RZ/RD 350 & Misc. 2-Stroke Tech BBS

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PostPosted: Sun Jan 21, 2018 3:48 am 
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Location: Alberta, Canada
So, I got me a new motor and, in my infinite wisdom, I decided to modify it.
The mods are certainly not extreme, but I think it’s safe to say that the engine is No Longer Stock.
Next year, when I get it on the road and start exploring the limits of speed and acceleration, I’d really like it to… not blow up.

In order to gain a bit of information on modifying 2 strokes, I’ve been reading, and reading, and re-reading a couple of 2 stroke tuning books. One written by Graham Bell, and the other by Gordon Jennings.

I’ve learned quite a bit of information on pistons, rings, piston speed, piston acceleration, combustion chambers, squish bands, expansion chambers (and the history of expansion chambers), tuned length, diffusers and baffle cones, cylinder porting, time area, angle area, intake tract length, resonant frequencies, cylinder scavenging, carburetion, carb size, ignition timing…blah…blah…blah.

I’ve read all this stuff and I still know next to nothing about tuning a high performance 2 stroke engine.
However, I did find tons of interesting items. Like the section on maximum piston speed and maximum piston acceleration.
After doing the math, I’m going to set my red-line (and rev limiter) at 10,000 rpm. (and that rpm is still at the upper limits of how fast a piston should be moving)

Another thing that both authors mention over and over again is that heat is your enemy.
And just about everything that you do to increase the engines power, will also make it run hotter.

The good news is that there’re all kinds of things you can do to reduce the temperature of the piston.
More fuel, less spark advance, less compression, different port timing, or a different exhaust design.

Like most amateur engine builders, I don’t have the knowledge to redesign heads, cylinders and pipes, or the physical skills needed to do the work.
So I can only make changes to the jetting and ignition curve, to keep the engine/piston temperatures within a safe limit.

When I decided to get a go-fast motor for my bike, I did what most people do, I let experienced, knowledgeable people do the design work, build the exhaust, and port the cylinders.
I just bought some parts and bolted them all together.
OK, I bought some really trick parts, and when this engine is tuned correctly, it should be a fire-breathing rocket.

Repeat, when the engine is tuned correctly.

Unfortunately, there aren’t a lot of experienced 2 stroke tuners in my part of the world.
So, for better or worse, it looks like I’m getting a promotion from assembly-guy to tuning-guy.
What could possibly go wrong with this plan? :smt005

All I have to do is find the correct Carbs, Jetting, PV Settings, and Ignition Timing in order to get maximum performance out of the motor, from idle right up to WOT, at an altitude of 3500’, without ever overheating a piston, causing it to seize up and die.
No pressure.

For now, I’m sticking with the stock carbs, so jetting shouldn’t be too difficult. I’ll go bigger, and slowly work my way down.
As a starting point, I’ve gone up 2 sizes on the main jet and put a washer under the needle clip.
I’m now running 440 MJ, 25 PJ, Needle on clip 4 with a washer under the clip.

I still have the stock 5K1 needles in my carbs, but I also have a set of the original Toomey needles.
The Toomey needles are definitely thinner than the 5K1’s, which should richen the mixture at most throttle positions.
After I finish breaking in the motor, I’ll do some testing and see how the Toomey’s compare to the stock 5K1 needles.

There’s lots of info on PV settings, and I can do some experimenting without causing any damage.
To begin with, I’ve set them to fully closed until 6000 rpm, then start to open and be fully open at 9200.
I’ve also set the power valves so that they have an extra ½ mm of travel, to account for the higher exhaust port.

The tricky part will be the ignition timing. From what I’ve read, you should retard the timing on a stroker motor.
My motor also has the compression raised to about 150 PSI. More compression equals more heat, and you should retard the timing to offset this problem.

Gordon Jennings also talks about ignition timing:

Jennings 2 stroke Tuners Handbook, Page 15

“Generally speaking, modifications that tend to raise engine output without increasing the crank speed at which maximum power occurs will require that the spark be retarded slightly from the standard setting. Major upward relocations of the power peak usually require a more advanced spark."

Jennings suggests that you retard the spark by 5*, and slowly give it more advance.
But he wrote this in 1972, before you could buy programmable CDI’s. Back then, you set the timing to X mm before TDC, and that’s where it fired for the whole rev range.

So, I should probably retard the timing. But, how much? And at what RPM?

Damned if I know. But I’m on a mission to find some answers before Spring.
I’ve already searched the forum to see what other people have done, and found that opinions vary. Who’d have thought?
Some people recommend that you retard the ignition timing on a stroker.
Other people have posted curves with the timing advanced.
Other people say that it’s fine to run a 4* advance plate on a stroker

viewtopic.php?f=1&t=5779&p=47393&hilit=ignition+timing+on+a+stroker#p47393

viewtopic.php?f=1&t=5850&hilit=zeeltronic+ignition+curves

viewtopic.php?f=1&t=9821&hilit=stroker+ignition&start=0

There are more examples, but you get the idea.

At this time, I’m thinking about programming these 2 curves into the Zeel.

For Curve 1, I’ll use the stock curve with 2* of retartd.
I’ll keep an eye on the EGT, and if all is well, I’ll run this curve for the first 200 to 300 miles, while I break in the motor.

2000 rpm - 15*
3500 rpm - 25*
9000 rpm - 15*
9900 rpm - 7*

I’ll put the stock RZ curve in the Zeel as Curve 2. I can then do some testing to see how the EGT compares to my track bike, which is also running stock ignition timing.

2000 rpm - 17*
3500 rpm - 27*
9000 rpm - 17*
9900 rpm - 9*

Here’s an Expanded version of the stock curve (I calculated the ignition timing for each 1000 rpm).

2000 rpm - 17*
3500 rpm - 27*
4000 rpm - 26*
5000 rpm - 24*
6000 rpm – 22.5*
7000 rpm - 21*
8000 rpm - 19*
9000 rpm - 17*
9500 rpm – 12.5*
10000 rpm - 9*


Depending on what I learn from running Curves 1 & 2, I’ll start advancing the midrange, from 3500 to 6000 rpm.
Maybe try something like the following as Curve 3.

2000 rpm - 15*
3500 rpm - 25*
4000 rpm - 25*
5000 rpm - 25*
6000 rpm - 24*
7000 rpm - 23*
8000 rpm - 20*
9000 rpm - 15*
9500 rpm - 14*
10000 rpm - 7*



I’ll do some more research, program some curves into the Zeel, hope for the best, and go have some fun.
Fortunately, I’ll have the EGT gauge to give me more info on what’s happening inside the engine, as I advance the ignition.
And I have free time on the dyno at work, which will also be very useful.


I’ll end this post with a couple of quotes from Mr. Jennings.
(I added the underlining because that’s the part that I found most amusing)

Jennings 2 stroke Tuners Handbook, Page 156

“Finally, with experience you'll learn to give your very close attention to all aspects of the mundane task of selecting jets and plugs, and spark timing, because in these things you ultimately succeed or fail as a tuner; all the rest is mere mathematics, surgery and wrench-twirling.”


Jennings 2 stroke Tuners Handbook, Page 129

“No greater futility exists than to invest heavily of one's time and money in building an engine with mirror-smooth ports, the best available pistons, rings, etc., and then approach the problems of carburetion and ignition armed only with enthusiasm.
It is an observable fact that some inherently mediocre engines have been made to prevail in racing simply because the men charged with selecting jetting and spark timing knew their business, but never in the history of the sport has there been an engine so splendid in its internal arrangement that a tuner's ineptitude could not render it entirely ineffectual.”


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PostPosted: Sun Jan 21, 2018 9:53 am 
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Location: Wolfenbüttel, Germany
The RZ engines can bear a lot of advance.
15 deg at 9k with decent pipes seems a bit low'ish and certainly would waste quite some of the potential.
Also 25 at midrange can easily be 28.

Check out the pipe building and PV mod manuals on http://rd350.gmxhome.de
The first one contains a method to determine the "right" ignition map, the second one covers the PV.

Generally spoken a stronger ignition and higher advance requires a bit richer jetting. If you've been on the rich side before, it may be good now.

What I did was changing the slope of the falling part (up from 5k in your last curve).
Start with what you have and then make the falling slope less steep (i.e more advance at higher rpm).
Ride it and feel how it revs. If it's better then repeat with more advance until you feel the engine runs worse than before.

Then optimise the overrev by introducing a second steeper slope starting from the poin bof max performance. My guess is that with a limit of 10k, there's nothing to optimise here.

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PostPosted: Wed Jan 31, 2018 8:01 pm 
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Hi Martin.

Thank you very much for your advice, and the link to getting the correct ignition and PV settings.
This information will help alot.


MK wrote:
Generally spoken a stronger ignition and higher advance requires a bit richer jetting. If you've been on the rich side before, it may be good now.


When I had the Stock motor in this bike it had the following parts / specs.
Zeel ignition, Dynatek coil, Toomey Pipes and Y-Boot, O-Ring head (130 psi comp.), stock carbs with 420 main jets and the needle on the 4th clip.

The new stroker motor is using Slingers Pipes with the Y-Boot, a new head with 150 psi comp., stock carbs with 440 main jets and the needle on the 4th clip with a washer under the clip.

The stock motor wasn't running overly rich, but just to be safe I went up a couple of sizes on the MJ as a starting point.
I've also got some 430 and 450 main jets so I can make some changes as needed.


I'm not sure if I have a Stronger ignition or not.
On the stock motor, I was using a set of 8 mm Taylor spark plug wires. On the new motor, I'm using a set of 8.5 mm MSD plug wires.
The Taylor wires have a resistance of 425 ohms
The MSD wires have a resistance of 57 ohms.

I measured the resistance of a couple of used NGK BR8ES spark plugs, and they measured 3800 ohms.
From an electoinic standpaoint, the voltage produced by the coil is dropped proportionaly across the resistance of the spark plugs and the spark plug wire.

The math says that, with the old 425 ohm Taylor wires, 10.1 % of the voltage was dropped across the wires and only 89.9 % of the voltage supplied by the coil was dropped across the spark plug.

However, with the new 57 ohm, MSD wires, only 1.5 % of the available voltage will be dropped across the wires, and 98.5 % of the coil voltage will be dropped across the spark plug.

Will the low resistance plug wires give me a stronger spark and improve the burn rate of the fuel mixture? [smilie=dunno.gif]


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PostPosted: Wed Jan 31, 2018 9:04 pm 
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Location: Calgary, Alberta, Canada
You should have a stronger spark, but it should not have anything to do with the burn rate in the combustion process. Compression and fuel and air control that, along with the shape of the combustion chamber. The stronger spark will ensure that you get ignition of the fuel at the time that the spark is generated.

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