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Mysterious 406 malady

 
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gunner34275
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PostPosted: Sat Nov 02, 2013 11:56 am    Post subject: Mysterious 406 malady Reply with quote

Could use help from some of you engine gurus...

Purchased a running Chevy 400. (Owner claimed around 8K miles and subsequent examination indicates this is probably close.) It had been sitting for some time and it sounded like it had a pretty good miss but nothing clanked, rattled or smoked so I thought just a bad plug/tune-up...no big deal.

When I lit it up at home I found number eight was not firing. Pulled all the plugs and sure enough, eight was grimy black, the rest pretty normal. Put in a new set, nothing changed. Checked for voltage with a timing light, spark checker and just laying the plug against the block. All agreed there was fire going to the cylinder. Changed the wire anyway, same results. Tried different cap/rotor, no change.

(Later, just to be sure there wasn't some anomaly with the HEI, I moved the distributor over a tooth and re-timed moving the wires one terminal over as well. Figured if there was a problem, the next cylinder would go out and 8 come to life. Nope, eight's still out.)

Time for a compression check. The results ranged from 160 to 175 warm. Number eight was 175. Took a look into number eight at TDC and lo & behold the piston is still shiny. The rest showed normal carbonizing. Uh-oh, bad camshaft. Wondered why there wasn't any tappet noise but figured that was it. Pulled the valve covers and rotated through the order watching the valve events. All, including eight, seemed to have he same amount of action. Later I would use a "windowed" valve cover to watch the valves while running. Even "strobed" them with the timing light. All appeared same and normal in motion.

Decided something must have been left in the intake tract or a major gasket problem existed. Pulled the manifold. Nothing there. Took a look at the lifters etc. at the same time, all looked fine. Running out of ideas. Decided to check vacuum and got rock steady 21 inches at idle, drop to zero when revved, return right back to 21 inches steady.

There were some other clues involved besides the shiny piston. (Which turns out to have a small area showing some carbon. See pic) This motor has been trying to force oil out every possible opening. There is evidence where the previous owners heave repeatedly tried to stop leaks even from places like the distributor gasket and crank bolt. That would seem to indicate the cylinder charge is not exiting the valves but being forced into the crankcase. In addition, the number eight upper bearing shell shows signs of being hammered by detonation. (See pic)


Starting to clutch at straws here. Had already been to machine shop and several local builders to compare notes. General consensus began to be a faulty cam grind index. Pretty far fetched and disassembly doesn't show any overt evidence. I know seven and eight look out of sync anyway, but comparing the camshaft, (General Kinetics #115850), to stock photos shows no gross misalignment. Don't yet have a new, or another old, cam to compare with. Am down to the short block now. Number eight piston is out of the hole and those valves are also disassembled. There is no further visual evidence, (which I can detect,) of what the problem may be. I did rotate the assembly and confirm norm indexing of the aftermarket crank just for the sake of form.

Any suggestions or thoughts appreciated.



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SMOKEmUP
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1979 Chevrolet Camaro

PostPosted: Sat Nov 02, 2013 2:18 pm    Post subject: Reply with quote

Sounds like it's all torn down, so why not completely pull it apart and rebuild it?
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clay
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1972 Chevrolet Nova

PostPosted: Sat Nov 02, 2013 8:34 pm    Post subject: Reply with quote

Don't know if I qualify as a guru but here's some things I would look at. If this engine has a dual plane intake are there any vaccum fittings in this runner? Maybe a vacuum leak external somewhere. Did you ever try another carburetor. What did the piston on #4 look like? It would have been paired with #8 in the intake plenum if it was dual plane and they may have been strarved for fuel if there was a carb problem. Since this appears to be a fairly fresh engine I would knock another piston out and see what the bearing looks like and also check the area just under the top ring. It should be clean with no signs of carbon if the rings are sealing. Are there any shiny places in the cylinder that line up with the head bolt holes? This could indicate an out of round cylinder from not torque plate honing and could be causing blowby. Check the valve cover breathers to make sure they are open. There are gromets that are closed for some reason - make sure everything is open. If it has a pcv system it should be able to pull a vacuum on the engine at idle unless there is a serious blowby issue. Wish I had a definate answer but keep digging. Welcome to Smokemup! Clay
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Paul P
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1971 Chevrolet Chevelle

PostPosted: Sat Nov 02, 2013 11:18 pm    Post subject: Reply with quote

That bearing shell looks like it needs attention. I would rebuild it and measure up everything again. Use a single plane manifold if you can fit it under the hood. There is plenty of torque in these motors and it will give you quite more power on the top end.
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1971 - Chevelle 408 SBC N/A
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Skunkworkx
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1968 Chevrolet Camaro

PostPosted: Sun Nov 03, 2013 10:53 am    Post subject: Reply with quote

Another far fetched idea.
Is the #8 exhaust tube clear ? Nothing out/nothing in ?
You already checked everything I could think of right now.

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af2
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PostPosted: Sun Nov 03, 2013 8:13 pm    Post subject: Reply with quote

I really want to wait till the next post..... Too long of a first post to even start.
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squeeezer
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1991 Chevrolet Camaro

PostPosted: Tue Nov 05, 2013 6:50 pm    Post subject: Reply with quote

Valve seal,valve guide,wrong finish on #8 bore
Sounds like its consuming oil
And signs of detonation
Hhhmmmmmm.....

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Knarley Darley
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PostPosted: Wed Nov 06, 2013 3:33 am    Post subject: Reply with quote

The valve adjustment was barely too tight, and it compression checked OK at cranking speed, but running the lifters pump a bit more, and it has no compression.Just curious, if you tried backing off the valve adjustment on that cylinder while it was running? Did the valve adjusters seem over tightened when you disassembled it? Did the cylinder sound dead when it was running? You pretty much covered everything else.
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gunner34275
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PostPosted: Wed Nov 06, 2013 9:37 am    Post subject: Reply with quote

Thanks for the thoughts guys. Sorry for the delay getting back.

It is a Chinese dual plane. There aren't any vacuum fittings on that runner. Tried two different carbs with no effect or change. Number four looks just like the other six, pretty normal. I will be looking at a few more pistons before starting the rebuild. Crankcase ventilation is not a problem with the exception of the #8 cylinder apparently venting its charge "backwards". There isn't any exhaust obstruction either. The valves have been tried at several different "settings" with no change.

As I said, I involved the best gearheads I know here, including one who was formerly a Mopar Team Pro-Stock engine builder and now owns our local machine shop. And another who was formerly part of the Chicago street racing scene. We're all stumped.

I'm going to extend the tear-down and order appropriate parts for a rebuild. When the mystery is solved I'll post the info here...Thanks again.
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Paul P
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1971 Chevrolet Chevelle

PostPosted: Wed Nov 06, 2013 11:34 am    Post subject: Reply with quote

Get rid of the chinese intake and put a good single plane intake on it. Small Block 400s do not really lose much on the bottom going to a single plane intake. They already have enough displacement to make up for it. This is of course if hood clearance is not an issue.
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2001 Focus 2.0 Zetec
stock cams, bolt-ons and tune
15.63@87 MPH 1/4mi

1971 - Chevelle 408 SBC N/A
6.86@102.5 MPH 1/8mi
10.78@122 MPH 1/4mi
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