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Author Topic: Questions on tuning Powerdynamo (MZB) ignition  (Read 1339 times)

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Offline Matty

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Questions on tuning Powerdynamo (MZB) ignition
« on: June 08, 2011, 07:52:06 AM »
So I finally got my powerdynamo ignition installed in my RV90, and it works, and puts out a nice 12V to all my lights.  I am having a bit of trouble with tuning though.

For starters, the powerdynamo flywheel is much lighter than the stock RV90 unit, so you give up some torque right away.  The motor revs faster, but on a bike like the RV thats not really an advantage.  Cant really do much about that though.

The other problem is that I think I have the timing really close, but not close enough.  I rode the bike all weekend, so its definitely rideable, but it seems to really fall off up top, and not want to rev as high as it did.  I'm thinking adjusting the timing a bit could fix that, but I'm not sure which way to go with it?

Should I retard it or advance it to pick up more on the top end?  I'm asking before trying, because I cant just rotate the backing plate, I have to pull the flywheel and time it from scratch at 22.4mm btdc give or take some advance or retard in the ignition.
75 Kawi F11 250 - Vintage Supermoto
72 Suzuki RV90 VanVan - Visit the VanVan Wiki!
1972 CTRV90 (CT90 Engine in RV90 Chassis)
2006 Suzuki DRZ 400SM

Offline poppaweely

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Re: Questions on tuning Powerdynamo (MZB) ignition
« Reply #1 on: June 08, 2011, 08:15:09 AM »
up-jet
2- RD400D
GT380K
B105

Offline m in sc

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Re: Questions on tuning Powerdynamo (MZB) ignition
« Reply #2 on: June 08, 2011, 08:30:38 AM »
jet up, verify timing. retarded is better up high.
get off my lawn.

Rock Hill, SC

Offline Matty

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Re: Questions on tuning Powerdynamo (MZB) ignition
« Reply #3 on: June 08, 2011, 09:09:52 AM »
Jet up even though there were no other motor changes?

It ran fine before the powerdynamo install.
« Last Edit: June 08, 2011, 09:12:41 AM by Mattydiah »
75 Kawi F11 250 - Vintage Supermoto
72 Suzuki RV90 VanVan - Visit the VanVan Wiki!
1972 CTRV90 (CT90 Engine in RV90 Chassis)
2006 Suzuki DRZ 400SM

Offline Coyote

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Re: Questions on tuning Powerdynamo (MZB) ignition
« Reply #4 on: June 08, 2011, 10:29:58 AM »

[slight flight of fantasy]

Mattydiah says
Quote
For starters, the powerdynamo flywheel is much lighter than the stock RV90 unit

AND..but it seems to really fall off up top
.
If you can never get your power back with timing and jetting, you may have to either make the crank heavier or raise (or widen) the exhaust port.
.
What?
First let me say that this is the last thing to think about, but it is a real effect, and is pretty cool to think about.
.
This can be an issue, especially if the engine has a rather high compression or is oversquare (has a short stroke and relatively big piston).
.
Huh? It seems that a 2 stroke engine changes RPM throughout each rotation.
The RPM slows down during compression, the RPM speeds up after combustion.
Allot of small racing engines have a race kit that includes a heavier flywheel or a heavier crank.
.
What happens is the engine speeds up while the piston is descending from combustion forces, right when you are needing blowdown. This robs you of blowdown time*area because the piston goes past the exhaust opening, to the transfer opening too fast, reducing the blow T*A,  thus it will not support the same power at the same high revs when you lighten the rotating mass. (Actually I think it is the moment of inertia, IE... you gain angular momentum if the flywheel mass is further from the axis of rotation, or some shit like that, but you get the idea).
.
You Gotta be... YGBSM (look it up wild weasel)
Yes, a CR 250 changes RPM by something like 700 or 800 RPM at 8,000 RPM or something like that.
Many race bikes have heavy flywheels or cranks to smooth out the change in RPM over each cycle.
.
Anyway, just a possibility, maybe not a probability.

[/slight flight of fantasy]

Hope it is just timing
Dave
<-- A real handful, Nitro Express, only 900 grain  -at the cutting edge of Insanity-

Offline Surewin

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Re: Questions on tuning Powerdynamo (MZB) ignition
« Reply #5 on: June 08, 2011, 04:34:06 PM »
Mattydiah,  Does your RV90 have a tuned pipe exhaust?

Dave,  Were did you find the numbers regarding the CR250 engine speed fluctuations?

« Last Edit: June 09, 2011, 08:05:46 PM by Surewin »

Offline Matty

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Re: Questions on tuning Powerdynamo (MZB) ignition
« Reply #6 on: June 08, 2011, 04:42:22 PM »
Nope, stock
75 Kawi F11 250 - Vintage Supermoto
72 Suzuki RV90 VanVan - Visit the VanVan Wiki!
1972 CTRV90 (CT90 Engine in RV90 Chassis)
2006 Suzuki DRZ 400SM

Offline Surewin

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Re: Questions on tuning Powerdynamo (MZB) ignition
« Reply #7 on: June 09, 2011, 08:05:13 PM »
As I understand it retarding the ignition timing help increase the power after the powerband.  It works by raising the EGT which makes the pipe appear to be shorter, or tuned for a higher engine speed.  So I'm not sure how much this would apply to a 2S engine with out a tuned pipe, where the delivery ratio may be a stronger function of other things, i.e. not the tuned pipe.  Sorry, not much help on your RV90.