TOPIC:

Hello, just to give you a bit of background I own a couple GT750's which are really just shaftie kz750's, on my bike I've rebuilt the engine done cam chains/starter clutch and rings and valve lapping etc so pretty ok which this motor 1985.

I bought a nearly identical bike as mine as a project but just a few years older (later 1983)and a few more kms on the motor (about 70k) and it has always been smokey on deceleration but thought no bother she'll be the valve stem oil seals like any other motor when they go hard with age so didn't think any more about it as I planned to refresh the top end anyway.

Recently finished doing the top end by doing the rings and valves, the pistons and bores were just within the wear tolerances noted in the Haynes manual so just did the rings themselves and did the oil stem seals and lapped the valves until I had blisters which is usually how I know I've done them well enough

Anyway just taken it for a quick run with a mate following me after running it in for a few hundred kms and he said it was still smoking on decelerationgrey/blue

Bike pulls like a train across the rev range even though I'm on pod filters and it has great even compression across all 4 cyls

Its defo oil being drawn into the combustion chamber but how???

Uses a bit of oil and the plugs look a bit carboned up , not oily really more sooted appearance although this could be the carbs that just haven't been set up properly yet.

I have googled and I saw a couple of posts saying that 4 into 1 exhausts can cause this but not sure if I buy into that as the exhaust is nearly straight though.

Any ideas or suggestions would be welcomed.

Cheers

Rich

Did you hone the cylinder walls prior to putting in the new rings? Sounds like you may be running rich. If you put in a fresh set of plugs, go out for a 5 to 7 mile run and pull the plugs are they black and sooty?

Hi mate thanks for the reply, yes I did hone/de glaze the bores with a nice even cross hatching.

I could very well be running rich as I've yet to check float heights after re building the carbs after fitting a set of carb kits, I kinda just forgot to do that!

Not sure if that would cause oil to enter the combustion chamber though, the oil level has gone from full to low in about 500 miles.

I've got a gunsen colortune 14mm on order so will double check the burn when that turns up.

Cheers

Rich

Smoking on deceleration = vacuum in the combustion camber = sucking oil over valve stem guides/valve seals.
A fundamental error is installing new valve seals without protection of the thin valve sealing lip.
You have to use a plastic tube (condom) to protect the valve sealing lip during the install.
The valve keeper key groove can damage the very small sealing lip easily during the install without the tube.

Hey thanks for that, no never used a condom which has proved costly as a younger man and perhaps is costing me again

Just dipped the seals in oil and lubed up the shaft and slipped over carefully, could have damaged I guess.

Is there any test I can do without having to tear down again?

Cheers

Rich

You said: "Its defo oil being drawn into the combustion chamber but how???"

I missed the part about the large oil consumption, my bad. I read black and sooty plugs, not oily plugs which is why my thinking went to rich AFR. With you, has nothing to do with oil consumption.

Color tune will definitely help you dial it in and a vacuum sync as well. Also has nothing to do with oil consumption but it will run and start better.

Yes the seal lip can be damaged and not must be with a lot of oil during the install.
It is dependent of the engine mileage,, the oil returning rings on the piston can be end of life too and let pass the oil on deceleration.

Did the colortune work for you