Cylinder porting problem?

[Chainlube]

I purposefully ended up with several door stops, but I knew I was doing something outside the box. It was more to push the limits to see where it passed the limits of practicality or functionality.

Yea, yea, 1 mm is good right? Lets see what 2 will do kind of thing.

 

[ImpulseRocket]

Yea, yea, 1 mm is good right? Lets see what 2 will do kind of thing.

More or less, yeah.

 

[Ivan168]

So far the engine is working fine, I've already burned 2 liters of petrol, I have 4 more to go before the complete breakdown as you said, but I still have the feeling that the low end is worse than expected, I have the feeling that the original gasket is not sealing well enough and the engine is losing compression, I plan to buy brass or copper sheet and make my own gasket with a thickness of 0.3-0.4mm, which is enough to not hit the spark plug piston.

 

[NeedSpeed]

On the G4s, I like to texture the runner then roll the edges of the intake port a little but leave it alone otherwise. I've found noticeable loss of low end and minimal high end gain when widening/dropping the intake. I believe this is mostly due to the velocity of the mix entering the engine. Get a degree wheel! You will see your port timings, record, make adjustments, and record again. After a while, you may find some numbers that you like to shoot for, as well as port shapes. I like to widen the exhaust to about 65% of the bore and shoot for about 165° exhaust duration and about 18° blowdown. I like to use a shape like this with less dramatic angles and the roof highest, slightly to the right of center.

Any more duration/blowdown, and you will be spitting your mix all out the exhaust before it can burn. Don't give up! It's pretty simple, once you grasp and understand the theory.  Research! Just because I like these numbers does not make them right for you. I would approach those numbers as a general max, not exceeding them until you want to try some stuff. Yes higher numbers are possible but I think mine are more consistent. Also, any time you port for higher rpm, you are going to sacrifice some low end. If you have alot of stop and go, maybe try something more moderate. There's a bunch of knowledge online. Check out some porting info from the guys that build chainsaws on youtube, then maybe go for some more moderate numbers. That should be a good starting place. Damien has a link for the book I recommend everyone read... I can't remember what it's called. Good luck!

 

[DAMIEN1307]

Damien has a link for the book I recommend everyone read... I can't remember what it's called. Good luck!

Yep, here it is, I found it again...lol...When you click on it, it will immediately download the PDF to wherever your computer downloads things to...lol.

 

[NeedSpeed]

Yep, here it is, I found it again...lol...When you click on it, it will immediately download the PDF to wherever your computer downloads things to...lol.

Thanks!

 

[Ivan168]

On the G4s, I like to texture the runner then roll the edges of the intake port a little but leave it alone otherwise. I've found noticeable loss of low end and minimal high end gain when widening/dropping the intake. I believe this is mostly due to the velocity of the mix entering the engine. Get a degree wheel! You will see your port timings, record, make adjustments, and record again. After a while, you may find some numbers that you like to shoot for, as well as port shapes. I like to widen the exhaust to about 65% of the bore and shoot for about 165° exhaust duration and about 18° blowdown. I like to use a shape like this with less dramatic angles and the roof highest, slightly to the right of center.
View attachment 218003
Any more duration/blowdown, and you will be spitting your mix all out the exhaust before it can burn. Don't give up! It's pretty simple, once you grasp and understand the theory.  Research! Just because I like these numbers does not make them right for you. I would approach those numbers as a general max, not exceeding them until you want to try some stuff. Yes higher numbers are possible but I think mine are more consistent. Also, any time you port for higher rpm, you are going to sacrifice some low end. If you have alot of stop and go, maybe try something more moderate. There's a bunch of knowledge online. Check out some porting info from the guys that build chainsaws on youtube, then maybe go for some more moderate numbers. That should be a good starting place. Damien has a link for the book I recommend everyone read... I can't remember what it's called. Good luck!

Unfortunately there is no degree wheel available here, that's exactly why I don't want to touch the transfer ports and as I understand from you even the exhaust port, what does that picture you uploaded show, how should the exhaust port look properly shaped?

 

[Gordy]

Unfortunately there is no degree wheel here, that's why I don't want to touch the transfer ports and as I understand from you even the exhaust port, what does that picture you uploaded show, how should it look properly shaped exhaust port?

You can print out a degree wheel and glue it to an old CD.

 

[DAMIEN1307]

Unfortunately there is no degree wheel available here,

You make a degree wheel yourself as Gordy just mentioned.

 

[NeedSpeed]

Unfortunately there is no degree wheel available here, that's exactly why I don't want to touch the transfer ports and as I understand from you even the exhaust port, what does that picture you uploaded show, how should the exhaust port look properly shaped?

Maybe try to find a "camshaft degree wheel". You can make the center hole bigger. Maybe you could print one on sticker paper and stick it to a flat circle. Wood cardboard sheet metal plastic. The pic is how I like my exhaust port shaped but the picture has more extreme roof angle just to show the idea. I like the roof a bit more flat than that.