Need help with carb

I have a 1976 360WR with a 38mm (measures 37.68mm) Mikuni VM carb. Here is what's in it
Main Jet 370
Pilot 45
Needle Jet Q8
Jet Needle 6DH3
Slide 2.0
Air 2.0

The problem it is running way rich, starts good, idle is fine and takes the gas well with no hesitation, but won't come on the pipe. Just brakes up like plug is fouled. I started with 390 main jet and came down to 370. New plugs all black and wet when I check them NGK B9ES. Do I keep coming down on Main jet or try something else?

I just checked timing it has not changed since rebuild bored 1st over size. jet needle clip is in second from top. Any help would be much app.

HAS THE BIKE RAN RIGHT IN THE PASSED WITH THAT JETTING ? IF SO WHAT DID YOU CHANGE?

i have learned to jet from the bottom up, especially since joining the cafe a few years ago...
but we could use some more information about running conditions. where are your problems the most pronounced? bottom, mid, top? and at what throttle openings?

starts about 3/4 throttle I believe. I never marked grip and housing to see. It seems fine just won't get to power band.

It ran last fall great on first run, Changed handle bars and gas and mix, yamalube 32:1

so it mostly happens wot..from the sound of things the main is a bit rich causing four-stroking but odd that is has started with no changes to bike. things that can make a bike run richer than normal "up top" are intake or exhaust restrictions, loss of compression, or even a broken reed can cause an issue like that. is the muffler real clean with no obstructions?

I'll check it all, but was all cleaned or new when rebuilt. I'm going down to 360 and I'm out of jets will let you know what I find. thanks

CHECK THE AIRBOX FOR DOGFOOD NOSHIT HAD THAT HAPPEN THAT DEAL LITTLER Y TOOK SOME DIGGING DAMN CRITTERS

i shot dog food out of the silencer one time leaving the bike at a friends house. one of the reasons i keep a plug in the silencer of everything, lol..

yamalube is clearly the culprit, it was the only way Yamaha could slow the huskies down

Sorry it took so long to get back. I got it almost back to it's 40 year old glory. I went down to 360 main, washed all oil out of air filter and raised clip on needle to top notch. Air jet out 2 turns and changed plug from b9es to b8es. It now comes on the pipe (what a thrill), but still breaks up a little at very top of power band. So I'll get 340 & 350 jets and try again. I hear you about the yamalube but have very good results on my other 2 smokers with it. I still have run away problem with choke sucking air I believe as with first start it revs to full throttle till choke is off then idles fine and after first start it starts first kick without choke. If will be a week or so till I get jets to try again but any help with choke problem is welcome. thanks guys.

maybe pull it to bits, it may have a perished o ring or rubber seal letting more air in than it should.

One of my old Mikuni's had that issue years ago. Tried everything, until some old boy said to buy a new choke plunger assembly. Never did see him again to say thank you !

I would try coming down one size on needle jet to Q-6. At this time a Mikuni VM is going on my 77' 390CR. Going to start off with your jetting numbers except Q-6 with needle jet. Don't have a Q-8 in my jet collection.

I ran VM Mikunis in vintage racing for 25 plus years and always used the needle jet size as the foundation for dialing in the jetting. Plese let us know how your jetting issue goes.

I'm very close, but plug was still wet when I pilled it. I don't have a 340 or 350 jet on hand. I do have a 330 which I think will be too lean, But I am going to try it with the needle raised to the second notch. My hopes are that it is too lean so I can get the 350 main. I've replaced everything in the carb except choke plunger and float valve and I'm having trouble with both. Another problem I have is the air adjustment screw is behind the air box making adj. very hard removing air box to turn screw 1/8 turn at a time.

Oddly enough I have the same bike with the same carb and same problem, runs pretty good til wide open throttle on the main then craps out. Running the same Yamalube R @ the same 32:1, it had a 410 MJ, Needle jet Q-8 166, pilot 45, needle 6DH3@ 2nd from top. Have some leaner jets coming, I checked the reeds they look OK. Keep posting your results so I can see your final jetting or PM them to me if you would. What is your elevation/ only 100 ft above sea level here. Also hard to start cold, not sure what it needs, going by trial and error. Bike is new to me, trying to get it sorted

be careful of unleaded fuels as they wont color the plug like the old leaded stuff.

reading plugs is mostly a waste of time with modern fuels unless non ethanol is used or racefuel.

Well here is where I'm at. Went all the way down on main jet to 330 with clip in all grooves one at a time, got plug to burn clean, but no change in power problem. Still won't top out smooth or get to peak power band. Rechecked timing, reeds look new (I think there boysen, but don't know what stock look like). I pulled exaust, the exaust port is very clean, but a lot of spooge around spark arrestor tube like gloobs of grease. Can't get spark arrestor out of pipe any suggestions?