Backfiring through pipes.

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Backfiring through pipes.


Post by bonham3 » Sun Jun 12, 2005 1:45 am


I was having a charging system problem (see gen/voltage reg. thread)
Thought i resolved it. Fired up my panhead 48/50 went for a ride with
lights on running fine till about 8 miles out, then when slowing down
wicked backfiring through pipe/pipes not sure which one.
and throttle slugish. Once in 4 th gear and moving at 35-40 plus it ran fine.
Then on the slowdown backfiring again. Was wondering if this can relate
to any voltage reg./generator problem? Still think i may have a coil problem (dynacoil 3.00 ohm) Have a good spark at one plug but a lesser spark at the other plug. Checked the ohms on the coil they were 3.5
om my meter. Still have that spark when i turn the ignition off also.
Just trying to figure out one thing at a time. Bike was running fine before
the batt./charging problems. Lights were bright while on the 8 mile putt
and did not dim. I put a meter on the batt. while at idle and it showed
12.6 volts Reved it a little and the voltage rose a couple tenths of a volt.
Not sure if the charging problem is related to the backfiring or what?
Appreciate any patience and help with this problem

Thank's Ron. Bonham3

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Post by Cotten » Sun Jun 12, 2005 4:23 pm

Was the weather cool and moist?

Most often such backfiring is the result of exhaust leaks, and sucking in cool humid air makes it louder, not only by maximum expansion (not good for exhaust valves), but the dense air carries sound better!

This the only application where I rely upon a silicone product.

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Post by bonham3 » Sun Jun 12, 2005 6:23 pm

Hi Cotten

Thank's for replying.
It was a warm muggy night but it has been that way for a week plus
here. These were loud spark flying backfires. Bike ran fine for half an hour
before. Also had to keep feathering the throttle to keep bike alive. It did not stall and restarted O.K. but at idle or low throttle backfiring. Exhaust
is fine. Just strange that it cleared up when i was doing 35 mph plus and
steady. Soon as i slowed down and came to a stop it started again.

Ron bonham3

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Post by Cotten » Sun Jun 12, 2005 6:50 pm

Next question:
Are you running a carburetor support?

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Post by bonham3 » Sun Jun 12, 2005 7:47 pm


I have a Zenith carb going to an S&S intake. A support runs from the block bolted to and through the carb and intake manifold. (one bolt)
I am trying to rule out some other things also.
One at a time.
Carb, Coil, Anything to do with the Gen. or Regulator as i stated in my other posts. Thank's for helping out so far. Hope we can get to the bottom
of this. One question in the meantime i would appreciate. In two parts.
1. Is it normal for the coil to fire the plug when the igntion is turned off?
I mean, pull the spark plug-attach to plug wire- groung plug and switch ignition (key) on then "off" (it sparks once) I can also kick bike over and
i get spark (with ign. on). That is fine.
2. Stupid but???. I keep forgeting which wires go on what leads on the coil.
Had them off so often trying diff. things. I get spark no matter which way.
Luckily i have a spare coil. (single green wire to ign. module on one post)
red ign. module wire plus red + on the other post terminal) or vice versa.
Coil is a dyna coil 3.0 ohm. I measured 3.5 ohms with coil off the bike.
use accel 8.8 MM wires (yellow) silicone/graphite. resistance on the longer
wire is 6.5 ohms, shorter wire 3.6 ohms. Is this normal/O.K.
I get a brighter spark from one coil wire post and a less bright spark from
the other post.
hope you can help:)

Ron bonham3

P.S.: I will ask about the gen./reg. later and what happened on that and what i did about it so far. New batt., new ign. module and why?:)
Pulled and ck. gen., polarized it etc.

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Post by Cotten » Sun Jun 12, 2005 11:09 pm

That's a modern coil, so I must leave it up to others to give an opinion.

Otherwise, I would just check everything, from the ground strap to the intake manifold.

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Post by mbskeam » Tue Jun 14, 2005 5:19 am


check the simple things first, spark plugs, points and condensor(cheap)

I did have a coil that did do this to me last year, ran good in the morning (cool weather)and in the afternoon (Hot) it would miss on one cyl. put on new dyna coil and no more problems.

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Reply Back


Post by bonham3 » Thu Jun 16, 2005 2:29 am

Thank's for the note. Ignition module is fine, carb is fine, I was left with the coil myself so i was looking for a way to test it. I did meter it for ohms
It is listed as 3.0 but i read 3.5 Problem?
plugs are fine, wires tested fine. I also hoped i could get my answer to
why the plugs fires once when i turn my ignition key off and also which
leads go to which coil terminal (the Dyna is not marked) I have tried both
ways and i have spark but do not want to destroy anything.

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Post by SkyHogg » Sat Jun 25, 2005 7:43 am

Just be aware that any electrical device can develop problems when hot that don't show up when cold. It is possible to bench test a coil as OK, but when it heats up the expansion is enough to create an open (or possibly a short) condition. The next time you went out, did it run OK at first, and then after about a half hour do it again?
The spark at your plug is created when the magnetic field inside your coil collapses due to the opening of the points. I've never heard of turning off the ignition switch causing a spark, but I haven't tried it either. Are you running a stock wiring job? I'm trying to picture how this could happen, but it's almost 3:00 AM and I'm a little slow at the moment.



Post by Guest » Fri Jul 01, 2005 2:41 pm

bonham,are you saying the dyna coil does not have a + or - mark on it?

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Post by 64DUOGLIDE » Sat Nov 05, 2005 7:33 pm

Cotten, is there another post in the forums that describes your use of silicone? If not, can you elaborate?


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Post by kell » Sat Nov 05, 2005 8:36 pm

Yes you can make a spark by turning the ignition key off. That is perfectly normal if the points are closed when you turn the key off.
Don't worry about the 3.5 ohm measurement. Most cheap meters read a little high when you try to measure low resistance. The leads and what not have some resistance. Try touching the meter leads to each other. If you get a nonzero reading you can correct your coil reading by subtracting that amount from it.



Post by Guest » Wed Dec 28, 2005 2:07 am

MY pan did the same thing you described.I had two things wrong.NUmber one i had a coil that would malfuntion off and on.I didnt catch it right away because it still had spark when id test it but it would cause cutting out a certian speeds at different times so i replaced the coil and it stopped The second time my bike started backfiring a few years later i found out that some merry prankster had leaned out the needle on my linkert carb and that when i would cruise along and left off on the throtte or gave it more throttle that id get a crisp back fire.I finally found out my needle on my linkert had been turned inward and it leaned out my carb causing the backfire.The coil mishap was intermittant.The leaned out carb was letting off or on throttle.

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Re: Backfiring through pipes.


Post by 58flh » Fri Aug 10, 2012 12:43 pm

Check the S&S intake for a VAC-LEAK!---It will happen as they warm-up!--Just bubble-test it & brush plenty of soapywater on the manifold!---Also are you running a auto advance breaker or original grip set-up!--If yes to the Auto Advance, make-sure the weights are not sticking!-(clean up real good & lube the pinshafts with light grease or wd-40)-Also a return spring could be broken!--This is a symptom also when letting off throtle & BANG-out the pipes!---Easy enuff to check about 5min. to pull the points & cond. cover & have a look underneath.-----Richie, Also look at your plugs!--If ones leaned out looking you found your BUG!-Manifold LEAK!---Good-LUCK----Richie 8)

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