OS 40 FP Mixture problem

[Roger Dyke]

Hi All,

I have an old OS 40 FP in my latest model. The engine was purchased in the early 90's but has only had about 20 hours use. It is in perfect condition and has excellent compression.

My problem is that at mid throttle it runs rich (4 stroking), and upon increasing the throttle takes about 7 or 8 struggling seconds to increase back to peak revs again. At peak revs it is fine and also fine at idle. It is just the pick-up from about half throttle that is the problem.

Peak revs were set with it 2 stroking, with 2 or 3 clicks backed off to slightly rich. Idle is also set for a constant 2500 rpm.

Prop = 11x6. To keep the rpm down to 10k to reduce noise.

Fuel = Model Technics Laser 5.

Glowplug = OS # 8

(The above set-up works fine with my Irvine 40)

Any ideas?

[Alan Gorham_]

I'd check the air bleed hole on the front face of the carb hasn't become gummed up with oil. That will make your low end mixture rich as it will stop air from being drawn in and will give the symptoms you describe.

[Roger Dyke]

Hi Alan: Thanks for your reply. The whole engine is spotless. I've had the air-bleed screw in and out several times and the air hole is clear. As mentioned in my previous post, the idle is fine at about 2500 rpm.

[Cuban8]

Must be thirty years since I ran an OS40FP, didn't have much to spend on engines then, so the FPs were ideal. Couple of things worth checking - are you properly leaning out for full revs? Three clicks back richer sounds quite a bit, perhaps you're overdoing it. We found Enya No 3 plugs to work well. The carb is quite a simple device on these engines with a strong suction from the small bore. Tank height too high perhaps? Might run better without exhaust pressure. Air bleed only effective up to a little past idle position so unlikely to affect mid range. Needle not damaged?

Edited By Cuban8 on 29/08/2019 20:12:01

[Alan Gorham_]

Roger it is good that you have eliminated the air bleed hole. Can't be that.

Note that the low mixture setting does not control idle speed so the fact that it idles at 2500 rpm is irrelevant as it could easily do that running with a rich or lean idle mixture, you just wouldn't know until you banged the throttle open.

I always found that my OS 40 FP ran much better on an Enya 3 plug and that was especially noticeable with the pickup from idle.

Edited By Alan Gorham_ on 29/08/2019 20:22:42

[Paul Marsh]

I have loads of 10FP's to 60FP's. Bought a 40 and 35FP last weekend for £5 each. they are brilliant engines, powerful enough for most models and good runners, and never seem to wear out.

[john stones 1 - Moderator]

Yep used em when I was a nipper, put a 10x6 on see it's any happier ?

[Denis Watkins]

Roger, you mentioned an airbleed screw in and out, clean and clear, but the hole at the front can still be blocked

Check that carefully with a pin, as this hole can become blocked again.

It's ok reducing noise, but use a 10 x 6 prop to find for now

And get a 2000 rpm tickover to be sure of the bottom end

The motor would find it hard to unload on an 11 x 6

Once you know the tickover is ok, then 100% the problem is the main needle

Again, this needs to be clear, and the "O" ring in good condition on the needle

[Roger Dyke]

Cuban8:

1) Full revs leaned out spot-on. Always check every time I fly.

2) I was thinking about using an Enya 3 and have sent for a couple.

3) Tank height looks fine. It cannot go lower anyway as it sits in the bottom of the fuselage.

4) Hadn't thought of trying it without exhaust pressure. Might try that.

5) Carb needle is perfect.

Alan:

1) I know that the air bleed idle screw doesn't control the rpm, but it does control the rich/lean idle mixture. It is set okay for a steady idle.

2) Enya 3 glowplugs on order.

Paul: I really like the old OS FP engines too.

John: I really don't want to go to a 10x6 as I'm trying to keep the rpm down.

Dennis:

1) Airbleed hole is perfectly clear. Checked again last evening when I took the carb apart.

2) I may try a 10x6 next time I go flying but I don't really want to go that way if I can help it.

3) I do believe that I can get down to 2000 rpm but prefer a little more.

4) As mentioned earlier, the engine is in excellent condition and the main needle is fine. There is no "O" ring on the needle. Just where the carb joins the body (and it's okay).

I have a feeling that it may be over-propped although the paperwork supplied with the engine does indicate that it's suitable for a 11x6 prop. Also I will see what difference an Enya 3 glowplug makes (if any).

Thank for all your replies.

[Jon H]

It might go against the grain but you could try running it without exhaust pressure. This would lean it out across the board and might at least be worth trying even if you dont fly with it like that. If it works like this lowering the tank may well be the answer.

I agree with the other comments about the fp series being good engines. I used a pair of 25's in my first twin, my dad still has another pair of 25's in a twin and i have used 15, 20 and 35's in other models with great success.

I current have a 15fp that runs brilliantly on laser 5 with an OS8 plug so that combo should be sound. I am personally not a fan of the enya 3 but it should still work as should the os8 and i think the a3.

I would lean it right out as cuban suggests. If its not sagging then its still got more to go so just keep leaning a click or two at a time until the revs drop a bit and then go back one.

if you like post a video so we can see the beast in action

 

Looks like i was beaten to the post. I also missed cuban suggesting no pressure. Im getting slow in my old age 

 

 

 

 

Edited By Jon - Laser Engines on 29/08/2019 21:33:08

[Alan Gorham_]

Sorry but with an air bleed carb there is no clever control over the mid range mixture so you will only find out if the mixture is wrong when you throttle up. Your comments about a steady idle mixture are a red herring. There is such a wide range of tolerance to mixture on a simple 2 stroke glow that your idle is telling you nothing in this case.

I would be sure I had a known good plug and known good fuel before making any other guesses.

[Roger Dyke]

Jon:

I can't lower the tank as I mentioned in Cuban8's reply. I could try it without pressure though. I do like the idea from Cuban8 and yourself that maybe it needs to be leaner at full revs. Perhaps I am being over cautious.

Alan:

I fully understand that the idle has no control over the mid range mixture. I was just pointing out that the engine seems to run absolutely fine at minimum and maximum revs. The fuel is good and quite new (works okay in other models) and the engine has had two new glowplugs in it.

[Martin McIntosh]

I have never had a problem with the old FP motors. Are you sure that you have opened or closed the slow running needle enough and not just closing the carb. too much to get a slow idle? If the motor starts to slow down on idle then it is too rich, v v if it speeds up after a few seconds it is too lean. You could also try a slightly cooler plug and use 10% nitro which will make the motor much less sensitive to settings.

When I restarted the hobby after an enforced lay off I stupidly bought an MDS 48 after reading the (then) glowing reports about them. The above fuel/plug combination totally sorted it.

[SIMON CRAGG]

As you seem to have tried most things, I would suggest taking it apart as either the bearings are slightly rusty and (dragging) or the liner has peeled on the inside. Hope this helps.

[Roger Dyke]

Martin:

The idle is set perfectly and the idle is no problem. I do realise that the idle speed is set by the throttle opening and not by the bleed screw. I have not tried 10% nitro. I use 5% on all my engines which seems to be fine although I do take your point that it could probably make a difference. I do not have any 10% but probably something to try for the future. Regarding the plug, I am going to change to another type on my next outing.

Simon:

I can see where you are coming from, but this engine is 'as new' in condition and has been stored well with absolutely no signs of corrosion, rust, flaking, or oxidisation. It's probably the best engine I have for condition.

Thank you all for your very helpful replies.

[Cuban8]

Tank position was only a long shot - TBH it'd need to be way out of whack to give you problems (if at all) other than at idle. I really wouldn't worry over an extra shot of nitro, 5% is more than enough and many users would happily run them on straight and the devil's venom (20% castor) back in the day.

The engine will always be somewhat of a compromise in performance to keep it simple and cheap to manufacture - it should operate satisfactorily though, but will never really have the power and flexibility of the ball raced, twin needle motors.

[Roger Dyke]

Cuban8: I do realise that the 40 FP is not a complex piece of machinery which will perform perfectly no-matter what the conditions at all throttle openings. It is quite a simple engine and I fully accept that. I was just curious as to why it chooses to 4 stroke at mid-throttle openings. Perhaps I have really answered my own question.

[Andy Blackburn]

Many years ago I had a 46FP that did exactly this; it was running on a #8 plug and the only way I managed to get it to run acceptably was:

• go down a prop size (to a 10x7, I think)
• up the nitro to 10%
• adjust the bleed screw by trial and error until the mid-range pick-up was acceptable (I think it was initially running a bit rich, but it was a long time ago), and
• accept a slightly higher tickover of about 2800 RPM

The problem is, it's an air bleed carb - your room for maneouvre is quite limited. Suggest you try the same process and if you want to keep the revs down with a smaller prop (maybe try a 10x6 for the 40), you could always limit the rpm by putting a switched mix on the throttle so that full throttle on the stick is only about 85-90%, or whatever will give you the revs you want (10,000?).

Edited By Andy Blackburn on 30/08/2019 08:12:25

[Denis Watkins]

Roger, you are our useful analyst, you have acknowledged a couple of things to solve this simply

For now

Fit a 10 x 6 prop and leaning out the high needle should suffice

[SIMON CRAGG]

Might be worth changing the carb for an ASP / SC twin needle job. Plenty available at the moment!.

[Paul C.]

I remember struggling to get the set up right on one of these engines many years ago , took me ages to realise that the idle needle worked backwards to the mail needle IN - rich OUT - lean . Not at all sure that this is your problem but is a bit of info 😂, have you checked the back plate for a good seal ? The plastic one on the LA series caused me some grief to get it to seal correctly. When all else fails take the engine out fit it to a test stand and use a new tank, I find that you can see what's going on, easily get at all parts of the engine and try the tank at different heights.

Best of luck, Paul.

[Engine Doctor]

The air bleed carb should work fine through the range on an FP engine. An odd cough at mid range can happen as the FP's carb is a simple bit of kit. Assuming your fuel and glow plug is good then the trouble you are experiencing is caused by fuel collecting/puddling in the crankcase while engine is idling. This can be  caused by an over rich mixture on the main needle as well as the air bleed setting.You say your richening the mixture two or three clicks at full throttle ? Try one click rich . If it exhibits any symptoms of  rich running at full throttle then it will be too rich at idle This excess fuel puddles in the crank case due to the inefficiency in the transfer process at low revs . . As soon as you open the throttle the engine tries to transfer/scavenge the fuel in the bottom of the crankcase as the system efficiency increases to the cylinder and the rich mixture cools the plug often causing the engine to cough and splutter or stop . The mixture should be est as follows .Set the air screw /air bleed screw half way across the opening , this can easily be seen. Start engine and when warm run at max revs and adjust mixture on main needle until it starts to go lean then richen the mix enough for normal Wide open throttle running with no excessive smoke etc. probably one or two clicks. Now close throttle adjust the idle mixture screw by allowing the engine to idle for ten seconds or so at an acceptable speed , usually around 2000 rpm ish . Open throttle smartly , don't snap it open . If engine hunts with little or no smoke then mix is too lean so screw In air bleed screw half a turn .and re-try. If engine coughs and splutters with lots of smoke or fuel being emitted from exhaust then its too rich. so open air bleed screw half a turn and re- try.. Continue until running is ok

If after a few cycles of this the problem persists then there is another cause

If your engine spits any fuel from the rear of the prop driver then the main bearing bush is probably worn/damaged and a rich idle setting would be needed to maintain Crankcase. pressure . This can happen after a long lay up if old fuel was left in case and any acid formed attacking the shaft or bush.

Hope you get it sorted as the FP is a good reliable engine if not the most powerful .

Edited By Engine Doctor on 30/08/2019 08:59:44

[Cuban8]

Posted by Roger Dyke on 30/08/2019 08:00:04:

Cuban8: I do realise that the 40 FP is not a complex piece of machinery which will perform perfectly no-matter what the conditions at all throttle openings. It is quite a simple engine and I fully accept that. I was just curious as to why it chooses to 4 stroke at mid-throttle openings. Perhaps I have really answered my own question.

Just remembered that I've got some ancient video footage shot in the 80s by a mate with, IIRC, a Ferguson Videostar camcorder of my President Hi-Boy trainer and its replacement OS40 FP - previously had a Merco 35. I haven't watched that particular video for ages, but I'm sure the sound of the engine clearly goes off-song compared to full whack (sometimes needed with the heavy Hi-Boy) when throttling back to cruise in preparation for landing.

[Braddock, VC]

Cut a piece of silicon fuel tube to fit over the spray bar thread, with the main needle out of course. It should be long enough that the needle valve compresses it from about 2 turns in, iirc OS used to put a piece of fuel tube in the box for new engines just for this purpose.

Also check that the main jet is clean and that the idle mixture hole goes right through into the carb air inlet.

[Roger Dyke]

Andy:

1) I was thinking about trying the 10x7 but thought that it would have similar loading to the 11x6. I do not really want the extra speed from the increased pitch. The 10x6 would give me higher rpm (more noise).

2) I don’t have any 10% fuel to try and do have to send for my fuel by mail order. I’d rather hang back on that for the moment.

3) That’s an interesting idea to limit the throttle servo throw but would rather keep everything as standard as I can.

Denis: An easy one to try (which I will) but don’t want to leave it that way.

Simon: An interesting solution but don’t want to go to drastic measures just yet.

Paul: Both idle and main adjusters are working fine. I don’t think that I have reached the stage yet that I have to remove the engine. I am only talking a minor problem here and if I have to live with it, it’s okay.

Engine Doctor: Thank you so much for the explanation and all that information. I hadn’t given a thought to ‘puddling’. I would think that puddling could happen at mid-throttle too. I rather favour that this could be my problem. I will try out what you recommend. The engine is not spitting fuel from anywhere. In fact it’s the most clean running engine I have.

Cuban8: Thank you for the information.

Braddock VC: I have tried the silicon tubing dodge on the needle valve and treble checked the air bleed hole. Everything okay.

My thanks to you all.