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Back firing question

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doc308
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Back firing question

#1

Post by doc308 » Sun Nov 07, 2010 3:02 pm

My '50 pan backfires quite often when letting off of the throttle between shifts. In examining the spark plugs, the front cylinder plug is absolutely perfect-looking, whereas the rear plug is burning excessively rich--very black/carboned-up.

Q's: Could the backfiring be function of excessive unburned fuel igniting erratically on thr rear plug unpon throttle let-off? I always thought that a lean condition would more typically cause backfiring.

I'm running Champion J12YC plugs. Is this plug roughly equivalent to Harley 3-4 heat range? I'm thinking of trying out a hotter plug in the rear cylinder to see if this helps. Which Champion plugs are equivalent to Harley 3's and 2's ?



Bosheff
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Re: Back firing question

#2

Post by Bosheff » Sun Nov 07, 2010 4:30 pm

Lean condition = backfiring through the carb, while a rich condition usually causes backfiring through the exhaust....bosheff

ozwick86
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Re: Back firing question

#3

Post by ozwick86 » Sun Nov 07, 2010 4:53 pm

You might want to check for holes in your exhaust which will cause this. My rear has a 1/8" hole and I get a little pop.

Cotten
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Re: Back firing question

#4

Post by Cotten » Sun Nov 07, 2010 4:54 pm

Doc308!

As Ozwick86 points out,
Backfiring when letting off the throttle is classic for exhaust system leaks in cool weather. It is hard on the exhaust valves, as cold air drawn in with the de-accelleration suddenly expands, sending a shock wave out both ends.

Your plug circumstance indicates another problem which is much more sinister. I would bet upon a vacuum leak to the front cylinder (at least).

Good luck with the bubbletest,

.....Cotten

doc308
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Re: Back firing question

#5

Post by doc308 » Sun Nov 07, 2010 5:52 pm

Thanks for the replies. Cotten--"bubble test"?

Cotten
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Re: Back firing question

#6

Post by Cotten » Sun Nov 07, 2010 6:39 pm

Doc308!

A discussion on pressuretesting with soapy water can be found at http://virtualindian.org/11techleaktest.html.

Please note that a #9 stopper or cork works as well as a testplate, if you have three hands. And it is the only way that the nipples can be tested separately from the nuts and assembly.

And with Panheads, it is wise to test the pan cover screw over each intake port as well.

Good luck!

....Cotten

doc308
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Re: Back firing question

#7

Post by doc308 » Sun Nov 07, 2010 10:56 pm

Thanks again, Cotton.
I failed to mention that my heads were redone to use the later-style rubber band manifold set-up. A long time ago, I recall seeing a mechanic spraying a mist of starter fluid around the manifold clamps/nuts while the bike was running to see if the idle increased. Is this method sound?

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Re: Back firing question

#8

Post by Robert Luland » Sun Nov 07, 2010 11:22 pm

Don’t worry! That’s what burn centers were designed for. Hell they can recreate your whole body at this point if you got a few years? Come on; let’s use a little common séance. You’re spraying a volatile liquid on a running engine with a loaded fuel tank above it. Give what I just stated thought for a moment.
Bob L

Cotten
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Re: Back firing question

#9

Post by Cotten » Mon Nov 08, 2010 2:52 pm

doc308 wrote:Thanks again, Cotton.
I failed to mention that my heads were redone to use the later-style rubber band manifold set-up. A long time ago, I recall seeing a mechanic spraying a mist of starter fluid around the manifold clamps/nuts while the bike was running to see if the idle increased. Is this method sound?
Doc!

You may have noticed the leaktesting article began with a warning against using ether.

The whole notion of spraying anything around a manifold on a running motor is inconclusive, unless there is an enormous leak. Bubbletesting, however, not only eliminates the variable vacuums of a running motor, it displays exactly where even the tiniest of leaks may be.
No leak is an acceptible leak.

O-ring conversions have trashed many a Panhead! You will want to test each nipple separately if the assembly shows any leaks at all.

Again,
Good luck!

....Cotten

duoglide58
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Re: Back firing question

#10

Post by duoglide58 » Wed Nov 10, 2010 1:40 am

When I bought my bike, it had big time leaks through the carburetor's throttle shaft bushings.
You might want to check those if you have not already.
Doug

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Re: Back firing question

#11

Post by Scrap » Thu Nov 11, 2010 5:33 am

Cotten wrote:Doc308!

A discussion on pressuretesting with soapy water can be found at http://virtualindian.org/11techleaktest.html.

Please note that a #9 stopper or cork works as well as a testplate, if you have three hands. And it is the only way that the nipples can be tested separately from the nuts and assembly.

And with Panheads, it is wise to test the pan cover screw over each intake port as well.

Good luck!

....Cotten
excuse my confusion, what is path the test would take to get from the air pressure supply and exit at the cover screw?

Cotten
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Re: Back firing question

#12

Post by Cotten » Thu Nov 11, 2010 2:30 pm

Scrap!

Over decades of abuse, many Pan heads have had the cover screw over the port drilled deeper and re-tapped. Sometimes they are drilled all the way through, or sometimes too long of a screw has cracked through to the port with little visible evidence.

So when pressure is applied to the manifold and ports, the air can come up and out the screw hole.
(Even casting porosities show up occasionally, between fins where you would never suspect.)

The fix can be as easy as teflon tape upon the screw, however it would always be best to have it welded up within the port.

....Cotten

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Re: Back firing question

#13

Post by Scrap » Fri Nov 12, 2010 1:43 am

Cotten wrote:Scrap!

Over decades of abuse, many Pan heads have had the cover screw over the port drilled deeper and re-tapped. Sometimes they are drilled all the way through, or sometimes too long of a screw has cracked through to the port with little visible evidence.

So when pressure is applied to the manifold and ports, the air can come up and out the screw hole.
(Even casting porosities show up occasionally, between fins where you would never suspect.)

The fix can be as easy as teflon tape upon the screw, however it would always be best to have it welded up within the port.

....Cotten
OK, Thanks, explains a lot, it's clear this is not your first rodeo. Do you know as much about women as you do about old bikes? I have a 57 Pan that I've owned for 35 years. I know and understand quite a bit about her. I've been married to a 56 Pam for 25 years and I still don't have a clue, can you help?
Regards
Scrap

Cotten
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Re: Back firing question

#14

Post by Cotten » Fri Nov 12, 2010 1:55 pm

Scrap wrote:OK, Thanks, explains a lot, it's clear this is not your first rodeo. Do you know as much about women as you do about old bikes? I have a 57 Pan that I've owned for 35 years. I know and understand quite a bit about her. I've been married to a 56 Pam for 25 years and I still don't have a clue, can you help?
Regards
Scrap
Scrap!

Well into my second marriage, I have finally learned to take it like a man!
I shuffle my feet, I scratch my head, and I say "Yes, Ma'am,... Yes, Ma'am."

....Cotten

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Re: Back firing question

#15

Post by George Greer » Fri Nov 12, 2010 2:28 pm

Cotten's got it right

Yes mamm, it the correct response....well..........most times

George

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