I have a 1971 RT1B that sat for over 20 years and I finally decided to get it back on the road.  The bike starts easily, idles well, and doesn't bog but it 'blubbers' over 3000 RPM and smooths out some after 4000 RPM. It seems to be running rich.  The spark plug shows wet after low speed runs.  

I checked the timing with both static and strobe timing lights and it was spot on.  I removed the air cleaner to eliminate that as a problem but that didn't help.  I have a new NGK BPRES sparkplug gapped to .024".  I also cleaned the old BP9ES (non-resistor) and that did not make a difference in how it runs. It appears I have good spark.  I have good fuel flow from the petcock and I loosened the gas cap in case that was not venting.  I tried adjusting the air screw at multiple settings.  

I removed the carb to check the float setting and it was good with float arms parallel and set at 8.5mm to the to the jet setter surface.   I have a #240 main jet, #30 pilot jet, 6CF1 jet needle, (all stock items) the o-ring on the jet needle looks to be in good condition but has some wear.  I have a stock 1.5 cut away slide.  The jets and passages are clean.   The starter pump piston appears to be fully closing when the enrichener is off.  Cylinder compression is at 115, not the best but would that cause it to run rich?   I guess crank seals could be an issue with a bike this old but would that cause it to run rich?

I'm stumped at this point.  Does anyone have any suggestions on how to get this old girl running smooth again?

Looks like I'm first up for a guess. Oring number 7 in the partzilla pic. Maybe not if it gets past that point & revs out. Exhaust restriction mainly in the internal spark arrestor down in the tail end. Possibly a worn needle jet where the slide needle runs, the metering part of the emulsion tube ie needle jet. Possibly someone's sat the slide needle on top of the plate instead if retained under it. Lot of possibilities but I'm only confusing the issue sorry. Try dropping the needle. Clip up. Love RT1's. Just add a bit more, idle mixture screwed right home which can cause richness up a bit higher than idle. Choke accidentaly left on. Lever up for off.

Not sure if you have several typo's on spark plug type but you should not be running an NGK "BPXXX" series plug.  The "P" stands for "projected tip" and it is not correct for your engine and could potentially cause serious damage.

The other glaring item you're mentioned is the jet setter o-ring that seals the float bowl to the needle jet and prevents unmetered fuel from entering the engine.  I can't imagine that after being parked for 20 years that o-ring is going to be anything but rock hard and not sealing well.  You've even mentioned it looking "worn".  It seems like with your stated "rich" symptom and how long the bike has been sitting that the o-ring would be an obvious part to replace?

Final comment is that carbs have several "jetting circuits" that overlap and are dependent primarily on throttle slide opening.   Not so much "rpm".  If a problem is happening consistently at a certain rpm, independent of throttle position and load, then it might be an electrical issue.  

My guess in this case is the jet setter o-ring.

Try closing up your spark plug gap to .020 . Another possibility is wear in your needle jet and needle .
Don't use a aftermarket carb kit , as these parts  frequently bear little resemblance to the OEM parts 
they are supposed to be replacing . Stick to genuine Yamaha or Mikuni parts .

I’m with Swoop on the worn nozzle and the needle. Replacing those two items on my DT400B completely eliminated the blubbering it did once you planed off at around 40-45 MPH. It worked so well that I also changed those two items on my 250B with similar positive effects. I would do the same on both of my 360As but I cannot find the nozzles. But I did find and purchase new needles.

Schu,

There have been reports that it's possible to use a very common 159 needle jet in place of the 164 series if that's what the 360A has.   I kind of remember no mods were needed, but you could shim if needed.  



(Apologies for the hijack!)

I would replace that jet setter O ring that seals in the bottom of the carb as Mark has mentioned. That little guy can cause richness and after 20 years of sitting im sure its shot. You should feel the bowl go on a little snug when your pushing it up against the body. Cheap and easy to replace. Takes a 007 O ring which you can find at your local hardware store or just order by part #.

I appreciate all the great comments. I'll definitely replace the jet setter o-ring. The needle clip is in the top position. I tried it in the middle position and it ran worse which helps to confirm the rich condition. The needle clip is below the plate. Choke lever was in up position, decompression lever was closed. That was a typo on the spark plug, actually a BR9ES. I'll check the exhaust for restrictions. What is the best source to find a Yamaha or Mikuni needle and needle jet? Thanks again to all who responded. I'll post an update when I get it running right.

I had one other question; at the bottom of the carb inlet there are three holes, the left one if factory blocked, the middle one appears to provide air for the pilot jet and needle jet. What is the hole on the right for? I believe the starting jet air passage is separate at about the 3 o'clock position.

Just another 2cents worth--Yamdog says,
"it 'blubbers' over 3000 RPM and smooths out some after 4000 RPM".
Which is kind of why i thought Oring might not be the total culprit because if that's faulty it'll limit out the revs & won't go past it due to richness.
In saying that--it's the first thing i'd replace regardless. But does sound like Emulsion tube is worn out egg shaped plus wear on the needle. #7 Oring also #17 & #4 needle & jet.
www.partzilla.com/catalog/yamaha/motorcycle/1970/rt1/carburetor

There was a carburetor revision on the RT1 .
Both appear to use the same needle jet P/N  275-14141-34 ( Mikuni  188 O-4 )
The early needle used was  P/N 275-14116-01  ( Mikuni 6DP1 ) ,
The second version is likely what you'll have P/N 284-14116-01  ( Mikuni 6CF1-2 )
Google Ebay , all three are available at reasonable prices .
PS . I'm not sure on the needle  clip position on the first version ,
but that may not be relevant anyway .
If you do replace these parts you may well have to play with the pilot air screw to balance
everything up with the new parts .