R90s rapping sound

[KNS]

Hi All
I have been on the form asking various questions about putting my R90s back on the road. I am happy to say it starts, idles and runs quite well, the plugs are consistent with each other and the right colour. The compression is 130 each side exactly so on the outside it should be great. I find it very low in power but I had not ridden it in a number of years so my memory might be off. The problem is that is is making a heavy rapping sound under medium load not necessarily loud under heavy load. Apart from taking out the engine is there any way to diagnose the sound?
Thanks and sorry for rambling on.
Ken

[Chuey]

Can you be more descriptive of the sound? Could it be an exhaust related sound? Is it from inside the engine?

A R90S shouldn't feel slow. They'll do a 13 second quarter mile. That's not fast by today's standards but it isn't slow.

I have heard talk of a "turkey gobble" sound coming from the crankcase breather. Could that be it?

Chuey

[Bamboo812]

Another possibility is pinging. Does it sound like rocks rattling in a can? If so, check your timing with a strobe. Too advanced causes pinging and would definitely rob you of power. You want the F mark to show up at full advance, around 3000 rpm.

[Major Softie]

Old gas could contribute to pinging and low power as well.

[Rebel]

How are the diaphragms in the carburetors?

[khittner1]

No diaphragms in slide-throat Del'Ortos, Reb.

[Major Softie]

No diaphragms in slide-throat Del'Ortos, Reb.

At least a B+ for effort though: it's certainly an explanation for power loss with Bings.

[ME 109]

Is there any beat box associated with the rapping?

(Just my 50 cent)

[Major Softie]

Is there any beat box associated with the rapping?

(Just my 50 cent)

[Rebel]

How are the diaphragms in the carburetors?

My bad, I forgot about those having the DelOrto's on them. I'll try to think of a more "Italian" explanation.