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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: tsbt on August 16, 2012, 11:17:14 AM
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Working out the bugs for a weekend trip. Balanced the throttle bodies changed the plugs, had the bike out to get new tires yesterday and noticed the engine in first gear runs rough around 4krpm and backfires more than it did before. Not a lot but some, I can still ride it hard. Today I'm going down my checklist of what might be the cause so far I've got:
- Need new sparkplug wires.
- Pulled a MacGyver: When replacing the rubber hoses I decided to swap out the hose from the back of the throttle bodies to the air-box with a cut piece of engine breather hose, temporarily until the backorder hose arrives. Thought maybe the curvature in the original may affect the air flow? Not sure.
- Idle reg valve need adjusting?
- TPS? not sure I'm equiped to mess with that yet :yes
I'm sure I'm missing something.
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Something you did must have caused it. I'm guessing a bad plug or wire. New ones can be bad.
Or maybe they are routed so the voltage jumps from one to the other? My plug wires seem to
have lots of aluminum oxide inside but they are hard to clean and cost $70 each! There is a
rumor that they have a spark gap built in to quelch ring back sparking so it is recommended
to use the stock ones....
Which hose goes from the throttle bodies to the airbox? There's one that goes to the fuel
regulator and one that goes from the crankcase to the airbox...
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There is a rumor that they have a spark gap built in to quelch ring back sparking so it is recommended
to use the stock ones....
Randy, the plug wires for the 3cyl K75's do have a spark gap in the stock cap. The wires for the 4cyl K100's and 1100's do not have this gap.
Some owners have used Magnecor brand wires, I have 2 sets of the wires sold by Realm mfg, the same company that makes the shocks and springs.
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I assume the aftermarket ones do not have the spark gap, do they work ok?
I like the Magnecor's had them on my jeep. I'd go that route if it works but
someone said it ran a bit weak at low RPM's with nonstock wires...
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Plugs and leads are a good starting point with that sort of backfire.
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Magnecor's rock.
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I assume the aftermarket ones do not have the spark gap, do they work ok?
Duck could probably answer that better, as he's used both the wires and has had 'hands on' with K75's that I haven't.
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These symptoms sound pretty much like what I've experienced of late. I finally fixed the intake leak and my 1985 K100RS ran fine as I began my much-anticipated northwest US tour. On day three all that changed. :confused:
It backfired in the silencer on trailing throttle, lurched and ran very erratically at low throttle settings. Highway speeds were seemingly okay though. I though perhaps the TPS has slipped out of adjustment, my that proved untrue. It was very difficult to ride the beast when I got caught in stop'n'go traffic in the Portland area -- the temp was crazy- hot and the bike was acting like a rodeo bronco.
Back home now, I can't find any intake tract vacuum leak, so I'm stumped as to the cause of this misbehaving. Where do I begin??? The spark plugs were newly installed before the ride, the HT wires have less than 25 K miles on them.
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Lawrence,
What do the plugs look like now? Do the tips all look the same?
When was the last time the throttle bodies were balanced?
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Noticed I might have a hole in my exhaust or a leak near the baffle. Also need me a set of Magnacors regardless I noticed some electric tape on one of the wires. Could also turn the idle reg up a notch...I leave tonight for a lake in the woods, first big ride since I put her back together, wish me luck!
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Had a great trip north to a friends cottage, post photos later, noticed however the bike vibrates more at 4000rpm than any other number. Guy who put my new tires on said the rear wheel must've hit something as its a tiny bit warped. Corrected for it when he balanced the tire somewhat. Said the new rubber will also smooth it out. Maybe if I'm lucky I can drive it out West to the Duck for a new rim. shipping on those to Canada is pricey. Not as much coughing this time coulda worked out some bugs on the highway. Oh and I definitely need a better seat my a** is grass :crutch: :crutch:
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vibrates more at 4000rpm
Synching K Bike Throttle Bodies (applies to all K bikes) (http://www.motobrick.com/index.php?topic=364.0)
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Synching K Bike Throttle Bodies (applies to all K bikes) (http://www.motobrick.com/index.php?topic=364.0)
This was the very first thing I did. Must be the warp in my back wheel, causing it.
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..........however the bike vibrates more at 4000rpm than any other number......
So does mine, and I do keep my TB balanced.
If I get a couple of hundred rpm either side of 4k the vibes quit.
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I get the vibes north of four grand for the next couple thousand. In any gear. Pull in the clutch and it's smooth as silk. Release it, and it's the high speed vibes again. Balancing didn't seem to fix, unless Im doing it wrong. I'd love to figure this out, it's really my only persistent annoyance.
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I've seen people report on this numerous times but I've owned three of them and a K1 and never experienced it on any of them. Are you sure the boots between the TBs and cylinder head aren't leaking air?
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Well I'm really glad to hear you say that, gives me hope. here's what I'm thinking and I'm willing to re-try all..
What are your thoughts ?? Missing anything or is anything below irrelevant to the vibration issue?
1) TB not sync'd
2) air leak in TB to intake
3) Some physical connection of engine (or exhaust) to frame? Missing boot, bolt et.
4) Bad or poorly gapped plugs (guessing here - thoughts?)
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On my K100RS it's turned out there are still leaks at the junctions of the rubber connectors between the TBs and the plenum. I tried to uncover their locations using the butane method. No workie! I then squirted electronic cleaner on them and that disclosed the problem(s). Man, those things are a big headache. I'm convinced that many of the complaints posted for poor running, backfires in the silencers and jerky-ness when rolling on the throttle are mostly the result of intake vacuum leaks.
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If he's getting the vibrations at 4000rpm with the transmission engaged, but not getting the vibrations at 4000rpm with the transmission disengaged, how could bad air/fuel processing be the problem?
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If he's getting the vibrations at 4000rpm with the transmission engaged, but not getting the vibrations at 4000rpm with the transmission disengaged, how could bad air/fuel processing be the problem?
An engine under load and an engine not under load are very different beasts.
Not a bad idea to retorque the header flange nuts and exhaust mountings.
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Another way to diagnose them is to sync the TBs. If one (or more) of them can't be sync'd then a TB boot to cylinder head leak is a likely cause.