Engine basics > Four strokes only

Saito 72 with OS throttle

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ash:
Hi guys...

I've just picked up a used Saito 72 Golden Knight fitted with an OS throttle. It was replaced after the original carb was damaged and no spare parts were available.

Do any of you have thoughts on the pros and cons of keeping this OS throttle and modifying it as per Bob Reeves' method, versus making a whole new venturi and needle valve assembly along the lines of perhaps the UHP or Beringer types.

I'm equipped to do either to the required precision without much trouble.

PS. That brass piece is just a tube pressed onto the carb as a duct for the model it was in. Will be coming off either way.

Robert Zambelli:
Hello, Ash.
You have a most interesting setup there.
I've been using four cycle engines for around 25 year but I've never heard of swapping a carburettor like that!
First off, are you sure that's an OS carb?
I have a SAITO 62 and the carb is nearly identical (?)
In any case, the SAITO 72 is a great engine. I've had a few and they are very powerful and reliable.
For your setup, I would suggest that you try it as is. If you plan on doing C/L and don't need the throttle, just wire it wide open and see how it performs. I have done that many times with flawless performance.
I did make a number of plain venturies for the 72 which worked quite well. I used a single taper with the vena contracta at .30 inside diameter. I used stock OS Max and Super Tiger NVAs, both yielding very good results.
Keep in mind that the four-cycle engines are very sensitive to air leaks - make sure everything is well sealed.
Also, check your valve tappet clearance - should be around .002.
In general, the four cycle engines run best with a 20/20 nitro/synthetic fuel and an OS 4-S plug.
The 72 will easily turn a 13-6 or 13-7 prop.

Use it on the biggest plane you have - it will easily handle things like the Patternmaster.
Remember, one test is worth a thousand expert opinions!

Bob Z.

Robert Zambelli:
I think you're right - it does appear to be an OS carb.
Tests should be very interesting.

Bob Z.

ash:
Thanks for the tips, Bob.

I'll give it a test run as is before deciding how to go. In fact I'll probably test fly it with the throttle fixed until I know what it wants in the prop and RPM department.

It'll be going into a rebuild of my old Zone 2 designed by Brad Walker for the Saito 72.

Robert Zambelli:
I forgot to mention.
When you have the rocker covers off, flood the valves, rockers and pushrods with oil.
Also, squirt some oil into the crankcase and shake it up well to insure that everything is well lubricated.
Too much oil won't hurt - it'll just blow out through the vent.

Bob Z.

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