Here are a few pointers about 4-stroke engines that I have either personally experienced/discovered or learned through feedback on my 4S mailing list. As I’ve often stated, what works for me may not do so for others so use your best judgment.
When a 4S engine is mounted inverted, it is best to start it that way.
Why? A 2S engine has no exhaust facility in the head. When it’s inverted, any fuel in the head will stay there and can cause hydro-lock. When turned upright, the fuel can either drain out the exhaust port or back into the crankcase.
A 4S, on the other hand, has the valves in the head which, when inverted, is the lowest point of the engine.
After choking or priming, you should always turn the prop gently and if you feel it locking, just turn it back gently until the valve opens. This will force some fuel back up the intake tract and eventually out the exhaust – you’ll see fuel running out the muffler. Then, turn it gently back and forth until it will go over TDC without resistance.
Look at it this way – if a 4S engine is mounted upright, a slug of fuel can ride up and down on top of the piston and since the slug is below the valves, it will not clear out.
4S engines like to start wet but if too wet, they will run backwards AND, here’s something to be aware of:
If you are running muffler pressure and your pressure line is submerged (as in uniflow), there is a very good chance that the engine will continue to run backwards. In other words, the muffler becomes a carburetor and the carburetor becomes an exhaust. Now, your engine is running backwards, discharging exhaust gases into the engine compartment. Possible fire? You bet.
But, it’s simple to stop – just put your finger over the muffler outlet. Stops instantly.
As different fuel systems start and behave differently, you must experiment to find which routine works best for yours and stick with it. Go to an electric starter if it makes you feel more comfortable but remember to clear out any flooding.
I find that, for the most part, my 4S engines seem to run better on their sides – not only the run quality but also the fuel draw seems better. An engine that requires muffler pressure when inverted or upright may not when profile mounted.
Many people have asked me about oil. I NEVER run my engines on less than 22% oil. I know that many use 20/20 but it just makes me uncomfortable. Since a 4S engine derives all its lubrication from blowby, a combination of a tight ring fit and low oil content may spell disaster for your gem. So it smokes a bit more – no big deal. As I recall, the 4S failures I’ve witnessed were on (larger) engines running less than 20% lube. Once again, your results may vary.
For what it’s worth, the best all-around fuel I’ve found for both my 2 and 4S engines is the Brodak 10/23/50-50.
Props? My best results have been with APC and Master Airscrew. I know that a lot of people have used more exotic carbon props but I have no experience with them and therefore cannot comment.
I have found that lots of pitch is the ticket – I use mostly 6 and 7. Buy yourself a few of each and experiment to see what best suits your situation.
A little rule of thumb for 4S props that may help:
9 to 10 inch diameter for .20 to .30 engines
11 to 12 for .40 to .46
12 to 13 for .52 to .56
13 and above for .72 and larger
Regarding three-blade props, I’ve only run one on a 4S engine and that was a 12-6 on the SAITO .62 and it seemed to work just fine.
Finally, 4S engines seem to need lots more break in time than 2S. You may need two to three HOURS of running before realizing full potential.
Hope this helps,
Bob Z.
ps - Should anyone wish to be on my 4-stroke mailing list, just e-mail me and I'll make it happen.
<rzambelli@moog.com>
Topic: A few things that may help if you're considering a 4-Stroke engine (Read 1975 times)