An engine question....

[Monocock]

I know this is not a Private Flying topic per say but it has been in the Engineering forum for a few days with no response at all.

Can someone give me some advice?

I have a Continental A65 in my a/c. It runs smoothly and is cleared for mogas. I use avgas and mogas depending on what is easier to get hold of.

As I open the throttle on take off or after a descent, the power increases but not as evenly as I think it should.

As I apply full power with a steady and progressive opening of the throttle to the stop, the power increases smoothly to 2000 rpm and then stops increasing. It will then jump to 2200 rpm and then continue to rise smoothly.

What is the reason for this "flat spot" and is it something I should be concerned about?

[Flyin'Dutch']

M,

I am not an engineer but this sounds like a carburetor issue.

FD

[magpienja]

Sticky/binding throttle cable maybe.

Nick.

[DubTrub]

Mono,

what's your carb heat status during the throttle application?

[edited to add: how much further above 2200 does it go? 2300 is max rpm, so you must be at cruise speed?]

[Atlas Shrugged]

One possibility may be that your mixture is set too rich at idle. Try testing it on the ground by setting minimum idle RPM, then leaning until the engine stops. Watch the tacho for a rise just before it does. The more rise, the richer the idle mixture setting. Then have LAME tweek it.

A

[Milt]

Sounds like you are equipped wit FADEC or do you change RPM manually ?

[QNH 1013]

My suggestion would be carb ice. The A65 is very prone to carb ice, and our A65 fitted aircraft will ice up if idling over damp grass. In both cases mentioned, the engine will have been at low power prior to the throttle being opened. I normally taxi out to the runway with carb heat applied and only push it to "cold" after fully opening the throttle.

I expect you already have carb heat applied in your descent, so I wonder if the carb heat is working correctly?

Do report back to us when you solve it.

[david viewing]

Please don't be offended by what might be an idiotic question, but are you sure that it's real? It would be very easy for the Tacho to stick at the same place on the dial. It's also quite easy to perceive symptoms that agree with faulty instrumentation.

[Monocock]

No, it's definitely real. There is about 3/4 of an inch of throttle travel that produces no increase and then it suddenly jumps up.

DubTrub - Carb heat is not applied. If it is it makes no difference.

Definitely not Carb ice.

I shall get it looked at next week.....

Thanks for suggestions.

[DubTrub]

Mono:

Apart from your current throttle/engine issue (which now sounds like a dodgy throttle cable or carb butterfly arm), you should really be using carb heat pretty much all the time at low power settings.

This is particularly relevant on approach to land...these small Continentals (up to the 6-pot O-300) have a different installation of the carb in relation to the oil sump, so are more prone to icing at lower power settings.

Personally, when operating my 65-powered machine, I use full carb heat from the commencement of the approach until shut down, and, like QNH1013, all the time when taxying on the ground before take-off, and then I only go to cold after full throttle has been applied.

Good luck, keep us posted.

[Monocock]

QNH and dubtrub

You are both correct I think!!

I used the "taxy with carb heat applied" method tonight and had a smooth engine speed advance on take-off.

I used the carb heat far more than ever before and it seemed to work. After a descent (with carb heat applied) I deselected carb heat AS full power was applied (ie both plungers in simulataneously) and the problem had gone!!!!

I played around for a while and tried descending for about 30 seconds at 1500 rpm without carb heat and then going into a climb. It had a flat spot......

Thanks chaps.

This just leaves me wondering one thing........


























......was my EFATO 6 weeks ago caused by this?:ugh:

[v6g]

Good to hear your back in the saddle Mono.....

[Mike Cross]

One other thing

I believe the C85 in our Luscombe has no accelerator pump in the carburettor and the 65 is probably similar.

The function of the pump is to squirt an extra dose of fuel into the venturi when the throttle is advanced and temporarily richen the mixture.

This is because opening the throttle allows more air through the venturi, momentarily weakening the mixture until the fuel overcomes its natural inertia and starts flowing at a higher rate. Opening the throttle a tad too quickly without an accelerator pump can cause a momentary reduction in power.

[DubTrub]

The bog-standard Bendix or M-S carb as fitted to most A-65's does not have an accelerator pump, Mike , as you correctly suggest...

This is indeed why the throttle should not be advanced too smartly....and another reason for keeping carb heat HOT for the duration of the throttle advancement, in that the mixture is inherently richer during carb hot air.

(incidentally, the old adage about "going to cold to prevent dirt ingestion" only applies to carburettors with intake air filters...mine has none, and I have never had any problem with cr@p causing carb problems...now if I was in the Arizona sand desert, that might be another matter!)

[edited to add that this was posted after my PM to Monocock, but the offer still stands, Mono.]

[Monocock]

First of all, apologies for the time of this posting. My 3 year old son decided to start singing the theme tune to Maggie and the Ferocious Beast just now and I know I won't be going back to sleep.

Thanks guys for the suggestions. It all makes more sense now and I am amazed at how the permanent use of carb heat at anything below cruise power made a difference last night.

The "gap" in the power rise was particularly worrying. It was enough on short finals to warrant a missed heart beat.

Thanks also BRL for allowing me the use of this forum for what some might deem to be "off topic" discussions!!!

[Kolibear]

Interesting though that Mono's symptoms where not those classicly associated with carb icing.

He said in the revs rose to 2000, paused, then jumped to 2200 before increasing.

I would have epected the revs to either be slowly decreasing, or remaining static whatever the throttle movement.

[LowNSlow]

DubTrub

I'm bit confused about:
(incidentally, the old adage about "going to cold to prevent dirt ingestion" only applies to carburettors with intake air filters...mine has none, and I have never had any problem with cr@p causing carb problems...now if I was in the Arizona sand desert, that might be another matter!)

An aircraft with no air filter on the main air intake, interesting. What aeroplane is it?

The dust ingestion wouldn't do much adamage to your carb although sucking in a pile of loose grass would spoil your day. However, dust ingestion would play merry Hell with your cylinder bores and piston rings over a period of time. I used to work for an air filter manufacturer and have seen the results of feeding controlled quantities of Arizona red dust into a running engine.

When I had a Cub with it's ice generating, accelerator pump free A-65 Continental I used to taxi to the runup position, run the engine for a minute or so to a) warm the exhaust thus making the application of carb heat more effective and b) to blow the loose grass, dust etc away before opening the unfiltered hot air intake. Applying carb heat until the symptoms of icing are clear is then effective and also unlikely to result in sucking in dust.

Smooth and relatively slow application of the the throttle is the order of the day with carbs with no accelerator pump. Slamming the throttle open WILL result in the engine stopping. I've always worked on a count of "one thousand and one, one thousand and two, one thousand and three" to take me from the idle setting to full power.

[david viewing]

Mono

Doubtless you're right about the carb heat but you might want to check the cable anyway. Reading your account of the 3/4 inch of travel reminded me that I once had a 'flat spot' on the carb heat (O-320) which defied engineer inspection and finally gave way somewhere over the Western Approaches, fortunately to 'hot'. Wouldn't have been so funny if it had been the throttle. The little pinch bolt in the butterfly arm had been stripped by constant engineer meddling. The problem (the cable slipping) only happened with the engine hot, hence they never found it.

[Kingy]

Hi Guys,

I always use carb heat when on the ground, often in the air and for a short while at cruise power before I pull the power back in the decent, then all the way in - I've never had a problem.

Mono - are you using a metal or wooden prop? I have found a really marked difference in throttle response with an A65 when fitted with a metal prop - On my old Taylorcraft it was easy to make the motor falter if the throttle was advanced too quickly, my Cub with a light ash prop is much more immediate.

Regards

Kingy

[DubTrub]

LownSlow,

It's how the aircraft has been since the early forties. 3500 hours later (of which 600 by me) no problems; at least none that are apparent. It's a bog-standard A-65. No major cylinder bore issues during last major. If your Cub was a J3, chances are it also had no filter when manufactured.

As I stated, there is no Arizona dust around where I fly, and I won't let you inject even controlled quantities of it near me!

Mind you, I do not do high static run-ups when operating from the beach, I always get the aircraft moving somewhat to avoid sand ingestion.

Loose grass just gets sucked in to the bores and gets burnt.

I agree with you on the thottle opening technique, my early hours saw a few sweated brows (cure: close throttle before prop stops, open again slowly).

[A and C]

Have you checked for excessive play in the butterfly valve shaft ?.

[LowNSlow]

DubTrub, can't argue with those numbers :ok: . My Cub was an L4 with a filter on the cold air but, as normal, no filter on the hot air.

Regarding the grass, I was thinking about clumps of cut grass rather than the odd bit.

PS We used Arizona red dust as it was available in a range of different, closely controlled, grain sizes. The fine stuff was good for polishing out scratches in sunglasses when mixed with a bit of water!