Hot Rod Roy

That's correct, assuming the throttle is partially open during the test.

I wouldn't consider that to be a valid test. We need an accurate analysis!

Someone needs to explain that one! That doesn't make any sense to me. Compression is higher because of bad rocker arms??? You did say compression was good in all cylinders, didn't you?

C409

..... What are the spark plugs gapped at ... did you gap them or install 'em right out of the box ? What spark plug wires did you buy ? Is there spark at the plugs ? .....

A Peter C4

It's possible you have a bad camshaft if spark is verified to be good at all plugs, a worn cam lobe will cause a skip. Maybe you should bring car to a GM tech private repair shop or corvette shop before you go crazy trying to diagnose this any further or waste money on process of elimination of possible parts.

Tom400CFI

I think someone is onto something here.

Bfenty

Yes it's good on all cylinders. I'm honestly not able to adequately explain what he meant as it's beyond my knowledge.

Spark plugs were installed straight from the box. I wasn't aware I would need to gap them.

Funny you mention the camshaft, that's what the first shop I took it to said. He thought it had an aftermarket can in it but I'm fairly certain no one has been that deep into this engine.

A shop is starting to sound really good right about now. Last owner threw a bunch of parts at this car, I don't want to do the same. I also don't know that I can afford a shop right now-should've thought about that before thinking I could afford a Corvette I guess...

A Peter C4

When you find the problem, witch maybe a simple fix, you have fun with it and you will be so happy. It's part of the Corvette sickness that most of us have, pain when it's down, but love when it's good. A good shop is a blessing when you can't find the problem on your own, atleast that's been my experience.

ElisTwoCents

Which 3 cylinders are misfiring? Just a basic check:

From the misfiring cylinder-
Pull the spark plug wire from the "bad" cylinder/cap and swap it completely with a working cylinders spark plug wire - see if the problem follows, if so, it's the wire. If the issue stays on the same cylinder, then the same test could be done with the spark plug that isn't firing, swap it into a known good firing cylinder and see if the issue follows or not. While the plug is out check the gap. That's a way, although not the most time efficient, but effective nonetheless and I'm sure you want peace of mind at this point.

At night, or in a dark garage, run the car and look under the hood on both sides, wire/insulation shorts can sometimes be seen. Just another way to easily check

All this could be done with a scope to track down firing, waveform characteristics and spark strength (kV) etc as well but I don't think you have one, most people don't at least and probably overkill. I would think it's something simple or overlooked hopefully.

Triple check the firing sequence (cap to cylinder matching up)

Good luck keep us posted

VikingTrad3r

85 flat tappet cam, possibly 3 wiped lobes on intake valve, means no air getting into cylinder because cam is wiped and doesnt lift the lifter pushrod which opens valves.

^^^ is that what you are meaning here?


is it possible just the lifters are collapsed?


i think u should u rule out ignition related potential before removing valve covers to witness heights achieved with each rocker arm.


can anybody confirm that if the rockers were too tight the result would be backfiring? That can't be causing the misfire right?

JoBy

You can also do a test with a timing light on those cables. If the light is flashing then you have a spark. Sometimes if you have a weak spark you can see the spark when the plug is out of the cylinder but not with compression pressure.

Bfenty

So we first need to rule out ignition-got it.

I've checked the engine in the dark, no arcing so no shorted wires. Good suggestion.

I wil try swapping around plugs and wires, but both have been completely replaced with the problem persisting-I'm confident they're not the issue, but as you said, peace of mind.

I can confirm that when I checked spark outside the cylinder, those cylinders had a reddish spark. Honestly I had kinda weak spark all around from what I could observe, but I'm not certain and I'm not an expert on such things. That's why I replaced the ignition coil though.

Would a timing light not flash if compression was keeping the plug from firing? Hadn't thought about that.

From this conversation I'm wondering if it's a combination of things. Weak spark in all cylinders but compression in these three cylinders is different and causes no spark?

Thing is, PO replaced entire ignition system not long before selling (I have the receipts). I suspect it was in pursuit of this problem. I agree that double-checking the ignition system needs to happen, and it may even be the issue, but my suspicion is that it's not.

BTW quick history of the car as best I know-PO did a ton of work fixing up this car, let it sit for a few years, then gave it to his son who almost immediately sold it to me. Son knows nothing about cars in general and I've never spoken to the father, just got the receipts for the work he had done.

Bfenty

Swapped wires around. No change. I think if there's an issue with ignition it's in the distributor.

JoBy

It detects the current in the cable. No current -> no flash.

If you have internal arcing in the distributor you can see a flash when connected to wire from ignition coil, but not wire to spark plug.

If you have a shorted spark plug it will still flash. So it does not have to be correct if you get a flash, but if you get no flash, then you know that it is a problem.

Tom400CFI

That is a great test^^^. Another way to test the same thing is to use a spark tester (or an old spark plug as a tester) plug it into the boot of your test wire, then ground that tester to the tip of your spark plug which is still screwed into the head. Now, if you see a spark....you KNOW it's also sparking inside the cylinder under compression -has to, to get to ground. Having said that, it's pretty unlikely that you have strong spark on some cylinders and weak spark on others; same coil is making all the sparks.



That exactly right. Wiped lobes will do exactly what the OP is complaining about; you have spark, fuel, compression...no fire (or so weak it appears as "no fire"). Could be intake or exhaust lobe.



Not likely. The lifter plunger has a meaningless amount of travel compared to cam lift; the lifter could be totally bottomed and still, the valve would open well over .350". And there would be an accompanying racket of valve noise, which the OP didn't mention.



It's not very likely ignition. The problem can't very well be before the rotor; the same p/u coil, ICM, coil and rotor are supplying spark to all cylinders. IF it were an ignition issue, it would have to be in the cap, wires or plugs of the affected cylinders. But he says he has spark to those cylinders...



Depends on the rocker that is too tight, how too tight it is...but yes, that could cause back firing OR misfiring.



.

Steebme

Is the bolt for the distributor bracket right? Is there any play in the shaft, does it wobble? Are any of your distributor wires cracked or frayed possibly grounding out? Not the plug wires, the harness wiring.

Tom400CFI

None of those things are likely to cause a consistent misfire on the same 3 cylinders.

Bfenty

Is there a way to check for a flattened cam without taking apart the engine? If I were to pull valve covers would that tell me anything?

To be clear I'm willing to try things but we are well beyond my expertise.

Anything before the cap and rotor in the ignition system would affect all cylinders, is that an accurate statement? To reiterate, whatever is wrong is affecting these three cylinders consistently, it's not traveling or random.

My gut is telling me it's not the ignition system. I know I at least have spark on all cylinders.

Need A Vette

There is but you're going to need a dial indicator, in order to measure the amount of lift, on both intake, and exhaust valves on those 3 cylinders. You would be able to do this with just pulling the valve covers, BUT, you need a dial indicator, a magnetic mount, etc, etc. and as you've stated this is well beyond your expertise. Now is time to seek out those mechanically inclined friends of yours.

D.

Steebme

I would think a ground out input wire could cause weak spark among all cylinders as op mentioned initially. A bent distributor could def pull the rotor away from the posts consistently. ICM could be on its way out too if it's more prevalent with the engine hot. Is it worth it to assume it's good or is it worth a check?

Tom400CFI

No you don't. If the cam is wiped to the point he's got three misfiring cylinders, anyone should easily be able to see that w/the naked eye. Don't need a dial indicator to see that.

Remove a valve cover on the offending side. Rig a remote starter "button", crank the engine while watching the rockers...rock. The ones that are working should be depressing the valve/spring about 1/2". The cylinders that are dead -if dead due to cam, will barely move the valve at all...or not at all. Easy check. Maybe 1/2 hour total, to check this.

Tom400CFI

Yeah...but we're grasping here. We're venturing into "very unlikely...but possible" territory when the OP hasn't check the "very likely" stuff yet.

A bent dist shaft would move the rotor, but it would still follow a circle w/in the cap that is equi-distant to all posts. Anyway, and HEI can throw a spark up ~1"...so the rotor ain't gonna get that far away from the posts. Finally...what forces exist that would "bend a distributor"?

ICM isn't going to affect the same 3 cylinders every revolution. ICM doesn't even "know" what cylinders it's firing. All it does is go; "on-off-on-off-on-off...." over and over again.