exhaust port timings

[nelson pk]

As regards to the exhaust port.....a famous two stroke tuning book states that 2 checks should be made to the piston and exhaust port.......the bottom of the port window should be level with or lower than the piston crown. The second check is irrelevant in this case as thats about bridged ports.
so....ideally you want it level. If the crown sits above the bottom of the port window it is nothing to worry about. Comments saying your piston will destruct if you leave it like this are not true. The piston crown is going to come into contact with the exhaust heat wherever it is as its the piston that stops the charge going further down the barrel and directs the gas flow out of the port. That is why if the piston crown is lower than the bottom of the port window it will not run so well as it disrupts the gas flow. If the crown is higher that the bottom of the port window it is not nearly as bad.
If it did go bang it wont be that that causes it.
Transfers are another thing and someone else can advise you better on that. At the end of the day you have an Indian 190 kit and it wouldnt surprise me if some things are off but your exhaust port seems to be causing you more worry than it should.

[nelson pk]

Just saw the pics of how far over the port it is. Its not great. Ignore my previous post and take it back to your tuner for advice.

[eden]

tv crank ?

wouldn't make any difference to the exhaust port as its the stroke that determines how far down the piston crown is at bdc.

[hendy]

Maybe it was that way to start with?

Surely most tuning raises the roof of the port or widens it?

I've learned a lot from this. As a four stroke chap none of this stuff has ever concerned me before and in a few days of thinking, measuring and reading I reckon I have a pretty good idea about a decent set up for a lambretta.

That said, the barrel is junk now and the tuner/supplier has agreed to supply and tune a new one.

I have no idea how it ended up so wrong, but at least I can spot good from bad easily now. Last weekend I didn't even know what port timings were.

It's been very interesting, thanks for all your help gents.

[Adam_Winstone]

TV crank (116mm rod) would make a difference if it was a non-TV compression height piston... but that would push it 9mm higher. The same could be said if you were sold a crank with a 110 Jap rod but that would push it 3mm further up than the standard 107mm rod.

These comments are based solely on rod length (for no change to piston compression height... like for like), nothing to do with stroke. Changing stroke would further change relationship of crown to port positions, but again, we want to consider rod length and stroke separately.

Adam

[eden]

TV crank (116mm rod) would make a difference if it was a non-TV compression height piston... but that would push it 9mm higher. The same could be said if you were sold a crank with a 110 Jap rod but that would push it 3mm further up than the standard 107mm rod.

These comments are based solely on rod length (for no change to piston compression height... like for like), nothing to do with stroke. Changing stroke would further change relationship of crown to port positions, but again, we want to consider rod length and stroke separately.

Adam

but you would need to add material to the top of the barrel as it would also push the crown out of the top of the barrel and the crown would then be partly covering the transfers at bdc!

[Adam_Winstone]

Correct but we don't know what the tuner was working to or had in mind... as if working to one set of measurements without twigging to the measurements relating to a different setup that had been done before.

I must add that I've worked on a number of barrels where the piston crown was above the bottom of exhaust at bdc but never to that extent before. I've often seen a couple of mm over the port but never so much, including ring land.

Something has gone wrong somewhere.

Adam

[eden]

I always do mine a bit lower than the crown

[Adam_Winstone]

Nowt wrong with that.

[eden]

if that barrel in my pic was used with a 58mm stroke the crown would be sitting much higher at bdc. it has a 62mm stroke so ive lowered the port quite a lot!