I am breaking in and tuning a 91 4 stroke engine. After adjusting both the low end and high end needles, the throttle advances smoothly to 3/4 open. But past this, the rpm drops and wont pick up again. So I only get maximum rpm at 3/4 open throttle.

Appreciate any suggestions for this.

Thanks,

Chris

Idle too rich and the main needle too lean. Open the main needle a turn or so more and back the idle off a bit. Don't start a new engine at idle, start at about 1/4 throttle. But be careful as it will be running much faster right away. Open very very slowly to full throttle. You should be able to tell if it is starting to run lean or rich if you open the throttle slow enough.

I would open the low speed screw some also before running it again.

If the low speed screw is too lean it will effect the high speed to where it has almost no effect.

[ul]Needle setting sequence[*] peak high speed[*] close low speed 1/8 turn at a time and check transition from low to high[*] When the transition to high speed starts to deteriorate, open the low speed 1/8 turn at a time to recover the transition to high speed[*] After satisfaction, enrichen the high speed needle 3-4 clicks
[/ul]

Thanks for the inputs. I will try them tommorow and see how it goes...

Chris

One more thing, I attached a 1.5 ft rubber hose to the muffler exhaust to deflect the oil and smoke.
Will this effect the engine tuning?

Chris

I doubt if it had much effect on your tuning
Run the engine for a moment without the hose and see for yourself?

It's weird how it took a while for me to figure out how the needles worked. It was never intuitive that the two were as fully interrelated as they are. The surprising part was that leaning one would make the other follow, basically moving the transition point around. Once this concept starts to jell, tuning became much easier.

Taking a carb apart and figuring out exactly what everything did was an eye-opener. This 30 minute task alone significantly boosted my tuning ablilities.

Jack

The voice of experience

I tried again today. Still the same problem.

The engine idles fine. When I opened the throttle, it ran smoothly to 3/4 throttle position, then when I past that point, it spluttered and rpm drops. Even when I close the main needle, the rpm won't pick up.

It's as if something is block the air, or the mixture became rich.. too much pressure from the pressure hose?

After that point,while engine is running irregularly,pinch the fuel line.The reaction of engine will tell you the problem.Over rich or lean.

Chris,


Before meddling too long with the adjustments, see if there is anything restricting the fuel flow in the lines (clunk pressed against back of tank, fuel-line kinked, Etc.), or inside the carburettor...

If opening the needle does not causes the engine to run very rich, something else is limiting the flow of fuel.

Do not use a fuel-filter in the line between the fuel-tank and the engine!


If you cannot find anything in the fuel-lines, you may need to disassemble the carburettor, to clear the jets.

Before you do this, connect a piece of fuel-tubing to the carburettor nipple and turn the high-speed needle, to ascertain it has direct control over the amount of air you can blow through the nozzle.
You should feel the resistance to blowing decrease, as you open the needle up to 4-5 complete turns...

After making sure the entire fuel path is clear of any obstruction, check the adjustments.
Start way rich on both needles... A four-stroke engine cannot be run so rich that it would 'eight-cycle'...

Start the engine and slowly advance the throttle to full and there, start closing the high-speed needle, until peak RPM is reached.
If the engine begins to slow lean; closing the needle more may cause it to backfire and throw the prop!

Find peak and open the high-speed needle ~1/8 of a turn.
Make sure you can close the throttle to idle and return to full-throttle.

Then go to the low-speed needle and adjust it for ideal response/transition.

Hi!

In a situation like this you must open the idle needle fully (much) so there is no restriction there. Eliminate all things that can restrict the fuel flow.
Then you start the engine and adjust the high speed needle at full throttle.
If the engine can hold the rpm everything is fine! If not open the low speed needle some more. If still the engine cannot deliver full rpm at full throttle something is restricting the fuel...Or wrong fuel is used, first check carb and fuel lines.
...Then set the low speed needle (idle needle).

Using the pinch method is not that exact! A much better way of setting every engine is by ear! listen to how the engine sounds and respond from there. This is very easy to do.

Thanks for all the comments.

I just took the carb and inlet manifold off - no obstructions there. Also took out the fuel tank and checked the lines - looks ok. I will get the lines changed tomorrow.

When the engine rpm decreased from 3/4 to full throttle, I noticed a lot of smoke from the exhaust. So the high end needle was probably not set lean. Closing the high end did not increase the rpm much, but once I closed past the 3/4 throttle, the engine surged and rpm increased dramatically... weird. It looks like something is restricting the air flow at beyond the 3/4 throttle, or there is a fuel surge (rich) at full throttle.

Just wondering if the pressure return tube from the muffler could be a cause, due to over-pressure at full-throttle?

Chris

Chris,


The amount of smoke emitted by the engine cannot (despite what some modelers think) be used to judge if an engine is running too lean, or too rich.

When running too rich, an engine would discharge a relatively large amount of droplets from the exhaust.
This 'vapor trail' rarely looks very much like smoke... It will rapidly wet and oil your hand, if you put it into its path.

Smoke would not...


An engine running too lean will emit smoke, from the actual burning of the oil in the mixture.


What you are describing appears to be fuel-starvation, since a four-stroke engine would not stall from a little too much fuel.

And as to muffler pressure increasing at high throttle; it does not double the fuel pressure and would at most increase it by about 0.05 psi, over the ¾ throttle setting...
This will only have a minor effect on the mixture strength.

...Not such an effect that would dowse the engine to death...


If you have an experienced modeler living in your vicinity, that you can turn to, this person may be able to notice something that you did not.

From what you did tell us, we can only assume that the problem is fuel-starvation at high throttle...

Have a sneaky hunch that your 18 inch exhaust extension may be causing too much back pressure. Try tuning without it. That length may be acting like a reverse effect tuned pipe, the resonant pressure wave is pushing exhaust back in the cylinder instead of extracting it.

It's too lean. Fuel supply is fine until you open the carb all the way, then it runs out of fuel and dies or acts lean (RPM's drop)


Close throttle to idle
Run the high speed needle all the way in
Now back the high speed needle out exactly 5 complete turns

Set throttle to 3 clicks above idle and start it.
Go to full throttle
While it is running at full throttle, lean the high speed needle until peak RPM (use a tach)
When it stops gaining RPM, quit leaning the high speed needle and back it up about 1/4 turn rich

Now adjust your idle the same way.
Turn it off
Close the throttle
Run the idle screw all the way in until it stops
Now back it out 3 turns

Start it
Idle speed
LEAVE THE GLOW IGNIGHTER ATTACHED
Lean the idle screw and observe the engine. It will gain RPM's as you lean the low speed needle. Just like the high speed needle picks up RPM as you lean it
Don't bother to transition it yet
Just keep leaning the low speed needle and observe how the engine picks up RPM
Eventually you will lean it too far and it will start to slow down
STOP
Richen it about 1/8th turn
Take the glow ignighter off and see if it stays running. It should drop RPM just a little bit but it should stay running

Now go back and double-check the high speed needle
Full throttle
RICHEN about 1/4 turn
Now lean to peak (use a tach)
When you find peak RPM, richen it up about 1/4 turn

Go fly it.

Hi!
18" exhaust extension...!!!??
Get rid of it!!!!

Good call. I second that theory!

Thanks for all the suggestions!

I will try them all today and report back. First, I'll have to buy another bottle of fuel.
These 4-stroke engines are thirsty with all running in.

Cheers.

Next time when running at the maximum throttle opening possible, pinch the line to see if it picks up speed or stalls. If it picks up speed it is running rich, if it stalls it is running lean. How much smoke is more of an issue with the glow plug heat range and the percentage and type of oil. With a hot glow plug it may actually smoke more with a lean setting.

Hi,

I have got the engine back to almost peak rpm. Here's what were done:

Removed the 18" extension - idle to mid was smoother, but still stalling at full throttle (7000 at 3/4, 5000 at full throttle).
Removed the wire-mesh fuel clunk/filter and change fuel lines - no effect.
Changed from 14x8 to 14x6 prop - slightly increased rpm at full throttle but still stalled at full throttle
Opened idle and high end needle and readjusted - not much effect.
Then, change to a small 12x8 prop, engine raced to 11,000 rpm.
Change back to 12x6 prop - engine reached 8500 rpm at full throttle after some adjusting to the high end!
(Peak rpm in the manual for this prop is 8,800)

This running at 11,000rpm - did it un-stuck something inside the engine or the carb....? The engine was initially run about 6 year ago on the stand, then left on the shelf until now.

Chris

Quote:

ORIGINAL: pauser98

...change to a small 12x8 prop, engine raced to 11,000 RPM.
Change back to 12x6 prop - engine reached 8,500 RPM at full throttle after some adjusting to the high end!
(Peak RPM in the manual for this prop is 8,800)

This running at 11,000 RPM - did it un-stick something inside the engine or the carburettor....?

Chris,


It seems something did clear an obstruction that was 'at large'.

It is, however, much safer to locate the problem by diagnosis, then to hope it becomes rectified on its own...


However, the situation you are seeing; with a lower load prop (12x6) being spun much more slowly than the higher load prop (12x8 - 11K is 1.38 HP; a respectable number), is more than a hint that some kind of a problem still exists...


Rectify it before you try flying!

EDIT: Typographical...

Hi Dar,

Sorry, the numbers should be:

Changed to a small 12x8 prop, engine raced to 11,000 rpm.
Changed back to 14x6 prop - engine reached 8500 rpm at full throttle.

Actually I have flown it on 14x6 prop yesterday. It's a high wing at 5kg weight, and flew fine.
I am hoping to fly it again in two days' time and see if the problem will come back.

Chris

8,500 is mighty slow for a .91 four stroke turning a 14 x 6. Except a Laser perhaps. What fuel and glow plug are you useing?

Hi!
8500rpm is a bit low! You should get around 9500-10000rpm if the engine is Ok.
You said you haven't run the engine for 6 years...!? If this is correct I think I know what the problem is. Have you checked the ball bearings?
A hiatus of 6 years is not good for an engine which has been run on Castor oil. This usually means rust and lots of yellow/orange oxidized Castor oil in the ball bearings.
I would take the engine apart and have a look at the bearings if I were you.