70cc porting

[DougW]

The two small auxiliary exhaust ports on an Airsal T6 Speedfight barrel are about 0.5 mm higher than the main exhaust port.

Internet browsing would suggest it should be the other way round, at least on a properly tuned engine.

Would it make any difference on a moped running a 17mm carb and a de-restricted but  legally silenced Technigas exhaust ?

I have the freshly scrapped by junior last barrel to practice on !

[Martin77]

Depends on port timing... if lifting the main port makes it too tall then I'd leave it personally. What's the port duration now, and if it was lifted? Someone else might chip in..

 

[2T Institue]

On 6/12/2020 at 5:30 AM, DougW said:

The two small auxiliary exhaust ports on an Airsal T6 Speedfight barrel are about 0.5 mm higher than the main exhaust port.

Internet browsing would suggest it should be the other way round, at least on a properly tuned engine.

Would it make any difference on a moped running a 17mm carb and a de-restricted but  legally silenced Technigas exhaust ?

I have the freshly scrapped by junior last barrel to practice on !

Does it have a CVT,if yes raise the main to about 200* and raise the aux's to 0.5 under that

[DougW]

6 hours ago, 2T Institue said:

Does it have a CVT,if yes raise the main to about 200* and raise the aux's to 0.5 under that

According to my measurements the main exhaust port opens 24 mm from tdc which gives 171.6 degrees of exhaust duration ( if it is 92mm rod 40mm stroke) , to get 200 degrees of exhaust duration it would need raising to 18.66 mm , 5.34 mm higher than current ?

apologies if I have got this all wrong, but it seems an awful lot !

It has a cvt, currently revved to 10 000 rpm according to Junior before it seized as way to lean. It has a Naraku ‘deristricted” cdi, I have a capacitor to try across the pick up to see if that retards it at high rpm, I’m sure I have a timing light buried away  somewhere!

Edited June 13, 2020 by DougW

[Martin77]

Plenty of scope to lift it, but I'd stick with about 185 duration (21-22mm from the top), porting to 200 degrees in one hit without altering anything else would be too much. 2T would probably disagree, but he always goes all out.. 😅

 

 

[2T Institue]

no use mucking around little Dougie will get the hang of it fast enough 🤣

[2T Institue]

On 6/14/2020 at 12:43 AM, DougW said:

According to my measurements the main exhaust port opens 24 mm from tdc which gives 171.6 degrees of exhaust duration ( if it is 92mm rod 40mm stroke) , to get 200 degrees of exhaust duration it would need raising to 18.66 mm , 5.34 mm higher than current ?

apologies if I have got this all wrong, but it seems an awful lot !

It has a cvt, currently revved to 10 000 rpm according to Junior before it seized as way to lean. It has a Naraku ‘deristricted” cdi, I have a capacitor to try across the pick up to see if that retards it at high rpm, I’m sure I have a timing light buried away  somewhere!

The maths looks correct

[DougW]

33 minutes ago, 2T Institue said:

no use mucking around little Dougie will get the hang of it fast enough 🤣

I'm sure he will, as he rides a 250 KTM 2t offroad, but I'm not so sure about Mrs DougW who likes to ride it to work occasionally !

Then again, she copes with her RM85...

[DougW]

Playing with scrap barrel to raise ex port.

Aldi needle files, access ok, going to take forever.

Dremel with die grinder bit, chipped plating.

Dremel with sanding drum, works ok if a little slow.

Waiting for new cylinder to arrive.

Base gasket is 0.5 mm as is head gasket, so thinking of 2 base gaskets and lapping cylinder head on, no head gasket, (stag wellseal) and raising ex port.