Knocking gt8o

Discussion in 'General Questions' started by Jdh, Sep 9, 2016.

  1. Jdh

    Jdh Guest

    I have a gt80 reed valve engine with keihin clone carb and a sbp expansion chamber. Engine was running pretty strong and I had no complaints until it started knocking. I pulled the head off and the chrome had been coming off the cylinder on the exhaust side. Anyhow top end needs to be replaced. Can anybody tell me what might have caused the knocking and the flaking. Also what replacement parts are compatible with that engine. I've got a 29er with 40 tooth rear sprocket and the most I could get out of it was 37 mph. What should I expect out of that set up? Thanks for your help.
     

  2. Frankfort MB's

    Frankfort MB's Well-Known Member

    Probably to high of compression...

    Aftermarket head???
     
    Timbone likes this.
  3. zippinaround

    zippinaround Active Member

    Most likely too high compression did you test it after you made all the upgrades? If its too high will destroy bearings and like you said peel the chrome from the cylinder.
     
    Timbone likes this.
  4. jaguar

    jaguar Well-Known Member

    running in the pipe powerband creates extra compression which means you needed to retard the ignition timing, which you didn't.
    I highly recommend getting my Jaguar CDI with high voltage coil which is suited for modified engines.
     
  5. Jdh

    Jdh New Member

    Thanks for the info. I just ordered a new gt80 ported cylinder, piston, rings, needle bearing and wrist pin. Do you think this will fix the knocking? I've been reading about the Jaguar cdi, and I like what I've read. Didnt think i would get to talk directly to Jaguar though. Is anybody familiar with the gt80 from kingsmotorized bike. Is it a p.o.s or what? Does the head that comes on the gt80 have to much compression? If so what head do you recomend? What kind of rpm can I expect out of the engine with the Jaguar cdi? I re jetted the carb with 70 was the number on the main jet Im using and I was using redline synthetic oil at 40 to 1. What's the chance of the bottom end being hurt? Thanks for everyone's input.
     
  6. Jdh

    Jdh New Member

    Thanks for the info. I just ordered gt80 ported cylinder, piston, rings, wrist pin and needle bearing. Do you think this will fix the knocking? I've been reading about the Jaguar cdi and I like what I've read. I didn't know I would get to talk directly to Jaguar though. Is anybody familiar with the kings motorized bike gt80? Is it a p.o.s or what? I re-jetted the carb with 70 was on the main jet running redline synthetic oil at 40 to 1. What kind of rpm can I expect out of this engine with the Jaguar cdi. And can I still run the high compression head without destroying another cylinder? What's the Chance's of the bottom end being hurt? Do you think that's where the knocking is coming from?
     
  7. butre

    butre Well-Known Member

    compression causes the stock chrome plating to flake, so here's a quick primer on compression

    for easy math, lets assume a 66cc with a 6cc head and 1.2:1 crankcase compression. basically a stock engine with a fred head

    while the exhaust port is open, the piston isn't compressing anything, so any displacement below the top of the exhaust port isn't relevant to our calculations yet. lets say, again for easy math, you have 50cc of compressible space. to calculate your compression you find the ratio of compressible space to combustion space. in this case it'll round to 8.3:1. assuming you're at sea level and everything seals good, that means if you multiply (8.3*14.7)*1.2 you'll get your compression in PSI. in this case, that'll be 146.4 PSI. not too bad. the problem is when you get on the pipe, that cranking pressure spikes, ideally to 176.4 PSI in this case. this is because the expansion chamber effectively works to seal off that exhaust port.

    176 is already sounding nasty, but then you gotta remember we're just talking cranking pressures, not combustion pressures. combustion pressure works on a logarithmic scale, at least to a certain point. a 10 PSI increase in cranking pressure can mean a 1000 PSI increase in combustion pressure. there's no easy way to determine what the exact combustion pressure is, but it could easily be a difference of 20,000 PSI in the cylinder. as you can imagine, that's not easy on these cheap plated cylinders.

    in my opinion, if you want to cram a ton of compression in these, you need to either get them replated by a company with employees that are paid more than 10 cents an hour, get them sleeved, or simply be prepared to replace the top ends every few hundred miles.

    if I remember right, last I checked my engine was turning 204 PSI. I get away with it thanks to the help of a replated cylinder and bushings instead of rod bearings. plus a little bit of avgas.

    the CDI isn't going to help you with RPM either, but jaguar CDIs are tested to 11000 if I recall. if you want more RPM, that's in the port timing. personally, I use a stock CDI with an adjustable magnet, just have the timing retarded for the whole rev range. when I built my bike jag CDIs weren't yet capable of my RPM requirements and I've been stubborn about switching.
     
  8. jaguar

    jaguar Well-Known Member

    butre, where did you get your cylinder replated at?
    you're the first! congratulations
    Jdh, all the stock heads give low compression. Unfortunately all the aftermarket heads give too high compression so with them you have to use two gaskets. But I think the stock head is fine after taking off about .8mm from its mating surface.
     
  9. butre

    butre Well-Known Member

    a company called millennium technologies. for $209 its a bargain compared to getting it sleeved.
     
  10. jaguar

    jaguar Well-Known Member

    that's the only real solution unless you keep the low stock compression.
    check out US Chrome also, anyone interested in getting your cylinder replated
     
  11. zippinaround

    zippinaround Active Member

    209$ just to replate? Yikes!!! Haha something I'll never be doing I guess
     
  12. Jdh

    Jdh New Member

    How do I correct the port timing for higher rpm?
     
  13. Jdh

    Jdh New Member

    The cylinder has a third transfer port and I have the arrow reed vale. What mods to the piston do you recommend?
    I've heard oof stuffing the case, what's the benefit of stuffing the case and how do I do it? Jaguar I'm going to buy one of your cdi's. How much is it and how do I get it? I want optimum performance out of this engine is the sbp expansion chamber sufficient or would I be better off with a kx 80 or some other dirt bike chamber?
     
  14. Jdh

    Jdh New Member

    I have 29 inch wheels and a 40 tooth rear sprocket. 37.5 mph is all I've been able to get out of this set up.what can I do to increase mph. Is this a respectful speed for this combo?
     
  15. Jdh

    Jdh New Member

    Will drilling holes in both sides of the piston help performance of a reed valve engine? I've heard of milling intake side of piston to match intake port do you recommend this?
     
  16. Jdh

    Jdh New Member

    Can I get the damaged cylinder replated or do I need to replate a new one?
     
  17. butre

    butre Well-Known Member

    for the intake side, go ahead and cut a big chunk out. leave the exhaust side as it is. if the exhaust is ever open to the crankcase the engine will run like garbage if at all.

    it's cheaper to replace a damaged cylinder than to get it replated, and often times damaged cylinders can't be replated.
     
  18. Jdh

    Jdh New Member

    I just replaced cylinder, piston, rings, wrist pin and needle bearing. Engine is still knocking though. Does anyone have any ideas of what it could be?
     
  19. Jdh

    Jdh New Member

    I replaced cylinder, piston, rings, wrist pin and needle bearings. Engine still knocks though. I was hoping the new wrist pin and needle bearings would fix it. Any ideas what else it could be?
     
  20. Steve Best

    Steve Best Active Member

    No, 37mph is slightly low for what you have.
    Yes, you need a ported piston to run a reed valve efficiently. It can be drilled on the intake side only.
    Knock, knock... What is your squish measurement? Measure with 0.060" (1.5mm) electronics solder.
    It should be 0.020" to 0.060" (0.5mm to 1.5mm). Ideally o.030"
    You might be making head contact.

    Is your sparkplug aimed to the rear? It helps eliminate detonation in close cases.

    Try running a colder plug.
     
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