I have owned a 1976 CB 750F Super Sport since new in Sept. 1975. I rebuilt the carbs in the early 90's due to gum deposits. The main needles were set at the middle notch and the main jets were 105's. The bike has always run rich, causing low mileage and black plug deposits. somewhere along the line I replaced the chain and front sprocket - I went with a 1 tooth larger sprocket. this increased my freeway/trip mileage from 30 mpg to 33 mpg. The engine was still running rich as noticed by the slightly black smoke in the exhaust. I re-jetted to 102.5 main jets to no avail - still smoking. I decided to go BIG, so I moved the main needles one notch lower. The engine runs OK except it will bog at full throttle as it is now too lean. I get almost 40 mpg, though and there is no black smoke exhaust and the plugs look like they are supposed to - a nice tan color. I am sure that the lean mixture is possible causing the cylinder temps to skyrocket. The exhaust pipes are now starting to turn straw color because of the excessive temps. I would like to use larger main jets to compensate for this. Any ideas? Do I need to go to 110's?
I would suggest new carb-to-head hoses for this one. If you have not replaced them, they are rock-hard and after several carb R&R cycles they don't seal very well, if at all. The darkest plug(s) will be the leakiest one(s).
Another thing: today's gasolines, with ethanol aboard, burn MUCH slower and colder than 1970s gas did. To boot, the modern D8EA sparkplug is too cold for even 1970s-era gasolines, and the D8ES-L plug made for these bikes is only available now in Resistor form (DR8ES-L) for 1990s-era superbikes, among others. You can get the proper heatrange plug in the X24ES-U from Nippon Denso, or even run the D7EA unless touring at interstate speeds. The D8ES-L was developed expressly for the CB750 engine, circa 1970, and is about halfway between the old D8ES and D7ES in heatrange, as an imaginary D7.6ES plug might be envisioned.
Also: consider pulling out the emulsifiers (one more time?) and enlarging the holes in them a few thousandths of an inch. It can be done with just a strong pointy tool, because the brass is soft, followed by hand-turning a 1/8" drill bit up their center to clean out the burrs. This will improve the emulsifer action and better aerate the ethanol-laced gas, which requires more air to burn than plain old gasoline.
Finally: don't run premium gas unless you are out on the interstate, at interstate speeds. Today's midgrade burns slightly slower than 1970s premium gas did, so it is a version of old-time 102 octane rating gas. More octane causes slower burning: too-slow burning causes unburned fuel that both washes the cylinder walls of oil and fouls plugs.
And: set your air screws inward to about 7/8 turn instead of 1 turn. The seats have widened from both corrosion and previous re-seating of the screws: this narrower opening will help lean out the bottom RPM range to more closely match the transition from idle-to-mainjet operation and reduce that 'flat spot' feeling.
