As most of you know, I’ve spent a great deal of time with these remarkable powerplants. I’ve come to the conclusion that, adhering to a few simple guidelines, one will find them as reliable and consistent as their 2 stroke kin.
I sure have.

One of the most prevalent complaints I hear is lack of consistency – difficult/impossible to get smooth repeatable runs.

Through a substantial amount of empirical study, I’ve come to the conclusion that there are really two gremlins that affect consistency:
   Non-uniform induction and heat.
I believe that the latter is too often ignored and can cause some very aggravating situations.

Most of us who fly stunt like the appearance of a neatly cowled, nearly hidden engine to complement the lines of our planes.

Now let’s think. Yes, we like to have a nice air intake to direct as much cooling air as possible toward the engine fins. And, of course, that air has to get out, so we create some nice outlets for that function. So, we have a nice smooth pass for air to come along and remove engine heat. BUT-----------------------are we sure that’s what’s really happening?

Here’s my take on it: the engine heats the incoming air. I’m sure that due to the turbulent nature if the flow, it will randomly swirl around within the cowl. This warm air will, over some period of time heat various other parts of the engine – like the induction tract.  Furthermore, the engine will breath air that is getting warmer as the flight progresses. As the air warms, the density decreases and the mixture changes.
I have in fact proved this simply by removing the cowl after an unacceptable run – sure enough, the run improved greatly.
To insure that the intake air temperature undergoes a minimal (temperature) change, I now insure that all my cowled 4-stroke engines breath air from outside of the cowl - this is done by incorporating a simple aluminum tube that takes air from the top of the fuselage and sends it directly into the carburetor air intake.

Regarding non-uniform induction, this seems to vary with different engines but all that I’ve seen has been on the larger sizes - .56 and up and mostly the SAITO engines. Even thought the tolerances may be well within limits, there is sometimes a small gap between the carburetor barrel and the bore, resulting in air leakage.
Simple to fix – buy a venturi or apply RTV sealant at the external junction.

Bob Z.

Bob - that is a very good bit of info. 

Although rare, one will find an occasional air leak in the OS NVAs. 

Since the 4S engines seem more susceptible, any fixes that can eliminate leaks should be implemented.

Bob Z.

I agree 100%.   

Although I was not in favor of venturis in the past, I was just being too closed-minded.  I’ve come full-circle and I agree that the venturis can and will solve certain problems.

As stated, most of the people that I know who had problems with carbs had them on the larger engines, with which I had minimal experience – remember, I fly mostly small ones (.20 to .40) and I’ve had remarkably good results with the carbs. I have two .20s, three .26s, three .30s, and two .40s flying with carbs and running quite well. BUT, as I’ve stated, your results may vary.

Pat Johnson makes an excellent venturi – I’m not sure if he offers them for the smaller versions but, if problems arise with the carbs, I would suggest calling him to see what he has to offer. You won’t be disappointed. 

I’ve heard of other venturis but I have no names to refer to.

Bob Z.

Thanks, Ron - I'll pass that on to my 4 stroke group.

Here is the method I use for cowled engines.

Bob Z.