G'day guys,
I love your forum and have used it silently for some years, but this post is seeking some advice from better advised and more experienced eyes!
I have a lovely little 74 DT100A that has 1700 original miles on it, I purchased it in early 2013 from the original owner. It is mainly a garage piece, didn't run when I bought it but I got everything sorted at the time (tank was rusty inside, carb heavily varnished, etc) and since then it has been a fun little bike to buzz around on when I visited my parents in my hometown.

A few years ago, I noticed when riding it that the bike almost seemed to begin to 'struggle' and had a slight bog at 3/4 - Full throttle, which seemed to eventually 'creep' down to 1/2 throttle after multiple rids. Due to me riding it rarely, I didn't worry, it still ran fine enough.

Eventually, I didn't ride if for a year or so, and the carb had completely gunked up - the tank had rusted again. A total tank restore and carb strip down and clean, and she was running nicely, however this 1/2 throttle and over bog was here to stay.

Nowadays, the bike kicks over very nicely, revs fine on the stand, but when riding up a hill near my current home (seaside, so elevation 30m or so max) the bike struggles and bogs hard, and barely runs - it doesn't 'die' and will keep lugging in low RPM, but even at WOT she won't climb in the revs.
Pulling up to a stop I can feather the throttle to keep it alive, and eventually it settles into a happy idle again, and blipping the throttle slightly assists this. The Bog doesn't seem to be RPM dependent, as it will rev up fairly high on the stand, but under load in gear and when applying throttle, the BOG is very pronounced and it begins to die quickly.

I've cleaned the carb (no joke) 10 times now, every circuit is clear, and now I'm running no air filter as the original foam disintegrated and I believe may have blocked jets previously. I wonder if this is creating a lean condition... although, it ran the same with the old filter in it.

I was wondering if any gurus may have some jetting specs I could try, for this little bike... or any hints as to where to go next. I thought the high RPM spark may be an issue... but the coil checks out nicely (1 ohm, 6 ohm) and resistance everywhere is perfect, 6 ohms from the points in behind the flywheel rotor up to the sparkplug lead. New 6V battery is charged and horn works well - blinker bulbs were removed by me as all had blown after I foolishly removed original dead 1974 battery and revved the bike - blowing all the bulbs and headlight in short fashion!


Current stats:
74 DT100A, original Mikuni VM22SS carb (100% clean!)
No air filter, no air filter lid under seat. New air filter currently ordered.
150 MJ
Air Screw, 1 1/4 turns out
Float Height 21mm
New B7ES plug (I believe a B8HS is required, have ordered)
Elevation 30m above sea level - Sydney NSW Australia

Any and all help, would be excellent!
Cheers, Mitch.

Admin/Moderator, please move this question to its appropriate location if required.

I wonder if the exhaust has got bunged up somewhat with carbon and unburnt fuel/oil?

Not joking here, but would you be able to put the exhaust on to a nice, hot open fire or bbq to burn off any crap?

Hope it's a typo...  do not run it with the B7ES.  Thread reach is too long!  If it needs a B8HS, you must use the H spec plug.

Thoughts that come to mind are timing and air leak.

First & only thought at the moment is exhaust, in the tail section most likely. If you can stand the noise remove the complete tail muffler & go for a ride & give it a good rev to clear it out, see how it goes. If it goes as it used to then pull the end baffle out & put the rest of 'that muffler' in the log fire, plus the baffle if its badly carboned up. They block up deep down in the spark arrester in that muffler & isn't removable. Secondly--or maybe first-- check the points have good gap at full open. If gap is almost invisible then timing will be way late about tdc & kill the power. Need 15 thou & no more. Ok, & as MarkT said , a short reach HS plug which i never would have thought of!!. Lucky it doesn't hit the piston, or maybe it has & almost closed the gap.
www.partzilla.com/catalog/yamaha/motorcy...a/crankcase-cylinder
www.partzilla.com/catalog/yamaha/motorcycle/1974/dt100a/exhaust