Low rpm misfire on #8 cyl, SBC

I have a lightly tuned 350, mild cam, 11:1 CR, HEI dizzy. Intake is some sort of early Chevy alloy stocker, with Speedway T bucket headers. It used to run smooth, but had a lifter clicking loudly occasionally, #5 intake valve. Couple of weeks ago I pulled the intake and stripped and cleaned the lifter, seemed to work okay, until the engine got to full temp and it started clicking again erratically. Cam lobe was good.

Also had a little game modifying the HEI to get a little more mechanical advance, as it only gave 12 degrees mechanical as supplied. Now it gives about 20. I'm running the vacuum pipe to manifold vacuum, not ported, though I've swapped it back and forth and it makes no difference except the idle needs adjusting. I run it at 12 static (so about 30 full mechanical) and an extra 15 so from vacuum at idle.

However, at some stage of all this I noticed the idle was rough and traced it to #8 cylinder. This cylinder gives about 130 psi on compression test, others are all 140-150. Header temp on #8 is about 300f and the others are 500f at idle.

I did a home made leak down test, ran 120psi into the cylinder but could hear no hissing from exhaust pipe or carb, so it seems to seal quite well.

I swapped the plug for another cylinders, no different, swapped the HT lead too, still no difference.

The valve clearances have been checked several times, both running and static, trying to get rid of the clicking lifter.

When driving it sounds like it is has that offbeat idle of a race car, as that #8 cylinder chimes in at about 1100 rpm.

Any suggestions? It must be linked that this is the low compression cylinder.

And just to ruin my day, and add to the conundrum, I've noticed some white gooey emulsion in the breathers and under the valve covers, like you get in England in winter, except the temp here in Philippines never drops below T shirt weather.

an old hyd lifter cure

when oil got low we would put in qt of auto trans fluid. Drive several days and change oil. Theory, trans fluid build pressure and blow out blockage.

Assumption, you`ve started in as dark surroundings as possible and look for spark jumping on bad wire.

Well I've swapped plug wires over to another cylinder, and plugs, the misfire never moved. Plug wires have done 500 miles at best.

I can't imagine the lifter has a blockage, as it has been removed, stripped, cleaned in gas, blown out with airline. Could be a hairline crack or chip, but that is on a different cylinder to the misfire anyway.

Sounds like you have a head gasket leaking or a cracked head.

I agree with Old Iron get a pressure gauge that you put on the radiator , and you pump up to what ever it says and let it set if you have a water leak , Gasket / head the pressure will leak down if you have white stuff like condensation , id sure say blown gasket , should be on the dipstick too .

Had that problem with my Nailhead & it turned out to be a vacuum leak. Replaced the intake gasket & that solved it! Also brake power booster had a vacuum leak... nearly drove me krazzzzy!!!

BoB

Thanks guys, something to work with. Seems to be several problems all overlapping each other.

My 350 in my dually had a strange light throttle miss i could not nail down. the trans was weak so when it started to shudder off the line i assumed it was the trans
it was the head gasket
I had slick 50 in it which let it get hot enough before i noticed a drop in power to burn all the paint off the head and melt the rear main.
i was zombie tired and didn't check the temp guage until the BBQ was ready.

I'm thinking a complete tear down: possible head gasket, low compression, water in oil, tapping lifter, misfire..... [S

If the head gasket was anything to do with the low compression I'd be seeing loads of blow-by from the breathers. Or the dip stick.

if the miss goes away as the rpm goes up, it would point to a small vacuum leak on the lifter valley side of that intake port.

seen it many times, the leak percent to flow at low rpm is large, as you increase the rpm the percent of the air leak shrinks in comparison to the flow and the cylinder can contribute to the power flow.

try a new intake gasket before tearing the heads off. use some Permatex "Aviation Gasket Cement" around the intake ports on both sides of the gasket and you'll never have a leak there again.

King Herald said:

I'm thinking a complete tear down: possible head gasket, low compression, water in oil, tapping lifter, misfire..... [S

If the head gasket was anything to do with the low compression I'd be seeing loads of blow-by from the breathers. Or the dip stick.

Click to expand...

Not if it was between cylinders like mine

I read once sprayng starter fluid around possible vaccum leaks is a good trick- listen for the engine to rev up a or smooth out

I'M thinking cracked head ben there!

I need to pull the intake again to mess with this lifter, so I'll change the gaskets and use some sealant at the same time.

Plus I'll swap the HEI cap, or even the whole HEI, see if the misfire goes away.

I have a small 'tube' of some local radiator leak stuff that I may throw in water to see if this water/oil thing will go away too.

Unfortunatly I still have three weeks to go offshore, so no answers for a while gents.

If this is your 2nd go around on the lifter ,, I think id go with a new lifter .. if its collapsed or bent push-rod maybe ? I had a bent rod and didnt know it till I tore it down to re-due the heads .

First time I've done the lifter, and I just stripped it and cleaned it, put it all back together. I have another used lifter, same type, so I'll swap the internals with the original and see how that goes.

What is the score with putting a new lifter on used cam? Do you need to do the running in for 20 minutes thing again?

I checked the push rod, nice and straight, ends are solidly fixed in place.

If this next try doesn't fix it I'll probably rip the cam out and fit a roller cam, if I can find one where I am.