Hello,
I just rebuilt my '70 RT1 motor with a new top end. Come to find out it wasn't in good shape and the carb wasn't working properly either. Now that it's back together and seems to be running like it should, I had a very thin foam sheet in the stock air box and have now replaced it with a stock foam formed piece. Much thicker and denser foam that was oiled before installing it. Well, the bike takes a nose dive at about 5k rpm as if it is starving for air.

Has anyone had this problem with the stock air box and filter? I'd like to keep it stock appearing. Is there any remedy that you've come up with? (drill more holes in the air box, ???)

Thanks,
Steve

I had the same problem on my '71 RT1B. I modified a thin foam filter with no oil and it improved the performance. Then I installed an airbox from a '73 RT3 but had to make a custom top cover so it would clear the seat. The stock element is still available from Yamaha. This made it run even better. Had to install a carb boot from a '73 as they are different. Also installed the RT3 battery box so a MotoBatt MBT6N4 AGM battery would fit.

What main jet do you have and is it a genuine Mikuni?

I had a friend go through the motor recently and he went through the carb too. I haven't been the carb after that so I think it has a 250 main jet and i believe it is a Mikuni jet. I'll look  into the '73 air box, sounds like it might have bigger openings for better air flow? Did you have to change the battery box, or was that just your preference? Did the '73 air box have the same mounting points as your '71?

Thanks,
Steve

The '73 air box fits without modification.
I prefer the AGM battery so I changed the battery box. Although there are other 6V batteries that fit the stock '71 battery box.
Yes the '73 airbox breathes better. And too much air filter oil will restrict it. I only ride on the street so I use very little oil.
 

Yes, I'm 90% on the street too. I had a really thin foam that wasn't very dense before and I switched it out for a more standard foam (thicker and more dense) and added oil, that seems to be really sticky now. So, maybe to much oil. I just found a RT3 air box on ebay for a couple hundred shipped in pretty rough shape. The mounting points (3 on mine) are not on it, so I might try modifying mine first to see if I can improve the flow.

Thanks for the input.
Steve