I've been trying to track down an idle problem for as long as my car's been running. Obviously not trying too hard. I'm almost positive it's an electrical problem (except when I'm not), but I'm not sure how to test in a logical fashion. I have no problem buying the parts I need. I just don't want to buy the ones I don't have to. I'm not looking for an answer so much as a plan. How can I rule out or isolate the remaining components without swapping parts? Kind of how you'd start pulling fuses to track down an electrical problem. Hell, even narrowing it down to an internal or external problem would be a start. The problem: Bad idle. Not particularly rough. More like it's alternately dying and surging. Evens out as the RPMs come up. I meant to grab some video last night but I forgot. The symptoms: Ammeter wanders with idle RPMs Engine cuts out and catches back up Spark is erratic - misses correspond to falling RPMs Timing wanders Dwell wanders Minor vacuum fluctuation, but still good (between 20-21") "Last of the milkshake" sound at the carb corresponding to falling RPMs I've checked/swapped all the usual suspects -- spark plugs, wires, coil, condensor, points, distributor, fuel pump, carb, grounds, and intake/vacuum leaks. Not saying it couldn't be one of these, but it's looking less likely. Would a slipping fan belt cause these symptoms by not spinning the alternator properly? Is there any way to test a distributor to see if it's grounding out somewhere? I ***ume a bad timing gear would affect timing, but would it cause the spark to cut out as well? What am I missing?
YIKES! You've mentioned a lot of problems. It's a ***** trying to fix **** over the phone, or like this. Just for ****s and giggles, try covering the carb opening with your hand a little at a time progressing towards almost fully covered SLOWLY and see if a sweet spot occurs. If yes, you're too lean a mix at idle or a vacuum leak. Be careful, as if this is the right approach, you can easily p*** the sweet spot by moving your hand too fast
Sounds like carbueration to me....If you feed a minute amount of propane down the carb at an idle and it speeds up its too lean,stalls its likely too rich.
Yeah, that's why I was looking for more of a conceptual approach rather than a fix. I'll try the hand thing, but I've gone through 3 carbs and it hasn't solved the problem. Runs much better, but it just makes the miss more noticeable. For what it's worth, the plugs are a perfect light tan.
I had a bad-but-tested-good wire to the coil act sort of like this once. Stranded wire, would p*** current (and continuity check) but would not p*** /enough/ current. Got an intermittent spark. Found out only when it burned through the wire's insulation. So, maybe try a new hot-wire from battery straight to coil as a troubleshooting method.
Huh. That sounds promising. How would you do that safely? Just fire it up as normal, then touch a jumper from battery positive to coil positive? Or start with the jumper in place?
I would try putting the extra wire on the coil, with the other end of the wire stripped but taped up (or carefully held by someone else - don't want to short things out). Start normally, then touch the other end of the wire to the battery and see if things get better. Either it would get better quickly or not, I would guess. Also check for all types of vacuum leaks - around intake manifold for example. Simple squirt gun of water may help in some spots, or a long rubber tube held next to your ear, put other end near potential vacuum leak spots and listen for tiny hisses that should not be there.
A quick shot of WD40 or starting fluid at suspected vacuum leaks will cause the engine to speed up if there is an actual leak. This is really helpful if your exhaust is loud enough to prevent hearing the cl***ical hissing sound.
Curiosity got the better of me and I ran home to try the hand test and shoot some video over lunch. I forgot and left the camera on the workbench, so no video until I get home again, but I think it worked. It basically acted the same as when I step on the gas. Sped way up and didn't seem to miss as much, though it still sputters occasionally. I also checked voltage at the coil. It fluctuated between about 12.4 and 12.8 volts, but never dropped below that. I'm going to feel like a dope if it really is a vacuum leak. I've checked about a hundred times and was never sure either way. I can't hear anything over the engine and starting fluid causes the engine to speed up, but only when I'm right next to the air cleaner.
Don't overlook the ground circuit. Some builders think to provide a nice fat ground connection to the framerail but neglect the fact that the ignition/engine needs a ground path. A friend of mine had an old Chevy pickup with a mul***ude of electrical gremlins and ****py running and until he found his throttle linkage almost GLOWING when he cranked the starter because everything had to ground THROUGH his throttle. A nice hefty ground cable from the engine block to the frame took care of all his concerns.
Yeah, I went through that last summer, partly for this and partly to solve a turn signal problem. I pretty much went from front to back adding ground straps to any spot that didn't make my multimeter beep. It's grounded.
Can you hold the puker a little closer to the car ??? My eyes aint what they used to be.. My hearing is ****ed too.. Looks like its at least running to me Good Luck Dave
The hand test is a good one, but it's not 100% conclusive of anything other than a lean idle mixture. Because if you correct mixture, it'll gain RPM. In your case, RPM fixes the problem, so correcting the idle mix will increase rpm, and the root cause may still be hiding. Without knowing more about your combo, I'll throw out a couple possible suspects- worn timing chain that's tensioning and loosening, or something with the carb's float level. When intakes leak into the valley and hide the whistling, the carb can have a weird sound, perhaps that's the "milkshake" you describe? Good Luck!
What do you mean by " timing wanders and dwell wanders"? Are the distributor bushings loose? Lots of sideways slop? What kind of engine?
Well, it's official. I am a dumb***. I was running about two turns lean. It runs great now. Totally smooth and way more power. I must have worked the other kinks out trying to find this one. In retrospect I'm amazed it ran at all. The mixture screw was right where everyone recommends you start with a new carb, it always fired right up, had good vacuum, good plugs, and never backfired. I never even heard it ping until today, about halfway to getting it to run right. Thanks for the suggestions.
Lemme guess,from the looks of your avatar you have a chevy six? 250,230,292. Do 1 thing for me,remove the distributer cap and hand crank the engine over till the points are fully open,now wiggle the dist. shaft back and forth,is the point gap varying alot? If so the dist bushing is shot which is common on chevy 6's. Your dwell is all over the board varying your timing and coil saturation time. Remeber"dwell varies timing but timing does not affect dwell.....Put a flipping H.E. I. in it and reap the rewards of better running 6
It's the first time i've done something like that. I can't believe it "bit me on the ***" THAT fast! Did you ever find out the REAL story on that water pump deal? Squealing belt on "whopping" the throttle on a "stick" car with a cast impeller? That ****'s ****ing hilarious! LOL. PS-is "whopping" something Italian?
The Jalopy Journal
