The saga continues...., #3 cylinder not warming up, lean AFR Options

[era vulgaris]

So after being without my car for a month due to a vacuum leak on #2 cylinder, and then rebuilding my DRLA's, I spent monday tuning the car with a quick test drive, and then drove for about an hour and a half yesterday without issue. The car was fantastic on back roads and on the interstate. All the while my head temps and AFR were spot on perfect. I was stoked to be back on the road!

I go to start the car today, and of course, the AFR is way off to start. I figure I'm just going to have to tweak the carb sync a bit. Then as the car is warming up I see on my 4 channel head temp gauge, that cylinder #3 is conspicuously cold. The other cylinders all come up to their normal operating temp, but #3 is staying around 200 degrees and won't go warmer. The engine itself sounded 'off', like that cylinder wasn't firing.

I tried changing spark plugs. Nothing. I tried blasting out the idle circuit several times with compressed air from both the idle jet side and the mixture screw side. Nothing.
What else can I try?
I don't know what possibly could've happened overnight for this to happen. The car sat in the garage untouched.

[ThePaintedMan]

Stupud question, but did you check the idle jet itself? Also, not that it matters, but what were your compression results? Never did see that data.

[era vulgaris]

Stupud question, but did you check the idle jet itself? Also, not that it matters, but what were your compression results? Never did see that data.

Yeah I blasted out the idle jet out too. I even swapped idle jets from 3 to 4 with no change.

I haven't gotten around to the compression test yet. After I got the carbs back on and tuned them, it ran better than it ever did so I didn't bother.
I guess I should get around to checking compression, but I don't feel like that's what the problem is.
If I smother the intake on #3 it behaves similar to the vac leak I had on previously on #2. The idle doesn't increase like it did with #2, but the AFR comes right back into the 12's. It can't be another vac leak, right? Every single gasket from the heads to the top plate is new, and every nut and fastener is secure and tight.

I also looked down into the carb while the engine was running and there's no fuel leaking onto the throttle plate.

Does this sound like an idle circuit issue? BTW, my plugs and wires are brand new.

[r_towle]

Did you ever change out the spark plug wires? Distributor cap?

[era vulgaris]

Did you ever change out the spark plug wires? Distributor cap?

I replaced the plugs and wires, which had been on the car for who knows how long.
I haven't replaced the dizzy cap or rotor yet, but the ones that are currently on the car I replaced a little less than a year ago.

[zipedadoo]

It's got to be electrical, if you have a dead miss in only one cylinder on a carbureted engine. You might have moisture in your dist. cap . Spray some wd40 in there and I'll bet it clears up.

Well it's got to be electrical if compression is ok....(IMG:style_emoticons/default/wink.gif)

[r_towle]

If you have a timing light, clip it onto that wire to see if it's firing.
Just cause it's new does not mean it's any good.

Check them all.

[era vulgaris]

If you have a timing light, clip it onto that wire to see if it's firing.
Just cause it's new does not mean it's any good.

Check them all.

I'll give that a try tomorrow. Those wires better be good, they cost me $75 from Tangerine!!!

I don't think it's moisture in the dizzy cap. Dry as a bone in there.

[76-914]

With the engine running, turn off the lights in the garage and look for arcing around dizzy, wires and the plug itself. (IMG:style_emoticons/default/beerchug.gif)

[porschetub]

With the engine running, turn off the lights in the garage and look for arcing around dizzy, wires and the plug itself. (IMG:style_emoticons/default/beerchug.gif)

(IMG:style_emoticons/default/agree.gif) great reply done it many times,you really can see a lot going on.

[JOEPROPER]

You can also try spraying with soapy water. (old timers trick) but will create an easy path to ground.

[rhodyguy]

Unisyn reads the same on all 4 venturis? Not just the 2 forward throats? You can easily raise the idle by using 1 of the idle speed adjustment screws. The other side will follow suit and raise the idle a bit so the pulsing of the needle smooths out. Try closing all of the bypass screws to start with a common baseline ( note the current setting on all four). Is the plug wire continuity the same on all 4? What effect does completely closing the idle air mixture screw on the bad cylinder at a stable idle have? After closing you should note a stumble. Perhaps a faint snapping back thru the Venturi or exhaust as you're forcing that cyl to go lean. No change? Suspect that idle circuit. All o-rings and metal caps in place?

[struckn]

Highly recommend you check compression. I had a simlar condition due to an open exhaust valve and a dropped valve seat in the head. With Quad Exhaust I could actually see droplets of gas blowing out one of the pipes.

[ThePaintedMan]

Highly recommend you check compression. I had a simlar condition due to an open exhaust valve and a dropped valve seat in the head. With Quad Exhaust I could actually see droplets of gas blowing out one of the pipes.

(IMG:style_emoticons/default/agree.gif) With all of the issues you've been having, we need to follow one of the three paths (spark, fuel, air/compression) to completion. With new wires, even given their quality, I'd move onto checking spark first, at least at the plug wire. It's the simplest test, and the thing which has been modified most recently. Next,we need a compression test to rule out that path. Finally, we'll deal with the carbs. But if you follow Kevin's test with the Unisyn, it'll also indicated issues with compression or something mechanical causing the issue.

[era vulgaris]

(IMG:style_emoticons/default/agree.gif) With all of the issues you've been having, we need to follow one of the three paths (spark, fuel, air/compression) to completion. With new wires, even given their quality, I'd move onto checking spark first, at least at the plug wire. It's the simplest test, and the thing which has been modified most recently. Next,we need a compression test to rule out that path. Finally, we'll deal with the carbs. But if you follow Kevin's test with the Unisyn, it'll also indicated issues with compression or something mechanical causing the issue.

Compression results are in. Engine was warmed up, however I don't have anything I can use to squirt oil into the cylinders. Not sure if that matters. Readings were with WOT.

1) 139
2) 142
3) 144
4) 138

I think that's pretty good. I'm feeling a bit relieved to be able to rule out big $$$ issues. (IMG:style_emoticons/default/biggrin.gif)

I also tested the resistance of the wires from Tangerine.

1) 1.47 kOhms
2) 1.04 kOhms
3) 0.88 kOhms
4) 0.66 kOhms

Makes sense since 1 is the longest wire and 4 is the shortest, right?

If you have a timing light, clip it onto that wire to see if it's firing.
Just cause it's new does not mean it's any good.

Check them all.

I put the timing light clip around all 4 wires and I got an RPM reading from all of them.

With the engine running, turn off the lights in the garage and look for arcing around dizzy, wires and the plug itself. (IMG:style_emoticons/default/beerchug.gif)

I didn't see any arcing or spark scatter.

Unisyn reads the same on all 4 venturis? Not just the 2 forward throats? You can easily raise the idle by using 1 of the idle speed adjustment screws. The other side will follow suit and raise the idle a bit so the pulsing of the needle smooths out. Try closing all of the bypass screws to start with a common baseline ( note the current setting on all four). Is the plug wire continuity the same on all 4? What effect does completely closing the idle air mixture screw on the bad cylinder at a stable idle have? After closing you should note a stumble. Perhaps a faint snapping back thru the Venturi or exhaust as you're forcing that cyl to go lean. No change? Suspect that idle circuit. All o-rings and metal caps in place?

All 4 air bypass screws are fully closed. I can get pretty similar readings with the unisyn on all 4 cylinders. I haven't tried turning the mix screw all the way in. I'll give that a try next.
What I find confusing is that today I could warm the car up and it sounded like it's idling normally, and I can get pretty close readings with the unison on all 4, but I can't get the AFR back under mid 13's, and #3 cylinder will still not warm up past ~200*. If I let it idle until it's warm, it seems ok. But then if I rev it up anywhere past 2K rpm and hold it there the AFR will be back in the 12's momentarily, but then after a few seconds it'll start to pop like it's lean and the car starts to shake. Then if I get off the throttle it'll no longer idle at all and the AFR will be way up at like 17, and it sounds uneven like it did yesterday when I thought #3 wasn't firing. And after that there's no getting it to idle again until you shut it off and let it sit for a while.

I'm thinking it's got to be an issue with that carb on the passenger side. I'm going to pull it off tomorrow, take it back apart, and blast out every nook and cranny with air. Are the new gaskets I put on it ok to reuse since they've already been compressed?

[ThePaintedMan]

I would say that's about the right direction to be going in, given the new data. One thing you might try before pulling the carb though is to pull the #3 spark plug wire with it running and check for arc between the wire and the engine. If it's definitely sparking, I'd suspect the carb. Still would try one last inspection/blowout of the idle jet on #3 first. If still there, then pull the carb. The gasket should be fine to reuse.

[ssuperflyoldguy]

iffy plug? switch or put a new one in the problem cylinder

[type2man]

The next time it happens shut it off and remove the idle jet and look at it (don't blow on it) to see if there is crud on the tip. What happens sometimes is the idle jet gets clogged but when you shut it off, the crud drops back down in the hole so when you take the jet out it's clean.

Had this happen to me on a road trip and what I would do when it would misfire is shut it off and coast and then start it back up and it would run ok again until the crud would come back up.

[rhodyguy]

You might be surprised with the vacuum readings after turning up the idle. While it's nice to have every thing equal at idle but that can change when the working linkage comes into play. It's a 3 minute test that is undone by turning a screw and you really don't disturb the current linkage setup.

[era vulgaris]

Mixture screw on #3 went all the way in with no change in the idle at all. Looks like I'll be pulling that carb apart. There must be something stuck in one of the passages.
This is so irritating because it worked perfect before I rebuilt it.

But that would explain why it ran great one day and then not the next. Some little speck of something was just lying in wait to clog up the fuel passages. (IMG:style_emoticons/default/mad.gif)