Heavy mystery time...really wierd engine behavior

[BTF/PTM]

Kudos to anyone who knows where "heavy mystery time" comes from.

so I discovered this neat little gremlin today while trying to find a carburetor leak after getting the fuel tank reinstalled. This is WAY more interesting than a carburetor leak.

With the engine idling and warmed up, I can pull any or all of the FRONT three spark plugs and there is zero difference in the way the engine idles. Not so much as a hiccup. If I pull any one of the BACK three spark plugs the engine begins to stutter and heave like it should cuz it's running on one less cylinder. The idle, at least to me, seems smooth and easy. No smoke, no stutter, nothing that I could see.

Valves are adjusted to spec. Battery and charging system are less than two months old. Ignition points are clean (they look new) and gapped at about 0.018". Cap and rotor are healthy, spark plugs are 200 miles old and gapped at about 0.035". Spark plug wires are brand new, I put in my set of Taylor 8mm's I've been hanging onto just to see if it made a difference. It didn't.

Engine has decent compression, it's an L28/N42, even with the throttle fully closed (forgot to pin it open during the test) the compression from 1-6 is 160, 155, 140, 155, 155, 155.

I have spark at all six plugs, I used the screwdriver trick to ground each wire to the valve cover to check for spark.

Really freaking strange. It does not feel at all like it's running on three cylinders while it's driving, it has plenty of power and is very smooth. Throttle response immediately off idle is sputtery and rough for a second or so, and then it seems to pick up and run just fine. Choke is fully open (whatever "deactivated" is called) during all this.

Any ideas? I worked with Motorman7 over the phone on this for quite a while, and my dad as well who's got 40+ years of gear head experience. This one's got us all stumped.

Edited June 7, 2010 by BTF/PTM

[EScanlon]

Unbalanced carburators.

E

[kenz240z]

The engine is idling on the rear carby only. As E pointed out, the carbies are unbalanced. You need to adjust them so that they are flowing evenly at idle. A Uni-syn or other flowmeter will make the job easier, although you should be able to get close by ear.

[BTF/PTM]

Motorman7 helped me understand this idea, I think I get the concept. He helped me sync the two carbs, and when I got home I moved one idle screw in a tad since the engine was stumbling so bad I had to blip the throttle sitting at stop lights on the way home. I reset that back carb's idle screw to where the front one is set (didn't change the front one from our initial tuning) so I'll make sure to adjust both idle screws when I run it tomorrow.

As for the part that doesn't make sense, I'm confused as to how one carb can dominate the other since each carb receives its own fuel feed. How does having a quarter turn worth of additional idle air flow through one carb end up cutting off air/fuel mixture through the other carb so completely that there's zero power produced by those cylinders? Sorry, I guess this is a question for the carburetor systems forum.

Thanks for the quick help on this, guys, I guess it wasn't such a heavy mystery after all

[tlorber]

It's strange that the car idles smoothly running only on cyls 4,5, and 6. However, I could see how the car would not idle well but could still run well at higher rpm. The idle adjustment only regulates where the throttle plate stops at idle. Once off idle, that adjustment has no effect. At higher rpms, the only adjustments that matter are the mixture (large round "nut" at the bottom of carb) and the bracket adustment that syncs the two carbs at higher rpm.

You may also want to check where the fitting (banjo bolt) where the fuel enters the float bowl. These originally had a fine copper screen in them as a final filter before entering the carb. These can get plugged with grit and starve the carb. However, this would seem unlikely if, as you say, the car runs smooth and has plenty of power.

[Zedyone_kenobi]

They both receive independent gas, but even if one is not working at all, the balance tube may be able to support the entire engine under certain circumstances. You can see how the balance tube receives metered air/fuel. Granted you its a fairly torturous path, but it is very possible.

[BTF/PTM]

I figured I'd update the thread in case this information comes in handy to someone later. I have my uni-syn now, but a couple days ago I was adjusting the idle screws without it in the hope of at least getting a response out of the front three cylinders. Long story short, I discovered two things:

1) something is wrong with the front carburetor other than being out of adjustment, the slide sticks closed when I blip the throttle (the back one responds nicely) and also hangs up going back down.

2) I had to turn the front idle screw in almost two full turns just to get the engine to idle on all six cylinders, at that point I was finally able to pull any one of the six spark plug wires and get the same (and this time much more mild since were six bangs instead of three) stumble when a cylinder stopped firing. Obviously this makes things horrifically out of tune, I was just trying to get all six cylinders firing.

So basically, the engine idling on three cylinders isn't the only problem, I've got bigger things to figure out. I'm also armed with the ZTherapy "Just SU's" DVD, so I'll be educating myself and learning to tune properly and also taking apart my second set of 4-screw carbs to hopefully have a good working pair.