Vintage Dirt Bike Q & A

Anything and everything about Vintage Dirt Bikes! After you Register, email nathan@alp-sys.com and let me know so I can activate you.
It is currently Fri Mar 29, 2024 1:47 am

All times are UTC - 7 hours [ DST ]




Post new topic Reply to topic  [ 6 posts ] 
Author Message
 Post subject: CR250 Carb Problems
PostPosted: Mon Apr 07, 2003 5:01 pm 
Hey I figured out that my 1976 CR 250 has a 36mm carb instead of a 34mm carb. The manual says that this was a dealer option with the addition of a high performance head. Now I don't know if I have this option or not the head looks like a stock head and the thing runs really rich. Does anybody know the difference or anything about this optional equipment? The guy I bought it from says that his bother might have changed the carb, but isn't sure. Anyway to tell if it is stock?


Top
  
 
 Post subject: Re: CR250 Carb Problems
PostPosted: Mon Apr 07, 2003 5:01 pm 
Derik,

Just thought of something else although I give it a low probability of being the culprit.

When you stick a larger carb on you're trying to push more fuel/air into the combustion chamber. If the exhaust ports are not large enough to move the spent charge out, or there is a large amount of deposits built up in the exhaust, or your expansion chamber has restrictions or holes or big dents, then more of the spent charge remains in he chamber to be mixed with a fresh charge. The fresh charge can not acheive opmital combustion due to the presence of all this burned fuel still hanging around so it's effect is similar to running rich. (Remember your expansion chamber's role is to move spent charges out and also to push the fresh charge back in with the returning pressure wave.)

Loren


Top
  
 
 Post subject: CR250 Carb Problems
PostPosted: Mon Apr 07, 2003 5:02 pm 
Derick,

I run a 36mm Mikuni carb on my 73 CR250 and it works great.

Have you checked the float height? Also check that the float needle valve is not leaking.

Are you sure it's rich? Have you checked the plug reading? Sometimes people get crappy performance and it's really leaning out. Can you run a pressure test on the motor? (Should maintain 9psi for two minutes. I think that's the correct number.)

Loren


Top
  
 
 Post subject: Re: CR250 Carb Problems
PostPosted: Mon Apr 07, 2003 5:05 pm 
Loren, I am replacing my stock carb with a 36mm mikuni. Can you provide me with some base-line jetting?

Thanks in advance.

Ed Blount


Top
  
 
 Post subject: Re: CR250 Carb Problems
PostPosted: Mon Apr 07, 2003 5:06 pm 
Derik,
If you're not going for an accurate restoration I wouldn't worry that the carb isn't stock. The problem with it running rich could be due to a number of things. First, the manuals of that era say to mix the fuel at 20:1. That was fine back in the day of leaded gas but today it doesn't cut it. I have some '73-'74 CRs and I run them between 32:1 and 40:1. I use Klotz Benol and it smells just like the old days. The other thing that could cause it to run rich is the jetting of the carb and/or the carb may need a rebuild kit. You can ge them on ebay easily. Is it running rich all the way through the powerband or just at the bottom or top?


Top
  
 
 Post subject: Re: CR250 Carb Problems
PostPosted: Mon Apr 07, 2003 5:07 pm 
It runs rich at all RPM's and I tried running the gas oil ratio from 20-1 to 40-1 with little difference. The jetting is exactly what the owners manual says and I dropped it all the way down to a 140 main, dropped the needle all the way turned the air screw out all the way and it still ran really rich. Then I measured the intake of the Carb and found out it was 36mm instead of 34. I ordered a new carb/ rebuilt 34mm and am praying this will fix the problem. Points, Coil, and timing are all in good shape, and the exhaust is clean as is the air cleaner.


Top
  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 6 posts ] 

All times are UTC - 7 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 36 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group