Engine misfiring att WOT

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Odmark

Well-known member
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Dec 1, 2015
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Location
Sweden
My Car
1972 Mercury Cougar
Okay so here is the deal. 

Yesterday i left the car idling to build temp. It started perfect as usual and idled smooth. Then after a while it started to run rough, and then rougher and rougher and eventually it died.

After this it became incredibly hard to start, it felt like it was drenched in fuel. Eventually i got it running (still barely idling) so i took it for a spin to burn som excessive fuel.

When i gave it WOT it misfired BADLY, and the bangs where extremely loud and sounded very odd. Not like any misfire i heard before.

Took the car home, checked timing, adjusted the carb a bit. And it smoothened out. Took it for a drive again and it ran much better. But still misfiring a bit starting from like 3500rpm and also a bit hard to start when warm.

Home again, notes that some of the sparkplugs are cracked so i removed them all and installed new ones today. The plugs removed were all perfectly brown indicating good combustion.

Now it runs slightly better once more... Easy to start when hot, works perfectly for normal driving.

Still when i floor it it gives 2-3 small misfires and then it hauls. So it only misfires/hesitates at WOT.

I also noted a small amount of steam coming from the passenger side bank crank breather that i havent seen before. No pressure though.

Do you guys have any idea? The day before it started acting up i got a call from a man who wanted to do a trade with me for the cougar, so i really need to adress this problem or at least find the cause of it so i can call the deal off...

 
What is your timing set at initial and all in? Is your vac advance on ported or manifold vac. What you described in your run sounds like detonation to me, most likely caused by too much advance in the timing at WOT, so either too much initial timing or too much mechanical or vac advance.

 
Are you running points in your distributor? If so, did you check the dwell?

Inside of distributor cap clean and dry? And while you're there, how does the rotor look?

What carburetor, mechanical or vacuum secondaries?

Fuel fresh?

Fuel filter?

 
I think in a previous thread he said he was running an HEI style distributor. That's what pointed me in the direction of advance being too much. He has also posted a while back about switching from Ported to Manifold vac for his advance.

 
Yes i have a GM HEI style coil-in-cap distributor. I now have initial at 12 and running ported vacuum. Total seems about 34 ish? A bit hard to see.

I have checked the cap and rotor and they both look fine.

I run a edelbrock 750 carb right now so mechanical secondaries.

Fuel is fresh, havent checked fuel filter but its just about 3 weeks old. Its one of those edelbrock metal filters so you cant see through em. Still earlier i used a see trough filter and never had any debris in it.

The misfires happening now can be described almost like you would hit a rev limiter for just under a second (and in this time it pops about 2-3 times) then it just goes. The only sound that comes to mind are "snap, crackle and pop"  :shootself:

It also crackles a bit when i rev it up slowly.

Note that this does not happen all the time, sometimes it just runs as normal.

And if i floor it from a stand still it just roasts the tires up to 3500rpm and then a snap and a crackle and then continues to roar up to 5.5K as usual.

I feel no real loss in power either.

And the crankcase smoke worries me a bit... even though its not so much.

 
Plug wires that are in poor condition or that have been pulled on can have conductivity problems that show up as load increases.

I had very similar problems last summer when one plug wire somehow got pulled from the plug just enough that the boot was on and the connector was not.

Plugs shouldn't crack, it suggest mishandling of the plugs, which suggests to me mishandling of the wires.

You could use a multi meter and check the resistance of each wire.

 
It sounds like it could be going lean during the transition from cruise to WOT. If thats the case, the Edelbrock calibration kit may be the fix. It has stepped metering rods

 
It sounds like the mixture is not right during transition from cruise to WOT. The Edelbrock calibration kit may allow you to fix the problem. The kit contains stepped metering rods, springs, and jets. Each kit is specific to the number of the carb. Chuck

 
EDIT: Measured the wires, they are OK! Also replaced fuel filter.

I think im going to reinstall the old stock points distributor tomorrow.

The wierd thing is, i have driven like 600 miles with this carb so far, and as long as i have had the dist at ported vacuum it has had no misfire problems at all. And then suddenly while idling its like everything falls apart.

It feels while driving that this problem is fuel related somehow though, but judging by the plugs its neither rich or lean so i guess not....

I noticed now, if i hold the car at 4000 rpm, and floor it to upp to 5000 rpm, then let off back to 4k, and up to 5 again, and so on. The more times i do it in a row, the more it starts to misfire. This also makes it feel that it is fuel related, at least to me...

On this crankcase smoke, how worried should i be? It does not seem to be any pressure, if i hold a piece of paper over the hole it does not move.

But it sort of "puffs" out smoke. It also comes a little out of the dipstick tube if i lift the stick with the engine running.

I have run this car about 4000 miles since the last oil change and the oil almost looks brand new, and it hasnt lost any either. About 6K miles since it was rebuilt.

 
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I have now replaced fuel filter and distributor with cables, maybe a little difference but not much, still the same problem...

 
That's not smoke, but is vapor from moisture and volatiles. Because of the miss-firing you may be getting some additional raw gas in the crankcase. Make sure your PCV system is working properly. As long as it isn't puffing out (caused by bad rings and bypass) you should be fine. It's​ normal to get some out of a warmed up engine.

Re-read Chuck's post, and rebuild your carburetor while you're at it.

 
Well it sort of puffs out, but there is no pressure. If i remove the breather the smoke comes out in small puffs, but if i put a thin piece of paper over the hole the paper does not move.

Anyway, pulled the carb apart today. Did not notice anything out of the ordinary. Cleaned it up and put new gaskets in it and put it together. And of course nothing.

I think this problem is internal....

 
Check your fuel pressure and delivery rate.

I agree with Mike, probably time to check your compression. With your engine running and PCV connected you should be sucking air in through the oil fill. I'd also check the valve train if everything else checks out.

 
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uuugh i hate to compression test a cleveland, there is so little room. I cant even get my screw-in compression tester down there...

Would anything in the valvetrain cause this smoke? Im thinking since the bottom end is rebuilt and the heads are not stupidly enough. Since this car ran for quite some time without an oil pump, all 8 cylinders where badly scratched and all rod and main journals too. So i would gess the heads are not in so good condition.

Hopefully the bottom end is ok

 
I don't find the compression test to be too bad on a Cleveland. The guage pops off of the hose on my tester. So I just fish the hose down by itself, turn the hose, let it catch and then hand tighten it. I snap the gauge back on and test.

 
Worn exhaust valve guides will cause excess pressure in the crankcase. Pull the valve covers, and it'll be obvious if they are.

Check the rocker arms for correct and full movement.

 
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My gauge does not pop off, therefore its very complicated if not impossible to do a compression test. But maybe i can get hold of one thats better.

Then i would be almost 100% sure that it is valve guide related. I really need to get new heads if i keep this car...

Will check this out..

 
No compression test yet but today i adjusted the idle mixture a bit leaner. And the misfires are gone. But it runs like crap when the engine is at low temperature though.

Also it drops a bit in the rpms both at idle and when driving now and then.

When idling it can drop from 900 rpm to 600 rpm, and then up again.

Have checked for vac leaks again, but nothing...

Also, it almost doesent respond to the idle mixtures screws at all. Same with the last edelbrock 600 and the holley 650 i had. I can turn them all the way in, almost no change in vacuum or idle, and i can back them out many many turns without a difference.

 
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