Rear cylinder running leaner than front cylinder

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57pan
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Rear cylinder running leaner than front cylinder

#1

Post by 57pan » Mon Nov 06, 2006 6:32 pm

The front plug is dark brown (not black) and the rear is nearly white.
I have pressure tested my manifold numerous times and detect no leaks at the o-rings or valve cover screws. Compression is dead even on both cylinders at 105.
Where else should I be looking for an intake leak???

Or could it be a timing issue (I mean that the timing on the front cylinder is not the same as the rear because of worn circuit breaker parts)?



Pantony
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Re: Rear cylinder running leaner than front cylinder

#2

Post by Pantony » Mon Nov 06, 2006 11:13 pm

Nope the circuit breaker is dual fire they will fire both plugs usually right on, you have a hairline crack in your head somewhere (which i think is opening up when it gets hot), or do you have a dual point single fire ignition, what year bike? spray the thing down with brakecleen when it's running see if you can isolate it that way

Cotten
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Re: Rear cylinder running leaner than front cylinder

#3

Post by Cotten » Tue Nov 07, 2006 1:23 am

When testing the manifold, I often then turn the motor through to pressurize each combustion chamber. Then bubbles will tell you if you have a headgasket or sparkplug leak, and you can crank the pressure for a really thorough crack search.

A worn points cam is certainly something to suspect.
But don't forget to try simply to switch wires from plug to plug.

....Cotten

57pan
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Re: Rear cylinder running leaner than front cylinder

#4

Post by 57pan » Tue Nov 07, 2006 4:43 pm

Pantony - it's a 57, single points. What's the theory behind the BrakeCleen test? What are you looking for? The possibility of a hairline crack crossed my mind too - sure hope I don't have to go down that road.

Cotten - I have rolled it over to pressurize the cylinder and found that there were bubbles around the rear plug. I was able to resolve that by using teflon tape on the plug threads. I will try that test again though and pay more attention to the head gasket seal this time.

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Re: Rear cylinder running leaner than front cylinder

#5

Post by Pantony » Wed Nov 08, 2006 3:45 am

The Brakcleen gets sucked in a vacuum leak and the motor will change rpm, sometimes an intake leak won't surface any other way than the old way of finding it, as in when it's idling (maximum vacuum) spray the intake joints and if it stumbles you have a leak.

57pan
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Re: Rear cylinder running leaner than front cylinder

#6

Post by 57pan » Fri Jan 19, 2007 9:59 pm

I thought I would update this thread and see if you guys think that I have found the problem.

I did try to check the head gasket for leaks but I was using a small portable (1 gallon) compressor and I just don't think it had enough volume to adequately test the head gasket. At any rate, I did not find any bubbles that indicated a leak.

I removed the heads anyway and found that the rear head gasket was in fact leaking right over by the rear exhaust spigot.

Here's a picture of the plugs showing the rich front cyl. and the lean rear cyl.

Image

Here is the evidence of the gasket leak on the cylinder:

Image

And here is the evidence of the leak on the head:

Image

So, what do you guys think... is this enough of a leak to cause the difference in plug readings that I am seeing?

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Re: Rear cylinder running leaner than front cylinder

#7

Post by Cotten » Sat Jan 20, 2007 3:27 am

Absolutely, any source of oxygen will affect the combustion.

Those aren't copper gaskets, are they?


...Cotten

57pan
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Re: Rear cylinder running leaner than front cylinder

#8

Post by 57pan » Sat Jan 20, 2007 6:42 pm

Yes, copper gaskets. I have a set of James teflon-coated gaskets for when I put it back together.

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Re: Rear cylinder running leaner than front cylinder

#9

Post by Panacea » Sat Jan 20, 2007 9:25 pm

Were the head bolts tight when you took it apart? sometimes they don't get re-torqued soon enough to avoid problems. Does the lobe on the breaker move at all when you try to jiggle it? Mike

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Re: Rear cylinder running leaner than front cylinder

#10

Post by Cotten » Sun Jan 21, 2007 4:36 am

Panheads are aluminum with steel inserts cast-in to accept the headbolts and sparkplug. Even the seats are retained by having the molten aluminum poured in around them.

Both metals are malleable, and after millions of compression strokes (or overtorques) the inserts commonly "pull" slightly beyond the rest of the headgasket surface.

The star-like impression around the headbolt hole shows its protrusion.

To make things worse, the aluminum casting itself will recede or contract where the gasket surface is closest to the ports.

So the headgasket surface should always be flattened before assembly, or these high and low spots can easily become a breach.

....Cotten
PS: I do not believe the Factory ever spec'd copper gaskets for Panheads.

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