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Guest Message by DevFuse

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rebuilt twin carby EA81in gyroplane isnt quite right, ideas?

ea81 twin carb

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8 replies to this topic

#1 flying high

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Posted 21 April 2014 - 11:08 PM



My gyroplane has an EA81 twin carb engine that was overheated to siezure and has now been rebuilt but it  runs rough and does not develop the power it should.


The rough running does not allow an idle below 1200 RPM as it causes the gearbox to chatter, previously it would idle down to 800 RPM. The engine seems to smooth out as the revs increase but the power is down as the new engine will not rev past 4800 RPM whereas previously it would go to 5400 RPM with the same pitch on the propeller.  The engine was 700 hours old before this rebuild with compression significantly lower than has now been achieved.


The work done

Cylinder heads professionally machined and rebuilt, new valve seats, valves and valve springs

Cylinder blocks decked, bored 0.5 oversize.

New pistons and rings

Pushrods checked for straightness

Pistons, rods and crank rebalanced

Camshaft checked, it is believed to have been mildly ground in the past but I did not measure it up when I had it out.

Reconditioned carburettors with butterfly shafts bored and bushed, Hitachi carbs, float levels correctly set.

New sparkplugs, distributor cap and points, points were set with dwell meter.

New spark leads were not replaced as they were only 30 hours old. One lead measured high resistance and was replaced. All 5 leads were then exchanged with a set from another engine that is running well without improvement of symptoms.

Alternator reconditioned.

New oil pump

Waterpump approximately 30 hours old.

Battery is 30 hours/12months old and in good condition.

Engine was rewired approximately 30 hours ago, some was heat damaged and repaired.


Spark plugs,coil and points have been replaced yet again in sequence without improvement.

Currently cylinder compression ranges from 175 psi to 195 psi.

There is no water in the oil or other indication of head gasket issues.

Ignition timing is set at 30° BTDC at 3500 RPM, this results in 4°BTDC at 1200 RPM (with vacuum line attached). Distributor has not been rebuilt but timing light gives a very solid and stable visibility of marks on the flywheel up to 4000 rpm, and was not tested any higher.

Valve clearances have been checked and rechecked at 0.25mm inlet and 0.35 mm exhaust.

I have sprayed carby cleaner liberally around all manifold and vacuum connections looking for a leak without success.

I have balanced the carburettors with a vacuum meter.

I have disconnected the carburettors one at a time and sealed the air intake forcing each carburettor to be the only carburettor in turn, no change in symptoms.

Carburettor idle solenoids were disconnected in turn with a significant deterioration of the idle in both cases.

I run the engine on 98 octane fuel with a valve seat protection additive.

The engine does not overheat when running.

Oil pressure is good.

I cannot hear any pinging or knocking but engine is run without a muffler so these would only be heard at low rpm.


Is it possible for symptoms to be caused by missaligning the camshaft by one tooth? Would the compression suffer if this error was made? I don't believe this is at all likely as I remember viewing the alignment mark through the large bevelled hole on the crank when reassembling but if somebody knows the symptoms of this error it would save some hair pulling on my part.


Otherwise I am devoid of ideas having replaced or swapped out everything I can think of.


Any suggestions out there?


Cheers and thanks




#2 jono


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Posted 22 April 2014 - 04:16 AM

is everything set as it was on the last engine build ?


You most likely know an engine in a vehicle it was designed for likely runs at 10 or 12 DBTDC at 750 - 800 rpm.


I have recently comp tested one EA81 at 510,000km and it was all 190 in 3 pots , 180 in the remaining.


Another of unknown age was 180 in 3 pots, 170 in other both were idlers about 750 rpm

An EA82 I have just built up not touching short motor returned 20 psi less on one pot than the other three, 185, 165 - it still idles fine.


Have you tried an AFR meter on it to see if it can guide you somewhere further ?


Seize mid air ??


Ever seen the watchdog TM2 or TM4 from the Signals people in Brisbane? Helps avoid some overheats.

You've obviously run the twins before and theyve been fine ? Do you know whose kit went through them ? Fuel miser ? And that each gasket was replaced with one like for like, and not a mix up.


Never had any trouble with my twins, never thought to shut one down either, but when you did - how ? Shut of its fuel supply as well as block its nose ? Can still draw air in through fuel jets once bowl is emptied, via chamber vent, no, you blocked off at the top .


Is it a std EA81 with the twins or a true EA81S with intakes at the centre of the head rather than intake valve at each end of th heads?

Some may also ask if it is solid or hydraulic lifters as all Oz delivered were solids, imported used engines scored hydraulic.


Did it feel right in the begining since the build ? Nice and tight and torquey ?

Talk to engine builder ?


Did balancing the twins make a difference ? Thought balancing was really for multiple carbs that were not interconnected in the manifold. Think I used HT413 and picked through the gaskets to cover a wide range of Hitachis to kep everything the same when it went back together.


Was the carb rebuild chasing this problem or done at same time as engine rebuild ? making it harder to diagnose :)

It is a different specced engine now isn't it, but should rev and idle - must be in the carbs - the idle, surely ?


I run my EA81's with a 16/56 cam and got engine to op temp and gacve both in and ex 4 thou

Edited by jono, 22 April 2014 - 04:29 AM.

#3 Dirk


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Posted 22 April 2014 - 05:29 AM



I know nothing about Gyroplanes but If it were me I would be looking at the ignition timing.


My thoughts are:  Ditch the VAC advance and advance the timing to 36 degrees at 3600 rpm +-.  If this helps you may choose to dial the timing in by feel/ear.


Personally I think VAC advance is a waste of time.  The mechanical advance does a more than adequate job.  With modified carbs and a mild cam you may not even be getting a reliable VAC signal.


Also check to see if you have the right Dizzy cap.  Is it identical to the old one?  I have noticed that depending on the year of the engine some are taller or even have the terminals offset.





#4 Dirk


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Posted 22 April 2014 - 05:32 AM



Forgot.  Also check the new points are identical.  Again these are not always the same and if they are wrong they could be breaking out of sync with the rotor and cap terminals.





#5 flying high

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Posted 23 April 2014 - 09:12 PM

Hi and thanks for the responses,


I'd really like to hear from somebody who can tell me the running characteristics of this engine if the camshaft is re-assembled one tooth "out".


Trying to do this in order,

Jono, it didn't quite seize in midair , power dropped, oil pressure dropped, my wife landed it in a farmers paddock and it stopped just on landing. Carburettor done by old reputable firm in Sydney, I don't want to use the name, and yes I did block off the carburettor, an idea born in desperation which did indicate that either both have the same fault or neither has a fault. Standard EA 81 with twins. Carby's were done chasing the problem as they were not adjustable when adjustment was attempted. The twin carbs on the single manifold still need to be balanced. Problem was there from beginning after rebuild and did not change with the rebuild of the carbys so I would expect that neither carby has a fault.


Dirk, my feeling is that it is in the electrics as well, one of the recommended methods for doing the timing in aircraft is to take it over 3500 RPM and set it between 28 and 32° before top dead centre, this amount of advance is considered safe. At this RPM the vacuum advance should not be doing a thing anyhow but yes you're right it could be taken off and possibly give me a slightly harder start but I can't imagine the vacuum is the problem as it exists throughout the rev range. The timing light flash onto the flywheel is rocksolid indicating that the distributor is not worn in the shaft and the advance does not change between 3500 and 4,000 indicating that the weights are fully extended. The distributor cap, points and leads were the ones off the old engine and were only changed trying to chase down the problem and they do look identical.


I'm planning to go to the gyro this weekend with a vacuum gauge and the dial gauge to check a few more things.


Jono, not too sure about the reliability of your compression gauge, those numbers do not fit with any of the aircraft rebuids I know of and somewhat out of kilter with my workshop manual. According to the manual my readings are high and the variance is well within range.


Thanks, any more ideas?

#6 Dirk


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Posted 24 April 2014 - 04:51 AM

Good evening!


Have a look at this.  It may allow you to do a rudimentary check on the cam shaft.  http://www.kelford.c...reeing-your-cam


If the camshaft was re-ground at some point, maybe they used a different reference to the original marks........maybe.  Don't suppose you happened to check the alignment when you first stripped the engine down?


BTW.  I have chatted to the guys at Kelfords.  They are very nice and quite generous with their time and free advise.  Could be worth asking them.


Other thoughts:  Is there any way that your timing marks have changed?  I changed my flywheel once and didn't notice that the timing marks were about 20 degrees retarded compared to the original flywheel.  This had me stumped for days.


Is your VAC advance connected as original?  Is it direct to the manifold or ported through a carb?  Judging by your timing numbers it is ported to the carb (zero vac at idle). If so, is it connected to the correct port on the carb?


Clearly you had this engine running well to begin with.  Other than reconditioning and fine tuning parts, you have not (knowingly) changed anything.  Must be something simple....


Cheers and good luck.



#7 flying high

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Posted 25 April 2014 - 03:43 AM

Thanks Dirk

I was going to fumble through this process with a fair degree of ignorance, I expect that link will give me a much greater chance of success.

I'll try to give a decent report after the weekend.

Many thanks


#8 ivans imports

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Posted 25 April 2014 - 10:04 AM

Replaced valve seats is big red flag this is almost impossible without subarus tooling just went thru this with airplane ea81 the valve seats had burned under the sides of them causeing them to drop out and smash valves. The cam being out one tooth would cause this to did you line up the chamfered hole on crank with dot on cam ? could also be decking the heads and block messed with valve geometry but doubt it

#9 flying high

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Posted 27 May 2014 - 11:14 PM

Hello all, apologies for being so slow in getting back to this I've just been flat out.

I went the hangar and checked everything I could including the cam and I concluded all was good. So that left me with only one thing to do so I started pulling it down. I intentionally removed the reduction gearbox first and restarted the engine. It purred like a kitten. So after all that time and fiddling about it wasn't the engine at all.

I've pulled the gearbox down and found the sprag clutch inside had just started to fail. As the sprag is not available that's the end of the gearbox.

I'll sort out the next gearbox over the next couple of weeks.


Sincere thanks for the help, I learnt a great deal about cam timing and other bits and pieces so it wasn't a total loss.



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