Pinking.

[Jimmyjames]

My early 225 mugelo had comp ratio too high , ran great to start with then the heat used to just grow and the pinking would start , I reduced the ratio and its still running 12 years later same piston and no replates .

How did you reduce the ratio?

[Jimmyjames]

My early 225 mugelo had comp ratio too high , ran great to start with then the heat used to just grow and the pinking would start , I reduced the ratio and its still running 12 years later same piston and no replates .

How did you reduce the ratio?

My early 225 mugelo had comp ratio too high , ran great to start with then the heat used to just grow and the pinking would start , I reduced the ratio and its still running 12 years later same piston and no replates .

[Adam_Winstone]

JimmyJames,

This is not a quick or easy one to cover but you might be able to find info already on-line.

If you've not stripped the top end down to measure the volume of your existing combustion chamber then this might not be the issue. If you were to strip it and find that the volume of the combustion chamber is too small then you would need to increase the volume. Whether you did this by modifying your existing head or buying a new, lower compression head, would be up to you. If you choose to increase the volume of the head yourself then you would need to measure accurately, work out how much metal you need to remove to get the compression ratio that you're after, then modify / measure /modify / measure until you're happy.

When it comes to removing material from the head to increase volume (reduce compression) then you need to decide where it is coming off from. You can deeper or wider, or a combination of both. NB: Going wider means eating into the squish band area and making the function of the squish band less effective. You would approach an early Mugello side-squish in a different manner to the way that you would approach a later central-squish Mugello head.

I have modified lots of heads, however, I wouldn't race to do it if you don't have experience of it as you could quite easily bugger it up. When increasing volume of the combustion chamber I choose to use a dremel with the quick clip (something like that!) head and cutting discs. The cutting discs do a good job of allowing you to keep a nice rounded 'bowl' shape to the combustion chamber.

If you're going away on this bike and don't have time to address it, you might want to consider retarding the ignition a little more as this will keep the motor a little cooler and less likely to hole a piston.

Sorry for skipping over this quickly but that's all I have time for at the mo'.

Best of luck with it.

Adam

[paul213]

I reduced the comp ratio by dremeling out the head until the combustion chamber was the correct volume then smoothed it out with a flap wheel . mine ended up at
9.3:1 geometric and 6.5 : 1 corrected or very close

[Jimmyjames]

Thanks will see how we get on.