Midrange issue - glow plug?
 

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.

RE: Midrange issue - glow plug?
 

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

RE: Midrange issue - glow plug?
 

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

Try some different things

RE: Midrange issue - glow plug?
 

Maybe a tank(or tank position) problem.

RE: Midrange issue - glow plug?
 

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.

RE: Midrange issue - glow plug?
 

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.

RE: Midrange issue - glow plug?
 

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

Chip

RE: Midrange issue - glow plug?
 

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

RE: Midrange issue - glow plug?
 

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.

RE: Midrange issue - glow plug?
 

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.

RE: Midrange issue - glow plug?
 

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.

RE: Midrange issue - glow plug?
 

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.

RE: Midrange issue - glow plug?
 

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

RE: Midrange issue - glow plug?
 

Harry- wake up!!!:D

RE: Midrange issue - glow plug?
 

..

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

RE: Midrange issue - glow plug?
 

Quote:

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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..

RE: Midrange issue - glow plug?
 

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...

RE: Midrange issue - glow plug?
 

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.

RE: Midrange issue - glow plug?
 

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

RE: Midrange issue - glow plug?
 

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.

RE: Midrange issue - glow plug?
 

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

RE: Midrange issue - glow plug?
 

Ihave to agree with Konrad (wow this makes what twice this year at least ) http://www.rcuniverse.com/forum/js/f...wink_smile.gif
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.

RE: Midrange issue - glow plug?
 

Quote:

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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.

RE: Midrange issue - glow plug?
 

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.

RE: Midrange issue - glow plug?
 

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? :eek:

All the best,
Konrad

RE: Midrange issue - glow plug?
 

mid range issue= hot plug and increase nitromethane... you ll be surprised how good your engine is...

RE: Midrange issue - glow plug?
 

Quote:

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ORIGINAL: daggets

mid range issue= hot plug and increase nitromethane... you ll be surprised how good your engine is...

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Daggets,


That's a 'walk around'; not a solution.

...Like 'solving' a rat infestation by moving elsewhere...


Adjusting the engine better is the proper solution.

RE: Midrange issue - glow plug?
 

right, doctor....
but i have many engine that you can tune forever, there is no solution...
if you lean the idle needle, you will have a good mid range, but you ll be to lean on full throttle.... you can then open the main needle as much as you want, there will be no effect as a low needle setting restrict the hight needle setting on the low side...
no matter the glow plug you use....
i ve read a french specialist article about nitro on glow engine...
it states that nitro permits a better combustion , especially on mid range, when the plug tends to cold quicker...
i ve a new rossi , a yamada 60 fr i ve never been able to tune..... and i m not a newcomer....
since i ve mix 20per cent nitro in theire fuel, they run great, at any rpm, and are easy to tune....
for sure, i m the only one in the world to say that, and the mid range problem is a mith for specialist.....and a reality for idiots like me....
this is written by me for what is worth...

RE: Midrange issue - glow plug?
 

Daggets,


I stand behind my words...
It is true that for some carburettors, it is next to impossible to tune for a proper mixture, over the entire RPM range of the engine...
Even several carburettors of very prominent companies are so effected; like the one of the OS.61FX, tested in May 2003 by MAN.
The older MVVS #3216/3219 carburettors are similarly affected.

When set for a good idle, the OS carburettor was diagnosed as being overly rich in the mid-range.
I diagnosed the older MVVS units as such.


It is possible to fine-tune, by 're-curving' the low-speed needle's taper.

Although not many modelers would dare that feat, I consider it to be the right solution; preferable in the long-run to using a hotter glow-plug and suffering all its consequences.

RE: Midrange issue - glow plug?
 

yes, you can modify the carburator of a so reputed brand such os, rossi, yamada... etc...
but, think...
they are able to design and build such a reputed mechanic and not able to make a tunable carburator?
in other word, you can also modify a car designed to burn premium gasoline, and make it run with regular....
but why?
os engine, like yamada are designed for us market, in this country, nitromethane is reasonably cheap, so it is used as an additve for glow fuel...
if you don t use this additive, you ll need a very high compression engine, but as soon as the compression weaken, the engine starts to be unstable...
thats why new engine are running strong on the beginning, and that after a short while, it s not the same anymore...
20 per cent nitro will solve that...
i m proud to talk with you, mister zeelon, it s constructive...

RE: Midrange issue - glow plug?
 

I humbly appreciate your words, Daggets.

For an engine with a particularly high compression ratio, such as a Rossi, or an MVVS; using a hot glow-plug and higher nitro fuel, are both out of the question, to improve mid-range reliability.
For an engine with a more 'general purpose' C/R, either can be used for this purpose.


But still, this is not the best solution.

RE: Midrange issue - glow plug?
 

I'm not afraid to crank in that low speed needle; all my birds are set up so I can tune the low speed without removing the cowl so it's easy. Also, on GMS engines, it's best to tune the low speed first, getting a stable idle as low as possible, then set the high speed. With the 1.20, when I leaned out the low speed needle, I'd end up getting to the point at which the engine would just flame out. I'd then start stepping it back in until it idled again, but then I would pick up the rough midrange and transition. The A3 is probably too hot for summer use, but with the cold air of winter, it's probably just right. Using the redneck heat test procedure, I'd say that cylinder head temps were reasonable. I'm happy with how it's flying now and will keep the hot plug for the winter. I've got an OS #8 standing by for the warmer weather.

RE: Midrange issue - glow plug?
 

I recently had a mid-range issue and it turned out that the throttle barrel screw had back out far enough that the barrel was starting to slide out of the carb. At full throttle it was in the correct position and at idle it was centered also so we didn't notice it but one time when it died at part throttle on the ground I noticed that the slide was half way out of the carb.

Don't overlook the obvious :eek:

RE: Midrange issue - glow plug?
 

Quote:

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ORIGINAL: Cpt Crash

...I've got an OS #8 standing by for the warmer weather.

---

Crash,


Your engine has a 1.20 cid displacement!

Virtually all medium-heat glow-plugs are too hot for an engine of this size.
The engine needs a cold glow-plug.

Also, the OS #8 is too short for this engine (and also for mid-large OS engines...) and is recessed in the chamber.

Some good cold glow-plugs are the Rossi R5/R6, the Enya #5/#6, but there are many others.


Using a plug which is too hot for an engine, requires that you retard the ignition by running a richer than necessary mixture setting (the needle is your ignition timing control).
This curtails your flight time and unnecessarily wastes your fuel.

RE: Midrange issue - glow plug?
 

I've been re-profiling the low speed mixture needles on carbs, if they are of the twin needle style as a matter of course. The OEM has no idea how I'm going to use the engine and can't possibly design for the whole range of operating conditions. I often flew at 1800 meter altitude. It is very easy to re-profile the needle with a stone honing stick. This modification to the needle should be thought of as be no more complicated than finding the proper heat range of a glow plug and replacing the glow plug. This should be well within the scope of most engine enthusiasts to adjust (no special tools needed).
I find that if you should error on the low speed needle profile that the replacement needle is often less expensive than a glow plug.

Also some carb designs allow the fuel discharge port to be placed at different location in the venturi. This also allow for some tuning of the fuel mixture.
To my way of thinking these features, twin needle and adjustable spray bars (discharge ports), are tunable variable in the carbs you mention and account for their great reputation.

Now it is true that all manufacture can make a dud, such as the OS FX carb. I think I agree with you that nitro (20%)is a mask for a poor (design or worn out) engine as the cylinder fit and rod ends wear.

All the best,
Konrad

Edit for spelling

RE: Midrange issue - glow plug?
 

+1

RE: Midrange issue - glow plug?
 

Meant to hit the edit. Delete

RE: Midrange issue - glow plug?
 

i can understand your analysis....
anyway, as much as many modellers, i don t know how to reprofile a low needle mixture on a carb.... for me, it is a high precision part it is better to don t modify.... especially if you do it "blind", with no measuring equipment...
and again.... why do the engine manufacturer did not cure that? it is easyer for them than for us...
helicopter s engine, wich are the same as ours, in most of case, uses 30 per cent nitro... why? to cool the engine?
i ve also noticed that helicopter engine are more reliable than the same on an aircraft..... have i dreamed?
competitors flying f3a classic pattern were using a lot of nitro, same today with 4 strokes..... why?
i just want to understand, i feel like i ve used engine for 30 years, and still not able to tune them....without the recommended mix...

RE: Midrange issue - glow plug?
 

You know, I was hoping the OP would come back and tell us as to what 1.20 two stroke engine he had.
I think we all probably went over his head with things to fix the problem and scared him off.

RE: Midrange issue - glow plug?