Whats up with this?
 

540 ci, 4.5bore,4.25 stroke
merlin cast iron heads
cometic head gaskets
crane hydraulic roller
10-71 mooneyham
intercooler richard lee
7lbs boost

Notice skip in engine. hold throttle at 2500 and here a slight
break up in engine coming from 1,3,5,7 side exhaust.
Pulled spark plugs and checked spark. OK
all plugs looked nice and tan with the exception of # 7
looks like slight deposits of white specs on electrode.(Alumminum)
Compression check all 150psi.
Change plugs Ngk5673-8 run engine, all start to change color
at electrode. Except 5 and 7 they are cold and wet????
Are cylinders 5 and 7 firing? feel the water jacketed headers
after running on trailer all are hot to touch except 5 and 7, warm only. I am suspecting maybe head gasket? Can it be bad
with 150psi compression all cylinders? Put air hose in # 7 to see if it bleads into # 5 nothing???I also pulled valve cover to examine valve movement all looks good.

What do you guys think.

I will tear it down just want to make sure I have not over looked
anything.

Thanks in advance for your help and comments
Gerry

Re: Whats up with this?
 

Hi Gerry:

This might be a possibility. Due to the firing order 18436572 the #5 spark can jump to the #7 wire, so #5 doesn't fire properly and #7 fires too soon. This would fit with all your observations. The cure is to prevent this crossfire from occurring with good plug wires and not running the #5 and #7 wires too close to one another all the way from the distributor to the head. Those are nice engines, I hope this helps.

Re: Whats up with this?
 

I will take two wires off the other engine and try running them also.
Good Idea.

Is it possible to have good compression (150lbs) across the
1-3-5-7 bank and yet have a bad head gasket or burned valve?

Re: Whats up with this?
 

I wouldn't think so, but a leakdown test would be more accurate for detecting difference between cylinders. Even if you find lower leakdown numbers on #5 and #7 the root cause of the problem has to be discovered. Bad head gaskets are usually the result of heat/detonation aren't they?

Re: Whats up with this?
 

ok after working on this all day. I tried just about everything
I could think of inserted air in # 5 . No leakage into 7. Did detect some leakage int exhaust. But this occured on
even the firing cylinders. Very slight. Stuck a borescope down
# 7 couldnt see anything for $350 its pretty useless.

Well I decided to tear it apart.The cylinders look fine, pistons
little carbon on top. Pulled the exhaust valves nothing just a nice margin Only 10 hours on the heads. However #5 and 7 had about
1/4 full with fuel. Yes fuel. Flip the blower over and take notice that it feeds the back cylinders first. Dont even tell me its in the carbs. So I filled the carbs thru the breather vents. To my
suprise the fuel is comming out of the boost reference hose.
Bad power valve!! Duh!!! The boost reference hose attaches at the manifold near distributor. I think It was drowning out 5 and 7.
So I tru the head back on. Gotta love the Cometics. I need a set of manifold gaskets and Im good to go. It has to be it.

Lesson learned drink more beer and chill out!!!
Can you imagine Im probrobly the only one who pulls a
head to change the powervalves

I guess a little luck dosnt hurt either:D

I hope this is the end of this thread
Thanks Tomcat
Gerry:o

Re: Whats up with this?
 

Hey Gerry, it was a lot of work but you just added a new specification that needs to be followed for all blower engines with boost referenced power valves.

The line from the intake to the carb has to be clear plastic like what Mercury uses from the mechanical fuel pump to the flame arrestor. And it needs to be installed with a loop like a bilge pump line or fuel vent line. If you ever see fuel in the loop you have a blown power valve.

Thanks for doing the leg work on this; now we're all a little smarter!

Tom