F5's 300 head mod thread

[Jakobi]

You're inspiring me to throw a 300 jug back on at my next top end.

 

[motopsycho87]

I think if you ran the squish at about 1.4 you'd be well and truly safe.

I ran the bike up on the side stand (needs new wheel bearings), warmed it uo for a few minutes and gave it a few full throttle braps and it was fine, actually seemed to rev higher (I suppose the power gain exceeds the frictional losses). I only use super unleaded (98-99 UK octane, 92-93 US I believe) so should be fine. Worst case I'll shave a cc out of the bowl next week, or retard the timing a little. But no pingy noises yet though.

Also, googling ktm 300exc compression test and yz250, a few people seem to run fine with 200-215psi. I did a wet compression test (hard not to with the diesel in there) and it read bang on 200.

 

[Jakobi]

Wouldn't expect it to ping without a significant load on the engine.

As stated, both my 250's have been setup in the 13.5 - 14:1 ratio. I've always ran them on 98RON. No issues, even when having to mix half a tank at 95RON.

 

[motopsycho87]

In order to get 14:1 I would need a 22.5cc volume above the piston, so I'm at least 1cc out of that range. Camera has turned up today, so I'll have a video up Sunday/Monday

 

[motopsycho87]

Righty, rear wheel bearings have been changed so I took it for a spin on the road, maxed it out in 6th, no signs of detonation, doesn't feel flat at all and good power all along the range. I'm using Tesco Maximum 99 octane, which is about 93-94 (i think) to the US. 5 centigrade at sea level with jetting in sig

 

[Jakobi]

Happy Days then!

 

[(F5)]

I never trust compression gauges to tell me about compression ratio. I always measure with head on bike and flywheel held at tdc.

Dirt use you probably get away with a bit, on the road there is time with throttle open to show up borderline issues.

 

[motopsycho87]

I did it with oil in the head on the bike and with a compression tester, 2 sources are better than one. The classic racer / bike builder that owns the track I frequent was talking to me Sunday about how he believes people don't realise highly strung bikes need to be pinned to not seize due to the intake of cool air and fuel cooling the piston (putting the squish to good use).

I held it flat out in 6th for a couple of miles on the road though and no seizure yet .

 

[(F5)]

Quite often seize on closed throttle due to too small of a pilot - after a long straight, many gears down high revs closed throttle for long time.

Sounds promising so far though huh?

 

[motopsycho87]

It is definitely running nice. I was riding a bit slow last weekend as my tyres were knackered, and I'm considering getting the heavy flywheel back on also to help me ride mx a little smoother, below is a video from my mates camera, I'm only on it for the last 20-30 seconds (green guy, red bike, license plate) so you can't gauge a lot from it, will hope to have my own video up this weekend!


And my pilot is purposefully rich just for that reason

https://www.youtube.com/watch?v=1ZZEZdzbZkE&feature=youtu.be

 

[Anders]

Thanks a lot for all the detailed info here. This thread is mainly about your 300 head, although you refer to your 200 as well.
I just measured my '15 200 squish to be 1.8mm (OEM setup, doing the bike's first ever piston replacement now).
I haven't removed the cylinder yet, but it sure has a thick base gasket, at least one of the 0.5 ones.
If I go with one .15mm gasket, that should bring the squish down in the 1.45 area, a tad too high I presume (if there is a 0.5 gasket now). However, I do have a lathe (and have cut heads earlier) so if necessary, I'll cut it down.

I thought about aiming for 1.3mm. Should I go lower? I run 98 octane gas (known as Super around here).
What squish height did you end up with on your 200? and what about the CR?

 

[(F5)]

I took one of the base gaskets out, bear in mind mine was the '02 200, then I skimmed it so squish was less than 1mm. 0.8mm would be ideal. 1.3 is miserable. Then cut chamber to reduce compression ratio.
13:1 should be fine, maybe up to 13.5.
You should check if squish band is parallel or tapered and correct to parallel. Can't remember thought the 200 wasn't bad in that respect but different piston manufacturers may be different as may changes over the years.

 

[Anders]

I took one of the base gaskets out, bear in mind mine was the '02 200, then I skimmed it so squish was less than 1mm. 0.8mm would be ideal. 1.3 is miserable. Then cut chamber to reduce compression ratio.
13:1 should be fine, maybe up to 13.5.
You should check if squish band is parallel or tapered and correct to parallel. Can't remember thought the 200 wasn't bad in that respect but different piston manufacturers may be different as may changes over the years.

On my '15 200, I found the OEM base gasket to be 1mm. The OEM volume above the piston is 15.2 - 15.5 ml (measured twice, got different results) which should give a CR of around 14:1. So I will aim for unaltered CR as this works well with my fuel.

 

[(F5)]

Take the time to get your eye in with measuring technique. Flywheel needs to be held at tdc which it doesn't naturally stay in. But yeah if it works with your gas keep it there.