Misfire?

I built a SBC 350 for my car. I've got new bearings, rings, valves. 305 high output heads, Eddelbrock Performer intake, Eddelebrock carb, 262-272 cam. It's got 2000 miles on it since the rebuild, no problems whatsoever, until today.

I pulled off the highway and all the sudden it was running like ****. I pulled the wires one at a time off the cap. I found the number 6 cylinder dead, no change in running what so ever with the wire disconnected. The spark was jumping from the cap to wire on 6, just the same as the other cylinders. I swapped first the wires and then the plugs from 6 to 8, no change. I took off the valve cover, all the rockers are moving. I tested the compression, it's even from one cylinder to another.

There's no fluid mixing, no fouling of the number 6 plug, no audible vacuum leak.

What's left that I didn't check? There must be something I overlooked.

Thanks in advance, O great hot rod gurus!

 

See it the skip goes away at higher rpm. ( verify by pulling the wire at hi rpm) If it does, it most likely is a vacuum leak affecting that one runner.

 

If it is an ignition issue,your # 6 plug should be fouled from fuel.
If it is a mechanical issue your # 6 cyl would be low on compression.

Swapping wires problem stayed @6
Swap plugs problem stayed @ 6
Sudden onset .
Comp. Test good.

Long shot is you have something blocking fuel mixture from entering cyl. Or changing the mixture like a vacuum leak that hurts 6 more than the others.
More than likely one or more of the items you presented needs to be reevaluated.

Define " running like ****" is that like 7 out of 8 cyl with one having a dead consistent miss, which isn't that bad relatively. Or is it much worse. What exactly happend immediately prior to it running like ****? Like a wot blast, missed a shift, down shift from 9,000 Rpms.

FAST- Fuel, air, spark, timing in the right ratio and with compression.
IF You've pinned it down to one cyl. Something is missing from that one.

 

Hmm interesting thought. A leak in the runner. I'll re-check everything else tomorrow, (i.e. ignition and compression) and I'll pull the plug wire at high RPM. But it it's not ignition or compression, the intake's got to come off.

Are aluminium intakes prone to cracks in the runners? If there is a crack, presumably on the underside of the intake (since I can't hear it), could it be invisible? Or would I be able to see it or at least catch my finger nail on the hypothetical crack?

 

Don't overthink it yet...or pull the intake without knowing exactly what is wrong.

 

I only noticed the poor running when I pulled off the Thruway from a consistent 85 MPH to a stop at the toll booth. I can feel the misfire when accelerating from a stop or driving slowly in high gear. I pulled the wires to find the miss when I arrived at my destination about 10 miles from the highway (work).

I drove the same car with a 307 motor with a blown piston ring for a month, this isn't quite as bad. But it is a 100% dead miss at idle, at least.

 

I'm trying to avoid it, but other then re-checking everything I already checked, what else is there?

 

I don't think any other cylinders are effected, It does feel like 7 of 8.

 

my guess would be a collasped hydraulic lifter on the #6 intake...valves appear to move ok when at slow speed, but the lifter fails to pump up under load.

 

Please post when you find the problem.

 

then I doubt it can be a vacuum leak, because it would need to be a huge leak to feel it then......... A huge leak would not be able to idle at all, it would quit.

Like you said; go back through the basics again.

 

Thats an extreme long shot to have any vacuum leak that only hurts/kills one cyl. The entire plenum would be full of those extra un metered air molecules and the entire set of cyl would suffer. Not that you couldn't have multiple problems.

Blockage of the runner might be a cause and also a long shot.

Relax and run the basics again, I think somethin is off there but my hand aren't on it.

 

Yes, especially when coming off long term highway speeds....but, where is the loud clacking/knocking you'd have to have?

 

It's the opposite of that. It feels like it's missing at slow speeds, but feels almost normal when going faster.

 

You can usually hear when a lifter collapses but its possible.I have seen similar
problems caused by a broken valve spring that sometimes will not show up on a compression test

 

Any sort of ticking noise form the valvetrain? If not, the only thing I can think of is that it wiped the intake lobe off the cam. They very rarely go completely flat, so you might still see some valve motion.

 

Exactly there's no lifter clatter.

 

I watched the rockers moving with the cover off and the engine running.

 

could also be simply a bad spark plug, cracked and shorting to 'ground'...no spark in the cylinder...change that plug.

 

The rockers will still but the valve is not being closed quickly enough causing possibly causing a missfire

 

Valvespring usually gets worse with rpm. He said the symptoms were the opposite...

 

That's what I thought at first, but I swapped them between 6 and 8.

 

That could be it. I'll look at that tomorrow.

 

I had a 350 with a skip only at idle and it turned out to be a valve spring that was broken an the bottom coil.

 

How do I check for that?

 

I'd bump it overr till both valves are shut, then grab the rocker with big channelocks to compare spring pressures between both of those valves.


..or is there any chance you maybe missed that one of the valves really isn't moving as much as you thought?

 

I'll try that tomorrow. I thought they were moving, but I'll look again. I had a cam lobe go flat on the 307 and it stopped moving almost entirely.

 

Anyone have any other thoughts, or is that about it? Verify valve spring pressure and cam lift, or else intake?

 

Have you hooked up a vacuum gauge and checked that yet? and are you sure the miss goes away at higher speed or is it that the car is now up to speed and the load on the one cylinder is lower so you dont feel the miss as bad

 

A vacuum leak should affect all cylinders, although if the bottom of the gasket on #6 was gone, it would definitely be worse there.

I'm still thinking cam or valvetrain though...