4-stroke setup problem
 

Here's the setup:
-New OS FS95 4-stroke, mounted in an equally new H9 Sbach.
-Engine is mounted horizontally with carburetor about same height as fuel level.
-Engine has never been opened up to check valve timing.
-New Type F plug.
-Byron 4-stroke fuel
-Stock muffler, with pressure tap connected to tank

Engine starts easily at the default needle setting of 2.5 turns, and makes a reasonable amount of smoke.
After warming up, engine responds normally to throttle advance up to about 2/3, whereupon it sags and will die if throttle is not reduced.
Behavior is the same as needle setting is varied from very lean to very rich. Idle jet adjustment affects nothing but the idle.
Changing plug had no effect.
Pressure tests on fuel system showed no leaks or flow restrictions.

Most mysteriously, the sagging problem disappears almost completely if the pressure tap is disconnected, as if the muffler is providing too much pressure.

Suggestions?

-Engine is mounted horizontally with carburetor about same height as fuel level. Maybe your tank level to carb allows the engine to run OK with no pressure from anywhere ! Just a guess.............

(not knowing your level of experience) The factory needle position is just a starting point it sounds like you may need to lean it slightly with your setup.

Why not? Do it. This symptom can be many things, leave adjustments like this ruled out by checking them. Troubleshooting is methodically eliminating each system.
When you state the idle mixture screw has no effect but idle, that's not true. It handles the transition. What starting point did you use for that screw? IF you do change the position of the engine, hold it upright and vertical, does the engine behave differently?

No need to check the valve timing, it will be fine if never opened up. If the LS needle is set too lean it will make the HS needle ineffective. Make the LS needle one half turn richer and see if that helps I've had a .95v for about 6 months, great engine.

OK, that's interesting: thought it only affected the idle, which is OK. I had it opened up substantially at one point, but I may be changing too many variables at once. I'll check that out...thanks.

Sagging rpm tells me it's lean. As Dave suggested, Richen up the low end and I would Richen the high end a half turn or more as well and run it up.

A little back tracking - did you break the engine in to seat the ring or does the engine have at least an hours run time at a rich setting? If the engine is brand new and not broken in, it will sag when set for a normal flight mixture setting. They need to be set fairly fat until the ring seats and seals better. New rings don't seal well and as such need more fuel/oil to help the ring seal.

It's got something close to an hour on it, mostly rich...I'll try giving it some more.

Abe, it's best to peak the HS to full peak before attempting to set the LS needle, the LS needle control about 85% of the throttle range. The HS needle only controls full throttle and it's needle seat is fixed. Setting it first eliminates one of the variables you alluded to.

Dave,
Thanks for that clarification. I've been running O.S. 4-stokes for many years and have recently experienced this very same problem. It occured after changng out a crappy ARF tank on my AeroWorks Extra 260, running a well broken in O.S. 81a. I'm going through the very same "re-needling" to bring it's performance back to where it was. I went from a 15 oz. tank to an 8 oz. one and suspect that that may have caused the change. I've been following this thread with great interest. I'll let you know how mine develops after this Monday. Checked all the usual things, plug, fuel filter, tank and lines, valves. I'll check back in on Monday.
Karl

Trouble is, I can't *get* to full throttle to make that setting...the engine won't run above about 2/3. I'll have to experiment with LS settings until I can get there.

Will it run up to full throttle with the glow driver attached?

OK, think I have a handle on it now. I got a good run with the LS needle open one full turn, and the HS closed 1-1/2 turns, from the factory settings. Still needs a little tweaking, but it's flyable now.

I'm at 5000 feet, which would account for some of the HS change, but I've never seen an engine go that far from factory nominal before. Maybe the factory muffler was changed after the default settings were determined, in favor of one with more back pressure.

Thanks all for the help.

Abe, think of it like this, there are no factory settings, they give a rough estimate, (ball park) that usually works, but each engine has it's own set of turn numbers. Occasionally an engine comes along that is like a weird person, just different.

All engines regardless of the manufacturer suffer from a certain amount of tolerance stack up. This can result in very different settings on identical motors that is why some are telling you the factory settings are just a very general guideline.

When you start to turn the idle screw and the high speed screw it’s easy to get them out of sync and even if they almost appear to be set fine tuning can be allusive.

This method seems to work well for all non air-bleed carbs. These are the ones with the idle adjustment on the side of the carb usually in line with the high speed needle.

Turn the low speed out until it stops or almost comes out. At this point you do not want it to have any influence on the next adjustment. Turn the high speed in until it bottoms out but gently do not force you do not want to damage the needle or seat.

Now turn the high speed out the recommended turns in the manual. Usually somewhere around 2 to 2 1/2 turns.

Use a short length of fuel tubing and place it over the fuel nipple on the carb. With the carb open if you blow through it you should feel/hear air passing in the carb.

Using a 1/16 drill bit or piece of wire close the carb until the barrel just holds the wire/drill in place. Do not force or you will damage the aluminium barrel or throat of the carb.

While blowing through the fuel tubing slowly turn the low speed adjustment in. When the passing air stops back the low speed adjustment out until you can just detect air passing into the carb. You may not hear it but you will feel it with your mouth.

This is about your best primary setting to start your fine tuning from.

Start your engine and slowly advance the throttle to full. If the engine begins to falter clip the glo driver on. If it now will go to full throttle the high speed is to rich, if it quits with the glo driver attached its to lean. Adjust the high speed no more than a quarter turn at a time until the engine runs at full throttle. Richen it up slightly until a slight drop in peak rpm is detected.

To set the low speed reduce the throttle and bring it to its lowest reliable idle. Once it settles advance the throttle smoothly and observe how the engine reacts. Suddenly quits the low speed is probably lean back it out a quarter turn. If it loads up, smokes and struggles to get to full throttle rich so turn the low speed in a quarter turn.

Depending on the carb you may have to reset your high speed after adjusting your low speed. Checking it every so often while fine tuning is a good idea.

When you get to the final tweaking adjustments no greater than the width of the screwdriver blade may be enough to effect a change. Go slow and take your time.
When properly adjusted the engine should idle for long periods of time and advance smoothly to full throttle after and extended period of idle. Short periods of idle the engine should respond instantly to full throttle with a rapid throttle advance. The longer the idle the cooler the engine will be so go easier when you advance the throttle after longer periods of idle.

This works for me.

Dennis

Wow, thanks for the extensive treatment. I'm gonna spend some time with this.

A 4 stroke will lose power and waste fuel with every little bit of turn too rich on the low speed needle. The key to understanding the low speed needle is that it needs to be just rich enough to take full throttle accelerations and no more. Just right is usually 1/8 turn rich from the point where the engine is too lean.

The big thing most people don't consider is that flying the engine around and listening to it is part of the tuning procedure.

Dave & Propworn,
Thank you for all your input. I promised get get back this Monday after going to the field to work things out. In my previous post I said that I had checked all the usual things, to no avail. I've been running O.S. 4-stokes for years with no problem at all. I would always build the plane, install the tank correctly, set the needles correctly, and fly happily ever after. Well as I said earlier, I had replaced some crappy ARF tanks that were suspect with my usual Sullivans. Going from an oversized ARF tank to the proper sized Sullivan trough things off. By simply returning the needles to the "factory suggestions" and starting over corrected everything.
Sometimes facing a new situation in engines requires us to begin again at the basics. Setting the main needle slightly leaner and adjusting the idle one correct for the new main setting did it.
Thanks to all that helped.
Karl

A poorly broken in engine,
an improperly adjusted carburetor
and/or an improperly installed fuel tank will make your flight unreliable.

Always follow directions and break in your new 2-stroke or 4-stroke engine before trying to fly.

This requires several tanks of fuel and a controlled running of the engine to condition it for proper operation. What you want to do is obtain a reliable idle and a smooth transition from idle to full power. When it comes to power output, always run the engine slightly rich, not lean.
Every preflight check should be making sure the clunk pick up in the tank rattles and isn't stuck.

It was stated you don't have to check your valve clearance after I suggested you do. Not all engines leaving the factory are set correctly. I've had two OS engines (new in box) so far that were out of adjustment in the past year.
I got a lot better idle after correcting it to the specs in the manual as well as throttle response. http://www.osengines.com/faq/product-faq.html

You know what they say about the word assume? Depend on it to make an ass out of u and me.

Take nothing for granted check and recheck everything about a new engine, errors can happen even with the best engine manufacturers.

Before I start every engine new or used for the first time I make sure every fastener is tightened properly especially the prop/spinner assembly. Valve clearances and operation are checked. It’s only a couple of screws for crying out loud. Oil the rockers, springs and a little down the pushrod tubes while you’re at it.

I then mount it in one of my test stands in the upright position with the tank centerline on the centerline of the spray bar. I get the engine running perfectly in this orientation and once I am satisfied I then configure the engine and fuel tank as it would be in the airframe. When I run it now I note how much the reliability and ease of starting has changed if any. If your setup is good you should notice very little change and the engine should need little if any adjustment to duplicate the break in positioning.

If there is too great a change you should immediately rethink the positioning of the engine and fuel tank.

When you are satisfied with the engine being run in long enough to be reliable on the test stand mount it in the airframe. There should be no difference when run in the airframe remember you duplicated the positioning and spacing on the test stand. If there is a difference there is something just not right with the install. It could be something as simple as the cowl not having enough outlet area and its being pressurised when the engine is running. First is to remove the cowl and see if it corrects the problem. If it does you will have to provide greater openings to relieve the pressure. Maybe after a longer run the engine overheats then you will have to do stuff like baffle the intake to redirect air over the engine, enlarge the openings to let out the heat.

It’s not simple sometimes to chase down these problems but take the time to figure it out its well worth it in the end.

Dennis

Assumptions are educated takes on a subject and some of us are pretty well educated engine wise. Like 60 years worth of gearheadedness.

Your a big boy capable of making your own decision if you choose to not check that's your prerogative.

I have my set methods that have served me well in all aspects of many hobbies and life itself and none of it depends on assumption. You learn that early when someone of your group will be placing themselves in a position where their well being depends on others to check and double check the equipment.

Dennis

I hate starting a thread and then seeing it go passive-aggressive...I'm checking out of this one.

Be glad it wasnt posted on RCGroups. You'd have 6 pages of garbage and 1 page of helpful information. The guys over here are much more civil... Things still get wonky from time to time though.

Sorry Abe, the advice is still good, just not all the same and my set methods are not the same as Props but are based on what works.

You should see it here when somebody starts a (which engine is better) thread. Lines drawn in the sand get quite deep.