Motorcycle Repair: Honda CB400T, honda cb400t, air fuel mixture

Question
Mark, I hope you can help me or at least point me in the right direction. I bought a 1980 Honda CB400T Hawk about 6 months ago. It ran fine but had a few flat while cruising and accelerating lightly. Over the last few months I have installed individual filters on the carbs and re-jetted the low speed and the main jets which GREATLY improved overall drivability and resposiveness. I did not mak any changes to the needles though. It runs great all the way up to the redline but I noticed if I cruise on the freeway about 70mph (approx. 7500rpm) it will not go any faster. I can twist the throttle another 1/8 of a turn but it will actually slow down if I do this. If I down shift 1 gear, the rpm's come up and runs great at a steady 70mph, but still will not go past 70mph. My question is, is this all the power it's got, or is there some thing I can do to get it past 70mph. I don't plan on riding past 70mph, but would like to have the bike running at it's full potential. I didn't know if it needs to be jetted further or changing the exhaust would help (it has the stock exhaust with the power chamber) or if this is as fast as it's gonna go. There's not too much aftermarket support left for this bike, so any help would be greatl appreciated. By the way, I only way 130 pounds if that makes any difference.

Thanks for your time and any help you can give me.
Darrell

Answer
Hi Darrell,

Examine the appearance of the spark plugs to determine if the air-fuel mixture needs adjusted (jetting).

That MC will not do much better than that. You can chage final drive gearing to get better top end. Ratio for sprockets is 3:1 meaning: one point on the front sprocket to every 3 points on the rear. In other words, make changes to the rear sprocket in 3 point incriments. Front sprocket: 1 point. You need not change the front size. This effects engine speed. Leave it as is. Rear sprocket effects vehicle speed.

Respectfully,
Mark Shively