For a 78 DT250E, your carb specs listed in the service manual as being:

Carb Marking code = VM28SS
#150 Main jet
#50 pilot jet

Idle should be set between 1,200-1,300 rpm.

I am hoping you are reading the main jet number incorrectly - I would guess it is a 160? or 180 if it is running rich as we are surmising. What altitude are you riding at, as that will make a main jet sizing need variance.

My manual for the '75 DT250, shows a 150 main, and a 50 pilot, with a 60 starter jet.
Your Suzy should have bigger jets, because it is water-cooled, and CDI ignition, which means it can run higher compression. Plus it has an awsome flowing (and sounding) pipe.

Carb specs as shipped from Mikuni:

Great videos!

Main jet only affects 3/4 to full throttle.

Possible that since you had a bad seal someone installed a large pilot jet and/or raised the needle to compensate for the air leak?

What do you mean by the starter jet, i only have 2 jets in this carb i think ?

the air screw was 1 1/2 turn and the idle screw was also 1 1/2 turn.

Im getting confused because the jet that you can remove by removing the big screw under the carb only is the main jet right ?
because i had to remove the bowl to remove the second jet and its smaller than the first one.

So im thinking that the main is the first one i took out and the pilot is the 2nd one ?




Is the one on the left the main jet ? because its hole is bigger than the other one.
And i didnt remember that the other jet was soo small, the second jet in my Rm's mikuni is much longer.
Is that normal ?
I can barely read the numbers on the jet on the right side of the picture but i think its either 50 or 150.
the one on the left is 40 from what i can tell.
Weird.
I have yet to check the needle position

I believe that what 4eyes is calling the starter jet is the pilot jet.

You have a Main jet (L) and a pilot jet (R) in the picture. The pilots are a bear to read. Sounds like it has the cortect one.

Your main would probably barely run iif it were a 40 - I am again guessing it to be a 140 if so it should not be causing a rich condition.

If you go to the www.NWVCP.com site you will be able to pull up a schematic to help ID parts.

"Starter jet" is usually pressed into a well in the bottom of the float bowl and is not normally replaced... not sure you could if you wanted to?

Only affects running with the "choke" on... which brings to mind another reason for running rich. The "choke plunger" could be damaged/leaking and not sealing?

(I put choke in quotes because it really is a fuel enrichening circuit rather than a car-type "choke" that richens the mixture by restricting the air into the carb)

yamaha-enduros.com is the best forum on the web for vintage yamaha enthusiasts.
Im amazed by the quantity of good information i get here.

The website you provided is the best website ever for a DT250 owner.
It show that the small jet is indeed the pilot as i suspected and it looks like its not too small but its in fact the good size if i compare it to the picture on this lovely website. Although its not a 50 its a 40 so that would make it leaner.

For the main jet, im assuming i cant read the number correctly because as you say it doesnt make sense.
I'll have to pull out the jets from the RM because i think i dont remember the numbers correctly. In my mind it has a 55 or 60 main jet, but im thinking i might be remembering the jet of my newer 1993 Rm 250 which has a keihin carb.

The current main jet in the 1990 RM which has a Mikuni must be 180 or so.

So as you say the main jet in the Enduro must be a 140, which seem to be the oem size jet.
So perhaps the problem isnt in the carb.
I still have to check the points and timing.
I talked to my father about the problem, he use to ride these enduros when he was a kid.
He says that when the points are done, they are done. Is it true or they can be adjusted (my father isnt a mechanic but he use to work on his bikes)

Ive got a question for you guys, when i checked the cylinder head and piston in the enduro , i actually filmed myself to do kind of a tutorial video but i never uploaded it.
Would you be interested to see it ?

---Ben