Vintage Dirt Bike Q & A

Hi
I just acquired this bike and have it running but it doesn't like the low end. I have so far rebuilt the carb with a keystar carb kit. I think one of the main problems is the pipe is full of carbon buildup. Does any one have a good way to remove carbon from the pipe?
Thanks

Hello, I see that your Mt250 is a 1975 model and you used a Keyster kit. All of the Keyster kits I have ever seen are sized for the early 1974 carb with the 122 main jet, 60 slow jet. Stock jets for the 1975 model are 115 main, 58 slow. Even the needles are different. Early 1974 part number is 16012-358-024, after that it is part number 16012-358-034.
The main jet change can be confirmed by going to the factory shop manual page 105 - #8 carburetor setting. Stock jets usually do not wear out, I've heard of jet needles getting worn but never seen it first hand. I'm far from an expert on these carb's, but it bothers me that everyone is using this Keyster kit on the 75 and 76 models and they end up running rich, which is better than lean so maybe it's a good thing in the long run.

swoop,

Hopefully this post will display correctly:

http://vintagedirtbikeforums.alp-sys.com/viewtopic.php?f=21&t=3831&p=13327&hilit=oxygen&sid=05904e8d1a15d859befd361e99c1848f&sid=05904e8d1a15d859befd361e99c1848f#p13327

texasmark,

You're right, the needles are definitely different. The OEM '74 needle for the 004A carb (271002) is quite rich, and Honda made succeeding needles steadily leaner all the way to the end (092B FL250 carb). The FL250 Keyster kit is the one I'd get, as its D96 needle is super lean and it's got the cool big-knob idle speed screw. I've got specs for a bunch of the Honda OEM 70's 2-stroke needles, PM me if you need 'em.

I replaced the needle & jet on my friend's '74 MT with a NOS 16012-358-034 set, it made almost no difference at all. I believe leaner slides / needles / pilot jets would really wake this bike up, but I haven't had the chance to test any of them on an MT250 (although they worked great on my MR175). Something else is going on with the MT250's carb too, as many of them overflow for no apparent reason.

Ray

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

Thanks Ray, yes I tried a needle from an early 74 carb on my 75 and it did not like it at all. Turned a brand new plug black in less than a 1/4 mile. I spent a lot of money at Jets R Us and NGK trying to get that bike perfect only to end up with the stock jet sizes and needle back in it. I can still get Ethanol free gas here also, so I think that helps a tad. I'm running pre-mix Bel-Ray MC-1 @ 50:1.

Mark

Thanks for the reply. If I can find the jet from the original carb, where can I buy new jets for the carb. I have searched around and have found it difficult to find them.
I can't wait to get this bike running well as it is a blast to ride and looks great.

Here's the main jets - the range will probably be 118 to 125:

http://jetsrus.com/a_jets_by_carburetor_type/jets_keihin_99101-357_main_Hex_25-xxx.htm

If your pilot jets measure 32mm long overall, these are correct - range 50 to 58:

http://jetsrus.com/a_jets_by_carburetor_type/jets_keihin_N424-24_slow_29-xxx.htm

If your pilot jets are 28mm long - as it's likely they are - I'd strongly suggest using the 424-24 jets above and modifying your carb's float bowl for clearance. Ten minutes with a Dremel tool should suffice. The 28mm 99124-076-0xx0 jets are very hard to find in the sizes you'll need (Jets R Us has them in #45 and below, but this is way too lean).

The slide below really improved my MR175's throttle response, and will surely do the same on the MT:

http://www.ebay.com/itm/1982-HONDA-FL250-FL-250-ODYSSEY-CARBURETOR-VALVE-SLIDE-/141272433944?pt=Motors_ATV_Parts_Accessories&hash=item20e47e4518&vxp=mtr

Ray

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

Swoop, have you read your sparkplug, checked timing and compression? If you have a true 1975 MT250 carb you will have the N424-24 slow jet, the 74 had the shorter off to the side slow jet (CR125M). Also the 1975 MT250 came stock with a 115 main jet and a 58 slow jet.

Mark