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Thuban

Horse scratcher
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Hi All. I'm new here but have been lurking and reading. I have a barn find, 1978 CB750K with less than 8 K original miles. It had sat for years and years.
I stripped it to the frame and built it back up. Somethings needed attention and other than inspection and cleaning, the engine was in excellent shape.
I have about 300 miles on it now. It's like a sewing machine. But, it is terribly cold blooded. I hate starting the engine and have it rev way up till it starts to get warm. I guess it is what it is. The clutch rattles and less so when you pull it in. It has a whine while running that seems to me, loud. But it runs fine, plugs have a good burn. Timing chain adjusted. Clutch is solid, no slip. It's heavy rolling it around but once moving it's solid and stable.
I'm wondering, do these engines all have a clutch noise and whine on the left when moving?

750.jpg

Thanks,
Thu
 
If it is revving way up then it sounds like you need to adjust the fast idle, should only need to be around 1500 rpm when cold. They really shouldn't be that bad for cold blooded, a minute or two and you should be able to start riding and start opening the choke back up. The clutch rattle is probably the primary chains you are hearing. Which is normal on the 750 but if tune up is spot on it should be minimal. I have never heard a whining noise on the 750...valve train and primary chain noise is about all you should hear.
 
If it is revving way up then it sounds like you need to adjust the fast idle, should only need to be around 1500 rpm when cold. They really shouldn't be that bad for cold blooded, a minute or two and you should be able to start riding and start opening the choke back up. The clutch rattle is probably the primary chains you are hearing. Which is normal on the 750 but if tune up is spot on it should be minimal. I have never heard a whining noise on the 750...valve train and primary chain noise is about all you should hear.

Thanks, Dirtdigger. I may need to drop the choke rpm down then. If I pull it all the way out it runs up to about 3 K when cold. I never let it get that high. I push it in and more so as it warms up. I say cold blooded because it won't take throttle off idle until good and warn. The Acell pumps shots are good. I made sure I tested them when I rebuilt the carbs. I use the four gage setup for sinc. Son-in-law's Shadow climbs to 3 k on start, don't mean it's right.

Left side noise may be normal for the bike. ( I'm comparing it to my XS650, almost straight stock. The Honda is quieter that the YAM and I seems to hear everything.) I may just need to get used to it and put some miles on it.
Thanks Much,
Thu
 
3000 rpm is way to high, definitely needs to be taken down. You may need to do some jetting or try adjusting idle mixture. With the accelerator pump working it should be rideable after a couple minutes of idle time. There should be no different type of noise on the left. Only thing on that side is the starter clutch. Unless you have an issue with the starter clutch or the starter staying engaged there shouldnt be any different noise on that side.
 
Dirtdigger,
I'm hard of hearing anyway. Way back in 73 my Z1 had a whine that was common to them. This sounds kinda like it. Engine is solid, starter not running or staying engaged. Yep, clutch rattles at idle in neutral. Rattles less if you pull in the clutch. Common on these from what I'm reading. I know the history of the bike. It has less than 8K miles on it.
Jetting/idle mix is set up by the book. ( The valve clearances blew me away!) Plugs have a good burn, carbs synced, pressure at rear of pipes feel equal.
Weather is going to be hitting the 80's soon. I'll see how she goes then.
Thanks
Thu
 
I rebuilt my 78 4K about this time last year. I've got about 700 miles on it since then. I see the rubber carb mounting inlet manifolds already have hair line cracks in them. None seem to be leaking as idle is solid and steady. Do we have a better source for these not so prone to cracking?
Thanks,
Thu
 
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