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Post by stan on Nov 27, 2017 10:00:31 GMT -5
Hi all, this is my first post so if this is the wrong place please move it!
I have a 50cc scooter (Direct Bikes DB50QT-11) which I bought broken down from a friend. He had over-revved it and the valves had made contact with the piston breaking both valves, the valve guides, the piston and severely damaging the head and cylinder... I've replaced everything mentioned with new components and also fitted a 70cc bore kit as it was slightly cheaper than the stock cylinder. The bike only has about 5000 miles on it but I checked the bearings and camshaft tolerances anyway and they're all pretty much perfect. I've rebuilt everything correctly - piston rings are good, bolts torqued down right, timing is perfect, all electronics checked and working etc but it just won't start no matter what I do!
The starter motor and battery are both strong, the petrol is fresh, the carb is clean and fuel is flowing, compression should be okay at 130 psi, I've set the lash to 0.004" on both valves but I've tried many other lash sizes and I've tried various spark plug gaps on a C7hsa plug and a CR7hix iridium. Nothing works and I can't think of anything else... The only thing I've noticed is that sometimes fuel isn't actually getting from the carb into the cylinder but usually it's fine and the plug mostly comes out wet.
This is my first time working on one of these engines and I'm totally stumped so any ideas from you guys would be really appreciated! (Also apologies for the long post...)
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Post by tvnacman on Nov 27, 2017 10:11:42 GMT -5
Welcome Stan!!!
I would start with the basics. Did you static time it correctly? Are you getting spark? Are all the grounds back and tight?
John
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Post by stan on Nov 27, 2017 12:39:12 GMT -5
Hi, thanks for the quick reply! I haven't checked the ignition timing as it was fine before the breakdown and I didn't take off the flywheel. Does that need to be adjusted now there's a bigger cylinder on the engine?
I've tested the spark by touching the plug to the head and it is sparking pretty strongly and yeah the grounds seem to be good and I checked any electronics I could get at with a multimeter and it all seemed fine.
One other thing that I forgot to mention is that the bike originally had a kickstand switch but at some point it broke and my friend soldered the wires together instead of fixing the switch. It worked for him afterwards so I assume that it's okay? I did try it with the wires disconnected as well but the starter still worked and it still sparked so I'm not too sure what that switch actually did...
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Post by tortoise on Nov 27, 2017 12:39:14 GMT -5
valves had made contact with the piston breaking both valves, the valve guides, the piston and severely damaging the head and cylinder . . timing is perfect If you have not already done so . . verify flywheel-crankshaft woodruff key is not sheared (which throws off ignition timing). Insert a probe into sparkplug hole, rotate piston to top, then observe if flywheel T-mark is aligned with case indicator. |
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Post by stan on Nov 27, 2017 13:15:51 GMT -5
Yeah the TDC mark on the flywheel aligns perfectly with the crankcase mark with the piston at it's highest point so it looks like the flywheel is where it's supposed to be. I actually hadn't thought of that affecting spark timing though - would've been annoying if it was something so obvious!
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Post by tortoise on Nov 27, 2017 13:48:38 GMT -5
Sooo perplexing . . especially when fuel-timing/spark-compression have all been verified . . and the engine will not fire-off on starting fluid! Hope the tank was not accidently refilled with diesel, exhaust is not obstructed, or vacuum petcock diaphragm is not deficient.
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Post by stan on Nov 27, 2017 14:25:38 GMT -5
Yeah I'm completely baffled! I tried it with starter fluid and also just spraying a bit of petrol in the back of the carb but it did nothing. I did drain the tank and put new petrol in but it would've been funny if I'd been trying to run it on diesel though lol. Exhaust seems fine and I've tried it without the exhaust attached as well. I did think it might be the vacuum petcock so I hooked the fuel line directly to the carb with a shut-off valve but that didn't help either... I'm thinking I might have to take it in for a service and let the local garage worry about it because I've been tinkering with this for over 2 months and got nowhere...
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Post by cyborg55 on Nov 28, 2017 10:05:24 GMT -5
Bad plug?
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Post by stan on Nov 28, 2017 18:12:30 GMT -5
I don't think it's that because I've tried several new plugs and they all spark properly when tested by touching it to the head with the boot attached. I have a feeling that it's something to do with the spark timing but the stator and flywheel are good so it must be the CDI unit causing the problem. I don't have a way to measure that accurately though so I'm bringing it to a garage soon to have it tested.
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Post by psmcmurr on Nov 28, 2017 20:22:36 GMT -5
Camshaft timing?
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Post by cyborg55 on Nov 29, 2017 10:03:04 GMT -5
I don't think it's that because I've tried several new plugs and they all spark properly when tested by touching it to the head with the boot attached. I have a feeling that it's something to do with the spark timing but the stator and flywheel are good so it must be the CDI unit causing the problem. I don't have a way to measure that accurately though so I'm bringing it to a garage soon to have it tested. Piston guy had a similar problem,,with timing jumping all over the place,,,it turned out to be the cdi,,,he swapped in a old known good one and the problem went away |
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Post by stan on Nov 29, 2017 16:32:03 GMT -5
Ahh brilliant, hopefully that's my problem then. I'll see if I can borrow a CDI from somewhere and try it with that
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Post by stan on Dec 9, 2017 7:06:17 GMT -5
Problem solved! Turns out that the CDI had water damage which messed up the spark timing and I had the valve lash set too tight. It'll only run with the gaps set at 0.01mm or bigger and I had them at way less than that.
Another problem I'm having now though is that when revving up on the stand it revs smoothly up to about 1/3 throttle but then stays at the same RPM until I open it up to around 2/3 throttle where it revs to roughly the RPM it should be at at 2/3. It then revs smoothly up to WOT. I've tried a bunch of different needle settings and main jet sizes but nothing makes much of a difference. Could this be caused by a wrong size pilot jet?
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Post by cyborg55 on Dec 9, 2017 13:07:04 GMT -5
Yep,, basically fuel starvation
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Post by chewbaca on Dec 9, 2017 17:54:12 GMT -5
is the constant rpm happening with or without the belt on
because with the belt on the rear wheel will absorb all the power
of the motor causing a flat spot in the rpm curve until the variator is maxed and then the rpm will start to climb up again
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