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Discussion Starter · #1 ·
okay guys my 468 has been missing out lately and running poorly, I checked the basics first made sure I had good spark and the carb was in working order. While checking this out I noticed a lot of smoke blowing out of the left side breather, so I performed a compression test on all cylinders and it looked like this. #1 200 #2 100
#3 195 #4 150
#5 195 #6 025
#7 190 #8 130
Anybody have any ideas what I'm looking at here is it broken rings or blown head gasket???
 

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okay guys my 468 has been missing out lately and running poorly, I checked the basics first made sure I had good spark and the carb was in working order. While checking this out I noticed a lot of smoke blowing out of the left side breather, so I performed a compression test on all cylinders and it looked like this. #1 200 #2 100
#3 195 #4 150
#5 195 #6 025
#7 190 #8 130
Anybody have any ideas what I'm looking at here is it broken rings or blown head gasket???
doesn't look good, for # 2, # 4 # 8 , and especially # 6, just asking, ? did you take all plug's out, unhook coil, and open throttle wide open and crank each cyclinder? just asking, as # 6 appears dead, 1,3,5,7, ok, #2,#4,#8, big difference between each?

to answer your question, ring's ?? ya, head gasket? any foam in radiator?
how's oil look?
 

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Discussion Starter · #3 ·
the oil looks ok and no problems with the antifreeze either, I did unhook the coil and held the carb open with a stiff spring. I pulled each plug and took my readings then replaced the plug, i also had a battery charger on to make sure the battery was keeping a full charge. Right after the engine was built it blew the fire ring out between cylinders #2 and #4 and it ran the same as it does now. I'm just worried since #6 is down so low.
 

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You should try it without any of the spark plugs installed, crank it until you get the highest reading, any reading below 190 psi add a little oil to that cylinder. If reading is noticably higher, suspect rings. Still low could be head gasket, valves, hole in piston, or ?. Hope this helps.
 

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I like to see no more than a 10% variance between all cylinders (highest to lowest). I just did a compression test on a 350 for my dad yesterday. This engine will go in a '55 step side that he's restoring. The lowest cylinder read 135psi and the highest was 180psi. Most were in the 140-145psi range. Needless to say the engine will be getting rebuilt. Make sure you have ALL the plugs out of the engine when testing. Otherwise you might not be able to spin the engine over fast enough to get an accurate reading. Try squirting some oil into the cylinder that is reading low and check it again. You may see the compression go up. A sure sign of fatigued rings/cylinder wear. You may also have some issues with the valve guides/seats. If you're only seeing the problem on one cylinder then look at the valve guides/seat. I suppose a blown head gasket could cause similar readings, but I would expect to see the readings low on adjacent cylinders.

HTH....
 

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Discussion Starter · #7 ·
Thanks alot for the insight guys, I'll go back and redo my compression test tonite and see what the deal is. I did put a boroscope down cylinder #6 and couldnt see any damage.
 

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So two things pop out of numbers like that:

First obviously #6 is way low to the point of being dead. Might want to do the old fashioned cylinder balance test where you pull the plug wire on #6 while the engine is idling. If the idle does not change that cylinder is dead. At which point it almost does not matter why as the engine has to come apart to fix it. You can do the oil check as mentioned to localize it to rings or not rings.

The other thing that jumps out on those numbers is the difference between left bank and right bank. It would seem that you have had something distressing the right bank of cylinders. The question is what/why? About the only thing isolated to a bank that way is the exhaust and maybe (long shot) cooling?
 

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Do you know an aircraft mechanic?
Get and use a leakdown tester, its how aviation recips are tested

has a calibrated orifice, you throw 80 PSI to the tool, it has 2 gauges, the rate of leakdown is how you can judge a cylinder, but a nice thing about it, is if you have a low cylinder, say 80/40 you can listen t the exhaust, or the intake, or the breather, and youll know where the leak is.'Only hitch on a car, is youll have to spin the cylinder to ITS TDC and watch the rocker arms to make sure both valves are closed.

Sometimes carbon or crap can get between the valve and seat, so we " stake " the valve with a rubber mallet, sometimes brings a cylinder way up

On a plane, 60 is the lowest you can go, 70-75 is really good
 

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Discussion Starter · #11 ·
It has a lunati solid roller everything is adjusted there, Im reasonably sure it cant be carbon buildup the motor has less than a thousand miles on it
 

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the oil looks ok and no problems with the antifreeze either, I did unhook the coil and held the carb open with a stiff spring. I pulled each plug and took my readings then replaced the plug, i also had a battery charger on to make sure the battery was keeping a full charge. Right after the engine was built it blew the fire ring out between cylinders #2 and #4 and it ran the same as it does now. I'm just worried since #6 is down so low.
That right bank cyl head seal is suspect.You have already had a head gasket fail and now possibly another.I would pull that head and use a straight edge to check your head and block surface for level.
 

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70IMP,GOT A NAME?

Anyway,not only is # 6 comp low but you need to realized cyl's 2,6 & 8 are way too low too when compaired to other cyls running in the 190's,thats way to much variation.

You have approx 190-195psi comp on the odds cyls which is good but then you have 100/150/025/130 on the odd cyls,something is definately wrong with that even bank cyl head/valves or piston/s & rings. You should see not much more then a 10% diff cyl to cyl when taking a comp test and your way off that by a longshot on 4 cyls. The cyl with 025 pld comp could be the cyl with all the blowby your seeing.

Incorrect valve adjustment(too tight or way too loose) but that would not explain the blowby unless a valve/s was adjusted soo tight one hit & broke a psiton,broken v/springs & bent valves (poss over reving/floated valves),broken pistons & or rings(poss due to detonation or too tight),wiped cam lobes/lifters,bad head gasket,was motor overheated enough to warp the even #'s cyl head or valves?

Any one of these or combination of these things could cause low compression & excessive blowby.

Does this motor run stock ex manifold with heat riser?

If yes maybe its been stuck closed or patially closed overheating that head causing your problem.

Or if running headers maybe the muffler for that side has partially collapsed inside casuing too much back pressure resulting in over heating that head. I have heard of this happening a few times over the yrs esp if the motor has backfired at one time or another with out you realizing it . That could blow/force/or bend one or more of the baffles inside the muffler shut creating too much back pressure. That would then overheat that head all the time prematurly burning the ex valves or warping them,blowing the head gasket,etc.

Did the motor run a little warmer then you think it should esp in hot weather?

But again the excessive blowby says there is something deeper going on and it may be a combination of a head problem along with broken or cracked piston and or rings.

It's looking to me like you will need to break down the motor to see whats really going on if you dont come up with anything after you check/try the things i and others have already suggested you should do.

Scott
 

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Discussion Starter · #15 ·
Well guys I finally pulled the right side cylinder head and found three melted pistons, looks like I get to redo a motor. geez this hobby is so much fun:noway:
 
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