For second time since buying the bike in 1977, I've replaced the jet needle sets in the '75 750F carbs. The first time was back in the 90's at around 35,000 miles when I noticed a gradual loss in performance. Symptoms were a big drop in fuel mileage, rich running, and no need to use the choke when starting, no matter how cold. Fuel mileage had dropped from the mid-40s to the mid-to-low 30s. I read at the time that needles and jets for these carbs were soft and subject to rapid wear. The story was Honda had recognised this, so later parts were hard anodized to provide longer service life. After replacing the jet needle sets, everything went back to normal.
Those old symptoms began reappearing earlier this summer. The bike was at over 112,000 miles and fuel mileage dropped from the low 40's where it has consistently been (while running on the potato juice they call gasoline here in the valley) down to the mid 30's. One day on the freeway, a cylinder started cutting out. I pulled the plugs when I got home and they were sooty black. I pulled the carbs and sure enough, the needles were visibly worn. I compared the old jets to the new ones using drill bits a plug gauges and they are worn, too.
With the new parts in, gas mileage is right back to the low 40s. It wants the choke when starting cold any time below 80 degrees. If your bike's mileage is around 35,000-40,000 miles and you aren't getting the fuel mileage you think you should, it wouldn't hurt to take a look at the jet needle set. Since the second sets went twice as far as the original sets did in my bike, it looks like the newer ones do last longer.
As a side note, I tried using parts from Keyster kits bought a couple of years ago. The kits have most of the small parts for a carb rebuild and cost the same as just the jet needle set from Honda. But it was no bargain. The kits contained only the jet needle and not the jet. The taper on the needle was visibly different than the stock needle. The gaskets look OK, but the rest of the stuff is not precisely made. I can't recommend them. Others here have noted the same thing, but unfortunately I read that feedback well after I bought the kits.
