300 head volume question

[32:1]

Hi guys
Right Ive got the 300 running best it ever has, squish now at 1.5(from 3.25) took 3mm out of head roof giving 20ml/cc of trapped volume, bike has excellent power, bit to much if Im honest and it stalls to easily in tight stuff

Ive read other posts but theres conflicting info, what volume would allow good lowend use, easy kicking and still retain good drive/power

Chris

[(F5)]

If there really is 20cc measured on the bike we'll do a little math

300+20=320
Divide by 20 and it goes 16 times. That's 16:1 compression ratio. On a 300.

So unless you are running methanol you should expect that would be detonating.

Look up a chart for allowable compression ratios for octane.

[(F5)]

PS here's my thread with how to accurately measure. It is important to measure precisely.

http://www.gasgasrider.org/forum/sho...highlight=head

[32:1]

Thanks FS, sadly I didnt measure anything apart from squish before I removed head, thanks for link and thats how Ive measured it, three times actually, 24.3, 25.2 and 25.00 so around 13:1 ??? To much still ???

No detonation at all and Ive jetted it correctly, I lost that lovely lowdown calm delivery since bringing squish down and its turned into a fire breathing wheelie monster, so I took more out of head today giving me 25cc of trapped volume, this has tamed it somewhat but its still snatchy on or just off idle and still stalls easily, I had to lean out needle one size and clip but motor feels real nice and pulls smoothly right through range, its just this low rpm chugg it never had which is hard to deal with in tight sections, stalling frequently

Now Ive tried the heavy FW weight (24oz) today too and if anything made the chugg worse, each revolution seems to thrust the bike forward a few feet each time.

Now could this be how the 300 is ? As Ive only rode 125mx bikes this seems hard to ride slow.. Ive checked ign timing and its stock with the plate still installed, I thought it may need retarding a little but on inspection this stator is fully retarded due to the forward screw hard against the slot, will remove and use just two screws later if it could be the ign. As Ive said Im not familiar with bigbore motors so this may be how they are, motor responds flawlessly with roll on throttle or snapped open, it idles perfectly and Im happy with carburation

Chris

[32:1]

image.jpg

My head now

[Jakobi]

Click it up a gear and ride the torque..

To me, it sounds like how I remembered my 300 cylinder all those years ago when I last had it on the bike. That instant pull/punch while great fun was very tiring to manage for me, and even more so if I was trying to show any aggression.

I personally found the 250cc engine a better fit for my style. Less off idle, and the more aggressive mid transition kind of entices and rewards more aggressive use of the clutch and throttle, while still having more than enough grunt down low to still chugg when you need to.

Have you cc'd just the head combustion chamber? Or only the total trapped?

[32:1]

That just could be the issue here

I have actually got the 250 topend that came with the bike, so I could experiment at a later date

Would like to try to get this 300 right or at least better before throwing the towel in
Head volume was 29.25cc with head laying flat, but piston dome would encroach when installed obviously
Chris

[Jakobi]

What gearing are you running on her at the moment?

If you've come off 125's it'll probably take an adjustment period to get used to the change in power delivery.

Some people have run G2 Ergo throttle cams to tone down the smaller inputs, not me though. I run quick pulls.

[32:1]

Gearing is 13/50
After initial ride I was going to try 13/46 as it is far to aggressive for my liking

I'll look into the cams but its more off throttle issue or at least nearly off throttle

[Jakobi]

http://www.g2ergo.com/store/g2-throttle-cam-system/

With that gearing on a setup 300 you won't need 1st gear. I run 13-50 on my 250 and it's very rare that I'll knock back to 1st even in the snottiest tight terrain. 2nd or 3rd with a slip of the clutch and you should be getting smooth drive. Too aggressive on the throttle and clutch and she'll punch around. No clutch and I find it very on/off jerky instant response. The 300 was a real arm stretcher though, given I'm only mid 70kgs in normal clothes.. it's more bike than I need.

I like to gear my bikes that I don't need 1st though as well. It gives me a real granny gear for goat tracks or putting along in the unknown. Leaves the other 5 gears to shuffle through without worrying about finding neutral accidently.

Jetting will also make a big difference off idle. A rich burble on a big bore will explode to life as it clears it's throat. The stock needles were bad for this.. blubber blubber braaaap! The Suzuki needles seem to tame things down and make them feel a bit more electric across the rev range, where the Yamaha/ktm series give a smoother bottom end, with a more aggressive mid transition on the taper.