250psi of compression in a CR250

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8/2/2020 7:58 PM

Hey guys,

I recently got my freshly built CR running, it is a 2005 with the following motor specs:

- New OEM crank / bottom end
- new OEM cylinder
- Wiseco piston
- Phathead cylinder head (H31 dome which is their lowest compression ratio)
- Lectron carb
- V force reeds


The motor sounds beautiful, however I was getting some noticeable detonation during heat cycles on pump gas (93 octane). I mixed in 50/50 with pump 93 and Cam 2 race fuel (pump 110 octane). I am still noticing some detonation despite some improvement.

So, I measured the compression with a snap on gauge and hit a pretty lofty 250 PSI.
I assume it is due to the head, and the owner of Phatheads is doing some research for me to help sort it out.
I really just wanted to raise the question and see if anyone is running 250psi in their CR's or comparable 250 two strokes?

I'd like to toss some 110 octane, or even 116 and send it for the rest of the season, but I'd love some feedback from the gurus on here! Put a lot of time and money into the engine so naturally do not want to do anything careless.

Thanks

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8/3/2020 11:42 AM

My stocker ('05) detonated on pump fuel from day one. 50/50 mix of C12 (and getting jetting dialed) took care of it.

It's a high compression motor by design. (It's actually tough for me to kick it over when the ring is fresh!)

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8/3/2020 12:55 PM

Just a wild guess, but the CR250 switched back and forth from flat top piston to domed piston numerous times over the years. Perhaps you're running a domed piston and that head was designed to work with a flat top piston?

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Powerband in every gear !

8/3/2020 1:14 PM

FGR01 wrote:

Just a wild guess, but the CR250 switched back and forth from flat top piston to domed piston numerous times over the years. ...more

Definitely a good thought, but I think I am good there. I am running a Wiseco 860M06640, which is essentially an OEM replacement spec. I will check that over with the head designer just to make sure!

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8/3/2020 9:49 PM

Cut with C12. C12 is made for higher compression two cycle engines. Should fix you up.

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Unleaded tastes a little tangy. Supreme is kinda sour, and diesel tastes pretty good.

8/4/2020 5:34 AM

Rickyisms wrote:

Cut with C12. C12 is made for higher compression two cycle engines. Should fix you up.

I am going to start with Cam2 race fuel from the pump. It is available right up the street vs. having to order VP.
Hopefully that will fix it up as well! Will report back

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8/4/2020 8:35 AM

Rickyisms wrote:

Cut with C12. C12 is made for higher compression two cycle engines. Should fix you up.

HatemOrLovem wrote:

I am going to start with Cam2 race fuel from the pump. It is available right up the street vs. having to order VP.
Hopefully ...more

What octane of Cam2? I see it's available in different octanes.

C12 is one of the best, if not the best, option for 2 strokes. It's all I run in my 2 strokes. VP110 is another good option for a little less money and both with take away any detonation.

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2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @2HRacing
Thanks to : Factory Effex, N2Dirt, Acerbis, DT1, Fasthouse, Matix, FMF, ASV, 100% & Mika Metals

8/6/2020 8:37 AM

mattyhamz2 wrote:

What octane of Cam2? I see it's available in different octanes.

C12 is one of the best, if not the best, option for 2 ...more

110 octane Cam2

I used to run it years ago in my TRX250R, never had an issue

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8/6/2020 8:57 AM
Edited Date/Time: 8/6/2020 9:01 AM

Did you check your squish when assembling?

Also, looks like that piston is domed (I couldn’t recall which series was domed and which was flat...looks like flat on 02-04 and domed 05-7)?

If so, and if the head was for 02-04 (flat piston), there’s your answer.

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8/6/2020 7:11 PM

^Correct. Pre-05 was flat and 05-07 are domed.

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8/6/2020 7:16 PM

omalley wrote:

Did you check your squish when assembling?

Also, looks like that piston is domed (I couldn’t recall which series was domed ...more

I did not measure the squish.

The builder confirmed that the head was intended for an '05 with domed piston - he is being very helpful and actually cutting two spare domes for me with lower compression / less aggressive squish.

Going to see how the bike performs next week on straight race fuel and pending that I may be testing out another dome.
Appreciate all of the feedback guys

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8/8/2020 5:33 PM

Had a 06 RM250 stock head with a Wiseco racer elite piston in it. It would detonate on 93 pump but ran great on VP110!

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