I agree RT325...  but generally won't be idling or starting perfectly.  Anything that causes it to "load up" would do it.  Or even simply expecting "computer controlled" consistency from an old two stroke that maybe isn't warmed up fully or something.  Doesn't sound like that would be the case here but I have seen it.  Easy to forget how "finicky" old cars and motorcycles used to be at times.

I was going to leave it to you to say but it's also (remotely) possible a broken baffle plate inside the exhaust is intermittently restricting the exhaust?  I've seen that once on a smaller bike.  I believe Yamaha listed it in one of the RT1 service bulletins on erratic performance?  Seems like a long shot but not hard to remove exhaust from bike and give it a good shake.

Exhaust--Yeah true 'that' MarkT. Can't believe i never thought of it haha.

+1 on the clogged exhaust. I have an RT1B that had a clogged exhaust when I bought it 4 years ago. My '71 DT1E had an internal baffle that came loose back in '71 after I had it for 6 months and it clogged up. Dealer replaced it under warranty.

If you have a spare exhaust that you can try to help troubleshoot. A '72/'73 exhaust will fit. Yamaha changed the design slightly.

What air filter do you have?

I've found that the aftermarket air filters can be restrictive if they are double layer with too much filter oil.

Some great suggestions this kinda problem can drive you crazy I see you rebuilt your top end retorque your head bolts when cold again. I had a similar problem with my RD350 ran real good the first day and crappie the second day then I remembered to retorque the head several more cycles and it fixed my problem. I was taught  that when I worked at Butler’s. 

Well, think I figured it out. New spark plug. Was told long ago, check the simple first! Pretty good at carb take down though! Thanks to all for the help . 

As above a new plug fixed the stumbling , but found the air filter as shown. This is weird, foam not burned (from possible backfire), not disintegrating from age, just gone. Nothing in the air cleaner runner to the carb or in the carb. I installed this element some 8 years ago and have checked it a few times but this time found this. Thought maybe a mouse, but no other evidence of that.  

Sure appears to be vermin chewing stuff
Sneezles61

I find it nothing short of miraculous almost every time I see issues solved here...  usually by someone like RT325 or one of many other great members literally located half a world away from the bike and by relying solely on a "written description"!  It's amazing.

Occasionally, even somehow I am able to provide an answer or two...  but mostly I just have experience gained from making a LOT of mistakes and screwing up a lot of stuff over the years...  as well as chasing my tail and replacing parts I didn't need to replace because I was convinced it had to be the carb or whatever. 

It took me decades to be more open minded and honest with myself and others when chasing down problems...  and to try the simple stuff first even if in my head I "know" it simply can't be "that". 

Congrats on finding the issue was the spark plug! 

I'm puzzled on the air filter condition since in one of the early posts this was said "Air cleaner is new, no vacuum leaks, new piston, barrel gasket, intake gasket, damn! Any help appreciated." 

It really does look like a mouse or something chomped away at the 8 year old filter...  and the "no signs" could be due to bike being ridden and the "evidence" has been digested by the engine?  If you haven't done so recently, I'd suggest removing the needle jet (aka "main nozzle") as often air filter debris clogs the air bleed passage to needle jet...  which can make it run lean at full throttle.  Some filter debris could be in the idle air and cold start enrichener air passage too.  And all of this could have happened very recently. 

Glad you caught the missing foam before ingested dirt ruined the bore and crank bearings. 



 

MarkT you are definitely one of those members it’s cool to see someone get there bikes on the road and keep the old Yamaha’s going 

Learning by mistake works for me haha, We may never make the same mistake again but there's so many new mistakes to make waiting in the wings.