Motorcycle Top Speed Problems?

Question

I recently purchased a 1982 Honda CB650SC Nighthawk. 17500 Miles. 5 Speed. 4 Cylinder Engine 650cc (probably actually around 627cc) Carburated. Earlier today (12:00pm), I warmed the engine up and tested the top speed it varied between 69-70 MPH.
Just a bit ago (around 6:00pm), I tested it again and it would not go above 64MPH at 5500RPM. The top speed earlier was probably because the wind was higher speed and i was downwind. I was also downwind this time, but the wind speed was not as high.
However...I would think that a 650cc 4 Cylinder would go faster than 64MPH. I only weigh 160lbs, so its not like I'm weighing it down a whole lot. The oil looks new, and the air filter looks new. I recently lubed the chain with the recommended gear lube as well. The tires are also properly inflated. What is keeping my speed down? Or is an engine like that just not that fast? I have a feeling it should at least reach 85 (like the speedometer goes up to) Please give me any suggestions

Yes, I was riding in top gear. And it Redlines at 9500 RPM. The gas in it is good, using the recommended octane and all (which is 86, and the pumps provide 87). Good suggestions so far, keep them coming and I'll check out each one.

Good suggestion about the leaded gasoline situation. Anyone have opinions on Marvel Mystery Oil and if 2oz. per 10 Gallons (half the recommended amount) would be safe and effective to help with top cylinder lubrication? Also, the owners manual recommended 80W or 90W Gear Oil on the chain over spray lubricants, don't know why, but I'm gonna follow it.

Answer 1

this bike needs a complete top-end rebuild for best results. that is rings, valves, lifters, plus the usual stage 1 mod of high flow air cleaner, free-er flowing pipes, and a properly jetted carb.
this bike should do alot more than what its doin. doin a top end rebuild will help immensely.

Answer 2

This Site Might Help You.

RE:
Motorcycle Top Speed Problems?
I recently purchased a 1982 Honda CB650SC Nighthawk. 17500 Miles. 5 Speed. 4 Cylinder Engine 650cc (probably actually around 627cc) Carburated. Earlier today (12:00pm), I warmed the engine up and tested the top speed it varied between 69-70 MPH.
Just a bit ago (around 6:00pm), I tested it...

Answer 3

I think you're misusing the term 'redline'. Redline is the maximum speed of the engine. If you're redlining in 5th/top gear, that is the maximum speed of the bike. Other than changing sprockets to increase the top speed, you've reached the limit of the bike. If you meant to say that the bike is hitting a wall at 40 mph but the bike was able to hit over 50 before, that is another story. There are lots of reasons why the engine just can't reach the speeds it was capable of before. Since you haven't even shared what year, make, and model of bike you have, this is all just a guessing game at this point...

Answer 4

You're getting no power, sounds like your motorcycle is not firing on all cylinders. Better check that out.

My 1986 Honda Nighthawk 450 could go 65 mph just fine, and even above although it accelerated sluggishly. It was a six-speed, though!

Answer 5

Like guardrail jim says, it's old and may not have been taken care of properly. Actually, it sounds like something is limiting your RPM, if it won't go over 5500. I don't think unleaded gas is your problem. Some of your settings might not be correct. Ignition timing, carbs out of synch, etc. I've got an '81 650 Custom with 36K. I remember that 4000RPM in 4th gear = 40MPH. And in top gear 70MPH was somewhere around 6500RPM with throttle to spare. That's with about 135lbs of me plus more in the saddlebags.

Gear oil on a chain? Absolutely correct. Every 300 miles.
If you change to an O-Ring style chain you can swap over to the spray.

Answer 6

Wow! I had to double check those numbers. My 1975 400cc enduro would go 80mph.
http://www.motorcyclespecs.co.za/model/Honda/honda_cb650sc_nighthawk%2082.htm

First check that someone didn't mess with the sprockets.
Front - 17
Rear - 38

The bike is over 20 years old. I think it's time for some engine work. Even though the mileage isn't that high, any previous owners may have abused it, or not maintained it properly.

Check the cylinder compression and perform a cylinder "leak down" test to check if the valves are sealing (not leaking).

Answer 7

Bike definitely is sick (in a bad way) all right. i think a complete tune up, compression check and valve adjustment are probably called for. before doing that tho - pull the drain plugs on the carbs and drain them out - and flush a little fresh gas thru - observe all cautions with the gasoline. sometimes water collects in the float bowls and the bike will run fine untill it gets to a higher speed and the water glob that is in the bowl sucks up against the main jets and you just can't go any faster.

Answer 8

In '82 they still used leaded gas. You can forget the wind crap. And if the plugs worked six hours ago, (Your 12 to 6 time.) they still work. The reason I mention leaded gas is I got a great deal on an old bike with low mileage also. Since the gas switched to unleaded, the valve seats are probably shot. That's what was was wrong with my "great buy". Valves would not seal on the shot valve seats, so I got no power. And crappy gas mileage to boot! I ended up doing so much head work that it took out my lower end just a couple of thousand miles later! (I did go a bit overboard on the top end. Figured what the %$&#, it was off! Don't mess with them old bikes that don't have the valve seats to deal with the unleaded gas. unless you are gonna do a complete rebuild on the old &%#$@^%$. ) And I would only recommend that on a hog. They at least have resale value. And don't forget to wear a windbreaker next time you check. You don't want "Vette" to hurt himself if you go over 80. (The grips don't matter on a Honda. Just on Suzis and Yammies. And the '87 and later Kaws. Shows you what HE knows!)

Answer 9

What kind of engine speeds are you observing when you are in the lower gears? Where is the engine redline on your tachometer? Have you been riding this MC regularly all winter? Could the gasoline be contaminated, or the fuel lines clogged up? When was the last time you had an engine tune up? I should think the Honda 650 would be capable of 8000 RPM partially loaded on a straight and level road. Could the brakes be dragging?

Answer 10

There is no way to answer this problem without having the bike examined by a certified motorcycle mechanic.

Answer 11

I have been riding motorcycles for 125 years and I never test my top speed at noon. You said you checked the oil and air filter but you said nothing about the rubber compound of your hand grips. Also make sure your valve stem caps are on tight. I also always wear a wind breaker jacket when testing for top speed that ways the wind is never a problem. I had 17500.1 miles on my night hawk with a 4 speed and I weigh 400 lbs. When I was leaving New Orleans during Katrina I was able to get up to 245 mph while riding against the wind, but I also was wearing my wind breaker. Do some more testing before noon. Make sure you wear a helmet just in case you fall down. I would not want you to hurt that highly intellectual brain of yours.