Appowner

First of all, which cylinder is it? Valves should be checked after each run during break in and at least at the start and end of the flying season after that.

I don't think the timing is off, though it's possible. If it were I would expect issues in the other two cylinders as well. But it's worth checking and a lot easier than doing a valve job.

I suspect one of the valves is either damaged in a way you can't see or it has some crap holding it open a little. Backing the adjustment all the way off will help determine this. Doesn't take much to make a valve leak. Takes even less on our tiny RC engine valves. Can you tell which stroke it's on when it sucks air? That could help determine which valve.

Back the valve adjustment all the way out on the bad cylinder and see if you have compression. If it still sucks, you may want to send it in for servicing. You can't simply pull the head off a Saito as it's a machined part of the cylinder. And because of that working on the head can be a real PIA. If you're handy with model engines you can give it a try. But if not, send it in.

There have been issues in the past with the Saito 3 cylinder radials. Cylinders running at different mixtures ultimately leading to one running either too rich or too lean. Supposedly they have corrected all this and there are a couple of shops out there with fixes as well. On the up side I don't recall hearing of any of these problems with the 120 R3. It's mostly with the one I have, the 33 R3. But it's worth taking note.

If you don't have an IR/Laser Temp meter, get one! Check all 3 cylinders at the same time in the same place. You might be surprised at the range of temps you'll see.

Good luck with it. Hope it's a simple fix.