Hello all,

Just putting the finishing touches on a full 1977 DT250 restoration. I've been breaking the engine in, but I feel like it's not quite as strong as it should be. Did a full test run today and have been checking the plug repeatedly. It's a nice chocolate brown color, but I get the feeling it's running slightly rich, so I'd like to ask here for suggestions on what jetting & air screw positions others are using or have used in the past with success.

Here is how the engine is currently configured:
Standard bore (Done by Eric Gorr Racing)
New Wossner piston
Crank rebuilt by EGR
Port work done by EGR
Running VAPE 12v conversion
(Carb was ultra-sonically cleaned and rebuilt with Keyster carb kit)
#150 Main jet
#50 Pilot jet
Needle in stock 3rd clip position
Air screw 1-3/4 turns out
Using the autolube system with Yamalube 2S oil.

I don't see exhaust burnt out--i mean Really Burnt Out--its the 'Main Thing' that robs 2strokes of performance.
Tail end muffler [complete tail muffler] is more likely than the main exhaust but still pays to do it all. In the bonfire overnight. Of course i could be very wrong haha. I was wrong once--i think!!.

The Carb rebuild with a Keyster kit rings alarm bells with me .
There have been many instances here , when new members have purchased rebuild kits , only to find their bike runs like s**t .
The needles especially often bear little resemblance to the Mikuni item they are touted to replace .
The jets themselves can be hit and miss components too . You're much better to use genuine Mikuni parts , either from a Mikuni
parts supplier or OEM Yamaha .

Yes, 1000x yes on what Swoop said on Keyster and using genuine Mikuni calibrated parts.

Its often difficult/impossible to get genuine Yamaha parts to rebuild an OE carb. In the case of things like main and pilot jets, the parts will be available from someone like Sudco off the shelf.

However, other parts like needles and needle jets are in many cases manufacturer specific, and in most cases are not likely to be easy to find.

Before anything else, fit the original parts back in the carb and see if that improves running. Also has the timing been properly checked?

Thanks to all who answered.

After some inspection, I noticed the needle that came with the jet kit is indeed wrong. It appears that most carb kits use the needle meant for the DT400. I ordered a 6F9 needle and now I can hit 75mph with no problem.

I've also got some OEM Yamaha jets on order, they'll just take awhile longer to arrive.

Mikuni was the OEM and often parts like the slide needle, needle jet, pilot jet, main jet, and float needle/set are cheaper from a Mikuni distributor than going through Yamaha... if Yamaha even still shows them... (Many Mikuni parts no longer sold by Yamaha are still readily available from Mikuni)