smoke out exhaust on decel

Hi everyone, I have a pretty good idea what it is, but I figured I should ask for some un-biased opinions!

On my 1978 kz650 SR, I have some smoke out of my exhaust on heavy decelleration, only after the bike is nice and warm.

First, the bike runs excellent! Has even compression around 165 each cylinder, all valve clearances in spec, and has been running great for a while now, this problem though has been getting progressively worse.


It only smokes out a white-ish MAYBE slightly blue-ish smoke on hard decel (i love riding out the gears and use engine braking a lot). For example, after a long highway run recently, coming off of the exit downshifting, I can see some smoke coming out of the pipe.

It does not smoke on start up, or acceleration, or if I pull in the clutch and glide to a stop.

Any suggestions on what it might be??

Thanks everyone!

valve guides/seals

PLUMMEN wrote:

valve guides/seals

x2

PLUMMEN wrote:

valve guides/seals

No way, oil never leaks past those... :laugh:

Yes, very well, or worse an oil control ring issue.

Either way, if its dropping to the idle circuit and its too rich the smoke would deffinately be black/dark. Light, blueish can only be oil burning.

Now, question is, how much oil is it burning?

May not really be an issue if its all burning off, and no fouling of plugs.

Then again, it does specifically say in the KZ Manual not to downshift the bike higher than 3500 or so rpm, as this causes undue stress on the engine components. But where's the fun in that! :laugh:

Good luck.

TeK9iNe wrote:

PLUMMEN wrote:

valve guides/seals

Then again, it does specifically say in the KZ Manual not to downshift the bike higher than 3500 or so rpm, as this causes undue stress on the engine components. But where's the fun in that! :laugh:

I couldn't agree more!

The valve guides/seals was my thought as well. Are valve guides difficult to do? I've replaced the seals on my kz1000 before. What is involved in doing the guides? Does it involve machine work?

Would I be crazy not to do the guides while I had the bike apart to do the seals?

It all burns off it looks like, and it doesn't hurt performance or anything at all. The bike is running so good right now, it's a shame to tear apart the head. It really doesn't lose oil either. So it's a pretty annoying problem.

flowbenchtech.com/porting/CRH
rings would make it smoke when accelerating ,not decelerating usually

freebyrd24 wrote:

...1978 kz650 SR, I have some smoke out of my exhaust on heavy decelleration, only after the bike is nice and warm.

First, the bike runs excellent! Has even compression around 165 each cylinder, all valve clearances in spec, and has been running great for a while now, this problem though has been getting progressively worse.


It only smokes out a white-ish MAYBE slightly blue-ish smoke on hard decel (i love riding out the gears and use engine braking a lot). For example, after a long highway run recently, coming off of the exit downshifting, I can see some smoke coming out of the pipe.

It does not smoke on start up, or acceleration, or if I pull in the clutch and glide to a stop.

Any suggestions on what it might be??

Thanks everyone!

does not smoke on start up, or acceleration

Worn valve stem seals typically result in exhaust smoke being worse immediately after cold start-up, and smoke much less as engine warms up (and seals get softer).

Perhaps an oil ring issue.

Would also check for oil freshness and correct level.

And sniff test for any gasoline odor in crankcase oil, and examine physical condition of crankcase oil.

Fuel intrusion into crankcase might cause the reported smoking.

But am uncertain why doesn't also smoke on acceleration.

Good Fortune!

Is your exhaust a 4 into 1? These draw oil past the rings on hard deceleration. If thats the case put me down for the rings.

OK, I just reread, you said "pipe" not "pipes". Pull the cylinders, it's the rings.

maybe just find another head to have done up to minimize down time

Valve seals have failed so many times in my 750 twin I blame them if my dog passes gas (regardless of the exhaust color).

The rings could also be marginal but I would think you could see some smoke on hard acceleration if they were.

A funky oil ring might leave it with good compression but still pass oil?

I do have a 4-1 exhaust. I really hope it's not the rings. What I really don't want is to tear down the head and fix it, just to have the rings still be an issue if that is the real culprit. Is there any way to check this?

At this point, I'm just tempted to run it until something bigger happens...