I read a couple weeks ago. MarkT was trying to help someone, he stated that some filter oils can be problematic. I thought to myself... the stuff I use is OK. I've been using the No Toil red for years. I decided to put a stock filter box on one of the 75 DT400's. I had been running a Uni pod and it was running good. I installed the stock filter box and grabbed an oiled filter off the shelf. It was a Uni 2215 with No Toil red. The bike ran poorly and I thought smaller main and maybe pilot. Instead I grabbed a new Yamaha filter.
I threw it in and
bike was right back to running good.
I then put some spray on Maxima Fab1 on the Yamaha filter I had just run. Gave it 30 minutes in the sun to dry . Installed and tested fine... still running good.
I then cleaned the Uni 2215 let dry ,sprayed with the Fab1 . Next day I installed it and running poor. 4th gear 5500 rpm at best.
The Yamaha filter and Maxima Fab1 ...4th gear snaps right to red line 7000rpm.
I now believe that not all filter oils are good and the cheaper Yamaha filter is the way to go.

The Uni pod filter flows fine the 2215 not so good
Thanks again Mark !!

"A man with a theory is at the mercy of the man with an experience."

Good trouble shooting technique.

great info thanks!

I have heard it said that "One good set of measurements is worth a thousand expert opinions".

Nice job and thanks for sharing your experience and results!

I think your test highlighted the difference between the stock Yamaha filter and the Uni 2215 ,
rather than any filter oil differences . Some people have peeled back one of the two layers on the
Uni , and have reported some improvement . Whether it matches the stock Yamaha is still unknown .
At least you can still buy your stock Yamaha Filter . We with DT1's are not so fortunate .

I just had a similar experience with my DT400C. I rebuilt it back to stock a couple years ago and have always run an original Yamaha foam filter. The only thing is I didn't know at all about filter oil. I just used some motor oil and squeezed it out. Then I noticed some drip, drip, drip, out of the filter box. Eventually it quit. Well fast forward to this last weekend and I rolled over 1000 miles on the bike. Decided to clean the air filter and change trans oil. Lately I have read some threads on filter oil. Mark T. mentioned TwinAir spray-on filter oil so I purchased some. Cleaned and dried the Yamaha filter. Sprayed on TwinAir, massaged it in and let dry over night. Put it back in the bike and fired it up. Bike ran better and smoother than last 1000 miles! Actually acted a little lean (more air flowing?) Lowered the clip one notch to raise the needle. Much better now! So long story short...more air getting through + more fuel getting in= more performance. I really learned there is BIG difference in how you filter your bike. Thanks all!

Edit...I'll be treating all of my filters to fresh oil!

The '71 DT1 I picked up recently has no air filter, just a cage in the stock airbox. And I see, no Yamaha or "Filtron" filters out there doing some cursory searches. But I do see the Uni NU-2203 is, from eBay seller classic-cycles. I searched a few threads on here, and see others have purchased the NU-2203 and had some success using the stock cage vs. the cage UNI supplies. Being that there doesn't appear to be many choices, I guess I'll get the NU-2203, use FAB1 (which I already use), and see what gives? Any thoughts?

I'm in a similar boat with my RT1B just completed this weekend. I'm running the Uni filter with one layer of foam removed and Twinair oil. I'm having carb problems currently (o-ring) so can't say absolutely if I'll be happy with it. What I have on order is the standard Yamaha filter for the RT3. It is taller by an inch or so. I have the correct filter cage for the RT1B so I plan to cut about an inch out of the middle of the Yamaha filter and slide the halves over the cage. As long as it is cut straight and has pressure on it when the lid goes on then should work ok. I'll see which one performs better when I get the new o-ring for the bottom of the carb.

I made a flat lid for an RT3 airbox and installed it with the Yamaha OEM filter (which is currently available) on an RT1B. The seat is different for the RT1B so that is the reason for the lid. I suspect Yamaha changed the airbox back in the day due to the DT2, DT3, RT2, RT3 starving for air with the original smaller box. Also had to replace the carb boots. Did this on a '71 DT1E. Both run much better.

I also installed an RT3 battery box with the Motobatt MBT6N4.