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OS 40 FP Mixture problem

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Roger Dyke29/08/2019 18:58:21
237 forum posts
10 photos

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_29/08/2019 19:23:11
1153 forum posts
133 photos

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 Dyke29/08/2019 19:42:19
237 forum posts
10 photos

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.

Cuban829/08/2019 20:08:37
2836 forum posts
1 photos

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_29/08/2019 20:21:35
1153 forum posts
133 photos

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 Marsh29/08/2019 20:24:04
3814 forum posts
1102 photos

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 129/08/2019 20:42:59
10867 forum posts
1490 photos

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

Denis Watkins29/08/2019 20:46:06
4077 forum posts
76 photos

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 Dyke29/08/2019 21:26:13
237 forum posts
10 photos


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.


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.


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 - Laser Engines29/08/2019 21:31:08
5090 forum posts
219 photos

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_29/08/2019 21:32:07
1153 forum posts
133 photos

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 Dyke29/08/2019 22:02:58
237 forum posts
10 photos


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.


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 McIntosh29/08/2019 22:43:19
3083 forum posts
1119 photos

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 CRAGG30/08/2019 01:38:52
528 forum posts
15 photos

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 Dyke30/08/2019 06:57:52
237 forum posts
10 photos


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.


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.

Cuban830/08/2019 07:32:21
2836 forum posts
1 photos

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 Dyke30/08/2019 08:00:04
237 forum posts
10 photos

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 Blackburn30/08/2019 08:09:17
510 forum posts
487 photos
1 articles

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 Watkins30/08/2019 08:15:50
4077 forum posts
76 photos

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 CRAGG30/08/2019 08:16:24
528 forum posts
15 photos

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

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