GN400 Kickin my butt! Literally!

Discussion in 'Thumpers' started by Andyvh1959, Jul 31, 2012.

  1. Andyvh1959

    Andyvh1959 Cheesehead Klompen

    Joined:
    Feb 16, 2012
    Oddometer:
    829
    Location:
    Da frozen tundra eh? 1.5 mile west of Lambeau
    I have an old Suzuki GN400 converted to an ice racer, looks like a Rotax flat-track racer. For this summer I have dual-sport tires mounted while I try to get this damn thing to start reliably. I have been struggling with it for months! Kick starting really is a pain, because most often it kicks back so DAMN hard, like it is off timing in fact. Please, do NOT send me replies about how to kick-start a single. I'm only 5'-6" and 185 lbs, and I can kick start my XL600 Honda (with no decomp lever) all the time, even when guys much bigger than me quit after one kick.

    So, again, I tore into the engine to confirm all the basics lets say. Here is what I have found/confirmed:

    1. Magneto rotor timing (its a 6-volt magneto system). Check, rotor key is in place, rotor is static timed
    exactly to the timing mark on the crankcase.
    2. Rotor position to piston. Check, rotor key is facing up and piston is at top dead center when the timing mark matches. I did replace the rotor with the one from my extra engine. The trigger magnet on the original rotor had somehow gotten chewed up at some time prior to me ever working on this engine.
    3. Cam timing. Check, with the cam chain tensioner out (per the manual)the cam "notch" faces to the intake side, and parallel with the top of the head. Both cam lobes face down as they should for the piston timing on the compression
    stroke going to the power stroke. Cam lobes, cam races, rockers, all looked normal/good, no scuffing.
    4. Valve clearance. Check, after I reinstalled the valve cover and reset the cam chain tensioner, with the rotor on the timing mark, I set the intake and exhaust valves per the manual at .004" for both.
    5. Spark. Check, when I clip the fresh plug, gapped at .028", to the head and kick the bike over I get a nice violet/purple spark, consistently. NOTE: this is with the brand new CDI/Coil unit (non-stock) I recently installed. It was hard to start before when it had the stock 30 year old Suzuki CDI unit installed, and I thought the CDI itself was failing. On the old CDI it produced a yellow spark so I assumed the CDI was dying. So I replaced it with a modern tech aftermarket CDI/Coil unit. I did get the bike running previously on it.
    6. Pulser coil. Check, the new CDI is wired to the Blue/Red wire for the pulser coil at 10 degree BTDC. Pickup coil plate is set at the normal position. NOTE: I did slot the holes further to give me the option of moving the plate for adjustable/different static timing.
    7. Fuel. Check, with the petcock on, fuel flows into the carb. When I had the carb tipped during working on the bike, fuel did flow out of the overflow. There IS fuel in the carb. Clamp to mount the carb to the intake is tight and the intake manifold is not cracked. NOTE: the carb is a Mikuni VM36 round slide carb, not the stock carb. The bike DID run on it previously. Pilot jet is a 15, slide needle on the 2nd notch from the top, Main jet is a 240.
    8. Air Filter. Check, its a foam aftermarket filter. NOTE: the bike did previously run using this carb/filter combo.
    9. Compression release. Check, slack in the cable after the decomp cam has flipped as I kick the bike over.

    I feel I have checked/rechecked/double tried about everything obvious I can think of and this little bastard is still kicking my butt!! I'm no mechanic by training, but I have been wrenching all my own bikes for 30 years. I know enough to make a running engine from a basket case (my Honda XL600) None of my seven bikes have been in a shop, and my 94 BMW R1100RS now has 169,000 miles on it with me as the sole wrench on the bike. So I do know a lot just by years of wrenching my own bikes.

    So,...after all this, I rolled the bike to the end of the driveway and tried to bump start it in 2nd gear. Nothing, some times the rear tire slides, other times I get a few "chuff, chuff, chuff" and that is it. I also sprayed starter fluid into the air filter thinking it would help, choke on/choke off, nothing but "chuff chuff". I am waiting for the replacement kick-starter to replace the bent/nearly sheared off original (it has kicked back that hard too many times).

    There it is. Anyone got any ideas, thoughts, guesses, random shots in the dark? When I have been able to start this little bastard, it's a ball to ride. Its geared way up with a 52 tooth sprocket on the rear. Supertrapp muffler with eight plates installed. Good runner, once it is running, but a real BASTARD to start. Once in a while in the past I have been able to start it, but it has gotten progressively harder to start all the time. Almost has me thinking of pulling the head to check out the valves and see what kind of carbon buildup there is on the piston.
    #1
  2. Andyvh1959

    Andyvh1959 Cheesehead Klompen

    Joined:
    Feb 16, 2012
    Oddometer:
    829
    Location:
    Da frozen tundra eh? 1.5 mile west of Lambeau
    UPDATE! As of 1/15/13 I have wired a Honda CH250 CDI to the stock coil and triggered off the P1 (red/green wire) from the timing plate. Kicks EASILY and starts on one kick. On to the ice I go!!

    The whole problem was I read the timing wrong and was trying to kick start the bike with the timing ADVANCED 25 degrees! NO friggin wonder it kicked me back so DAMN hard. DOH! Wired right it starts real easy.

    Latest update: as of Oct 2013: with the stock Suzuki coil and CH250 CDI the bike sputters above 4500 rpm. I suspected the stock coil had too much resistance, does not match well with the Honda CH250 CDI. I replaced the stock coil with a coil from a CH250. Seems a better match and the bike revs better now.
    #2
  3. sonic reducer

    sonic reducer Been here awhile

    Joined:
    Aug 19, 2007
    Oddometer:
    257
    Location:
    Seattle WA
    i used to have a gn400 that i made a little super minimalist hardtail out of. 6 wires on the whole bike. pretty damn fun bike but hard to start and very slow.

    I had a reply all written out but then read the second posting, ha ha. :deal good work.
    #3