Inlet Timing

[joeythescooterboy]

Been doing a bit of measuring and found I have 190deg inlet timing.
Read through my old Norrie book and it he says that you should have a max of 70deg after TDC, I have 110 before and 80 after with a Mazz 60mm crank.
Just how important is the 70 after rule?

[holty]

hi, if your inlet timing is too big you would be best to use a reedvalve as this will stop the inlet charge being expelled back out of the carb, this is the main reason that reedvalves are used, to increase inlet timing to much higher levels.

[joeythescooterboy]

It's not too big kid, just not sure how important the 70 after "rule" suggested by Norrie is.
Inlet was ported a while back for standard crank with 10mm cut off leading edge and it was fine (can't remember exact figure, I've slept since then )
I knew Mazz crank was a longer dwell but it's only now I've actually measured that I know I've got 80 degrees after TDC.

[bristolmod]

how does it ride??

[joeythescooterboy]

It rides well, very torquey, pulled uphill around the Amalfi Coast Rd in 4th 2's up but won't go over 6000 rpm....... but that's another story.
Really just trying to get clarification on the 70 after thing.

[bristolmod]

if it aint broke........

[Jack221]

It's all about rpm. The higher it revs the more you need. 90 degrees is probably good for 10,000 rpm

[sean brady scooters]

lol, more like the other way around i reckon ! ...as only by increasing the inlet timing will it rev higher ..
anything up to prob a max of 210 degrees can be used ,depending on use and gearing etc .
whenever i work out the desired timing ,i always start from the opening time ...usually around the point of transfer closing although it can be opened slightly earlier .
the main restriction of course on most rotary intake vespas is the area of the port ,t5,s have a wider crank web and so offer a better time /area .

[Jack221]

You've touched on so many subjects all at once, I don't know where to start This inlet timing thing is something that keeps on coming up though. I think Joey's 80 degrees ATDC is ok. Norrie wrote that book a long time ago. Like you implied with modern thinking, it could do with a bit more BTDC, probably about 120 degrees, as this would be compatible with any barrel that has peak power at somewhere 8500+ rpm.
Running 80 degrees ATDC on a barrel that doesn't need it will just make it a little over rich at low/mid rpm, should hardly notice.

I am one of those that think a well set up P200 rotary intake valve will give more mid/top end power than reeds up to a 30mm carb. Any carb bigger than 30mm and there just isn't enough area, in which case, it's reeds all the way.

[sean brady scooters]

just to add that the later the rotary inlet timing is closed ,the higher up in the rev range the power is made ...not always the best thing though as is often the case ..cranks cut that way can make a vespa to peaky !