Thumper Club Forum
Technical => Bike Problems/Questions => Topic started by: andy230 on June 30, 2008, 10:12:47 AM
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And I'm not talking about a trip to the dentist... ;D
Got the engine in, and the bike just about built (brakes, seat, tank, controls etc):
www.team-thumper.blogspot.com
Only to find that when turning the engine with the back wheel, that the valves were making sweet sweet love with the piston at the top of its travel...
So I stripped it, dropped out the motor, move the cam round one tooth, came up with a slightly coarse mechanism of turning the crank (even the valve springs make it tough to get the cam "over the top"!), and its ok now I reckon...
Back in tonight hopefully, and then this should be us nearly at the point of running....
cheers
a
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Ummmm. You have timed this cam in havent you ?. I really wouldnt start it untill you've checked its correct. A lot of the camshaft re-grinds dont time up using the standard markings (the one I bought being an exception), and the camshaft sprocket has to be slotted so it can be timed in. I bought a sprocket from ThumperStuff which has been already machined.
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Steve
The cam (St 4 megacycle) came with a cam wheel. The wheel has a horizontal (at TDC) line on it which aligns with the mating surface of the rockerbox / rockerbox cover. It only lines up one way (ie it can’t be 180 out).
The camwheel looks like its probably been slotted and filled with weld / composite such that it can only really go one way. So no real option to time it as such, and as the motor has run with this cam, and camwheel with a hi comp piston (now flat), I’m going to go with it as is.
Incidently, if this were not the case, how would one time the motor?? Starting with either a blank cam wheel or a standard one?
(sticking fingers in ears, “LA-LA-LA-LA”)
cheers,
a
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To some extent it depends upon how the manufacturers specify the timing. But the basics are
- A Timing disc, basically a graduated disk, which bolts ont the end of the crank, you then fabricate a pointer, and this allows you to mean crank rotation in degrees.
- Dial gauge
The basic process is
- Fit timing disc, I bolt mine to the flywheel using the flywheel bolt
- With the head off, or with an extension (down the plug-hole) use the dial guage to find TDC, there are various ways but the way I have used is
- Find TDC and zero gauge
- Rotate the crank forward untill its 1mm ATDC, record the degrees on the disc
- Rotate the crank backwards untill its 1mm BTDC, record the degrees on the disc
- Half between the two is TDC.
- Head back on, and assembled. Fit the dial gauge to either the inlet or exhaust valves, to measure the lift of the valve.
- The manufacturers spec will list a valve lift at a specific angle relative to TDC, you need to rotate the cam in the slots to suit.
Ideally when find the points 1mm before and after TDC you should approach the point so the piston is moving in the same direction, this will eliminate any clearances from the equation. I suspect there are other ways, but this is how I approached it. As it was the cam I had didnt need timing in, I just didnt beleive them so checked anyway.
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cheers steve, I am (fingers crossed) hoping its alright...
Motor is back in now. Some electrics, (and fluid in the front brake!) and we may be in business....
a