Hi folks.
Trying to get to the bottom of why my RT1 not running right.
The problem occurs when warmed up after about 10 minutes riding.
On throttle everything seems fine, when shutting off throttle the revs will drop to about 3,000 but jump up and down between 3,500 and 2,500 for three or four seconds before subsiding to normal tick over.
I've uploaded a video and at 1.42 to 1.48 I've pulled the clutch and I'm completely off throttle and it's clear what the problem is.
I've replaced the carb o-ring for the correct one and I'm pretty confident the carb flange is now sealing and the throttle slide is free moving.
I haven't checked the crank seals yet, any ideas what it might be?

Tried fatter pilot jet? Have you done a leak down test?

I listened to that section of the video maybe ten times... and also the whole video... It could be a small air leak or slightly lean I guess... Closing the air screw a half turn might make it better?

My dominant thought is that it sounds pretty normal to me and sounds like it's running good for a big bore Yamaha of that era.

I think we get spoiled by high tech computer controlled fuel injected engines.

Haven't done a leak down test yet, it does feel like an air leak but I'm not experienced enough to know, hopefully the video will help the diagnosis.
Mauser, I know what you mean about off throttle lumpy but this is far worse and not right.

Thanks Mark, I hope it isn't "normal", it really takes the fun out of riding it.
It certainly runs well on throttle and pulls strong.
I'll upload another video as there's another bit where the revs not subsiding is clear.
I've never had a fuel injected bike
Thanks for your help guys.
Oh, T-bone I'll check the pilot jet

I was just thinking I had seen a Yamaha Bulletin on this topic someplace... I think there may have been one on every version of the 360... I actually found it this time. I think Enduronut posted these years ago.

I also have to say again that this minor off-throttle surging is very much the "nature of the beast" of the 360's and 400's.

Thanks Mark that's an amazing bit of information.
I did actually laugh at the gearing suggestion for the "heavier American rider", If you read that from an international perspective it's a bit rude towards the American riders

I "liked" Mark T's post. If I could double-like it I would. I didn't know such info existed. I'd have to say my RT1-B runs very similar to Pete's. I finished restoring this summer and started for the first time. I ended up bumping the pilot up one size and keeping the air screw turned in as far as I can and still run. That's where the motor likes it best but still a few surges. I also timed it at 3.4 as manual indicated. Haven't considered bumping it back. I wonder if the piston modification described would help my DT400C? Still has minor surging. Reed valve motor though...

Just listened to the video & it's way too lean at about 1/4 throttle. danger is they love to seize if you're belting along hard out then roll the throttle back so be careful. So--first check the cutaway number. No sure what it's mean't to be but say it's 'now' a 3.5 & possibly should be a 2.5 so someone might've been trying to improve low speed running but over did it a bit. So check sllde specs against original. Then lift the needle a notch at a time until it goes opposite to what you have, like slightly flooded sound. Lift needle--'lower clip'. But i'd be very tempted to increase the pilot jet a size or two which 'in theory' should richen it 'just' above idle, where the problem 'is' & controlled by the compensating hole from the pilot. Compensating i think it the straight up hole inline with the jet & the idle hole is angled pointing forward. Good idea to give both a poke with something then a blow through. So--if a richer idle jet helps 'that' problem you have but then the idle is too rich, you can counter that by leaning out the air screw. Hope that's not 'too' confusing. Confused 'me' haha.

On the fuel injection I was talking about vehicles in general, not just bikes.

It's easy to forget the days when you had fiddle with the choke... pump the gas pedal "just so" to get it to start... have to worry about flooding the engine... warm up the engine before even thinking of driving off... deal with occasional stalling, bucking, and surging (usually at the worst moment).... that was normal driveability for many of the 1970's cars.

We've forgotten what "normal" was back then.