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Old 09-22-2012, 10:03 PM
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Impossible to find misfire

I have a 406 SBC with a misfire. I have tried everything I know how to find it and I have found next to nothing.


A little history. Its a new build with about 600 miles on it. It's a mild street engine with a street/strip 276HR hydraulic roller cam, AFR 195 heads, 9.8:1 compression, hypereutectic pistons, Holley 770 Street Avenger carb, Mallory HEI distributor. The entire engine is brand new from carb to pan with the exception of the block.

It misfires most when it's cold, then seems to almost go away within about a minute of firing it up. I have done a compression check, and all are between 155 and 170. I did a running compression check and all are 100-110 or so. I've tried a different set of wires, new set of plugs, and a new cap for the already new distributor. When I take an infrared temperature, there isn't any one cylinder that is low. For some reason, cylinders 5 and 6 are a little higher than all the rest, but I can't explain that and not sure if it matters anyway.

The ONE thing that I found when changing plugs is cylinder 3 looks a LITTLE wet. It doesn't look bad, but there is only 600 miles on the plugs. A fresh set of ACDelco plugs (gapped .040) misfire immediately also. I tried changing the plug wire, again, on that cylinder only and there was no change at all.

I took the carb to a specialist just to have it checked out and it is perfect. The exhaust smells a little rich, so I wanted to make sure there wasn't a problem with it. He bolted it on a test engine and it ran like a top.

I'm lost at this point. I have no idea what to check or even how to narrow down which damn cylinder it is.

Any help would be greatly appreciated

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Old 09-22-2012, 10:51 PM
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Curious why and how you did a running compression test. A reliable compression test is done with the throttle at WOT while cranking.
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Old 09-22-2012, 11:13 PM
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What kind of intake?
Could just be cold intake runners, fuel puddling, lean misfires till it warms up. Might still be there till it warms up fully.
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Old 09-22-2012, 11:28 PM
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Is the car under load when it misfires or idle?what rpm is the miss the worst? hows the power when warmed up?
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Old 09-23-2012, 10:41 AM
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I did the standard compression test also, but the carb guy I went to suggested also doing a running compression check, so I did that as well. It's just using the gauge with the engine running instead of cranking at WOT and the plugs out.

The intake is an Edelbrock RPM Air Gap.

You can hear it misfire anytime it has not been run. Under load, at idle, or a little revved up.
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Old 09-23-2012, 10:50 AM
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Do a cylinder balance test using a manifold vacuum gauge to find the weak/missing cylinder.
Dis able the spark one cyl at a time notting the vac drop.

Remove the valve covers and watch the valves move. Bad cam bad lifter
Try a different distributor.
Install a ground strap from engine to body firewall. (this is a must with a HEI dis.)
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Old 09-23-2012, 10:53 AM
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Typical no heat to the plenum syndrome. Cold intakes don't work well except for WOT.
JMHO
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Old 09-23-2012, 12:06 PM
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pull spark plug wires while its running and try to see if one is changing the tone more or less when you pull the wire.
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Old 09-23-2012, 08:45 PM
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How much drop should there be? The cam is a little lumpy and the vacuum gauge doesn't exactly stay steady.

I already tried a different distributor and I replaced the ground straps when I did the engine. I'll check them though. The battery is bolted to the passenger head anyway, so that should be a hell of a ground already.

Any way I can test the manifold theory before spending money again on a new manifold?

I will try disconnecting the plugs to see what changes.
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Old 09-23-2012, 09:32 PM
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Ford used to use a heat box under their carbs. You could look into one of these if the rest of your checks don't work out. Easier than swapping intakes.
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Old 09-24-2012, 07:04 AM
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Quote:
Originally Posted by Pre-Tuner View Post
I have a 406 SBC with a misfire. I have tried everything I know how to find it and I have found next to nothing.


A little history. Its a new build with about 600 miles on it. It's a mild street engine with a street/strip 276HR hydraulic roller cam, AFR 195 heads, 9.8:1 compression, hypereutectic pistons, Holley 770 Street Avenger carb, Mallory HEI distributor. The entire engine is brand new from carb to pan with the exception of the block.

It misfires most when it's cold, then seems to almost go away within about a minute of firing it up. I have done a compression check, and all are between 155 and 170. I did a running compression check and all are 100-110 or so. I've tried a different set of wires, new set of plugs, and a new cap for the already new distributor. When I take an infrared temperature, there isn't any one cylinder that is low. For some reason, cylinders 5 and 6 are a little higher than all the rest, but I can't explain that and not sure if it matters anyway.

The ONE thing that I found when changing plugs is cylinder 3 looks a LITTLE wet. It doesn't look bad, but there is only 600 miles on the plugs. A fresh set of ACDelco plugs (gapped .040) misfire immediately also. I tried changing the plug wire, again, on that cylinder only and there was no change at all.

I took the carb to a specialist just to have it checked out and it is perfect. The exhaust smells a little rich, so I wanted to make sure there wasn't a problem with it. He bolted it on a test engine and it ran like a top.

I'm lost at this point. I have no idea what to check or even how to narrow down which damn cylinder it is.

Any help would be greatly appreciated
Does the miss ever go away completely, like when you bring the rpm up? Is it consistent (same cylinder) or does it vary?

Check for arcing plug wires w/the engine running, hood up, in the dark to see.

What ignition system are you using? Cap spotless inside?
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Old 09-24-2012, 09:44 AM
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How about a lifter that is haning up?

I had a very similar problem with my 350 where every once in a while it had a intermittent miss and I went through the plugs, wires, vacuum lines, distributor and a few other things trying to figure where it was.

Add one bottle of Marvel Mystery oil into the crankcase and see if the problem goes away.

For me that was all it took and I never had the problem again.

It feels just like an electrical problem and until a good friend mentioned the lifter I had never even thought about it.

Hope that helps Jimbo
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Old 09-24-2012, 03:40 PM
406cu.in. of tire smokin' fun
 

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The spacer isn't going to work for me because I am already too close to the hood, even with a dropped air cleaner. I'm not sure what that would do for me anyway because the coolant would be cold still. Maybe I'm missing something.

I just checked the cap and it is flawless. All of the terminals look clean and nothing is built up on them.

I started the truck in the garage with the doors closed (don't try this at home) and lights off and couldn't see anything arcing. It was pitch black and I looked very closely.

I'm using a Mallory HEI billet unit with MSD wires. All is new.

The theory behind the Marvel Mystery Oil is that it will free up a lifter if one is sticking? At this point, I'll try anything.

I just tried unplugging each wire with a vacuum gauge connected and the problem goes away so fast I couldn't get to all of them before it went away. I'll try again either later or tomorrow.
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Old 09-24-2012, 04:14 PM
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[QUOTE=Pre-Tuner;1593539]The spacer isn't going to work for me because I am already too close to the hood, even with a dropped air cleaner. I'm not sure what that would do for me anyway because the coolant would be cold still. Maybe I'm missing something.

It warms up right quick. You tie it in to your heater hoses. Outlet from the intake.
When you get the motor up to temp check the plenum temp with a temp gun or your finger. See how cold it is! You'll be suprised!!
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Old 09-24-2012, 06:34 PM
406cu.in. of tire smokin' fun
 

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I looked into the mystery oil and it seems that is more for older engines that might have some sludge build up. This engine is new, but like I said, I'm gonna try anyway.

One thing I forgot to mention, and I don't think it would matter, but I am using pure gas (no ethanol). It is 93 octane still.

Also, I am running 20 degrees of initial timing, but I don't think that will matter either as it seems right in line with what I hear other people running with a 276hr cam.
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