Symptoms of weak mixture quiestion.

[nickjaxe]

Hi guys posted this on a yahoo forum just wondering if I could
get any more info from users here,

I re-built my 503 some months back its now done about 10hours, I
fitted new 2nd oversize pistons, the old ones were std,
I have noticed when I am on cruise revs 5000rpm it has a kind of
miss, max revs are 6200rpm, but saying miss maybe misleading its
just a very slight well lets say iregular murmer full power its fine all day long, the engine is a 503 twin ignition single carb on a trike,

The plugs are a light brown the mag end a bit lighter than the other
but both are in the tan range of coulors, I was thinking
with it having bigger pistons would it need a slightly richer mixture
with the bore size being bigger,

But thinking about it I suppose the pistons draw in as much mixture
as they need, I suppose I am just cluching at straws,

Anyways any of you guys give me any ideas of my prob with the
symptoms above.

Nick.

[Morph]

Light brown, or rather tan is the ideal colour the plugs should be. Do you have EGT's probes, this would give you a better idea of whether the engine is running lean or not. High EGT's indicate a lean mixture

A white colored plug is caused by engine overheating. Failure to repair this engine will result in severe engine damage. Common causes for this are:
Incorrect spark plug (too hot heat range).

Low octane fuel.

Timing is not set properly.

Cooling problems, (dirty cylinder fins, no or low water if water cooled, low or no engine oil).

Carburetor air/fuel mixture is too lean (too much air).

Leaking crankshaft seals, no oil, base or head gasket leaks, or crankcase leaks on two stroke engines.


My old 503 used to run rough at 4500 to 4800 rpm. The EGT temps would go up inbetween these revs so I tended to avoid them. I used to call it the "hot spot" maybe your motor, having been recently overhauled, has changed it's characteristics and the hotspot is now at 5000rpm.

apart from that no other ideas. Maybe Duck Rodgers can help

[nickjaxe]

Hi Morph thanks for your ideas all seals/gaskets are new, timing I am sure is ok carb its just as it was before the re-build,

I was thinking along the same lines as you its just developed a rough spot unfortunaly its at my HOT range,

I did re-pitch the prop after the re-build and I think it may be a bit to course now static 6000rpm, so I think I will reduce the pitch a bit and see if my HOT speed is in a smoother part of the rev band,

Many thanks for your ideas Morph.

Nick.

[nickjaxe]

Hi again Morph I was thinking again about your hot spot well you know what I mean,

Anyway I test ran my engine tonight on the ground and sure enough between 4500ish and 5000ish rpm there is a rough spot, when it was like this I gave it a small amount of choke and that made it worse,

So all I can think its a rich spot, thats the same prob I an having when flying, did you ever get to the bottom of it.

Nick.

[DarkHelmet]

Could it be a harmonic vibration that you are confusing with an engine problem? You did repitch the prop and it could induce a harmonic at a specific rev range. Did you set the prop back again?

Seeing that you are in the UK it is hard to pop over and have a look and listen

How many blades on the prop? What is your reduction gear ratio?

[Morph]

I repitched to get a higher static, my cruise rpm came up to 5300 to 5600 depending on load and speed. Had no more problems

[nickjaxe]

Hi guys, harmonic vibration Not realy sure about that one is that somthing I would hear rarther than feel through the seat of my pants as I do now,

Its a 3 blade warp drive 2.58 ratio, I will try like did Morph and back the pitch off 0.5 of a deg, my static revs at the moment are to low at 6000rpm, so fingers crossed.

Nick.

[nickjaxe]

Ok re-set the prop was static at 6000rpm now its 6200rpm, still same prob,

I have also noticed that the revs fluctuate a lot in this rev range by over 1000rpm,

I have posted a new message regarding the jet needle on here thinking the carb may be set to rich,

Its becoming a pain,

Any more ideas.

Nick.

[nickjaxe]

Its sorted, a rotax enginer told me if I was not getting the propper max revs ie 6500rpm in flight and I was prob only getting 6200-6300 on climb out the engine will run rich burble and fluctuate on the revs at part throttle,

It must have been because the engine was not revving to the correct RPM,
Because I flew it for 3 hours today after re-pitching the prop to give 6500rpm in flight and never missed a beat, and no more chasing the revs with the hand throttle set it at 5500rpm it just sits there rock steady,

Must admit I would not have thought an engine that was not making its max RPM would run rich,

But that's what I was told by this rotax engineer,

Hope this may help somebody else.

P.S. on my aircraft tach its showing 7200rpm max in flight now, but it over reads by 700rpm so if my maths are correct its giving 6500rpm now in flight, hope all this may help sombody else, you learn somthing every day,

Thanks for all your help and advice guys.

Nick in the UK.

[Morph]

Thanks for the feedback

[PMarshall]

nickjaxe It looks as if you are using a B Box as the ratio 2.58 is a B Box ratio. The 3 blade warp drive should only be used on the c or e box due to the mass moment of inertia of the warp drive being to heavy for the b box.

My 2 cents worth

[nickjaxe]

Hi PM I dont know about that, your right it is a B box but it came out of the mainair factory with that prop on so I dont know if its right or wrong.

Nick.

[DieselFan]

I repitched for 6500 static, EGT's at 5000 around 1200+ and upto 1300! 6000 - 1100. It was suggested that I service my carbs etc

BUT this was the problem

Becareful of setting pitch too low even tho static is ok for engine, as this unloads the prop in cruise flight and the engine does'nt work "hard" enough! Point is that you can under pitch even though you don't exceed the engines max rpm. Pitching should be a combo of EGT and RPM.

By simply repitching for static 6100-6200 it set EGT's at 1000 and back to norm. I also noticed on config at 6500 that in gusty wx rpm increased by about 1000rpm, in new config 300 max.

It can be very confusing when u see EGT's high yet you think, but my pitch is less which means engine is working less...but as you say it's a symptom of lean.

[Skimmer]

I had a very similar experience on a 503 with a prop set too coarse. Engine pops, spits and is generally unhappy - you think there is some major problem! Getting the RPM up to max 6400 or so ground sorted it all out. I think 2 strokes are more musical instruments than engines!

[nickjaxe]

Sorry with the late response but my internet has been down,

I have the prop sorted now it turns out the engine was actually running rich due the pitch of the prop being to course as somebody else has said,

I found this out after speaking to a rotax engineer over here in the UK,

He was asking what my revs were, when I told him they were 5900-6000 static he said right away this is my prob, the engine was under revving, and now I never Knew this but it causes the engine to run rich, and have this rich mis, if its over reving its makes it run weak,

I used my laser tach to set the static revs to 6200rpm and it was the fine the next time I flew it no burbling in the engine and holds the revs nice and steady no surging,

Not sure if I mentioned it but it turns out the aircraft tach is way out at higher revs but seems accurate at revs upto 3000rpm,

so if you are re-pitching your prop dont use the aircrafts tach.

Nick.