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

Quote:

ORIGINAL: daggets

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

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.

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...

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.

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...

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.

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.

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

Quote:

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.

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

+1

Meant to hit the edit. Delete

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...

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.

Hi All,

Original Poster here. Sorry for letting some wonder what's been happening. I've been traveling for the holidays and wanted to wait til I had a chance to try some things at my next trip to the field before replying. Thanks to everyone for sharing their thoughts though. Will update with results as soon as I can. BTW Earl, engine is a JBA 120.

Rgds,
Ken

Here is a portion of an engine manual that describes how to re-profile the low speed needle. This works for all Bodemann (Webra-TN carb) style designed carbs
http://www.rcuniverse.com/forum/fb.asp?m=10230872
As for blind if you use your engine you can determine if the corrections work or not. It is no different than adjusting the fuel mixture as you currently do it now. You don't (well I don't) measure the cross section of the needle that is in the fuel metering jet and subtract it from the jets cross section. Nor or do I use a gas analyzer to measure mixture strength. Just make the adjustment and see how the engine responds.


No, I don't think it really is easier for the engine OEMs to do this than us. Like I said there is really no way for the OEMs to know how we are going to use the engine. To make it worse there are no standards by which the fuel are mixed. So again the OEMs have no idea as to how the engine will be used. The high nitro in you heli is not contributing to the reliability of the engine. A lot of that is traced to the cooler operation of the engine because there is more oil flowing as nitro has less power per unit volume. So to get the same power you need more fuel flow. More fuel flow means more oil flow. Also most helis are only at full power for a very short period of time. Most air plane engines are at or near their full power setting for most of the time. When I flew FAI F3A I used low or no nitro in my ships because of the cost factor of nitro. To fly pattern I had to practice a LOT! So the cost of fuel added up FAST! The only 4 cycle engine I recall using (or needing) moderate nitro (20% to 30%) were the YS. She did this because the YS has a rather low compression ratio (CR) head and even less of an effective CR at partial throttle. The four cycle engine was at a disadvantage as for as power and weight going against the 10cc full wave piped engine so the use of nitro was an attempt (band aid) to boost the power levels. Note the rules committee had to give the four cycle a 2:1 displacement advantage even after they tried the logical 1.5:1 to begin with!

I hope this helps with the whys and hows to adjust the mid range by direct intervention with the fuel flow.
This thread might also help.
http://www.rcuniverse.com/forum/m_10..._1/key_/tm.htm

I've been doing this (toy engine stuff) since 1969, and compeditivly (well some might disagree[] ) since 1980

All the best,
Konrad

If anyone is going to start playing with the needle taper, they best have some extra needles on hand.

I think I said much the same in post#35.

The cost of a needle is often less than the cost of a glow plug. I also recommend that one have a full set of glow plugs that span a large range of heat ranges. One does need to make a bit of a commitment to this sport/hobby to get the MOST from it. A few glow plugs, needles and a tachometer are the minimum one should have.

All the best,
Konrad

Update on results: first of all, good news. Engine is now a different animal - what a world of difference (and great relief for me) - it is well behaved.

Changes made:
a) leaned low-speed needle 1/8 turn
b) changed to OS 6 (hot) plug
c) fuel changed from store mixed to Cool Power (10% nitro both) - not by choice - store is no longer selling their own pre-mix.

I believe all three elements played a part in improving the engine's running characteristics.

Anyway, thanks again to all for their worthy input.

Cheers
Ken

this is a good new.... a lot of nice guys feeled concerned by your problem.... and the discussion was very constructive....

http://images.rcuniverse.com/forum/u...45/Ec87152.jpg


wow! i ll never guess that!

Oops, didn't read second page before I got on my soapbox.