1973 Yamaha RT360

Discussion in '2 smokers' started by Wesley J, Jul 24, 2021.

  1. Wesley J

    Wesley J Hoser

    Joined:
    Jul 15, 2008
    Oddometer:
    831
    Location:
    Calgary, Alberta, Canada
    Hey Guys,

    I've been struggling with this bike off and on for years to be honest and have hardly ridden it. This winter I thought I finally solved my problems but it turns out I'm a still a miserable failure...

    The bike has never run right, especially in the upper rev range and has always been very low on power. So far I've done the following:
    • carb rebuild and replaced jets to stock
    • Rebuilt/repaired decompression mechanism
    • New crank seals
    • Timing adjustments.
    This winter I replaced the crank seals hoping that this was the culprit of the bike falling on its face at upper RPM's. It did not, but the bike does pull better at low-mid RPM's.

    Anyway, any ideas?

    Cheers,

    Wes
    #1
  2. ABee

    ABee Near Normal

    Joined:
    May 12, 2013
    Oddometer:
    165
    Location:
    North Georgia
    I have had a couple of these over the years. You might want to check your exhaust to see if it is clogged. Pull the baffle out and clean it. I have seen these almost totally plugged with carbon. Some baffles have very sloppy welds that restrict flow as well. (Assuming you have an OEM pipe.)

    Another thing to check is the reed valve. Even though the OEM reeds are metal, they still get fatigued and can crack and break. It could be that a previous owner replaced these at some point with fiber reeds which will crack, fray or break over time.
    #2
    Marvin S, jimroid and spokester like this.
  3. spokester

    spokester Long timer

    Joined:
    Jul 9, 2013
    Oddometer:
    2,003
    Location:
    SE USA
    For your general information, I rebuilt one a few years ago. Never did figure out why the transmission drain plug blocks the only crossover for oil between the clutch and transmission cavities. Could it be carburetor jetting?
    RT3 Motor in a Box | Adventure Rider (advrider.com)
    #3
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  4. Strong Bad

    Strong Bad Former World's Foremost Authority Supporter

    Joined:
    Oct 29, 2007
    Oddometer:
    13,897
    Location:
    Omicron Persei 8
    Air cleaner, reeds, and clogged exhaust are the usual suspects

    The bike starts, and idles fine? If it has a slow return to idle an air leak is suspect.

    "Stock jetting" means nothing. Each individual two stroke should be jetted for current conditions, stock jetting is only a starting point. What does the spark plug look like?
    #4
  5. msahern5

    msahern5 Jumping at Shadows

    Joined:
    Jul 31, 2005
    Oddometer:
    1,690
    Location:
    Upstate S.C.
    There are some things man was not meant to know. That is curious, though.
    #5
    spokester likes this.
  6. Wesley J

    Wesley J Hoser

    Joined:
    Jul 15, 2008
    Oddometer:
    831
    Location:
    Calgary, Alberta, Canada
    Thanks, I'll remove and inspect the stock exhaust baffle, I have not completed this yet.

    The reed valve is original and in as new condition but I'll revisit.

    It runs very well at idle and pulls well at all throttle positions up until about half RPM and then effectively refuses to pull any longer. It really wont even rev out if you hold it WOT.

    It does have a slight air leak/minor slow return to idle but I would expect that to act the opposite to what I have going on. An air leak should have less effect at low vacuum of WOT no?

    Plug is dark brown without any buildup.

    I have gone 2 sizes up and down on the main (no needle jet or jet needle changes) with no appreciable effect on WOT performance.

    I do have an oil leak that seems worse than before. I'll inspect, it may be that the case halves are leaking from the bottom end as well and is the source of a bottom end pressure leak at higher RPM

    I appreciate the assistance. I will:
    1. Check baffle
    2. Check air cleaner
    3. Track down my air leak (most likely throttle cable or cap related?)
    4. Confirm reed condition
    5. Cry tears of joy or pain depending on outcome.
    I have not performed a leak down test, does someone have a link to a good set of instructions on how to do this on a 2 stroke?

    I just finished a sympathetic restoration on a '69 CT90 so I have time to devote to the RT360 again.

    Thanks for the help guys, I appreciate it!

    Cheers,

    Wes
    #6
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  7. Strong Bad

    Strong Bad Former World's Foremost Authority Supporter

    Joined:
    Oct 29, 2007
    Oddometer:
    13,897
    Location:
    Omicron Persei 8
    An air leak can be diagnosed by giving a very small spritz of starting fluid directed at the reed block and at the carb to the reed block connection. The idle will jump up where the ether enters the flow. You could do the same to diagnose a leaking crank seal on the ignition side. The carb cap and especially the cable are very unlikely sources of an air leak.

    When jetting a 2 stroke, jets are increased and decreased until there is a noticeable effect. Keep increasing the main jet until the bike 'blurbles" from being too rich, then decrease the jet size until the "blurble" stops.

    Dark brown plug can also come from too cold of a plug. What plug are you using?

    Although rather rare, the secondary coil (the one under the tank with the plug wire) can fail yielding a weak spark. Over the years I've seen them occasionally do this. It can drive you nuts because it will show a moderate strength spark (yellow colored spark) while testing with the plug laid on the head, but not provide a spark under compression. You want a good blue colored spark with a nice strong snap.
    #7
  8. Foot dragger

    Foot dragger singletracker

    Joined:
    Dec 27, 2006
    Oddometer:
    29,191
    Location:
    chico,just below rag dump(nor-cal)
    I cant imagine a drain plug blocking oil access between clutch area and transmission area. 2 strokes have holes in the cases to facilitate oil flow or it wouldnt work.
    The oil fill hole is in the clutch cover case so oil must get to the trans pretty readily from there.
    #8
  9. Foot dragger

    Foot dragger singletracker

    Joined:
    Dec 27, 2006
    Oddometer:
    29,191
    Location:
    chico,just below rag dump(nor-cal)
    If the crankcase area had a leak at bottom,or anywhere it would be forcing oil out at high pressure when running,it would be a mess and pretty obvious. Wouldnt run well at all.
    #9
  10. V4Rider65

    V4Rider65 Christian Adventurer

    Joined:
    Jul 28, 2014
    Oddometer:
    63
    Location:
    Southern Ohio
    Wes, Don't give up on this bike. I have an RT3 360 and absolutely love this bike. I actually raced it a few times. Go to https://www.yamaha-enduros.com/ website. Those guys have all the solutions to your troubles. RT3 in Motocross.jpg
    #10
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  11. Wesley J

    Wesley J Hoser

    Joined:
    Jul 15, 2008
    Oddometer:
    831
    Location:
    Calgary, Alberta, Canada
    Thanks for the motivation!

    I checked the baffle, its in perfect shape and wide open.

    The float level was off so I adjusted to the correct level. Then I played around with jetting. Stock is 220 and I went as high as 240 and as low as 190. It sort of runs decent at 200 but gets quite hot quite so I'm assuming its lean. Also, it has much less power on the low end with the 200 jet than it does with the 220.

    Anyway, I write up a better list of what I've done and post it over on the yammy enduro website.

    Cheers,

    Wes
    #11
  12. Wesley J

    Wesley J Hoser

    Joined:
    Jul 15, 2008
    Oddometer:
    831
    Location:
    Calgary, Alberta, Canada
    I will perform the air leak test and grab a new plug. I'll also check the coil. I'm not familiar with these vintage coils, are they "universal"?

    Thanks for the suggestions
    #12
  13. Tim McKittrick

    Tim McKittrick Long timer

    Joined:
    Dec 29, 2003
    Oddometer:
    2,496
    Location:
    Wasilla Alaska
    I installed a powerdynamo ignition and charging system in my RT2 to both convert it to 12v and to eliminate the points. Their coil has about 4x the power of the stocker, and I expected to have to greatly increase the jetting to keep up with the hotter spark.

    To my great surprise I had to jet down a LOT to get the bike to run correctly. It would appear that with loop scavenging and the bore size of the 360 there is a fair bit of charge dilution from leftover exhaust, which confounds the mixture. I eventually settled on a 190 main and the needle down two positions.

    I’ve put about 5000 miles on the bike since, and it has yet to so much as foul a plug.
    #13
  14. Scootern29

    Scootern29 Long timer

    Joined:
    Sep 4, 2012
    Oddometer:
    1,075
    Location:
    In the hot, hot, dezert
    After reading through this.....I can't remember which carb is on that bike, but if it has the external main jet on the bowl, it may be the little oring on what is called the jet setter. If it is worn your motor won't do much after mid throttle. It's leaking fuel past the oring. It's common with the ethanol fuels.
    #14
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