an erasable PROM or the like. I checked on Digikey this morning and a MC6802, the last of that family you can still buy new, (not sure if that's the right one... for some reason I'm thinking it was originally the MC6809 which is NLA) is $159 or some such, so it's certainly not a trivial undertaking. Butt maybe you know something I don't, and that'd be great if true. Enlighten us.

Not sure also what the whole MAP sensor thing is all about. That sen-sore is just a potentiometer: at its heart; at atmospheric (no vacuum), the output is at 5V, and at deep-space vacuum (30" of Hg), it's at zero; (or perhaps the inverse of that, butt that isn't important) at any pressure in between, it's at some ... in-between value, in a more-or-less direct proportion. Not sure what changing its reference from 5V to any other voltage will actually accomplish, unless the ECM is programmed to understand that, and in which case, it can understand a 5V reference just as well, and be re-programmed to re-interpret any incoming values. As in, if the highest analog voltage the MCU can convert to a digital value is 5V, then any voltage you feed it will be interpreted as either something between 0V and 5V, or if the input exceeds 5V, either as 5V, or as the lowest possible negative value, plus a few more counts. Much like an audio ADC either "maxes out" at its full count when the input goes above its reference, or "folds over" to the lowest possible value and starts counting back up from there. The former looks like "clipping" in an analog amp, butt the latter is ... heinous. For, say, an 8-bit ADC output, the highest output value is 0xFF (decimal 255), but the next higher value, since there's no "carry" bit in that MCU, is 0x00 (zero). The next higher value is 0x01 which is MOS DEF not higher than 0xFF which leads to a MASSIVE distortion. Not that I can speak to any of this with any authority, because of course, it's just numbers, and I'm not a number. Just a mathematician that had to earn a living some other way because mathematics wasn't a marketable skill back in the early 70s.

I don't really know anything about this though. I'm a rank beginner, only having been an electronic engineer for the last 50 yrs or so, mostly in the broadcast, stage audio, metrology, medical, and various other insignificant dalliances, including quite a few ADC designs; so pretty much a n00b really. I'm always open to learning something new.

 

Thanks @sofakingdom . Someone back in the early 2000s put in a lot of work to make this stuff available to non-experts such as myself. Also, I guess it's technically not a MAP sensor, as lower voltage indicates low vacuum. I would have to do more testing but what I'm using the regulator for is to bump the idle vacuum output signal into the range the computer "thinks" it should be. It doesn't take much and I don't think it ever passes the 5.5v max voltage output.

I've done some more tweaking and I don't think there's an issue with the system - I just need to fine tune my settings both mechanically and via the prom to achieve a better idle in gear. The real question is how?? I've got my timing set to give me 29 degrees of advance at idle, I'm not getting nozzle drip, dwell is varying between 30-35 but the surges in gear are just irritating. I'm currently idling at 650-700 rpms - is it unheard of to idle in gear at 800 with a larger cam?

 

My brain is having a little problem processing the lower vacuum and surges. If the vacuum is steady but low, that's understandable with a longer duration lobe profile. My little brain is thinking that even at the low vacuum, if it is steady the control system should stabilize and compensate, resulting in a reasonably stable idle RPM, if you increase the idle RPM in gear, does it stabilize?

Of course, if there are other reasons for low vacuum, such as leaks, that creates a potentially uncontrollable variable.

Also, if the MC solenoid is disabled, can you adjust mixture for a more stable idle at the lower RPM in gear? I'm thinking the MC travel stops may be a little wider in range than optimal, even at the non-adjustable end, and/or the metering rods may not be optimal for your setup.

 

Well I spoke too soon... I let the car get good and hot last night and shined a light down the primaries. Unfortunately, I am still getting nozzle drip. It's not spewing but has an occasional drop that coats the throttle blade. I also tested for vacuum leaks and found that if I spray carb cleaner near base plate to carb body gasket the rpms raise and smooth out.

@Vader I'll have to check with the MC unplugged later but during open loop at idle the car runs better. idling in park at 850 rpms the car runs 98% smooth. In park I'm getting 15" of vacuum I'll have to check in gear, but I'm guessing the vacuum leak/nozzle drip isn't helping.

 

At least you've found a couple reasons explaining the symptoms. Hopefully you'll get those ironed out and resolve the problem. The tough part may be getting a carb kit in 2024. It used to be that every mom 'n pop parts store had cases of them. I suspect that's changed.

 

I had to open up the Idle Air Bypass to .1065 in order to stop the nozzle drip. I also opened up the DCR to .055 but the car is still not running optimally at idle in gear. I've decided to go a different direction and ordered Summit cam #8800. It's a bit of a step down but I think the idle will be smoother and hopefully be easier to tune... Just out of curiosity I pulled the plugs (200 miles) and they look brand new so it's possible the idle issue is just the nature of the combination.