Jump to content
Sign in to follow this  

Cr250 octane or jetting problem

Recommended Posts

So I have a 02 cr250...a little while back I had 0.030 shaved off the base and the squish set at 0.050 and I told my machinist that I wanted to be able to run 93 octane. So I put my bike back together with a brand new top end and also swapped on a pwk carb. I started off with 45 pilot 2nd clip on the a715 needle and a 175 main and 1.5 turns out on the air screw. So I break it in and at about the 2.5 hour mark I started getting some pinging when I was just putting around at low rpm. So I increase the pilot to a 48 and I'm still getting it maybe a little bit better but still there. Also I turned the air screw in to 0.5 turns. After that i increased the main to a 178 and rich end the needle one position to try and help it but it didnt. I have drool coming out the silencer and the plug is black. It does take a few kicks when cold but once it's warm it's a one kick bike all day. So any ideas? I'm thinking I might try a 50 pilot and if that doesn't work I may go to race gas

Share this post


Link to post
Share on other sites

Im thinking a machinist wasnt the right person to tell that you wanted to run 93. What would they know about compression ratios? 

Share this post


Link to post
Share on other sites

Well when I say machinist I mean like a local guy that does machining for bikes as a side business

Share this post


Link to post
Share on other sites

What premix ratio are you running?

 

A generic 1/2 turn on the air screw is a lot. Are you gaging off idle throttle response (whack the throttle method) when doing that?

 

If you shave .030" off the base, doesn't that by itself increase compression ratio? Then you set squish at .050". What is stock squish?

 

I see stock compression ratio is 8.5:1.  If your engine work didn't throw it over 11.0:1 -11.5:1, I would think 93 pump fuel would be fine.

Share this post


Link to post
Share on other sites

Premix ratio is 32:1 with amsoil dominator. The throttle response is normal snappy 2 stroke no hesitation. Yes by shaving the base it does increase compression the squish before was .064 so maybe that took enough area out but I'm not sure. I did a compression test on my old top end before I tore it apart and it was 120 psi warm, and I did one last week and it was 180 psi cold with 3 hrs on the clock. I should also note that my gauge is an old mac tools one

Edited by rippah

Share this post


Link to post
Share on other sites

Just my opinion, but I've never liked running such an oil laden ratio as 32:1. It's obviously not burning it all if you have a lot coming out the exhuast. If you keep that ratio, and the Amsoil, you might consider running the Amsoil Interceptor instead. It has a lower flash point of 183* compared to the Dominators 215*, so you'll burn more of it and get less spooge from exhuast.

 

If you drop the amount of oil in your premix to something like 40-42:1, you also richen your premix, which might make the difference with your jetting. Example: that 48 pilot you said was better than the 45 but still not quite right. A 48 pilot delivering a 42:1 ratio is delivering more fuel than that same jet flowing a 32:1 ratio. Something to think about.

Share this post


Link to post
Share on other sites

Oops I had a brain fart I meant interceptor...what I'm thinking is maybe when the motor was still in the break in stage the rings weren't sealing as m7ch as they are now and maybe now that they are sealing better against the cylinder wall it's putting the compression just over the edge of pump gas I dunno, but yeah it does make sense about the premix ratio

Share this post


Link to post
Share on other sites

I think before chasing jetting and premix much more, go ahead and try some higher octance race fuel, even if you mix it 50/50 with 93.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...