Hi Everyone,

Wanted some opinions on a mid-range issue I'm having. Engine is a 120 2 stroke which runs great at idle and full throttle. Needles are set fine at both ends (pinch test for idle, and backed off 1/8 turn/couple hundred RPM at full throttle).

At about 40% to 60% throttle though, on an OS 8 plug, engine runs fine for about 2 1/2 seconds, then becomes a little erratic - dropping rpm then up then down (surging). If I roll the throttle smoothly and quite slowly back to full after no more than 2 1/2 seconds, it's fine (otherwise it quits). For a while I thought the issue was with my needle settings (mixture) but yesterday tried a number of plugs with interesting results. First, OS F - again, fine outside of the midrange, but after 2 seconds at half throttle the engine suddenly dies. No surging. If I try to advance the throttle back to full after 2 seconds the engine still dies, no matter how gently I roll the throttle. I tried varying the needle settings at both ends with a number of combinations (richer, leaner) to no avail at all. Then I put on a high quality cold plug and can't remember the exact results, but I think similar to the OS F.

So the engine runs best with the OS 8 (at least I can still change the throttle no matter how rough it is at idle or how short a time I can spend at half-throttle). Wondering if I should try a different plug? Maybe hotter? Like ENYA 3?

Thanks,
Ken

P.S. Forgot to mention, engine is mounted inverted.

It sounds like the carb is way out of adjustment. Start by setting both needles to the starting specs in the manual. Tune the top end out as you have then work on the idle end. Set you throttle at you half way mark and the punch the throttle. If it stumbles any, lean the idle. Keep at this until it jumps to full every time.Lean it about 1/16th turn at a time. You will probably find that the idle is now dead on for full idle. I have a Thunder Tigre that was running as yours is now. I finally went through the above process and was susprised to find I needed to lean the idle almost 2 1/2 turns to put it right. The two strokes load the crankcase with fuel whenrunning to rich side and can give some conflicting indications of the problem.

Don

I've found that sometimes things like the prop size or muffler will cause this problem

Try some different things

Maybe a tank(or tank position) problem.

What specific engine do you have? it might help us to know which engine you had. Also does it have the carb that came with the engine, or did you change it out for a different carb?
What type of muffler are you using? A highly restrictive muffler on a inverted engine with a fuel tank too high in relation to the carb will cause these symptoms too.
Are you using a inline fueling valve? If the fuel line routing is complicated, try simplyfing it to see what happens. I see guys having problems all the time who try to use extra gizmos and gadgets to allow for easier fueling and defueling of a model airplane.

Turk may have a point.  A high tank position can cause this problem, especially if the muffler pressure is low.  In fact the muffler pressure is why a differant muffler can solve this problem.

Or a checkvalve between the muffler and the tank to ensure pressure stays up.

Chip

It wouldn't happen to be a GMS 1.20 would it...?

Ken,


Let us start with the idle mixture adjustment... It is not 'set fine'. The 'pinch test' gives you a good starting point from which to continue; and by no means is it necessarily the correct final setting for the low-speed needle.
So, you should start from this preliminary setting and adjust from there, to achieve a perfect transition from low to high throttle.

In a 3D plane, it should be carried out, with the model at hovering attitude and at hover power (as the 'low' point), to 'launch' (full power).
For a sport/scale plane or a trainer, high and low are from off-idle to full, with the model in level attitude.


The OS #8 is 'medium heat'... According to [link=http://www.os-engines.co.jp/english/line_up/plug/pluindex.htm]this OS Japan page[/link], engines larger than .60 need a cold plug, such as their own A5/#10 glow-plug.
The OS F is actually hotter, but 'behaves' slightly colder in larger two-stroke engines.

The Enya #3 is the hottest plug they have...
As I wrote, you need a COLDER one.

I don't think the inverted mounting of your engine, causes fuel to pool in the plug's cavity, but you should change to the more appropriate side-mounting position.

I've found the OS F and Saito SS plugs to run slightly hotter than the OS 8 in engines that need hot plugs. There was some difference with those two 4 stroke plugs, but not as much as the OS A6.

The Tower plug is hotter. I now think it is cooler than the A6.

engine dying at mid-throttle is a carb problem. Many carbs do not have the midrange OK. Get a decent carb like Webra, MVVS or OS. An Enya also is good, but the very narrow midrange slit fouls easily.
If the engine is an MDS, clean the spraybar and needle very well. There also is a hardly visible slit there (in the idle needle) that controls the midrange.

Heh! That's exactly what I thought when I read the post too.

To the OP (Ken): Whatever engine it is, the carb's midrange sounds like it's not correctly calibrated - quite common with Chinese engines unfortunately and there's no practical remedy without re-engineering the carb. An OS Type F plug is the best for staying lit in adverse conditions like this. If the F is not staying alight, a glowplug change is not going to solve it no matter what brand you try. I know you've adjusted the engine's needles correctly from reading your post, so it's not an operator issue, despite some suggestions to the contrary.

Try a new carb of another brand or better still, bolt an OS 1.60 FX into the model and be done with engine problems forever.

^^^ I should have saved typing a response and just referred to Pe's post!

Harry- wake up!!!

..

Since muffler pressure drops a bit during midrange and it is running rich keeping the pressue up is not a good thing.

Quote:

ORIGINAL: wcmorrison

...Or a checkvalve between the muffler and the tank to ensure pressure stays up.

No way, Chip!

This cannot work and never will!

Muffler pressure in the tank at higher throttle settings, is higher; and becomes lower as the throttle is closed.
For this to happen, the pressure-line MUST allow bi-directional gas flow, to allow the fuel pressure to stabilize almost instantly.

If a check-valve is placed within this line and the engine is run at full; and then the throttle is closed to idle, the pressure within the tank will be way too high for a proper mixture... It will be too rich!
Since the excessive pressure cannot be bled-off back to the muffler, because of the check-valve; it will only become reduced as fuel is consumed; and this will take 'years'... during which the engine will dowse itself to death..

The hotter plug idea worked for my GMS 1.20. I had run about a gallon of fuel through it, but it continued to have a really rough midrange. Idle was fine and full throttle was smooth, but when I'd notch it down to half, it would slowly drop revs and start smoking and sputtering. It never quit and gave me a dead stick, but I wasn't too happy with that. An old pro suggested I go to a hotter plug. One of the guys had an O.S. #6 (old A3) and we swapped it out. It changed everything: the motor started running smoothly from idle to full throttle. At midrange, it was smooth with no sag or sputter, and transitions were effortless. I took the plane up and the engine performed beautifully. Give it a shot...

Crash,


Leaning the mixture (low-speed) would have had the same effect... as a 'hotter than needed' plug.

Many, many modelers; even some experienced ones, are reluctant to touch the low-speed needle...
They seem to think it is like the tamper-proof low-speed needle in carburettor equipped, emission-controlled, late '70s and early '80s cars.

The OS A3/#6 glow-plug is way too hot for any (non-Evolution) engine larger than a .40 (even many .28s); and certainly for your 1.20.


Set the mixture much leaner to comply with a cold plug (OS A5/#10, Enya #5/6, Rossi R6/R7, Etc.).
A hotter plug is just a 'walk around'; not a solution.

Fuel consumption of your engine would be much higher than it should be.

No, not experienced one. [X(] This reluctance is a sure sign that the tuner does not has a solid grasp of the variables to properly tune an engine!

Remember that the tamper proof covers on the smog engines of 70's and 80's were a regulatory demand of the government agency and had no basics in reality.

All the best,
konrad

I've seen many experienced modelers shy away from adjusting the low speed needle. I think Dar is right. They get mislead into thinking the high speed needle controls everything.

OK, if you define experienced as high time or old, but no knowledgeable tuner would avoid adjusting the low speed mixture or a number of other variables.

All the best,
Konrad

Ihave to agree with Konrad (wow this makes what twice this year at least )
I have met a lot of flyers that do not touch the low speed needle and some that wont touch either needle once it is set(and a few that think it is factory set and dont touch anything). Iadmit Iadjust the high speed whenever Ifly but if the low speed is still working reasonably I dont touch it as often. Ifind the mixture changes dramatically during the first gallon or two but stabilizes after that to where only the most minor adjustment is needed to compensate for weather changes.

Quote:

ORIGINAL: jeffie8696

I have to agree with Konrad (wow this makes what twice this year at least ) [img][/img]
I have met a lot of flyers that do not touch the low speed needle and some that wont touch either needle once it is set(and a few that think it is factory set and dont touch anything). I admit I adjust the high speed whenever I fly but if the low speed is still working reasonably I dont touch it as often. I find the mixture changes dramatically during the first gallon or two but stabilizes after that to where only the most minor adjustment is needed to compensate for weather changes.

Jeffie,



Than you're actually agreeing with what I wrote in post #18... before people responded to it.

Cheers.

It's no different than how many experienced old timers won't use the rudder stick. Some just overlook things to do about tuning engines, etc.

Credit in this thread should go to DarZeelon, I just piled on.
No matter who said it most engine issue can be traced back to operators not knowing how to extract the optimum performance from their set up. This isn't limited to just engines as BLW has noted.

Why would anybody have an issue with anything I have posted, other than spelling?

All the best,
Konrad