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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Westone on June 16, 2013, 06:23:52 PM
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Though much improved after syncing the throttle body butterfly valves, balancing the TB vacuum with the idle air bypass screws, and adjusting the valves precisely so that all intake and exhaust valves are consistent with each other, my '88 K75S still does not idle smoothly. The idle varies 200 or 300 RPM randomly, between 1,000 and 1,300RPM, and it backfires periodically (though much less now).
When I say backfire, I mean ignition occurs back up into the throttle body, not that it makes a loud noise out the exhaust from the ignition of raw fuel. It does not do that at all. I know this because when balancing the throttle body vacuums with my homemade device, which has clear silicon tube connected to the vacuum ports, I could see the light from the ignition in the throttle body through the tube when it stumbled. It is not always the same cylinder that backfires.
In addition to the steps mentioned above, I have also verified that the TPS is set correctly and checked for leaks with MAP gas 3 times. So now I am wondering if some of this is just the nature of the engine and occurs on all K75s to some extent. If not, what do I need to look at next? Other than an imperfect idle it runs great, accelerating smooth and strong from idle to 6,000RPM with absolutely no hesitation, no matter how fast I get on the throttle. And when decelerating, it does not after fire at all, not even a hint. Should I be able to get it to idle more evenly with no stumbles?
ETA: Also, Installed freshly rebuilt Bosch Type II injectors.
ETA: Pulled the plugs and they have the ashy grey color associated with a good combustion mixture.
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it should idle evenly at 800-900rpm, could be a timing issue, if not the ignition then the cams. Ever had the chain off or is the tensioner still intact?
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I've only had this bike for a few hundred miles. The cam chain may have been off in the past. If one of the cam gears was off by one tooth, how badly would that affect the running of the engine.
The valve cover was off last weekend and I got a look at the cam chain and tensioner. While it looked okay and the chain was tensed, it occurred to me that if the cam gears are not synced correctly I wouldn't know it from looking.
The timing is easy enough to check. I will take a look under the timing cover and see how it is set.
Since posting this I found a couple of YouTube videos with segments of K75s idling. Mine is definitely not idling as it should. It would cut off at 800 RPM. It stays around 1100.
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did you replace the caps on the vacuum ports with new?
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I've only had this bike for a few hundred miles. The cam chain may have been off in the past. If one of the cam gears was off by one tooth, how badly would that affect the running of the engine.
The valve cover was off last weekend and I got a look at the cam chain and tensioner. While it looked okay and the chain was tensed, it occurred to me that if the cam gears are not synced correctly I wouldn't know it from looking.
The timing is easy enough to check. I will take a look under the timing cover and see how it is set.
Since posting this I found a couple of YouTube videos with segments of K75s idling. Mine is definitely not idling as it should. It would cut off at 800 RPM. It stays around 1100.
Should be easy to check cam timing. There are marks on the sprockets that line up with the surface of the valve cover
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Thanks for the replies. Yes the rubber caps are new. Thanks for the info on the cam timing. You may have seen another post I made after making this one, regarding the fact that I had propane leak tested this engine several times a month or two ago, and found no leaks. After making this post, I shot some starter fluid at the throttle bodies and found a leak right away in the hard plastic connection between the #1 throttle body and the engine. I'm thinking it has most likely been there since I bought the bike and I just failed to find it with the first round of leak tests.
New parts arrive Tuesday, and as soon as I am able I will be replacing that part under all three throttle bodies. The bushings on top I already replaced, which is what makes not having found the leak the first time frustrating. No leak, no need to go that far and replace $160 worth of hard plastic. $9 worth of bushings, sure why not. I was going in to clean the innards of the throttle bodies anyway, and look for problems thinking maybe that had something to do with the crappy idle since there were apparently no leaks.
I really hope this effort will smooth out the idle.