Port Timings
Background
Ever since I put a 60mm crank on scooter I have been suspicious that the port timing had been messed up. I remember about 10 years ago I rang Terry at Taff speed, when he was still alive, and he told me that I should stick with the 57mm crank. I had already put the long stroke crank in at that stage so I couldn't bear undoing all of the work (or money).
Symptoms
- Difficulty getting the low speed jetting right.
- Rough running just off idle (4 stroking)
Forward wind lots of years and I finally got a chance to take a proper look, hopefully anyone else with a similar kit will find my findings useful.
Current build
- PX200 engine
- Cast iron Polini 208 kit, port matched transfer ports in the casings.
- A Worb5 head machined for the long stroke crank (it has a 1.5mm squish and the plug is moved toward the centre).
- A 60mm racing crank
- SIP 1.5KG full size balanced racing flywheel
- 28mm Delorto
- SIP racing clutch (very nice)
- SIP clutch bearing actuator
- Max. CHT temp. 160 Degrees C
- Max compression 150psi (warm, WOT).
- Ignition timing 19 degrees BTDC
I was running the kit with no base gasket and no head gasket.
The squish was around 1.2mm which is about ideal for racing (Norris Ker recommends 1mm in his book).
The jetting was about right (apart from the 4 stroking bottom end, usually found due to being to rich). I've been playing with jetting and slide modifications for years and still not got it perfect.
Ignition timing was around 19 BTDC (or around 2.5mm piston movement).
Stock Polini port timings
"Apparently" the stock Polini kit has port timings of 123° transfers to 171°exhaust timing with the 57mm crankshaft, but that information does vary depending on where I find it on the web.
Another site said 161.2; 112.8; 21.2 which is miles off what I found and I do question.
My port timings
My initial port timings with this set up where:
- Exhaust port open: 262 degrees
- Transfer open: 238 degrees
- Transfer close:115 degrees
- Exhaust close: 91 degrees
The sums
Exhaust port open - Exhaust close = exhaust port duration
Transfer open - Transfer close = transfer duration
Exhaust port duration - exhaust port duration / 2 = Blowdown
- Exhaust duration: 171 degrees
- Transfer duration: 123 degrees
- Blowdown: 24 degrees
This is pretty much exactly what I had found the Polini 208 kit to be, so the long stroke hadn't really effected the port timing much, if at all. I did want to get the rough running sorted though so I elected (easy to vote when there is only one of you) to install the base gasket which did alter the port timings a little.
Piston just clearing the transfer port.
Finding TDC
Modified Port Timings
"To alter or not to alter"
I chose not to alter the height of the exhaust port, even though my calculations found that I my ports timings where very close to the edge of what is recommend. I put this down to Mr. Polini knowing best and left it there. Besides, I want a grunty engine for riding around town, I've given up on the super high revving 2 stroke scooters.
So all I ended up doing was cleaning up all of the ports up a bit, and installing a 0.3mm base gasket and a 0.7mm head gasket. This gave me the following set-up.
- Exhaust duration: 174 degrees
- Transfer duration: 125 degrees
- Blowdown: 24.5 degrees
- Squish clearance: 1.9 mm
Some greasy scribblings
I also backed off the ignition timing to 18 degrees BTDC
Test ride
With the slightly revised port timing, the relaxed ignition timing and the reduced compression the engine feels smoother across the rev range, and does tend to reach high revs more easily (not a surprise as I have marginally raised the port timings. Given the reduction in compression I was surprised it still pulls very well.
CHT measured at a MAX. of 151 degrees which is quite acceptable.
Conclusion
All in all it was worth while to go through this exercise, one to put my mind at rest about the timings, but also because the bike does go better.
It does still 4 stroke a little just off idle though...
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