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Katana 750 mechanical mystery

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  • Katana 750 mechanical mystery

    I own an 06' Kat 750 with just over 9k miles on her, bought with 7k about 4 months ago from a dealership. I love it, moved up from a ninja 250 I bought for $500 off craigslist and put 5k miles on before selling. While on the kat, noticed a leaking fork seal and decided to get both replaced since its an old bike and it only gets more expensive as it gets worse. I've only ever worked on engine mechanical stuff, I didn't have the tools or knowledge to work on forks, although I maybe could've figure it out. In comes the mechanic my riding friend and his dad, who happens to be a 10 year MSF instructor, recommended to me. I take in my bike and he gives me a quote for replacing both, seems like a nice guy, doesn't have a crazy price. One week goes by, I ask about the status, and the mechanic says the USPS shipping was off and it'll be a few more days before the parts come. A week after that, I contact him again, he says the seals are done, but it wont start... I don't believe him so I go to the shop and sure enough, she wont. I was very confused because I had been riding it everywhere, everyday, up until I took it in and it ran perfectly fine. He offered to look into why it wouldn't start and since I was at a loss, I said sure. Later that day, he gives me a call and tells me that he found that the spark plugs were loose, there was little oil in case, and when he ran a compression test, the cylinders came out to 75-75-40-50. I didn't understand how any of this happened, I have been working on dirt-bikes, scooters, and motorcycles since I was 12 and hadn't heard or seen anything like this happen. My bike was in the shop for just over two weeks. The only explanations I could come up with were that: this mechanic isn't such a nice guy and thinks that I'm just some dumb 17 year old who wonders what compression and a spark plug is, or that the dealership I bought it from ripped me off. I wasn't too sure to trust the mechanic, since I knew low compression to that level meant either new rings or new valves, AKA, not too cheap. I also didn't understand how the plugs were loose, maybe he loosened them prior to me coming in so it wouldn't start? Seemed like an awful amount of work since you have to take all the front fairings and the tank off. I picked the bike up and got it home, first thing i did was change the oil, which was very low, and a few drops of oil into the cylinders, then continued with a compression test of my own. Compression showed up exactly as he told me, 75-75-40-50. I was shocked and pissed off because I bought this bike to learn to ride a bigger bike, and planned on doing so for a while. I didn't have the money to go out and buy another bike. I did the ring compression test, putting a teaspoon of oil in each cylinder, and the compression stayed the same, indicating that the rings were fine, and hinting towards a valve problem. I decided to put the plugs back in and I tried firing it up and sure enough, she started. I set up an external fuel supply and let it run for a good five minutes, revving it up a little here and there. Idol was perfect and it ran all the way up to red line and back down as smooth as can be. I re-did the compression test after this, and all cylinders read 130+ psi with slight variation by cylinder. I'm now back to riding her everyday, problem free. I've heard of cars that have been sitting for a year or two, when compression tested, show up really low, because the valves are all gunked up by gas and etc. leaking in and not allowing proper seating, but my kat was only there for two weeks. I also wondered if it was some mechanic trick, like spraying something that would gum up the valves into the cylinders, so its expensive for the customer, but is easy work for the mechanic to fix. Anyone have any ideas what could've happened? Thanks, Dean.

  • #2
    Do a valve adj check.

    Do the test again on a cold engine, are you able to recreate the issue?

    do a leak down test if you are, verify where the loss of compression is going.

    93 750 Kat

    Modified Swingarm, 5.5 GSXR Rear with 180/55 and 520 Chain, 750 to 600 Tail conversion, more to come. Long Term Project build thread

    "I've done this a thousand times before. What could possibly go wron.... Ooops!"