Benjamin4456

Ok, I had a thread not that long ago that was titled "Idle/low rpm misfire", but soon I realized that my title was limited and that perhaps I should make a new thread to better re-address the current issue as a few things have changed since, in addition to the main focus. So anyway, here we go...

The car is a 1980 GS that I have done the following to - in no particular order. And note that none of these have seemed to affect my issues noticeably, although some were long enough back that I really don't remember. Of course there's more than this, but these are the things that could affect the running of the engine. That said: new engine, '82 dizzy and DLIDFIS, three different rebuilt carbs, carter fuel pump upgrade w/ relay mod, recoated fuel tank, all new rubber lines, basic maintenance, smog delete, free flowing (and need of replacement) exhaust, and probably more, but that's all that comes to mind.

Now, I built the new engine myself (it's a 1982 12A) and it is ported as follows: Stock intake opening, extended slightly beyond '74 spec closing. Stock exhaust opening, slightly earlier than T2 closing. Compression is fine at somewhere around 110psi +/- ~7psi on all faces. My DLIDFIS setup is using GM igniters (pn dr-100) and MSD High Vibration Blaster Coils. The plug wires were replaced within the last couple months with Taylor Spiro Pro 8mm wires. Trailing is running through leading position - new rotor and cap as of not long ago. Fuel pressure set at ~2.75psi and fuel is darned near center in the sight glasses.

So that's the relevant background I can think of, let me know if there's something I missed that may be of use.

The issues are these:
- The car will not idle cold. It will, however, always catch within a second of cranking if it dies, and even on the first go it's fast to catch. When cold, it is impossible to hold steady rpm under 2.5k. Anything below that and it dies or stumbles without warning. Sometimes I can get it to idle for a few seconds at normal speed (still when cold) if I had just been pumping the gas slightly and working it down to that lower rpm, but even then it will abruptly die after a little while. While attempting to hold the rpm under 2.5k when cold, the engine might sit there a second or two, but then suddenly dies (this is become a trend here) - usually I can save it by letting of and pumping the gas a little, but not always. Keep in mind that this behavior is true of my stock carb (although to a slightly lesser extent) and also my modded carb (see this thread for background on my carb and intake).

- Driving the car after it having sat over nine hours, unless the car has been warming up for at least five minutes, is practically impossible - stock and modded carb. This is true regardless of ambient temp, although it definitely gets more pronounced with colder weather. The issue is twofold: If I go to accelerate from a stop the car dies instantly (although this depends on how long it has been running for). And then when coming to a stop or letting off the gas in neutral after driving, the car will also usually die or at the very least bounce around at 200 - 500 rpm for a while. This dying behavior continues until the car is nearly up to operating temp, although it gets progressively better the warmer the engine gets.

- Misires, cold and warm. So when holding the car at any rpm, regardless of temp, it will misfire or 'bump'. This is usually worse when cold, but not always. Depending on how I have it tuned, sometimes the misfire is random, and other times it's cyclical. I can usually get the misfire to only occur every few seconds (yeah I know, still pretty often) at any given engine temp but at all others it is worse. Even if I can't hear an audible pop from the exhaust, I can always see the engine/shifter jittering sporadically. The mixture screw is also really touchy, although this is mostly on my modded carb that has the 120's for primaries, so that figures to an extent. Although, no matter the idle speed, I can never (even at operating temp) get all misfires to subside. On my modded carb - which is currently installed - I can go from cyclical rich misfires to random lean misfires in just over half a turn of the mixture screw. And there is no happy medium; it's a combination of the two or either.

- Bucking. Under 3k I get lovely jolts while decelerating. I usually just put the clutch in to avoid it, but there are many cases when it would be nice not to. New transmission and motor mounts. Drive line has some slop, but it's not absurd or anything (I discovered the majority is coming from loose spider gears, which is great...).

The latest progress I've made in possibly solving part of this issue is that I noticed very rare glitches in my leading ignition. As in, I was standing there for a couple minutes and my timing light only picked up the glitch twice. The issue might even be my timing light (it's old and second hand), but I checked to see if wiggling wires or the switch caused anything and nothing was observed. So maybe there's an issue with my ignition, maybe not. I'd still like to get a scope on it, but I'm not having much luck getting access to one. I have another, crusty, rusty dizzy I could potentially try - maybe when I have more time...


So that's my issue(s) to the best of my recollection. If I missed something that may be helpful to troubleshooting let me know and I'll further explain or get back with an answer. All theories are welcome; this problem has lived on - in some form - for as long as I've had this car running and driving, which by now is about two and a half years. Thanks in advance. And I hope making a new thread is fine and all, my apologies if I shouldn't have.

Benjamin4456

Perhaps I hoped for too much - I know it's hard to troubleshoot issues like these beyond the basics over the internet, particularly on a non-stock car - although I thought there might have a been at least a couple bites.

As of now my biggest concern is the misfire. The others are probably related to it in at least some part, and seeing as how it manifests itself regardless of the carb (unless I really just can't tune a carb's idle, but I'd doubt that) it's more than likely still ignition even though I thought I ruled it out. Like said before, I did notice a very occasional glitch last time I checked timing, but the misfires are far more often than what I noticed in the ignition. I have another dizzy - it'll need some cleanup before installation though - that can give a try sometime here, unfortunately probably not a for a month or two.

I may have a lead on an oscilloscope that I could borrow, but I'll find out for sure later today.

If anyone has ever dealt with issues that sound at all similar to these and you solved it, please share. I don't have too wide of a experience base with these cars so I'm not sure if there's some weird quirk that may be happening or if it's just something out of wack that I haven't pinned down yet.

Once again, thanks in advance. I'm sure someone has some ideas, even if it's a shot in the dark.

Jeff20B

iTrader: (13)

It might just be the engine. Maybe you have a stuck side seal.

Edit: sorry, I just caught the part where you said compression is fine, so scratch that idea. Ok, I think it could be ignition as well. Maybe some cross talk or maybe the leading pickup is wigging out. This would explain a lot and can lead to the behavior you are describing.

Diagnosing ignition issues can be a lengthy process. Brand new plugs, new wires, cap and rotor often will not fix it if the pickup and / or ignitor are screwy. Or maybe a bad coil that likes to arc instead of sending the energy down the wires to the plugs. Maybe the internal winding of the coil are breaking down and arcing internally either when cold or warm, depending on heat expansion and how close a not very well insulated coil winding gets to the outer steel shell of the coil. This problem usually shows up on a warm/hot coil in use but is often ok when cold when everything is smaller.

And you say it won't idle when cold, that is a tuning issue. It is best to set up the carb to be a little bit cold blooded while cold so it's not all pig-rich when fully warmed up. These are carbs that meter fuel depending on conditions. They aren't exactly a fuel delivery computer, but they do pretty well none the less. But some fine tuning on your carb may be in order here.

I hear ya when you say you went with 120 idle air bleeds and whatever size fuel jets you went with, but maybe try a little bit richer fuel jets and see how your idle does. Like 2 to 4 numbers more (from say 120 to 122 or 124 depending on your current jet size). I say this from experience and knowing that all three primary circuits are all intimately connected and communicate: idle, transition and mains or main primaries. You change one it affects the others. And because the transition bleeds can't be changed (those long narrow ones with the little 46 stamped on top), that only leaves idle air bleeds and main fuel jets for tuning.

Note: the primary air bleed/emulsion tube should stay at a stock 70 and doesn't offer any benefits to trying other sizes, but feel free to experiment. I've seen 60, 80 and of course 90 on various carbs with only the 70 actually running well in all conditions on hogged carbs whether boosted or NA.

Benjamin4456

First, thanks for responding Jeff, and my apologies for not getting back sooner, oh, it's only been four days, eh... whatever. I've been reading posts, just not enough time to respond.

Now then, I believe as well that it's probably ignition still. If I remember correctly, the leading pickup is actually quite sloppy on this dizzy - not sure how much is normal, but back when I rebuilt I took note of it. Hoping it's not crosstalk. Moving wires around and away from each other doesn't change anything so probably not that.

Indeed it can be. I'm thinking that the next day I have some time, I'll try running each leading coil through the cap and see if there are any changes - DLIDFIS is a redundant ignition system after all, might as well test both sides separately. Seeing as how I can get it to idle almost perfectly (not quite, but nearly) on just trailing with some tuning changes through the leading cap position, I'm going to assume that the cap and rotor are still in fine condition. Hopefully these tests will isolate a coil or ignitor if one is misbehaving. Or prove it's something in the pickup assembly, who knows...

And yeah, I know cold starting is mainly a tuning issue. Like you said, I too think it's running a little lean. Plugs are decent, but definitely not rich, and when cold starting and bringing it up to fast idle (1k-2k) it will idle a second and then die, as though it's temporarily running on the fuel from the AP and then doesn't have enough from the mains/transition orifices so it cuts.

Primary air bleeds and main primary fuel jets are 120's (fuel jets are closer to 118). I bought one extra pair of fuel jets to mess around with, so I think I'll drill them out to 122 or so and see what happens (they are currently 116's I believe). Hopefully I'll pick up a wideband around the holidays and some more jets at the same time so I can dial things in.

The primary main air bleeds are still the stock 70's. I'm looking at installing the tapped secondaries with 80's soon (the taps and all finally arrived), but I doubt that will happen tomorrow.


On a slightly different note, I might still be able to borrow an oscilloscope as I now know where one exists, I just have to figure out who to talk to. In other words, it might still happen, but maybe not too soon. This is a busy time of year for me after all...

Benjamin4456

Ok, this past Sunday I had a chance to swap some slightly larger primary fuel jets. In theory they're about 120, but they're probably closer to 122 or 123. My 125 bit doesn't fit, so I know they're smaller than that. And I say slightly larger than 120 because I drilled them by hand and had particularly bad luck this time compared to previous attempts.

Anyway, I feel like I can get the idle a little smoother than before when up to temp and driving around seems a little smoother as well. However, I also had to raise the idle slightly, as with these jets, the car is even less likely to idle when cold. Before it would idle low and rough if I had let it sit a few hours, but now I have to let it warm up for a couple minutes even if it's only been five hours or less (albeit the temp outside is 30's to 50's, but still). I haven't noticed much any change in the behaviour between idle and 2k - when cold I cannot hold it at any rpm in this range without it sputtering and/or quickly dying. And with that, even if I raise the rpm with the pedal by just 100 while cold, I still get this sputtering effect. It's as though my transition circuit just doesn't work when things have been sitting. Richening the mixture doesn't do much, and then it only makes the hot idle worse. The mixture screw is also really touchy (I believe I've mentioned that before) as in, less than 1/8 turn either direction and the idle is rich or lean misfiring. To me that seems off, but I don't have much experience with these carbs yet, so perhaps not?

One positive is that the car (when up to temp) seems less likely to stall when going from a stop, so that's nice. However it does smell richer than before (I get comments concerning the smell quite often - currently catless) and now I also occasionally get small smoke puffs on shifts doing wot pulls. Bucking doesn't seem radically different either - still present below 2.5-3k or so.

Ironically the car seems more powerful now. I don't know what it is, but this car has always seemed inconsistent in terms of power delivery. Most days wot is fun, but nothing crazy, especially in the lower rpm's. However some days (and they are seemingly more frequent since this jet change), the car just pulls. It's honestly a crazy difference. You can actually feel the power building as you go up the rpm range. I really don't know what's going on here (I thought most of the time it was my clutch, which does spin on occasion, but now I'm not as sure). It's probably a separate issue from the misfires and whatnot, who knows... I don't that's for sure.

I took an audio clip this morning of my cold start. It's sort of long so my apologies, but anyway, here's some context for it. Once I start the car I am pumping the gas slightly to work it up to above 2k where it will actually run. Then I hold it there a little and then try to lower the rpm but that fails. I hold it above 2k via foot until ~1:28 where I again try to lower the rpm. After I get past the stumbling here I am not adjusting my foot, the idle is fluctuating by itself around 2k. I then set the fast idle (I have it on a toggle switch) and that's that. Don't mind the exhaust note; my current system has disintegrated...

So that's what I have for now. Any thoughts or things to check out based on what I mentioned? I probably won't get a chance to check out the ignition until next week or maybe this Sunday.