rotary valve inlet timing on PE /PX range vespas

[sean brady scooters]

we all know that cutting back the crank to extend the inlet timing gives more power but at an elevated rpm range .
and thats what is usually done as per the instructions that you get with either malossi ,polini or pinasco kits ...
but is that the way to really go ?..
we have had great results in going the other way in fact ,opening up the intake more BTDC as opposed to after .
65/70 degrees after seems to be the optimum for after tdc,but extra timing can be got from opening it much earlier and so keep the motor less revvy ,usually i have so far just done these to open at the point of transfer closing but i,m well aware that most motorcylces with rotary valves open at least 10/15 degees before even that ..
so, question is ..have any of you done that ...?
that would equate to approx 130 /70...or more ...so 200 degees plus

[OzOAP]

I Cut the crank and elongating the inlet port forwards/backwards to get desired xº/xº timing

[sean brady scooters]

yes for sure.
but what i,m asking for really is the optimum opening point for rotary valve inlet timing ..in other words how many degrees before TDC or how many after BDC can it open .just wondered if anyone had done this and what effect it would have by say opening the inlet before transfers have closed .

[tha83]

I did just what Oz said. Cut the crank and opened slightly front and back intake, must have got lucky cause I didn't know what I was doing, just instinct. Malossi 166 and pulls like a freight train, even two up. I'm 230 and my gal is 130, no trouble pulling the San Francisco hills. Basically I square-ovalled just past where the factory drills down into the inlet. Smoothed out and pushed past a wee bit in each side front and back. Just being a fool with a tool and it worked. I reckon I went 2 or 3 mils past the factory opening each way,

[wack 63]

I've often thought of doing that so the inlet opens before the transfers close but didn't want to do it incase it didn't work or made the bike too peaky? Graham Bells book has this topic covered but it relates to disc valve motors so may not work with rotary induction. Tony OBrien would probably be the man to ask on this subject or some of the gurus' on the Smallframe forum .

[joeythescooterboy]

I thought disc valve was rotary induction??

[sean brady scooters]

I thought disc valve was rotary induction??

it is the same thing

[tony]

Kind of. The 'real' disc valve system is much less restrictive than the Vespa rotary system and allows a full circle crank. In terms of inlet timing then yes the same rules apply and in that sense it's the same.

[Diablo]

I ported my own Polini this way and it seems to do what the theory suggests which is increase bottom to mid range power with little in the way of over rev. This is because I knew the exhaust timing would be relatively low and also I wanted to pull taller gearing. It does 70mph at 6500rpm and has done 20,000 miles now so think its safe to say its a good mod.
I decided to do it after reading what Bell said about disc valves and just applied it to the Vespa.

Nowadays I tend to just flow and blend the port and just cut 10mm off the crank on most road going bikes. This is because I've found that before the bike goes onto pipe they are difficult to jet cleanly and get a decent tickover. To do this you end up running leaner settings but this causes problems when shutting off the throttle and coming back on from high revs. Couple this to people wanting autolube and std carbs and the benifits don't outway downsides for me. What I can say is when I overlay the graphs of my 207 against the 220s we do now the 207 boxes above its weight. But embarrassingly my bike has always been difficult to jet low down. I think next time I do one for myself I just won't go back so far and try and get a compromise.
On small block motors the inlet is so small that I often cut backwards to increase timing but still pull taller gearing.

[drunkmunkey6969]

Interesting stuff, thanks for response. It's a rotary valve type inlet and has the asymmetrical timing benefits, but due to the crank webs and narrow inlet on casing doesn't have the full flow benefit obviously.

Most cut cranks seem to come with too much ATDC and not enough before. For a top power motor I'd like to see a decent expansion chamber with strong draw, peaking circa 8k. A Malossi 210 kit is generally lacking in transfer timing on the B and C ports, but a 60mm crank goes a long way to correcting that along with some porting. Add a crank with inlet timing circa 200-220 degrees and I think it would would be pretty ballsy.

And for the touring option, I think Al has it spot on.....a 24mm carb, JL right hander, Polini kit and a crank cut to circa 115 BTDC and 65 ATDC....?