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Iv'e recently bought a Monza 160 and although the engine has allegedly been rebuilt, it smokes a lot soon after starting. I have had the top end and barrel off and the rings seem to be new, there is no visible wear on the bore and the piston has no slack movement. The head was a bit coked up, so I gave that a clean and took a look at the valve seats, which seem fine with no sign of burning. The tappets are correctly adjusted to 0.06mm. Now I'm stuck! Anyone have any ideas where I should look next to see where the oil is getting in?
Thanks,
Nick
(Lamberhurst)

Nick from Kent wrote:Iv'e recently bought a Monza 160 and although the engine has allegedly been rebuilt, it smokes a lot soon after starting. I have had the top end and barrel off and the rings seem to be new, there is no visible wear on the bore and the piston has no slack movement. The head was a bit coked up, so I gave that a clean and took a look at the valve seats, which seem fine with no sign of burning. The tappets are correctly adjusted to 0.06mm. Now I'm stuck! Anyone have any ideas where I should look next to see where the oil is getting in?

Hello Nick from Kent,

Ok, let’s get this one started ...

It's normally best to try an initial diagnosis of a smoker with the engine still assembled . But seeing as we're past that stage:

There's really only two ways for oil to get into the combustion chamber: down from above, i.e. down thru the guides. Or up from below, i.e. passing the rings.

1. If it smokes AT start-up but then gets less, it's most likely guides (basically 'cos, while the engine was stationary, oil was seeping down the guides and collecting in the combustion chamber).

2. If it smokes AFTER start-up and continues blowing, it's most likely rings (basically 'cos the movement of the piston is continuously pulling oil up the bore and into the combustion chamber).

The fact that the rings LOOK new means nothing. They need to be the right size, be fitting properly in the piston grooves, and be correctly gapped. These are best checked by someone with the necessary equipment, knowledge and experience.

As for the piston being slack or not ... well, you can really only ascertain that by measuring the clearance between the bore and the piston (as well as checking for any ovaling in the bore).

The condition of the valve seats tells you nothing, I’m afraid. But:

1. You could try wiggling the valves in their guides - the specified maximum clearance is .057mm.

2. There should be an oil seal on the top of the guides. They should have been replaced; but even then, they can get damaged during an engine re-build.

HTH, and over to others!

Craig

Many thanks Craig, what you say basically confirms my suspicions, and it looks like I'm going to need some professional help now! Any suggestions for singles specialists in the South east? I don't have the necessary tools or experience.
Cheers
Nick

A friend with Mach1 had a mystery smoke syndrome, which was found to be caused by fuel in the sump. Turn off those petcocks, folks.

Jordan

Nick from Kent wrote:Many thanks Craig, what you say basically confirms my suspicions, and it looks like I'm going to need some professional help now! Any suggestions for singles specialists in the South east? I don't have the necessary tools or experience.

Hi Nick,

My first port of call would be Pietro at Motori di Marino (in Sussex, West Chiltington).

http://www.dimarino.co.uk/

(If you do go over, can you please ask him for the frame and engine numbers of the Darmah SS he's selling? I keep a little register, you see ...).

Otherwise, I know a few private individuals (including a couple on here ) who might be able to help, if they have the time/inclination. I'll pm you.

Ciao

Craig

Jordan wrote:A friend with Mach1 had a mystery smoke syndrome, which was found to be caused by fuel in the sump. Turn off those petcocks, folks.

Yep, that will do it too ...

Thanks again Craig, I will start with trying Pietro as you suggest.
Not fuel in sump BTW. I drained and refilled the oil and always turn off the petcocks. Judging by the previous comments, it looks like it may be rings and/or bore problems. I'll update when I know more.

Nick

Ventodue wrote:

Nick from Kent wrote:Many thanks Craig, what you say basically confirms my suspicions, and it looks like I'm going to need some professional help now! Any suggestions for singles specialists in the South east? I don't have the necessary tools or experience.

Hi Nick,

My first port of call would be Pietro at Motori di Marino (in Sussex, West Chiltington).

http://www.dimarino.co.uk/

(If you do go over, can you please ask him for the frame and engine numbers of the Darmah SS he's selling? I keep a little register, you see ...).

Otherwise, I know a few private individuals (including a couple on here ) who might be able to help, if they have the time/inclination. I'll pm you.

Ciao

Craig

Just got my head back from Pietro, and what a fantastic job! Only charged me half the price he quoted too. His show room is an Aladin's cave of classic Italian bikes, it took me ages to tear myself away! Thanks for the pointer, Nick.

But what of the bore and rings? You said they were suspect?

Nick,

One point also worth noting is that even with a rebore and new pistons and rings, a motor can still use excessive oil if the it isn't run in properly. Running in isn't all about treating the motor with kid gloves.

Initially when riding use the motor for lots of engine braking. The throttle closed causes a high vacuum in the combustion chamber and oil is sucked up past the imperfectly bedded in rings. This ensues that the top ring gets plenty of oil.

After 100km or so, if the motor seems to be running ok, you should heavily load the motor for short periods. This should be done when the motor is at normal running temperature. By heavy loading I mean going up a steady climb but allowing the motor to stay in a higher gear than you would normally use. Slog the motor at about 3000 to 4000 rpm. The effect of this is to pressurize the rings and force them into heavier contact with the cylinder wall. A very good match of the ring to bores imperfections is thus made. If a motor is treated very lightly for 1000s of kilometres then there is the likelihood of the bore glazing. Once the bore is glazed no good seal can be obtained, and the engine will use excessive oil. To remove the glaze, the motor must be pulled down again and the bore re honed.

Cheers,

Stewart