Hi everyone,

My DT400 'gurgles' and doesn't want to rev above about 4k. I have cleaned the carb numerous times, fitted genuine new needle and needle jet, raised and lowered the clip, fitted new genuine Pilot jets of various sizes and increased and decreased the size.

I was at the point of 'giving up' when I decided to disconnect the rubber boot from the carb to the air box and instantly the bike took off, revving cleanly up to the red line and power that I've not experienced before on that bike. I then obviously thought that the air filter was the problem so I thoroughly cleaned it, it did appear to be over oiled. After cleaning and checking it was completely dry I decided to fit it without oiling it to allow for extra air into the carb. It goes back to how it was, struggling to rev.
I then ran the bike with the rubber boot fitted but no air filter and the air filter lid screwed down - it again runs better but is still struggling to rev cleanly, so I removed the lid from the air box and ran the bike - revs cleanly again!
What is going on?
I don't understand!
Would really appreciate your thoughts and help on this matter.
Thanks

Main jet too large?

Sounds like inlet to airbox must be restricted?

There's a rubber "snorkel" that goes up under seat.  Maybe something got in there and made a nest?  Or maybe bottom of seat or something under seat is blocking airflow?

I like Marks answer better

I have read a few times on here that removing the airbox lid on the DT360/400 aids running. However, I don't know the ins and outs or whys and wherefores as to why this is the case!

Ok after 16 refurbs. To make them run the cleanest you have to run a very free flowing air filter and if you remove the tool tray that has the cover over the snorkel on the airbox it helps a lot. I have not had 2 of these bikes run exactly the same. The yamaha big bore 2 strokes all have carburation problems of either too lean or too rich. The 250's are good the 360 400's 456's and 490's were all the same. If they run good at top end then the burble down low and if they run clean at low rpm they melt at high rpm. I am actually going to try some of the fixes at a company called dial a jet but Im not there yet. Just my 2 cents worth

Thanks for all the replies.

Has anyone modified the Tool Tray ie- is it worth drilling some holes in to it to allow for additional air flow - I'm a bit reluctant to leave a large hole under the seat.

I have found the air filter must have very little oil saturation or it wont rev out. If you want performance and you are riding in dusty conditions take the tool tray out and oil the filter. It still gets really dirty on top of the filter when you look in the snorkel but you just have to clean it more often. Really the air box is a restrictive unit by itself and the filter fills the airbox and makes it worse. When you sell the bike put the tool tray back in. Doug

A trick we did in the 70’s is to drill a hole in the air box and install 1or 2 vents as shown of course we didn’t care if it was original back then

Here’s a cheap box to play with if it fits your bike