Vintage Dirt Bike Q & A

Trying to learn more about the Keihin carburetor on my 76 cr250m

I noticed that there are three different part numbers for the carburetor slide:

16022-357-812 #2.5
16022-357-425 #3.0
16022-357-325 #2.0

What are the differneces in these slides, do they provide different amounts of air?

thanks.

I believe the -812 slide is the standard 2.5 OEM slide. The -325 is the optional (richer) 2.0 slide, and the -425 is the leaner 3.0 slide. The 2.0 etc. designations refer to the amount of slide cutaway in mm. The more cutaway there is, the more air can flow at low throttle settings, and the leaner the mixture is.

Here's a simple explanation link:

http://www.howtomotorcyclerepair.com/how-to-tune-throttle-valve-cutaway/

The slide cutaway can have a big effect on throttle response. The stock cutaway for my MR carb is 3.0, but I run a 3.5 below 80F, and sometimes a 4.0 above that.

A good test for proper slide cutaway is to accelerate up a slight grade at 1/4 to 1/2 throttle while in the powerband. Too rich will seem sluggish but steady, too lean will seem zingy-quick but lack real acceleration, correct will pull strongly.

Using a needle with a larger straight diameter has a similar effect to a larger slide cutaway. I believe at least some of the Keihin 66mm aftermarket needles are still available for your carb. Differences of .0005" (five ten-thousandths) of straight diameter can make a noticeable difference.

Does your carb have 010B stamped on it?

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks Ray for the response, that is good information. I will check to see what stamp is on my carb.

Ray,

My carb has 009B stamped on it, what does this indicate?

I would like to try to lean out the carb some, as I believe it is still too rich, I am running a 140 main and 50 pilot. Your test of accelerating up a slight grade accurately defines mine.

I haven't looked closely at the slide yet, but do they have a part number some where on it? I guess I will need to try and find another slide or possibly cut mine down, not crazy about doing that.

Also, when you mention a 66 mm needle, is that the length of the needle? Is that what's in my carb? Are there part numbers on those as well? How do I know what needle to try if I want go that route?

Thanks,

Ken

is your float hight set right?

_________________
80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Ken,

I second b-joe's float height suggestion - it really should be done before any jetting regimen, to give a proper initial baseline.

My notes indicate the 009A is the original '73 CR250M 34mm carb, so your 009B is probably nearly identical. The 009B in the link below looks very much like the original CR250M carb to me:

http://www.ebay.com/itm/Nos-75-76-Honda-CR250-carb-73-74-Honda-CR250-009B-/161115817517?nma=true&si=l9630cirqgyza42bSBU%252B0Ub%252FRKU%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557#ht_102wt_923

Anyway... my 34mm and 36mm Keihins both use slides with 190 stamped on the bottom, along with the cutaway in mm (2.5, etc.). Your slide should be this same 190 type.

I've modified a number of slides, to both larger and smaller cutaways (some at a machine shop, others by hand). The one I did at age 19 came out like crap but the others work fine. These optional vintage carb slides tend to be scarce and expensive, sometimes you have to go with Plan B, such as buying eBay carbs or throttles for the slides to modify (done that many times).

The 66mm (yes, that's the overall length) needle situation is kind of complex, especially if the needle and/or needle jet is worn. If you think that's the case with your carb, a new needle/jet set (Honda P/N 16012-381-004) should greatly reduce excessive richness from about 1/4 to 3/4 throttle, all else being OK (pilot jet not super rich, pilot air circuit clear, no choke leakage, etc.). I'd try that first before delving into the 66mm N427-48 Keihin needle jungle, but if you decide to try that route, I have a fair bit of info.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks again,

Great advise and information. Yes, I had previously checked the float height.

Ken