81 XS400 jumps/kicks on down shift?

[Phil_B]

Hello,

Just picked up a 81 XS400 in very good shape. Im very happy to have it.

Everything seems to be ship-shape. Except when I down shift, say from 3-2, the bike jumps forward. Im giving no throttle after the clutch goes in. So its throttle of + Back brake deceleration leading to the down shift, clutch in, shift down....and.....on releasing the clutch (no throttle) the bike jumps forward.

Is this a problem? On other bikes I've ridden (i'm a noob) a downshift procedure as describe above doesnt kick.

Any advice would be much appreciated...

 

[16VGTIDave]

Sounds like the throttle is not closing fully or the engine isn't dropping revs as quickly as it should. The latter is often caused by vacuum leaks or a lean idle mixture.

 

[Jeff0133]

....or an out of shape...kinked or sticky throttle cable

 

[Phil_B]

I spoke to a red seal bike mechanic. He suggested that the chain might need to be adjusted. He didnt see the bike, so who knows.

Does that sound right to anyone?

I mentioned the vacuum seal to him, and he said I would likely see other symptoms. Like problems with idling. Wich I dont have once the bike is warm.

I checked the clutch cable, seems to be nice and limber. No kinks, or pinches.

 

[Phil_B]

To add a bit more info.....

The Revs instantly jump up to 3-4k, when I release the clutch after a downshift. No throttle given.

And so the kick is some engine braking. But Im not giving any throttle.

Any thoughts or advice would be welcome.

 

[16VGTIDave]

Red Seal? Is that a beer? Whiskey? I only ask because a slack chain won't cause the engine to rev up or the bike to jump forwards. It will make the bike twitchy when transitioning from accelerating to decelerating and vice versa, that is it. But that is just my opinion, as a retired aircraft mechanic.

The engine revs will increase on a downshift, as the engine is being used to slow the bike. Just as the engine RPM decreases on an upshift. A slow downshift will exacerbate this. But the bike shouldn't jump forwards. Are you sure you aren't confused by the sudden deceleration caused by the engine braking (and resulting increased engine RPM) throwing you forwards?

Have you been licensed long? Taken a rider training course?

 

[Phil_B]

Experienced mechanic for sure. But he probably assumed I was not a total idot, and took it for granted that I knew how to downshift. I didn't mention the revving to him, just as I neglected to mention in my first post.

Starting to think I might be the problem.

Taking a training course now. Unlicensed but learning. The bikes we ride during the course don't seem to engine brake as hard or noticeably as my XS. But then perhaps I'm just going faster with my bike.

Okay, so that deceleration is exactly what I have been referring to as Jumping/kicking. In my inexperience I have just not understood how downshifting would afect the bike.

I thought the engine wouldn't break as long as I didn't use the throttle.....