Lurch when starting

[feliz]

My 75B has a distinct lurch when starting, my friend told me it's called "chain snatch" but I'm not sure it's the chain although it feels like it. Chain and clutch are adjusted properly and in good shape along with the sprockets. Everything was new at the frame up restoration 3,000 miles ago. It's worse when warmed up and sometimes is accompanied by a shudder. I had a CB750 that did exactly the same thing once but sold it before I rectified the problem. The bike runs fine otherwise but I'm concerned something may snap one of these times. Thanks for any insight.

 

[Classic]

Does this occur when using electric start, kick, or both?

 

[feliz]

Well, I may not have explained it properly. It happens when releasing the clutch and starting to move away, not when starting the motor. It's when immediately moving away from a stop. Sorry if I didn't explain it correctly.

 

[5twins]

I would examine your clutch assembly, in particular, the cush springs on the back of the outer basket. The 650 uses these instead of rubber mounts for the rear sprocket to dampen the clutch engagement. The springs can and do break. If you do find broken springs, I would buy a whole used clutch assembly off of eBay. They can usually be found for $20 to $30 which is cheaper than the cush spring repair kit that's available. I would also advise you to get the later '80 or newer clutch. It has a spring-loaded bottom plate that damps engagement and is easier on those cush springs.

 

[sreimer]

Fever up n pop the clutch. Plates need to be seated to each other

 

[XSLeo]

On my 75 it lurched like that.
As you engage the clutch the springs 5twins talked about compress, then as the bike starts to move they extend. This lurch is most noticable when taking off, once under way you won't feel it but it still does it just not as much.
The later clutches they changed the clutch hub to incorporate a spring washer under the first steel plate. They removed one fiber and steel plate to make room for this spring washer. This smoothed out the engagement so you don't feel the lurch.
I have the clutch from an 82 on my 75 now, smoother at take off. Other than that it feels the same as the 75 clutch.
Leo

 

[feliz]

Thanks to all for your responses. I'll start watching ebay for a newer clutch. Is it just a matter of taking out the old pre 80 clutch and installing the later one or are other mods necessary? Thanks again.

 

[XSLeo]

Yes, a straight up swap. The hub and the nut and lock method of the nut is the major difference between the two. All the washers, thrust bearing, basket, plates, springs, pressure plate, are the same. So you could just pull out the pressur plate, clutch plates and the hub, then just install the later hub and clutch plates and such. Without disturbing the basket and the rest of it.
I would take it all apart to inspect the parts and use the best out of both sets when reassembling.
Some reccomend replacing the stock thrust bearing with the new one Mike's sells. I found it didn't help on mine. Maybe my thrust bearing was still good.
When reassembling just follow the book. The book does have a mistake in it. On the thrust bearing there is a washer on each side of it, the book only shows one.
On the pressure plate be sure the hole lines up with the dot on the hub.
One good mod to do is drill out the stock spring screws and putting allen heads in.
I did, works well. Just be sure the bolts don't stick out past flush with the back of the hub when installed. My bolts stuck out just a bit and cut a shallow groove in the basket. Maybe .5 to 1 mm deep.
Leo

 

[5twins]

Use 40mm long Allens, not 45s. The 45s will stick out the back of the hub a few MM. After drilling the old screws, grind the tops of them flat. That will allow the 40mm screw to go in another MM or two and end up nearly flush with the back of the hub like the originals.