Need some help diagnosing.

Discussion in 'Motorized Bicycle General Discussion' started by JOBAfunky, Jul 3, 2014.

  1. JOBAfunky

    JOBAfunky New Member

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    I'd love to have a pro in town that I could talk to and have listen to my bike, but you guys are pretty helpful so I'll try my best to describe what's going on.

    History:
    Installed a 66cc Bikeberry kit on an old(and free) murray 26 inch mountain bike.
    Was noticing what I think is called "four stroking" except when giving it gas while going uphill.

    I replaced the spark plug with a ngk and gapped it up to .04 and I seemed to be not getting this as much, but I stripped out the head in the process.

    Got a replacement straight low compression head(Thanks Davezilla and mapbike) and now it back up and running but four stroking most of the time again.(minor worry) Engine seems to pop more (minor worry).

    Throughout all of this I was popping spokes, and at first I took it to the bike shop to get them fixed and wheel trued, got the whole thing back together pedaled 2 feet then "ping" so I got good at swapping spokes and truing the wheel, but now the wheel seems to have a wobble at the axle. (medium worry)

    The other thing is after the engine gets warmed up and going high rpm I sometimes get a raspy sound like a thin sheet of vibrating metal loosely attached to something. (medium worry) Or maybe the sound of a little gas escaping, kinda hard to tell. Power is less than original, but I expected that going to a low compression head(No worry)

    So, what should I keep or toss out of my worry box?
     
  2. Davezilla

    Davezilla New Member

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    Sounds like it might be overheating when it starts to get that tinny sound and loses power, it may be worthwhile to jet it up a bit richer on the main jet to help with the cooling, or it's time to swap over to a Puch or Fred head.

    If the power is just less all around that could just be a result of swapping over to the lower compression head but if it's losing power after warming up and when it starts making that noise, then it's overheating.

    best thing to do would be to get yourself a temp gauge so you can monitor the cylinder head temp so you can tell if it's getting too hot, just look for them on ebay or amazon.com, they're really easy to install because the sensor is pretty much like a spark plug washer with a wire coming off it. These are fairly cheap too and they will save your engine since you can always shut down if you see the temp going up too much.
     
  3. JOBAfunky

    JOBAfunky New Member

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    I'm usually traveling less than a mile at a time. It probably not getting to overheat temps that quickly right? The power loss is kinda overall, though I just took it out and it seems that the top speed is higher, but it takes forever to get there.
     
  4. crassius

    crassius Well-Known Member

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    these engines are not warmed up after only a mile
     
  5. JOBAfunky

    JOBAfunky New Member

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    Ya, that trip tonight was twice as long since it was to run back to work to grab something I forgot. It was on the way back that I was getting going pretty fast.
     
  6. Davezilla

    Davezilla New Member

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    I gotta agree on the warm up and the short trips, but add some WOT in there and a lean mix and things can heat up on the top end pretty fast... especially with a stock head... Now to completely warm up the engine where the crank wheels actually heat up to full operating temperature, yeah, that takes a bit longer... like around 15 minutes of riding time, not running at idle, but riding at a decent pace. The parts that are prone to overheating and tend to get destroyed by heat are the cylinder, head, and piston, they get too hot and bad things happen. I do doubt it can get that hot in just one mile tho, even at WOT.

    as far as having a slightly higher top speed with less compression, that is possible at higher rpm's where compression is less relevant and the engine can make slightly more power instead of less, but the rpm's have to be prety high and the increase is hardly noticeable except on a dyno or maybe you'll see another mph on a gps
     
  7. maniac57

    maniac57 Old, Fat, and still faster than you

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    Sounds to me like you have a leaking head gasket. Have you checked the head bolts since the replacement?
    All my engines get a retorque after a couple heat cycles. (Cold to full operating temp back to cold)
    Don't go nuts and crank them another half turn, just check and tighten ONLY if needed.
     
  8. JOBAfunky

    JOBAfunky New Member

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    Once or twice, but I should probably get another in.
     
  9. Nghtrider62

    Nghtrider62 Member

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    Being a backyard Bike Mechanic Check your axle It may be bent or cracked/split. "but now the wheel seems to have a wobble at the axle. (medium worry) " Replace as soon as possible if problem. Is the rim true? I true all my own. Have since MMI taught me how to back in the 80's, just transferred that knowledge to bicycles.
     
  10. Davezilla

    Davezilla New Member

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    Gotta agree about the head gasket and re torquing it frequently as it breaks in. I usually set the torque wrench to the same torque each time but I always break the torque and re torque, you can feel which ones are working their way loose when breaking torque, but I like to do it this way to get a nice even clamping each time. You can just run the torque wrench on each stud and tell if anything worked it's way loose, there's nothing wrong with that, I just prefer to break torque before retorquing, and always do it with a cold engine
     
  11. 2door

    2door Moderator
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    I always like to stipulate to a new builder that when we say re-tighten a fastener, what we mean is to 'check the tightness' preferably with a torque wrench.

    Tightening, to many, means feeling/seeing the wrench turn the fastener. That's where you can get into trouble with stripped threads. If you use a torque wrench, and you should be using one, you can 'check' the tightness but not exceeed the torque value you've set the wrench for.

    Too many times I've read this: "I re-tightened the head bolts like I read to do here but they stripped out".

    There's a world of difference between 'checking the tightness of' and 'tightening' a fastener. And that goes for any fastener.

    Tom
     
  12. JOBAfunky

    JOBAfunky New Member

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    How important is it that I change the head gasket with the placement of the new head? Do I need to get both a new head and cylinder gasket? Or am I OK using the originals?
     
  13. crassius

    crassius Well-Known Member

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    if the head gasket looks good, I first try just cleaning & flipping it over
     
  14. Davezilla

    Davezilla New Member

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    Doing this is perfectly ok...
     

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