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Author Topic: Timing and TDC for a RM  (Read 4633 times)

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Offline Slow Pete

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Timing and TDC for a RM
« on: August 04, 2016, 09:50:53 pm »
Hi All,
          So I have my new HPI ignition for my 81 RM250.   The instructions say ignition should be set at 2mm before Top Dead Center, but these are generic instructions that suit bikes across the board.   There is no mention of TDC in my manual does anyone know the  specs for a RM ?

I have read some stuff on line that pushing it out a bit will give more bottom end and less will allow it to rev more, but obviously there are limits.   Does anyone have any suggestions ?

Pete
« Last Edit: August 04, 2016, 10:20:18 pm by Slow Pete »
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Past owner of.  Bultaco 125,  Suzuki RM250A, RM125C, RM125T, RM400N, RM125X, Maico AW250 1977.   Presently 81 RM250X , 87 RM125H , 86 RM250G & 81 RM125X.   Current projects 80 RM400T & 78 RM125C

Offline Lozza

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Re: Timing and TDC for a RM
« Reply #1 on: August 04, 2016, 09:53:34 pm »
You want to know where Top Dead Centre(TDC) is ???
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Jesus only loves two strokes

Offline pokey

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Re: Timing and TDC for a RM
« Reply #2 on: August 04, 2016, 10:19:25 pm »
The answer is

Suck it and See


Your bike will be set up different than someone elses so start at factory setting and then  try your own  and see what you like .
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Offline Slow Pete

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Re: Timing and TDC for a RM
« Reply #3 on: August 04, 2016, 10:51:54 pm »
No  I know where top dead center is, what do Suzuki recommend is it 2mm like HPI recommend ?
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Past owner of.  Bultaco 125,  Suzuki RM250A, RM125C, RM125T, RM400N, RM125X, Maico AW250 1977.   Presently 81 RM250X , 87 RM125H , 86 RM250G & 81 RM125X.   Current projects 80 RM400T & 78 RM125C

Offline 80-85 husky

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Re: Timing and TDC for a RM
« Reply #4 on: August 05, 2016, 09:38:05 am »
TDC is Top Dead Centre. its the point where the con rod has reached max travel and the piston is at the top. timing is always done BTDC so that it allows enough time for the flame front to expand just as the piston moves past tdc and is heading back down.
Put a dial gauge in the spark plug hole. move the flywheel around (piston at the top) and find the point where the dial gauge stops climbing and starts falling that's tdc. move the piston "back" until the dial gauge says 2mm has been travelled by the piston in a down direction backwards from TDC...that's your timing point. try that and see if its ok. then 1.8 then 2.2mm to see if its better. capiche?
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Offline skypig

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Re: Timing and TDC for a RM
« Reply #5 on: August 05, 2016, 11:58:12 am »
2mm or 2deg?

My 81 125 just has lines to line up on the stock system. (Seemed to have a little more midrange when I carefully set it up compaired to where it was set during the, er, "full overhaul") The lines are  probably no use with an aftermarket set up. I suspect they don't mean anything like TDC, just references. I'm often wrong.

I bought a degree wheel at CD. Might never use it, but it's cool to have. I'll keep it with my 4cyl vacuume gauges.
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Offline sleepy

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Re: Timing and TDC for a RM
« Reply #6 on: August 05, 2016, 05:23:59 pm »
Quote from: Slow Pete on August 04, 2016, 10:51:54 pm
No  I know where top dead center is, what do Suzuki recommend is it 2mm like HPI recommend ?

It is hard to compare an aftermarket ignition's timing to the factory spec because both systems have an advance curve that alters the timing as the RPM changes. Older RM's had there timing marks line up at around 6000 rpm , so without having fully mapped the stock ignition with a good timing light and replicating as close to that with the HPI you will have to do a bit of testing or go to a Dyno shop.
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