I was wondering whether it is normal that a fourstroke clearly loses power where it is needed most. Overheads, top loop of the vertical eigth, hourglass...
Laptimes are a steady 5.2 secs. Max line length. 14-6 rev-up prop. 10% nitro fuel.
Could it be too lean a mixture? Any other thougths?
Thanks for sharing your experiences

Quote from: Paul Van Dort on September 29, 2026, 07:57:04 AMI was wondering whether it is normal that a fourstroke clearly loses power where it is needed most. Overheads, top loop of the vertical eigth, hourglass...
Laptimes are a steady 5.2 secs. Max line length. 14-6 rev-up prop. 10% nitro fuel.
Could it be too lean a mixture? Any other thougths?
Thanks for sharing your experiences

I don't have a ton of experience with FS's in general and none with the Saito 72, but I run a Saito 40 and OS 26 Surpass and I don't notice loss of power in maneuvers. Quite the contrary. Constant power until the tank goes empty. And not just me. I have seen another Saito 40 and an OS 40 Surpass in stunt applications and not seen them go lean in maneuvers.

Maybe try opening the needle a bit as a test. Also, not sure if you are using the stock carb or a venturi but if there is any air leakage going on it might result in the lean direction.

And I also have been running 15 nitro on my FS's.

On my Saito 40 (stock carb wide open) I find peak rpm on the needle then richen about 300 rpm. Same for the OS 26 Surpass.

Colin

Quote from: Paul Van Dort on September 29, 2026, 07:57:04 AMI was wondering whether it is normal that a fourstroke clearly loses power where it is needed most. Overheads, top loop of the vertical eigth, hourglass...
Laptimes are a steady 5.2 secs. Max line length. 14-6 rev-up prop. 10% nitro fuel.
Could it be too lean a mixture? Any other thougths?
Thanks for sharing your experiences

   What kind of tank?

    Brett

Quote from: Brett Buck on September 29, 2026, 07:37:09 PMWhat kind of tank?

    Brett

   And what is the venturi set up?? Or are you using the R/C carb locked down?

  Type at you later,
  Dan McEntee

Quote from: Brett Buck on September 29, 2026, 07:37:09 PMWhat kind of tank?

    Brett

It is a metal uniflow tank with fixed uniflow and clunk for the feedpipe. Uniflow tube is adjustable between full uniflow and standard. Today in full uniflow configuration. Steady run from start to finish. Except for the sagging .

Quote from: Dan McEntee on September 29, 2026, 11:12:14 PMAnd what is the venturi set up?? Or are you using the R/C carb locked down?

  Type at you later,
  Dan McEntee

I modified the R/C carb. It is fully open,The idle pivot is removed and finished with epoxy so that the venturi is smooth. There is a 5,2 mm restrictor in front of the carb. Adjustable via inserts.

   I would try changing to the standard vent, and then reducing the venturi. In my pursuit to find a good four stroke run with my Saito .56, I discovered that it doesn't like uniflow. It didn't like a normal vent to atmosphere either. Try a couple fop flights with it on standard muffler pressure. You might see a slight increase in RPM  for the last part of the tank, but very manageable in my opinion. Four strokes are all about fuel draw, and for some reason the uniflow interferes with that.
   Your venturi opening doesn't sound too large but  if there is no spray bar through the bore, and is more like a true venturi, it may still be a bit big. I have an after market venturi on mine with a spray bar through the venturi and the opening is about 6mm or less, and I installed a nylon screw that I can turn in or out as temperature changes to make the remaining opening bigger or smaller.. You probably don't have room for that, so some sort of slip in insert to take it down about another millimeter or more will get you there if running on muffler pressure to a standard vent doesn't changes things. I think you are close, just getting a little lean.
  Keep us posted, and type at you later,
  Dan McEntee

Quote from: Paul Van Dort on September 29, 2026, 11:27:56 PMIt is a metal uniflow tank with fixed uniflow and clunk for the feedpipe. Uniflow tube is adjustable between full uniflow and standard. Today in full uniflow configuration. Steady run from start to finish. Except for the sagging .

   Get a standard plastic clunk tank, set it up for suction.  Jim Aron had a very similar issue at the 2002 NATs, the solution was to get a standard Sullivan SS-4 clunk tank and set it up exactly like it shows. This was a suggestion by Brad "Godzilla" Walker. Jim was having to fly the airplane at 4.8 second laps just to have enough tension to get through overheads and high maneuvers, and it sounded like it was sagging - but it sounds the same rich and lean. $6 and 45 minutes later, 5.4 sec/lap with should-dislocating line tension everwhere.

  Brett

Thanks for all the feedback and suggestions. Experiments continue.

Quote from: Brett Buck on September 30, 2026, 06:39:19 PMGet a standard plastic clunk tank, set it up for suction.  Jim Aron had a very similar issue at the 2002 NATs, the solution was to get a standard Sullivan SS-4 clunk tank and set it up exactly like it shows. This was a suggestion by Brad "Godzilla" Walker. Jim was having to fly the airplane at 4.8 second laps just to have enough tension to get through overheads and high maneuvers, and it sounded like it was sagging - but it sounds the same rich and lean. $6 and 45 minutes later, 5.4 sec/lap with should-dislocating line tension everwhere.

  Brett

    This is pretty much what I did after finding Bob Reeves' set up in the four stroke column, and that may have what guided me to the stuff written by Brad Walker. Changing the tank made the biggest difference, and the adjusting screw through the venturi lets you find tune things.

  Type at you later,
   Dan McEntee

Quote from: Dan McEntee on October 01, 2026, 08:32:22 AMThis is pretty much what I did after finding Bob Reeves' set up in the four stroke column, and that may have what guided me to the stuff written by Brad Walker. Changing the tank made the biggest difference, and the adjusting screw through the venturi lets you find tune things.

   The adjustable-choke venturi was a great idea, too. The Berringers were the first to use it, and since they were machinists, made them from scratch. Bob's breakthrough was figuring out how to replicate the same idea with parts on hand and simple hand work.

    But the tank thing was BY FAR the most impressive single step I ever saw with 4-strokes, it was completely transformational.  It was also, unsurprisingly, one of the areas where the 4-stroke "promoters" gave out (and probably still do) *completely erroneous* information about how to set them up, along the lines of "traditional stunt values" (AKA bullshit).

   The only further development before 4-strokes fell off the radar was the extremely low RPM approach first used (to my knowledge) by Igor Panchenko. Bob Reeves did a great service putting together all the different pieces and putting them together in a credible way. We lost something when he left us!

    Brett

In my experience with 4S, when it lost a little power going up, the glow plug needed to be changed. The OS F did not last very long, as compared to glow plugs in 2-stroke engines.

So I suggest you try a new glow plug.

Martin

definitely using Bradly tank. Power master 20/20 YS fuel and a wood prop. I used 14x5,6,7 and 8 depending on the climate: cold/windy 14x5 (less pitch)..... muggy and hot 14x8 (more pitch).

Never felt the need to have an adjustable air intake. My OS 70 Ultimate would never change performance in a wide range of density altitudes. Same fuel amount, same needle position. I only changed my props depending on the climate conditions.

Quote from: Martin Quartim on October 02, 2026, 08:41:13 PMIn my experience with 4S, when it lost a little power going up, the glow plug needed to be changed. The OS F did not last very long, as compared to glow plugs in 2-stroke engines.

So I suggest you try a new glow plug.

Martin

Great input. Thanks. About 100 flights with the same plug. I will
try with a new one.

Quote from: Martin Quartim on October 02, 2026, 09:28:16 PMdefinitely using Bradly tank.

I don't understand the advantage of a standard venting over uniflow wrt to 4S engines. The only thing uniflow adds is a constant pressure at the venturi over the duration of the flight.
I am a bit stubborn (as many here on the forum :-) ). I hate to implement dogmas without proper understanding of the cause/ consequences, of the physics involved.   

Quote from: Paul Van Dort on October 03, 2026, 02:44:25 AMI don't understand the advantage of a standard venting over uniflow wrt to 4S engines. The only thing uniflow adds is a constant pressure at the venturi over the duration of the flight.
I am a bit stubborn (as many here on the forum :-) ). I hate to implement dogmas without proper understanding of the cause/ consequences, of the physics involved.   

I'm with you. No disrespect to the experts out there but I've only really heard that FS's don't like uniflow. But no good technical explanation of why.

The only 2 FS's I have run use Brodak metal wedge uniflow tanks. No clunks. And ram air pressure to the tank. Engines (OS 26 Surpass and Saito FA 40) have run quite well.

One thing I forgot to mention is that both of my models are profiles. Maybe that has something to do with my success.

Also, I have not modified the engines in any way. Stock carb run wide open. But I do have mine set up with a manual throttle control. I can start the engines at say 30% throttle for a brief warmup, then go to full throttle wide open for launch. Probably not really necessary on these smaller FS's, but that's what I do.

Colin

Quote from: Paul Van Dort on October 03, 2026, 02:44:25 AMI don't understand the advantage of a standard venting over uniflow wrt to 4S engines. The only thing uniflow adds is a constant pressure at the venturi over the duration of the flight.

   And reduce the feed pressure compared to suction.

   I don't understand it either, that doesn't mean it does not work. If it doesn't work, you can put it back.

QuoteI am a bit stubborn (as many here on the forum :-) ). I hate to implement dogmas without proper understanding of the cause/ consequences, of the physics involved.   

   Many people are prone to crippling themselves with that sort of reasoning - and of course your brand of dogma ("don't try anything unless I understand how it works") is just another variant of the same disease.

    But of course, as a known purveyor of baseless dogma, that's what you would expect me to say.

     I had to learn my lesson the hard way, too. Knowing why helps you figure out the next thing to do, and is important. But if you use that as a justification to limit what you do, you are going to really miss out and not be able to take advantage of everyone else's knowledge and experience.

   I have no idea if a plastic clunk tank is the solution to your problem or not - it sounds like identical symptoms to those we have seen before and had a solution to. But never trying it because you can't fully explain the mechanism is shooting yourself in the foot.

    Brett

   

When I was testing the Saito 40 in my Pathfinder, I tried a Brodak MW uniflow tank, and the engine sounded like it was working in 4-2-4! revving up a lot more going up. I don't know why I did only one flight with it and never made a video with this setup. I can't remember for sure, but I think it worked well.

Quote from: Colin McRae on October 03, 2026, 11:15:17 AMI'm with you. No disrespect to the experts out there but I've only really heard that FS's don't like uniflow. But no good technical explanation of why.

The only 2 FS's I have run use Brodak metal wedge uniflow tanks. No clunks. And ram air pressure to the tank. Engines (OS 26 Surpass and Saito FA 40) have run quite well.

One thing I forgot to mention is that both of my models are profiles. Maybe that has something to do with my success.

Also, I have not modified the engines in any way. Stock carb run wide open. But I do have mine set up with a manual throttle control. I can start the engines at say 30% throttle for a brief warmup, then go to full throttle wide open for launch. Probably not really necessary on these smaller FS's, but that's what I do.

Colin

   

   Well, I was experiencing the same problems that Brett has described and chased my tail for a coup0le of flying seasons, and it was either so fast it screwed me into the ground, or if I opened the needle to slow it down I couldn't get the pattern in using a 4 ounce tank. I finally read up on the Brad Walker set up, and installed a new RST 4 ounce tank set up like the instructions, and it worked right from the start.  Then I found Bob Reeves writings and added the screw to the UHP venturi, and that let me fine tune things. I always used a 4 ounce fuel load. I set everything in warmer weather to run about 7 minutes using Bob Reeves low RPM set up that maxes out at about 8000 RPM or a bit less. When the weather got cooler, I just turned in the screw somewhere between 3/4 and 1 turn of the 13-32 screw until I got the same run time as in warm weather. It was very repeatable. Fuel is usually YS-20/20, and prop is a 13"-6" or 7". I have read in a lot of places through all of this, that four strokes are all about fuel draw. Use as short a fuel feed line as possible, and the higher nitro lets you open the needle for more fuel, and the venturi diameters are usually 6mm or smaller to help with fuel draw They have that extra cycle that does not produce power and does not draw fuel. I think using a uniflow breaks up any negative pressure in the fuel line that should be drawing fuel, or maybe even crank case vacuum is blead off too easily??? Just a guess. A standard vent on muffler pressure, I think, will build up more positive pressure in the tank. Again , just a guess. I just know that I have tried it on a couple of engines and it works. You can get a tiny speed up at about or after the hour glass until the tank runs out, but it's very manageable. In some weather conditions I don't notice a thing. As the saying goes, you mileage may vary.

  Type at you later,
  Dan McEntee

Paul,
I used to run large Saito 4-strokes (FA-56, -62, -72) a long time ago.  I hope to come back to them soon; but for now I wanted to share my experience to help you out.
My initial runs used standard 6-7 oz uniflow metal wedge tanks left over from 2-stroke 60-size engines, and I was partially filling them to arrive at the desired run time.  The result was exactly as you described: sagging at the top of the hemisphere.
After reading a bunch of posts, I started noticing a pattern, where most issues came down to fuel draw.  Since 4-strokes intake only once for every 2 revolutions, but intake cycle lasts a long time, it is critical that there are no issues with fuel supply.  I think 4-strokes are much more sensitive to air-fuel mixture, and cannot tolerate either overly lean or overly rich settings.
My idea was that the reason for success with plastic clunk tanks was simply due to their proportions: they are are much shorter than typical stunt metal wedge tanks.  Since fluid pressure drop depends on the length of total fuel path (both rigid and flexible tubing) from the intake point inside the tank to the needle valve, the plastic tanks resulted in the shorter path, and helped "feed" the engines better.
With that observation, I abandoned the old 6-oz tanks, and custom made a uniflow tank that was just big enough for the flight (about 4 oz).  I arranged the tubing in such a way as to minimize the fuel path (see pictures).  I also created a very pronounced outboard cant, to guarantee good fuel head all the way to the end, regardless of airplane yaw, etc. The goal is to have no more than 110-120 mm from pickup tube to the needle valve.  Also no bends or loops in the tubing.
The result was consistent runs for the entire duration of the flight, just as expected of uniflow. 

One day, just out of curiosity, I decided to plug the uniflow vent, and converted the tank to standard ("suction") venting.  As expected, the engine got progressively leaner towards the end of the flight, making the last 1/3 of the pattern too fast to fly comfortably.

None of this negates the findings and experience of other people who have succeeded with plastic clunk tanks, suction venting, etc. I think this is yet another example where a technical problem might have more than one solution.
 
If you have questions about my approach, feel free to post here or PM me, I would be happy to help.
Happy flying!

Quote from: Dan McEntee on October 05, 2026, 10:05:28 PM[snip]
   maybe even crank case vacuum is blead off too easily??? Just a guess.
[snip]

  Type at you later,
  Dan McEntee

Dan, the 4-strokes intake through a head valve, not through the crankcase... I'm sure you know that.. 
Not trying to stir a debate; we're all trying to help Paul enjoy his 4-stroke runs, and I appreciate your input.

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