F5's 300 head mod thread

[(F5)]

Ha, yeah lost a gear lever a while back so had to complete a loop in 3rd which was tedious, fortunately no killer hills. Hence, a gear & brake, clutch lever. Split link, the tube in front of PV cover is a looped 4' length of gas tube. Black thing at top of pic is small bicycle pump tied to rad hose. All items very light.

[twowheels]

That's great work. Here's a question though - from your previous work with 2T engines does the shape of the chamber (taller shoulders to achieve volume) make much difference in how the engine will idle? Rev? Torque?

I'm getting excited to fire up the lathe and have a go first at cutting a mandrel and then at modifying a head.

[(F5)]

Most of my work has been on high rpm roadrace bikes so the power you want is a good spread before & past peak power to cope with gear changes without falling from power. We try to make the chamber real shallow. There has been a trend to toroidal shapes which are more bathtub, skinny squish areas & with flat top pistons even moving the sparkplug closer to the piston so it is protruding from the chamber ceiling. Makes measuring volume like I've done problematic.

Dirtbikes, esp Enduro/trail bikes you want a considerably wider spread. To be honest I don't know whether the same extreme shallow shape would help or hinder. What I do know is that the squish area is larger so the chamber will naturally be taller anyway. The most influence will be in the effect of the squish area. Get that working properly & you should be ahead of the game. I think this will have some reasonable effect at lower revs, lower throttle position & idle. But there should be less gas wasted in an unburning area. this should mean more power.

The std chamber is a bit odd in that it has a flat section by the spark plug. I think the reason they have such an angle on the squish area is to stave off detonation as uncertain which fuel the customer will use (who reads the manual huh?).

Anyhoo, I made another cut & that ended up as 13.1:1 & I decided to call it a night, slapped it back together & ready to ride sunday. Hopefully it is worth the effort, proof in the pudding & all.

[byost]

First off thank you for sharing your knowledge. I am a rookie when it comes to rebuilding modern 2 strokes but I enjoy tinkering with machine tools. I was re-reading your first post and I am confused about the 4mm hole above the exhaust port you were describing. Is this stock or something you have added? I don't remember ever seeing a hole in any pictures I have seen. Also, I always assumed that the top of the piston was flat. Is it actually domed? You said that there was a 4 degree difference between the piston and the squish band. I am just trying to picture this in my head. I am also wondering how wide the squish band is from the outside edge of the cylinder to where the dome starts? Once again, thanks for sharing!

[byost]

Ok, I found the technical article and the excellent diagram of the squish band shape by RB. It's all starting to make sense now, like I said , I'm a rookie. Now I want to go home and check my squish. I love this forum!

[Fred1956]

Originally Posted by byost

I was re-reading your first post and I am confused about the 4mm hole above the exhaust port you were describing. Is this stock or something you have added? I don't remember ever seeing a hole in any pictures I have seen.

That hole is on the 300 cylinder only, it is supposed to be a compression 'release' to aid kickstarting. From what I have read you want that hole there when kickstarting a 300. I have no experience with it, just what I've read on it

[twowheels]

So, just for the fun of it, where in the stroke do you start your displaced volume calculation?

Is it full stroke, so BDC? When you close the exhaust port? When you pass the compression bleed hole (on the 300)?

[(F5)]

Full stroke. That's what matters when specifying a com safe for the gas you will use. When the engine is running the pipe is pushing mixture back in when in its range. when out of range the effective compression will alter, but the power valve is moving changing that point so so called 'trapped ratio' is a moving target anyway.

If people still argue point out that foul strokes stated compression ratio never mention when the cam opens or closes the valve. Why should the position of the hole - side of bore vs top of head make a difference.

The hole in the 300 (wasn't one in my 200, haven't pulled apart a 250) helps kickstarting as it does lower the compression ratio as the engine is turning over so slowly & cycle not started yet.


Actually I hadn't read that tech article (or at least might have skimmed it many years ago). It is well written & his picture looks quite a bit like my end result, but tiny picture. He also seems to have polished the heck out of it which looks nice. I don't agree with everything he says, but not enough to argue. $75US is a very fair price to pay (about $400nz kidding, -actually our exchange rate is much better, about $95NZ) is a good deal for that existing development. Cost more in freight & time for me & I like doing things myself, but if yer stateside it would make good sense, he's not being greedy.

[Jakobi]

No comp bleed on the 250's.

[(F5)]

Well I'm impressed. At first I was wondering if the bike was just running great but it was what I love about a big bore 2 stroke. Maybe I've made noi change and I'm just making it up as I've done some work. As I went on I realised it had woken up at low revs and low throttle positions. It's much like my old GG trials bike.

So after a loop (37km), I swap bikes with my mate, also a 07 EC300. The difference is stark. I find myself blapping. We change back. He's impressed, more bottom end and sound different he sez. Mine feels like it's burning more cleanly.

Fuel consumption is identical though.

Might have to experiment with mainjet. Feels altered in requirements but not sure which way. Not getting on it for long today. Also idle is lowered to the point it will stall. Must try altering idle jet but it feels so perfect off bottom I'm not inclined to mess with it.