Hi guys, I'm slowly getting my rd250b 75' (euro model ) on road back again and have question about new VM28 carbs.
My original hi top carbs are in very bad condition (previous owners hack jobs....), bike was running poorly and couldn't get it to work right (one carb body does have damaged air screw seat by using hand made jet?? , carb body where needle jet sits is cracked...) . So I bought new set of Vm28-57 to use it instead of original ones.
They came with 5DP39 needle and 169-n8 needle jet.
Engine is (will be ) stock, except one piece YZ reed petals , reed spacers and K&N YA0700 (couldn't find the original paper one here) .
I know I have to modify them for oil injection that is no problem, and change pilot and main jets ( start with 30 and 150 should be safe) but will that combo (5DP39 needle and 169-n8 needle jet.) work ?? Or I have to look for some other ones? I have found that some guys here are running those new carbs in their bikes, so maybe someone could shed some light on that matter?
Cheers guys.
You will probably need to swap out the needle and NJ you have now. A common upgrade for the VM28 is the "Dave F Mod" which uses the 5DP7 and 169-P0 (or P2). This is in conjunction with a 5/64" drilled (or 2mm) airjet. These specs are usually applied to RD350s, but should also work for the 250 (hopefully someone can confirm that). Pretty sure the only change from the 350 to the 250 would be the PJ & MJ, which you are already doing.
i agree. id swap the internal bras over see how it goes, but get a new main jet and pilot.
should be fine. :twocents:
Thanks for replying.
I disassembled the "bad" carb and needle looks like this , you can't feel any steps but the coating is gone in the "midrange" , also atomizer after installation into the new carb with mainjet and the brass spacer/gasket sits kinda low in the body ? And I forgot to mention that slides in new carb are 2.0 and old ones are 2.5(they would need to be machined to correct length, they are in perfect condition outside ) .
I could try doing it like this but it don't like wear of that needle. Would I also need to change(drill and tap passage for jets) the air correction jet for using that old brass in new body? (Air would go straight from the Y boot , not the separate line from airbox) . I checked and that stuff for the Dave F mod is available in UK so I could get this here (Poland ) in few weeks I think .
that all looks kinda normal. but you need to run the 2.5 slide to run those jet tubes and needle. the loss of color isnt as much wear as it is 40+ years of being exposed to fuel vapors.
Great, so that's what I will do for start. Will do update when engine will be running. Thanks for the help.
Quote from: SoCal250 on September 17, 2021, 08:32:43 PM
You will probably need to swap out the needle and NJ you have now. A common upgrade for the VM28 is the "Dave F Mod" which uses the 5DP7 and 169-P0 (or P2). This is in conjunction with a 5/64" drilled (or 2mm) airjet. These specs are usually applied to RD350s, but should also work for the 250 (hopefully someone can confirm that). Pretty sure the only change from the 350 to the 250 would be the PJ & MJ, which you are already doing.
ive heard a lot about the DAVE F mod
what is the advantages of this mod
what does it do and where can i read more about it
Go to the "tech library"