Ive Just recently put an RT3 back on the road,but im finding it lacking in power,like a lot of missing power.It performs more like an 80s 250 MZ,rather than a 360,with a claimed 32 Hp.Ive treid to keep everything stock ,or as near as i can get.Stock carb,pipe airbox,although no lid,compresion good,new rings,healthy spark,new crank seals,intake boot,reeds seem okay and the plug is a light brown colour suggesting the fuel air ratio is about right.The only thing i can put it down to is that when assembling the engine i notice some work on the barrel to match the piston ports,nothing major and it looked to have been done well,could this be the issue?.the barrel,is on its original bore and they are hard to come by in the UK,so is it possible for me to fit a stock piston to this barrel,a bit of a long shot i know.

I think RT325 would ask if you burnt out the exhaust pipe…
Sneezles61

Ha, was going to type that lol. First thought for 'me' for lack of performance is always exhaust & particularly if it has a built in spark arrestor that traps carbon deep down in the end. Wouldn't be first time i've been wrong though. I'll go read the story again.
Ok, start again. Barrel is still on 80mm std? so is piston modified--you say can i fit a std piston. Hard to imagine any mods that would strangle the power by piston mods as reed inlet sort of decides when to suck & when not. Or has someone moded the crown to advance the ports-probably not if it has the Lshape top ring as no room to do that. One more thing i just thought of is the carb has a tiny Oring that seals the bowl with side main jet onto the needle jet extension. If this doesn't seal it'll go rich & not rev as bypasses the main jet. Doesn't sound like your describing that. Mind you a very restricted exhaust can cause 4 stroking like rich & not wanting to rev. One more poke--if the crank flywheel key is shorn of timing goes where ever it wants although would just stop or kick back harder than hard & break something. Flywheel needs to be tight, that's for sure. All wild guessing & hope you have some luck sorting it. I had one almost new about 900 miles or k's NZ this is. Was a front end straight on crash job where i worked & i tendered for it & got it. Think i was the only tender & what's more i'd likely assembled it new not long before, 'local yamaha shop'. Sorry , digressing & reminiscing lol. Good old days. Welcome if you're new.

There are owner's manuals for the RT3 and DT400B in the Tech Library. 

Under US safety law, the manuals had to contain vehicle passing time and distance information.  DT400B did better than RT3.

Also in the Tech Library is a DT400B test where the dyno showed 23.10 HP max and a 0-60mph time of about 13.5 seconds.  Based on the above, the RT3 may have had less "real" power than DT400B?

After the early 1970's Yamaha pretty much didn't list horsepower ratings...  in the early years when they did list HP, it was almost certainly "advertised" horsepower...  laws put an end to the "advertised HP" practice in the early 70's. 

Bottom line is these are not fast bikes by modern standards and unless HP rating includes an SAE Standard test number, it's probably "advertised" horsepower. 

 

We used to laugh at the advertised speed & miles to the gallon in the early days of all the new models. Must've been down hill with a tail wind. Trouble was the customer invariably came back complaining about all that--but but the advertising says!!

I think my MG1-T is supposed to get over 100 miles per gallon. 

We sold the first Honda 50's, pushrod 50's before that fangled ohc thingy. I'm very sure they advertised 240mpg. Probably damn near did that too haha.

Think ill go down the exhaust route first,it does have an after market exhaust baffle,so this might be restrictive,originals are impossible to find..The barrel is 80mm standard bore,with the original piston used.The piston ports has been shaped along with the barrel..The rubber o ring on the starter jet is new ,so its not this thats the problem.Ill report back after the exhaust investigation

Did you say you don't have an air box lid. What air filter do you have?

Sorry to be a pain, but just want to be sure we're both on the same page with the Oring but i'm sure we are. 'No7'. Not easy to see in the drawing. They get called by different names but yep--seals 'jet setter, i've seen it called' into the bowl, hence into the main jet in the side fitting. All good--ignore me haha.
www.partzilla.com/catalog/yamaha/motorcycle/1973/rt3/carburetor
Can we see the aftermarket baffle please when you remove it, just to be nosy & see how restrictive it is.