How to calculate port timings?

[Donnie]

From my thread about how to get a proper squish and Im wondering overnight now if I should go through the port timings etc and find out what they are as I still don't know what exhaust im going to end up with can anyone recommend an idiots guide on how to do this?

Ive googled and there seems to be an array of information but think im confusing myself more and more.

Is there any website / you tube clips that anyone on here would recommend?

Donnie.

[dickie]

Put a degree disc on your crank.
Mark tdc
Make sure barrel is fully seated but with head off
Use a very thin feeler gauge, say 0.1mm poke it through your exhaust port with piston at bdc
bdc.
Rotate crank until it is stopped by the feeler gauge and write down how far it is from tdc (should be around 90 degrees on your kit)
Rotate crank in opposite direction until it is stopped by the feeler gauge.
Write this number down, it should be the same as the first number if you've marked tdc correctly)
Add the 2 numbers together and that is your exhaust duration.

Do the same process for your transfers

Don't bother with your inlet as you've got a reed so it's irrelevant.

I think that's it but I'm fairly confident that someone will spot a mistake somewhere.

[Donnie]

Well ive used the scooterotica calculator here http://scooterotica.net/page19.htm

So, it's a 58 stroke and 107 conrod in there and a deck height of -0.3mm assuming you measure the edge of the crown to deck as opposed to the highest point of the piston crown?

So exhaust port wise im getting 36mm from top of the exhaust port to the top of cylinder giving me 166.73 degrees of exhaust duration

Transfer duration works as 47.45mm top of transfer to top of cylinder giving 113.14 degrees

Inlet duration? Piston skirt length (edge of crown to bottom of piston skirt on inlet side) is 63.06mm however there's a cutout where i have measured it and there's about 3mm either side of this still which is technically the bottom of the piston so hope i measured the correct part? Inlet Floor Depth (Top of Cylinder to Bottom of Inlet Port) is 98mm
anyway if correct the inlet duration is working out at 174.04mm

[Captain Pugwash]

Port measurements with a vernier gauge then Re-calculated into degrees is good and it will give you an idea of what port timings your running but the degree disc method mentioned above, will be more accurate and will give you your actual or at the least it will be close to your real port timings, With less chance of errors.

IMO The inlet port duration isn't too critical with the reed fitted but a larger than standard port timing duration and area may be beneficial........
But if the inlet is too wide and doesn't support the piston it can damage the piston skirt.

[dickie]

Donnie, those durations confirm what I expected. Your exhaust should be 173 and 124 for your transfers.

Raising the barrel by 2mm will get you to approximately that. You could easily check by adjusting the numbers you put in the calculator by 2mm, but I'll bet my left gonad that you're within 2 degrees when you add a 2mm base packer.

My wife says I should go out more.

[Donnie]

its funny, i read that the timing disc method was less accurate than measuring lol, ill try the timing disc version tomorrow evening and report back.

thanks all for your patience so far.

[dickie]

Timing disc is the daddy. You're measuring directly rather than working back from data.

Direct measurement is always best.

[Eden]

Well ive used the scooterotica calculator here http://scooterotica.net/page19.htm

how long ago did they rip off my script? lol
http://www.lambretta-images.com/archive/porttiming.php

ive just looked at their source code and its defo my code as ive got a few bits in there that don't do anything in the calcs and they have used it all. even the error in the corrected cc LMAO

sneaky stupid cunts!

[CHRIS in MARGATE]

Not necessary !

[Donnie]

ooops sorry didn't mean to cause problems/rows

[dickie]

Jesus christ Donnie!

This isn't tiddly winks we're talking about here. Don't you understand how deeply the appreciation of port timings on old shopping mopeds can affect all of our futures?

Please think before you ask such flippant questions in future.

[rossclark]

In the interest of balance this one might not cause such a stooshie...

[Donnie]

Jesus christ Donnie!

This isn't tiddly winks we're talking about here. Don't you understand how deeply the appreciation of port timings on old shopping mopeds can affect all of our futures?

Please think before you ask such flippant questions in future.

LOL

[Donnie]

OK so, (moving on )

Using the timing disc method

the exhaust port is 93 + 93 meaning the exhaust duration's 186 degrees.

Transfers are 117 +117 = 234 degrees??

What have I done wrong???

[dickie]

You're 180 degrees out

So, your timings are :

180-93=87
87+87=174 for exhaust

180-117=63
63+63=126 for transfers

These are pretty much spot on for a gt186

Did you get these timings with any base packers or gaskets in place?

[Donnie]

Doh, told you all this was new to me!

Yes, only one standard paper type base gasket in place with this, should I have done it with none?

[dickie]

No, you need to understand what they are when you're finished building. So you've done the right thing.

Now you need to check your squish but i guess when you did it originally it was no different to how it is now. Yes?

Am I right in thinking that at tdc the piston crown is 0.3mm above the top of the barrel?

[Donnie]

yes the edge of the crown's .3mm above the barrel top, if I put the head straight on now it'll hit like before. putting the 1mm head gasket only gave me a .5 squish.

[dickie]

This is odd. It seems as though you have a short barrel! I know I said it was the less likely scenario earlier!

Double checking, at bdc, is the edge of the piston crown perfectly level with the bottom of the exhaust and transfer ports?

[rossclark]

What type of casing is it?

There is some variability in the height of the base gasket face between Italian / Spanish /Indian cases, though usually it's too high in the latter examples.