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The Vintage Japanese Motorcycle Owners Group: Discussion Forums

Vintage Japanese Motorcycle Owners Group :: View topic - XL350 KO backfires


XL350 KO backfires
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05c50
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Joined: Oct 26, 2007
Posts: 2948
Location: Greensburg Pa

PostPosted: Tue Aug 08, 2017 11:11 am    Post subject: XL350 KO backfires Reply with quote

OK, I need some thoughts and ideas. I'm working on an well beaten xl350 that will start on the first or second kick, will idle smoothly all day, but as soon as you crack the throttle, it back fires out the exhaust...... not a little putt, but it bangs like an eight cylinder engine with the plug wires crossed or lots of water in the tank.
Here's a run down on the bike: Good compression
Valves adjusted properly
Spark plug replaced
New OEM coil, condenser, high tension lead
Points cleaned and set to .014
Timing set and advance checked and working
Fuel tank and petcock cleaned, new fuel
New fuel line and filter
Carb cleaned
Float set and jets all OEM
With it idling, I checked for vacuum leaks at the boot/intake, the idle mixture screw does affect the idle. If I turn the throttle Very very slowly, the revs will pick up, but any sudden turn of throttle results in an ear splitting bang. Although this bike uses a battery for the lights, it's currently running without one. The ignition switch has been eliminated from the wiring.
The spark plug doesn't show signs of being over rich and the exhaust doesn't smell bad. Although it seems like an ignition problem, it only occurs when the throttle is twisted.
I know that I missed a few things, but I've run out of things to try. Got any thoughts?

...........Paul

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crazyeights
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Joined: Oct 26, 2016
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PostPosted: Tue Aug 08, 2017 3:56 pm    Post subject: Reply with quote

It would be a good idea to check the exhaust for blockage if you haven't already. Maybe it could be a clogged air box? Does the choke change anything? Good luck Cool
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05c50
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PostPosted: Tue Aug 08, 2017 4:07 pm    Post subject: Reply with quote

Exhaust isn't plugged, in fact there's a hole in the pipe, air box on or off makes no difference, pulling the choke at idle will kill the engine and pulling it while trying to rev doesn't change the backfiring. Thanks

.......Paul

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�77 CB750a
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�80 CB650c
�75 GL1000-finally finished
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68 CB450K1-taking up all my time
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spurlock
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Joined: Jul 08, 2014
Posts: 831
Location: Vacaville, CA

PostPosted: Tue Aug 08, 2017 5:18 pm    Post subject: Reply with quote

Hi Paul, I agree with you that sounds like ignition - under increased load the spark cuts in and out, letting fuel mixture into the exhaust until the spark comes back and ignites it. But you've replaced several major ignition components. Maybe try running a new wire from the empty black female bullet connector at the stator plug directly to the coil and on to the points. And leave the kill switch out of the equation. Just thinking possibly an internal break in the wiring somewhere might be causing intermittent spark. I suppose a bad stator winding could be the culprit as well, only cutting out when load and vibration increase. Good luck!

-Bill

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05c50
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PostPosted: Tue Aug 08, 2017 5:37 pm    Post subject: Reply with quote

Thanks for the thoughts Bill. I've already eliminated all the wiring except for a direct wire from the stator connector to the coil/points connector. A resistance test of the stator coil winding while shaking the wires doesn't show an intermittent/broken wire or connection.

........Paul

_________________
�77 CB550f
�77 CB750a
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�80 CB650c
�75 GL1000-finally finished
�79 KZ200
68 CB450K1-taking up all my time
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spurlock
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PostPosted: Tue Aug 08, 2017 5:55 pm    Post subject: Reply with quote

C'mon Paul, how do you expect us to give the right answer when you come back with "I already tried that"? Razz

I guess there could be an intermittent break in the actual stator windings which your test of shaking the external wires would not show, not likely though. I wonder if hooking up a timing light would show erratic blinking and therefore lack of spark when the throttle is opened. You've changed or removed the spark plug resistor cap?

-Bill

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1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
1990 Honda GB500tt
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05c50
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PostPosted: Tue Aug 08, 2017 6:12 pm    Post subject: Reply with quote

I generally try to eliminate everything that makes sense and even some things that don't before I go looking for help. Wink I did replace the resistor cap with a known good 5 ohm, 10 ohm and 0 ohm cap. I also tried watching the flashes of the timing light to see if it cuts out, but it doesn't seem to coincide with the back fires. I'm still searching. Rolling Eyes

...Paul

_________________
�77 CB550f
�77 CB750a
�78 GL1000
�80 CB650c
�75 GL1000-finally finished
�79 KZ200
68 CB450K1-taking up all my time
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spurlock
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PostPosted: Tue Aug 08, 2017 6:48 pm    Post subject: Reply with quote

Might be worth a look at the points pivot. Make sure the movable points arm is free and not sluggish.

-Bill

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1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
1990 Honda GB500tt
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05c50
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PostPosted: Tue Aug 08, 2017 7:22 pm    Post subject: Reply with quote

Removed and checked the points closely and they appear to be OEM (TEC?) and although they show a little wear, the pivot is smooth with very little play and the actual contacts meet squarely and even. I may have stumbled on to something, I'll keep you posted.

.......Paul

_________________
�77 CB550f
�77 CB750a
�78 GL1000
�80 CB650c
�75 GL1000-finally finished
�79 KZ200
68 CB450K1-taking up all my time
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dorT500
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PostPosted: Wed Aug 09, 2017 8:18 am    Post subject: Reply with quote

05c50 wrote:
...........I may have stumbled on to something, I'll keep you posted.
popcorn
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spurlock
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PostPosted: Wed Aug 09, 2017 9:07 am    Post subject: Reply with quote


_________________
1975 Honda CB125S
1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
1990 Honda GB500tt
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dorT500
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PostPosted: Wed Aug 09, 2017 9:18 am    Post subject: Reply with quote

05c50 wrote:
...........I may have stumbled on to something, I'll keep you posted.
Smile I'm beginning to think that what he stumbled on was his collection of vintage Playboy magazines. This may take awhile.... Rolling Eyes.....or maybe just a couple of minutes. Laughing
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dorT500
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PostPosted: Wed Aug 09, 2017 9:33 am    Post subject: Reply with quote

Anybody read any good books lately?


Idea Mannn....I'm glad this isn't radio......they hate 'dead air'.

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GONE.......WITH A PUFF OF SMOKE AND A BLUR OF SPOKE
________________________________________________
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05c50
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Joined: Oct 26, 2007
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Location: Greensburg Pa

PostPosted: Wed Aug 09, 2017 1:25 pm    Post subject: Reply with quote

dorT500 wrote:
Anybody read any good books lately?
Idea Mannn....I'm glad this isn't radio......they hate 'dead air'.


Unlike you, I have other things to do besides sit in front of the computer and talk to my make believe friends! Wink Wink


Anyhoo, I've got it running good now. The moral of this story is NEVER trust the PO. Turns out that the timing was advanced by about 5 degrees. I'm really surprised that only 5 degrees would make such a difference, it was really exciting when it back fired! Shocked The PO had made a very convincing mark to indicate the F mark, but it turns out he was off by 5 degrees. When I set the points statically the mark was right on, but when i used a timing light, it was way advanced. Thanks to those that offered advise.........except dor.......

.....Paul

_________________
�77 CB550f
�77 CB750a
�78 GL1000
�80 CB650c
�75 GL1000-finally finished
�79 KZ200
68 CB450K1-taking up all my time
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View user's profile Send private message Visit poster's website
spurlock
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Posts: 831
Location: Vacaville, CA

PostPosted: Wed Aug 09, 2017 1:34 pm    Post subject: Reply with quote

Hey Paul, congrats in figuring out the problem. I wonder whether the backfiring was caused by the timing just being off (i.e. starting the fire at the wrong time), or whether it was caused by weak spark due to the flywheel magnets out of phase with the source coil and so sending weakened power to the ignition coil. Just one of those random questions that pop into my mind every once in a while.

-Bill

_________________
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1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
1990 Honda GB500tt
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