Topic: Honda Nighthawk 650 1983 - first drive

[tommygunxs]

I took out my honda for a first test drive. The idle was strange, i turned the idle screw and it responded, but after i while the idel was very high- it did go from correct idle to very high idle, this happend several times. Do anyone now why they go from correct idle to very high idle? (I had put the idle screw correkt)

could it be that I have the choke on and I turned the idletrottle, as it says on the honda instructions- but the thing is, the mc get a lot of fuel when the choke is on maximun, and there is not so much meaning to turn the idle throttle before the mc get warm?

or is it a airleak?

[M 750K6]

The 750s struggle to idle cold. Wouldn’t be surprised if the 650s are the same. Don't adjust idle until the engine is fully warmed through. 10 miles should do it.

[tommygunxs]

thanks! i will try that!

[fizzlebottom]

Your 650 is a DOHC motor, so it may be quite different from mine but I'll give you what I know.

Even when the carbs are running on a fresh rebuild and all adjustments have been done by the book, my bike is very picky until it is warmed up a little. I close the choke fully, start the bike, feather the choke until the bike idles at ~1500RPM for about a minute, then feather it down further to just above 1k. I'll ride off and fully open the choke maybe a mile down the road, if that.

But if I start the bike from cold and begin cracking the throttle right away it behaves like I've never maintained it. The 81-82 CB650s with CV carbs, at least, are not happy cold runners. They need to be warmed up some. I think a big reason is that the CV carbs rely 100% on healthy vacuum pressure, which relies 100% on both perfectly sealing valves and healthy piston rings. If you have worn rings and/or poorly seating valves, then those carbs simply do not behave as as optimally as they should. Synchronizing helps until the differences between cylinders is enough to outshine the sync screws. Hypothetically that's many many years and many many thousands of miles of riding, but everyone will be due for a top-end and carb rebuild eventually. Not saying yours is, but it isn't unheard of.

So, in short, make sure you do your maintenance. Sync your carbs and get their air/fuel mix just right. Then adjust idle off of that when the bike is at full operating temp.

[scunny]

IT will be a Sohc. thru to 84 I think.

[fizzlebottom]

IT will be a Sohc. thru to 84 I think.

Nope. Honda ended 4cyl SOHC motors after 1982.

[tommygunxs]

Your 650 is a DOHC motor, so it may be quite different from mine but I'll give you what I know.

Even when the carbs are running on a fresh rebuild and all adjustments have been done by the book, my bike is very picky until it is warmed up a little. I close the choke fully, start the bike, feather the choke until the bike idles at ~1500RPM for about a minute, then feather it down further to just above 1k. I'll ride off and fully open the choke maybe a mile down the road, if that.

But if I start the bike from cold and begin cracking the throttle right away it behaves like I've never maintained it. The 81-82 CB650s with CV carbs, at least, are not happy cold runners. They need to be warmed up some. I think a big reason is that the CV carbs rely 100% on healthy vacuum pressure, which relies 100% on both perfectly sealing valves and healthy piston rings. If you have worn rings and/or poorly seating valves, then those carbs simply do not behave as as optimally as they should. Synchronizing helps until the differences between cylinders is enough to outshine the sync screws. Hypothetically that's many many years and many many thousands of miles of riding, but everyone will be due for a top-end and carb rebuild eventually. Not saying yours is, but it isn't unheard of.

So, in short, make sure you do your maintenance. Sync your carbs and get their air/fuel mix just right. Then adjust idle off of that when the bike is at full operating temp.

thanks, actually I did start the motorcykle and started to turn the adjustment on the carbs, I will try to take the bike for a drive now and let it get warm and not touch the adjustment befere its warm
a carb sync would be a good ide, the motorcykle is 41 on years old, but have only 13000 kilometer on the meter, its like 8000 miles

[M 750K6]

No doubt about it, miles = wear. But, the highest wear rate is straight after start up. E.g. the oil is cold and not in all the right places. If it's 8,000 miles of once a month 100 mile ride, that's good. If it's 10 rides of 10 miles, not so good.

I've also seen lots of scratches on carb brassware, from poor cleaning practices. If the bike has stood for long periods with fuel in the carbs, likely they have had to be cleaned a few times.

That said, my CB750 had 40,000 miles on it when I bought it. It was the best I could afford. I have spent a lot of time and trickled out a lot of money, to get it to where it is now. I would definitely have preferred to start with an 8k bike like yours! Just not a complete guarantee of condition, so worth going through it to recommission it for reliable running.

[fizzlebottom]

I've also seen lots of scratches on carb brassware, from poor cleaning practices. If the bike has stood for long periods with fuel in the carbs, likely they have had to be cleaned a few times.

I learned this one in time. Used to clean my slow jets with a guitar string and only recently ended up just ordering fresh new Keihin jets cause I could no longer guarantee I hadn't chewed up the insides to death.

Adding to scratched up jets, the other thing I'd have to ensure is that the carbs have genuine Keihin jets and not some aftermarket junk. Gotta start from a good place, cause otherwise you're simply trying to polish a turd most of the time.