That's gold info! I will definitely try this plug if I can find one. I have a sense NGK eliminated the copper core plugs and are doing only nickel now (probably due to high cost of copper). Quick search shows a 5510 replacement for the B8HS. I'll keep looking.

I got my first LT2 cheap when I was a kid.  Started good but had no power.  I tinkered with it for hours and hours...  nothing helped.

Then I left the air filter off and took it for a ride.  Ran GREAT!  Filter was OEM Yamaha...  filter looked new...  (bike was a 72 first sold in 73 and I got it in 74).  It had been cleaned and oiled carefully at least twice when I was trying to figure out why it had no power. 

Went to Mike Patrick Yamaha with the filter and they gave me a new UNI under "warranty".  (There was no warranty, they were just nice people helping out a kid who had spent all his money on a used Yamaha and wanted to ride it.)

Apparently the pores in the foam can close up.  Filter looks perfect but won't flow air.  Just a thought.

LT2 low on power, it's because it's only 100cc's, it's a bike for 100 pound kids. I had a 1971 enduro HT-1B in 1971, and wasn't much faster than the mini bikes of the day. I did ride it probably 50 miles every day. Still have it. An adult on a 90 or 100cc enduro reminds me of the Shriner's parades. Have had both a LT2 and LT3. 

Trust me...this will be a bike for the kids, I have four bikes I'm working on at the same time. My 74' DT250 is the bike I will be riding once I finish with this one and the '79 DT100 but that one will likely be a full resto. Still seems I should be able to go up a small incline (not a hill climb, mind you) with a running start in something other than first gear. That said, with as many of these bikes as you have, you would know better than most. Beautiful collection by the way. Every barn and garage I drive by I wonder if there is a hidden Yamaha treasure inside. With 20 or so bikes you must have an amazing garage.

It should have plenty of power to go up a small incline. I had both a LT2 and LT3 and the LT2 was lacking power big time. Ran fine at idle and at slower speeds. It ended up having a restricted exhaust pipe. It was hard to spot too. Took awhile to unplug it but once i did it woke it right up. I weigh about 228 and it would take me anywhere after the fix. Fun little bikes! 

good luck!

Music to my ears. What did you use to unplug it? I said in an earlier post I soaked it with a can of B12, shook it, emptied it, soaked it again, shook it, emptied it then tried to heat it with a combination of heat gun and propane torch. it seems to be clear but also seems heavier than it should be. I've seen posts where people literally place the pipe in a firepit. 
 

1st, did you take out the pipe outlet and silencer out and clean them good? Number 2 and 3. Or see how it runs without them in the pipe?

 .

i remember using everything at my disposal. carb cleaner, brakleen, simple green and hot water, Soaked it, long aluminum rod, coat hanger and after all that i finally got chunks of carbon out with a 3200 psi power washer spraying in both ends until i felt the water went thru good and i started seeing stuff come loose. 

Here's the bike.

  

Never tried the fire pit method nor have i ever heard of it working until i read it here on the forum or i might have tried that! 

Damn that’s a sweet bike. Reminds me of how far I have to go but daily parts deliveries are bringing it back step by step. I did remove the end piece but I didn’t know how to get the silencer out. I’ll check the repair manual section.

Yep be sure to take out No2 as well, which might be well stuck in.

That's a nice bike . Basically it looks like a 100 cc version of the HT1.
I have never seen an LT first hand . Clean and simple .