Tappet Clearances

[Strada-Stu]

Hello All,

I got round to doing some winter maintenance on my '75 Strada today.

The tappet clearances on the rear cylinder were easy enough to adjust but I just couldn't get the clearances right on the front cylinder. With the flywheel mark in the right place, I adjusted them to .1mm and then rotated the engine through one revolution and found they were miles out. After a further revolution they were spot on again.

I confess I'm more familiar with two-stroke engines than four-strokes. Can anyone indicate why the above might be happening and what the fix might be?

Thanks

Strada Stu

[RHThomas]

Hello Strada Stu,

Sounds like you may have adjusted them at TDC on the wrong stroke....

As the camshaft rotates at half the speed of the crank, the TDC mark on the flywheel
corresponds to the piston being TDC on both the compression stroke and the exhaust stroke.

The tappets must be set at TDC on the compression stroke when both valves are closed.
You can nomally feel this by grasping the rockers and they should jiggle up and down a little bit.

At TDC on the exhaust stroke one or both valves will be partially open, where there should be no clearence as the valves are being pushed open. Were the clearances tight when you adjusted them?

I would suspect TDC on the compression stroke is where your clearences are now miles out.
Try setting the clearances with the engine in this position.

This is where 2 & 4 strokes differ the most....

Cheers,
Robin

[Strada-Stu]

Thanks Robin for the clear and gentle explanation. I've obviously been spending too much time in the office or fiddling with two-strokes and had forgotten basic four-stroke theory.

Armed with your explanation, I now realise that luckily I had adjusted the clearances at TDC on each cylinder's respective compression stroke so I'm all set and the bike is running nicely.

Thanks again,

Stu