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  1. #1

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    K&B 7.5cc outboard bearing life

    Since I have been playing with this particular engine and tunnel boat I have had nothing but problems, like fix it. The motor is an old one 8701 as well as the boat, but I thought I could run it and have some fun. I replaced the pto bearings last year and the time before I have replaced the front and rear bearings. I don't get a chance to run it much so I haven't gone through too much fuel. I'm learning to tune the boat slowly and experience all the problems. Now I'm fed up. I just got it running and then I heard the telltale sounds of a bad bearing. Is this normal for such little use. I use after run oil and always look it over each run. My receiver batteries and glow plug starter work fine after twenty years of recharging. I've learned the difference between stale fuel and fresh nitro fuel since I go a long time between runs. I'm just getting tired of having to order another bearing. I'll order a full set of bearings this time and then see which one needs replacing. Is this normal for these engines?

  2. #2

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    RE: K&B 7.5cc outboard bearing life

    Are you flushing the engine out real good? Pull glow plug and open carb all the way and spray some carb cleaner in glow plug hole and carb ,don't be stingy, spin engine with starter and repeat, now you have the fuel washed out and the after run oil will protect the bearings , I use transmission fluid for afterrun oil.......

  3. #3

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    RE: K&B 7.5cc outboard bearing life


    ORIGINAL: f16man

    Are you flushing the engine out real good? Pull glow plug and open carb all the way and spray some carb cleaner in glow plug hole and carb ,don't be stingy, spin engine with starter and repeat, now you have the fuel washed out and the after run oil will protect the bearings , I use transmission fluid for afterrun oil.......
    Very good advise I was just oiling my engines and found that I was getting little specks of rust that hurt the piston and bearings I now do a better job like stated above
    alcohol is for drinkin. gasoline is for cleanin . NITRO is for racin

  4. #4

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    RE: K&B 7.5cc outboard bearing life

    never lost a bearing since i started running that engine in the early nintys ran in salt and fresh watter but a would always flush my engine with WD 40 remove the plug open carb fill engine spin the engine over with starter add more wd 40 wipe engine down reinstal plug

  5. #5

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    RE: K&B 7.5cc outboard bearing life

    Thanks for all the advise. I was doing the WD40 flush & etc. but when I was having problems and only got a minute or two of a run I got lazy and put it away. I can't tear the engine apart till I get some money but that could be why a bearing went bad again in such a short run time.

  6. #6

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    RE: K&B 7.5cc outboard bearing life

    Ya these boats are a lot of work to keep em up but worth it.....................

  7. #7

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    RE: K&B 7.5cc outboard bearing life

    I tried to run today. Have sanded and rebalanced the props due to prop damage with a rock (my error). I have taken the engine apart and can't find anything wrong with any of the bearings. I greased the flex shaft with white lithium grease.I couldn't get past the cavitation and bearing (shrill) noise problems. I have Octura X447 and X450 and couldn't get more than an eigth of a tank use. This is getting to be too much for so little stick time. My glow plug element looked white/grey so I'm stumped. I have noise that sounds like bearing sounds but all the bearings I examined felt okay.

  8. #8

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    RE: K&B 7.5cc outboard bearing life

    since you have had the flywheel off could the noise be from contact between it and the case ? or it mighy be in the pto (where the cable conects to the powerhead ) try removing the flex cable start engine if the noise is still there you know its in the power head to fix the cavatation try lowering the engine 1/8 inch at a time

  9. #9

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    RE: K&B 7.5cc outboard bearing life

    what shape is the teflon liner in for the flex shaft if worn noise could be comming fron there

  10. #10

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    RE: K&B 7.5cc outboard bearing life

    Thanks for the help. I'm going to need to be more careful with my troubleshooting. I already took the flex cable out to troubleshoot. The pto bearings are new and feel smooth. The flex cable and teflon housing are new. I've greased the flex shaft lubed it with gear oil but the noise persists in a few minutes of running. I'll lower the prop by an eigth of an inch since these runs are after a new paint job and I am a greeenhorn at adjusting things. I have a few more hobbies so I have to wait a few days. It just seems perplexing since the bearings appear good as well as the other components.

  11. #11

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    RE: K&B 7.5cc outboard bearing life


    ORIGINAL: fmimages

    Since I have been playing with this particular engine and tunnel boat I have had nothing but problems, like fix it. The motor is an old one 8701 as well as the boat, but I thought I could run it and have some fun. I replaced the pto bearings last year and the time before I have replaced the front and rear bearings. I don't get a chance to run it much so I haven't gone through too much fuel. I'm learning to tune the boat slowly and experience all the problems. Now I'm fed up. I just got it running and then I heard the telltale sounds of a bad bearing. Is this normal for such little use. I use after run oil and always look it over each run. My receiver batteries and glow plug starter work fine after twenty years of recharging. I've learned the difference between stale fuel and fresh nitro fuel since I go a long time between runs. I'm just getting tired of having to order another bearing. I'll order a full set of bearings this time and then see which one needs replacing. Is this normal for these engines?
    fmimages
    I saw a picture that you posted of your boat I also watched the you tube video. What brand is the boat? What % fuel are you running? Happy boating Fred

  12. #12
    Dan S's Avatar
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    RE: K&B 7.5cc outboard bearing life

    what % oil do you have in your fuel?

    Dan.
    F-41 Stryker Cat 260 Full Mod - Whiplash GV NFS 260 - FireFighter 18 - Miss Vegas CVRM 18
    Futaba FASST

  13. #13
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    RE: K&B 7.5cc outboard bearing life

    An inner race slipping on the crankshaft could be causing the noise, but usually when that happens it will drop RPM's when it does it. It usually won't give you much time if that is the case to change the crank before it finally galls up enough to weld itself together.

    The top bearing seems to take the most wear on my 3.5SS.
    Contains only the finest ingredients, Methanol, Castor Oil and Nitromethane

  14. #14

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    RE: K&B 7.5cc outboard bearing life

    You can also get that sound from your propshaft assembly. It should be lubed everytimme you run and also flushed after running. It can also sause some cavitation if it is worn to badly.


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