OK. I've been tinkering with the DT400B (somewhat) for the last several months. It's got TONS of grunt down low and is really fun to ride. It starts first kick, and will pull the front wheel up on throttle alone in first and 2nd (and third if I lean back). I installed a sleeper pipe and results were awesome, but it didn't address my one issue with this bike. Right where I'm used to a 2 stroke really ripping, this bike feels like it hits a rev-limiter.

While tuning, I cranked it once without the side of the airbox/ air filter installed. It CAME ALIVE!!!! I did notice that the air filter wasn't perfectly aligned, so I took care of that and slapped it all back together, and now still experience the same loss of power up top, (possibly 4 stroking)? It doesn't seem restrictive at all and I'm pretty shocked that it makes such a difference. It's got an Uni filter on it that is clean/ lightly oiled.

Questions/ observations for the brain trust:
-Do these symptoms sound familiar and should I just free up airflow? If so, how is the best way to do so?
-Bike is definitely running a bit rich, but plugs are not fouling. I've got the air screw set close to 2 turns out, where I experience the best off idle/ return to idle. The top end won't get above ~275 degrees anywhere I shoot it with my infrared thermometer, no matter how hard I flog it.
-When I purchased the bike, it had been sitting under a tree (covered) for about 2 years. It was reportedly a "ran when parked" situation. Previous owner was trying to sell for $3500 and had the carb cleaned/ boot replaced, but shop told him not to crank it until he addressed rust/ failed tank liner. Seller seemed to be dependable and inside of carb/ tank was consistent with the story. I cleaned both well. Inside of tank is blingy and I'm running an inline filter just in case.

What thinks y'all? Thanks in advance!

You don't say which bike you have.

I have read here that the airbox lid on the 1974, 1975 and 1976 DT250/360/400 can restrict things somewhat and advice has been offered that it should be left off.

Thanks man. It’s in there but buried. Bike is a 1975 DT400B.

So I guess you’d cut a hole in the tool tray or something? Maybe I’ll snag another air box for hacking on.

My recollection of that earlier post was that the tool tray restricts air flow to the airbox .

I've found that the UNI air filters with oil restrict the airflow although you may be on to something with the airbox cover removed.

Also possibly the exhaust could be plugged and the inline fuel filter clogged.

Exhaust brand new, and Fuel filter free flowing.

Hard to imagine that Uni filter having any restriction, but I suppose I could try a factory filter before hacking up an air box. On that note, looking at the air boxes on eBay, you can see quite a few that are modified. It’s amazing that nobody made an aftermarket one, given that it’s such an obvious boost in performance.

Hello Hook, you have checked the O ring in the carb that seals excess fuel from getting around the main jet ? Main jet is correct ? I have been a fan of running the Yamaha air filter.
The spark plug will tell you if it’s running rich from too much fuel, here’s a guide

If its rich on top it will limit rpm , And four stroking is the symptom. How ever if its correct you may be expecting more than what it has . The big bores have

very mild port timing .

I don't think that they hacked them, they just left them off altogether. Nothing to lose just trying it.

Oh, and sorry for missing the model!

With that cylinder temp, sounds rich to me. What size carb and how's it jetted?