Hi Elmo, Everybody -
I solved my problem. Recall my bike's long drive chain was "bouncing", causing my bike to "pulse" forward. I figured that if I could stop the bouncing, the pulsing would stop. Here is my fix:
Went to Ace Hardware (spent LOTS of money there). I purchased a length of their large diameter plastic (silicon?) tube. See photos. I also purchased a 5/8 inch solid shaft, eight inches long. They had 5/8 inch diameter bolts for less than $3 each (Great! No cutting involved!). I already had the inexpensive Ace roller bearings of 5/8 inch internal diameter, which just happen to fit perfectly inside the plastic tube! Nice fit! The shaft rested nicely in the frame valley, the 2 inch length of tube rotated without friction, the chain rested on top of the relatively soft plastic, and placed a very slight apex angle in the chain when it was under tension. I fastened it all down with intent to make it easy to remove if necessary. So visually, it is not real pretty. But it works! (You may need to rotate the second photo; I had to upload it as you see it.)
During an 8-mile test ride late today, the chain rode over this "chain idler". The idler rotated at chain speed, and made not a sound whatsoever. And accelerate as hard as I tried, there was no chain bounce! Hooray! Another engineering problem solved! With readily available parts!
Now about that 79 cc HF engine... In the 2000 to 2400 rpm range, that little engine likes to produce some serious torque pulses during its power stroke. Given the length of my bike frame and its inherent flexing, my bike does quite a bit of shaking until the engine gets over 2400 rpm. Then the bike settles down and rides as expected. I noted some other unexpected additional random shaking. Then I realized that was from the imperfect road surface.
At my altitude (currently in Albuquerque) of 5500 feet, I look up derating an engine because of high altitude. At full throttle, gasoline engines are derated. One chart I found derated at 3 percent for every 1,000 feet above sea level. So I am at a slight disadvantage compared to sea-level bikes in all-out acceleration and wide-open throttle top end.
I added a cargo basket and some rear attention-getting lights. The brake light is wired to go full bright when I squeeze the rear brake handle; it is an LED truck light powered by a 12 volt sealed acid battery. The strobe flasher (from Radio Shack about 16 years ago) is a real attention getter in the dark. I safety-wired it down so it will not break loose if I hit a nasty pothole. Now I just need some headlights, red reflectors, and the air horn that I already have. Then I am ready for night riding!
The bike is in its break-in phase right now. It has only 18 miles on it. Nothing has broken (yet). It need more stress testing. In a couple of weeks, I may take it to the east to a town about 30 miles from here. There is a 7,000 foot pass between here and there. I think the bike and I will make it if I don't overrev the engine.
That is my long-winded story. I will probably have more to write in the future....
MikeJ