Slight and Intermittent Backfiring

[kris]

Hi all brickers across the globe! I have Heinz 90 per cent re-assembled, gassed her up and, after a lot of coaxing, got her to fire. It was like music!! But then I noticed that she would backfire just a little and at totally random intervals. That, and an odd and unpleasant mechanical clunk, when revving the engine a bit....again, all at irregular intervals. This bike has 61,000 miles (97,600 km) on her clock. She hasn't been run properly since 2005. At idle she does sound smooth. I've done the screw driver to the ear to the engine block and it really sounds good. So, any ideas? As always any help is greatly appreciated. I'm excited to get her on the road!

[johnny]

i gotts an idear... put some plugs in your ears and commence to riding that thing...

j o

[Chaos]

some minor backfiring on de acceleration is normal with the K75.  If it'd doing it at speed, check the wires and coils.  Run it at night and look for arcing. Also check the valves.  Good chance some of them are tight.  The knock, if it's right off idle, could be the alternator drive , aka know as "monkey nuts" little rubber balls that take up shock loads.  When they disintegrate you get metal on metal and will eventually destroy the drive. 

[rbm]

Afterfiring is indicative of an exhaust leak.  K75's leak near the junction between the header and muffler.  K100's don't have the same connection so you might have leaking exhaust gaskets.  Did you do anything to the exhaust?  If not, the gaskets may need replacement.

Random mechanical clunk has been talked about in other threads.  Could be monkey-nuts need replacing or maybe the output shaft.  Older K-bikes like yours had 6 rivet output shafts that were later changed to 12-rivet shafts.  The 6-rivet shafts tend to wear the rivet holes and the assembly becomes loose.  Might also be a split bushing around the throttle bodies causing air leak.  Have you examined the bushings (i.e. the rubber things that join the TB to the block) for their integrity?

[The Mighty Gryphon]

Backfire on closed throttle is an exhaust leak as mentioned above.  Misfire on open throttle could be any number of things.  Check that all the plug wires are tight on the plugs.  Check for air leaks around the throttle bodies although that mostly whacks the idle. 

I only have one year with a K, but in my limited experience I would say they do a lot of clanking and banging with no apparent ill effects.  I would check the monkey nutz if you haven't done so already.  Another offender is the anti-backlash gear and spring on the output shaft.  Makes a hell of a racket between 1200 and 2000 rpm on my K75.  Sounds like one of those big steel balls from a pinball machine rolling around in there.

[kris]

Thank you, gents. The backfiring occurs at idle and did happen a little on a short test ride down the street - throttle closed. I removed the exhaust system in the process of doing all other work on the bike. I realized later that there were gaskets that go into the block. I did not remove them with the exhaust system nor did I look for them. It's not huge bang backfire rather smallish backfire.

Jim, it's good to hear that things seem to rattle a bit in there. I found the service records for this bike and the PO faithfully followed the plan until about 48,000 miles...she's got 61,000 on her. She doesn't appear to burn oil, the splines were like new and everything else (except for an original - '86 - fuel filter) looks to be in good shape. I'll spend some time on her first. I was thinking I'd put some Seabreeze in the crankcase to do some internal cleaning. I'm also running some Marvel Mystery Oil through the fuel tank. That stuff has done some good things for my '99 Sienna.

So would that exhaust leak typically be at the manifold or at the unions between manifold and muffler? Jim, is the anti-backlash gear anything to worry about or does it simply make some harmless noise....with the soft touch of a German brass band!!?

Thanks again.

[The Mighty Gryphon]

There's an expression I've heard numerous times that refers to the sound of a brick engine...

..."box of rocks".

The good news is that they seem to smooth out at 65+ here in the States and 110+ up north.

[kris]

Thanks Jim. Always appreciated. Great job on the fuel cooler, but is that a necessary thing? BTW, I'm old enough to be trilingual.... Celsius, Fahrenheit and Imperial.

I had a bike mechanic do a little job on my Concours when it was new (to me). I asked him how the engine sounded to him and he said "great".  I was concerned because it's a little noisier on a cold start. The guy said he's not really interested in how engines sound cold....warm 'em up and then have a listen!

[kris]

I was able to tighten three of four exhaust clamps. I fired her up....no backfiring of any kind. I took her around the block a few times....no backfiring! It's a frickin miracle!! And I want to thank you all for weighing in on my request for info. I rode my 2004 Kawi Concours just before climbing on the K100RT. It's like being on a different planet. The machines are so different. The gearbox is very smooth. The clutch is also silky smooth. And the bike doesn't clunk when you put it into first....that's a nice difference. The Kawi definitely feels a little spicier in terms of throttle response. It'll all take some getting used to.

Happy (safe) trails!