Subaru EA81 Engine Low Speed Problems

[techie49]

Hi All,

 

Is there anyone out there that can assist with a problem I have setting up the engine in my Zodiac 601?

 

The engine is a Tom Wickers conversion with twin Bing 64 carbs. Prop is a new Bolly 72 x 60 three blade.

 

The prop is set at 13 degrees pitch. The problem I have is that the damn engine almost shakes the airframe apart at anything under about 2500 rpm. I've carefully dismantled the carbs to check for crap in the bowls - clean. I've removed the idle jet in each and made sure they're not blocked. I've checked the setting of the idle adjuster and set both to 1 1/2 turns out from fully clockwise. Still no good. Yesterday I took the prop off and ran the engine. I slightly adjusted the timing to give smooth running at 1500 rpm which gives a prop speed reduced by the rotax gearbox to about 700 ish rpm. Without the prop the engine runs ok but putting the prop back on causes the shaking. It's only at low speed though. My logic suggests the prop is ok as at high speed it's ok with little vibration. Anyone got any ideas? This three year build is tantalisingly complete but for this problem. Don't suggest buying a Rotax - had I the cash I would have!

 

I'm about to check the prop balance although the Bolly figures supplied suggest it's balanced perfectly.

 

HELLLLLP !!!!!!!!

 

Paul

 

 

[M61A1]

Hi All,Is there anyone out there that can assist with a problem I have setting up the engine in my Zodiac 601?

The engine is a Tom Wickers conversion with twin Bing 64 carbs. Prop is a new Bolly 72 x 60 three blade.

 

The prop is set at 13 degrees pitch. The problem I have is that the damn engine almost shakes the airframe apart at anything under about 2500 rpm. I've carefully dismantled the carbs to check for crap in the bowls - clean. I've removed the idle jet in each and made sure they're not blocked. I've checked the setting of the idle adjuster and set both to 1 1/2 turns out from fully clockwise. Still no good. Yesterday I took the prop off and ran the engine. I slightly adjusted the timing to give smooth running at 1500 rpm which gives a prop speed reduced by the rotax gearbox to about 700 ish rpm. Without the prop the engine runs ok but putting the prop back on causes the shaking. It's only at low speed though. My logic suggests the prop is ok as at high speed it's ok with little vibration. Anyone got any ideas? This three year build is tantalisingly complete but for this problem. Don't suggest buying a Rotax - had I the cash I would have!

 

I'm about to check the prop balance although the Bolly figures supplied suggest it's balanced perfectly.

 

HELLLLLP !!!!!!!!

 

Paul

1. Have you balanced your carbs?

2. If it's in that one rev range, it could be harmonics.......and that could be nasty, do you have a wood prop you could test it with?

 

 

[fly_tornado]

rubber damper failing?

 

 

[techie49]

1. Have you balanced your carbs?2. If it's in that one rev range, it could be harmonics.......and that could be nasty, do you have a wood prop you could test it with?

If you mean using a manometer - no

 

Both carbs are set as near as damn it identically and the engine runs ok without the prop.

 

No - wish I did have another prop to compare with. I'd thought of that one as well. I was thinking of trying a two-blade prop.

 

Paul.

 

 

[techie49]

rubber damper failing?

New and all tightened up nicely .........................(assuming you mean the engine mounts)

 

 

[fly_tornado]

do you have a damper between the engine and the gearbox?

 

 

[crashley]

hi have you checked the pitch is the same on all three blade and that there is no run out also something could be breaking down under load you could try setting the pitch a lot finer and see if it runs smoother under less load

 

 

[M61A1]

If you mean using a manometer - noBoth carbs are set as near as damn it identically and the engine runs ok without the prop.

No - wish I did have another prop to compare with. I'd thought of that one as well. I was thinking of trying a two-blade prop.

 

Paul.

I would look at borrowing a prop if possible, just to rule out or confirm harmonic vibes. Also try to balance the carbs with a manometer or vacuum gauges, the extra load of the prop could bring out carb balance issues that are masked without load.Good Luck

 

 

[techie49]

hi have you checked the pitch is the same on all three blade and that there is no run out also something could be breaking down under load you could try setting the pitch a lot finer and see if it runs smoother under less load

Pitch is set to within 0.5 degrees on each blade. I've got the prop off and I'm trying to check the balance. One of the guys at Tyabb offered the use of a dynamic balancer but I haven't caught him in his hanger yet.

 

The whole situation is peculiar as at speed it's smooth. Just at idle rpm I'm having problems..............................????????

 

Paul

 

 

[techie49]

I would look at borrowing a prop if possible, just to rule out or confirm harmonic vibes. Also try to balance the carbs with a manometer or vacuum gauges, the extra load of the prop could bring out carb balance issues that are masked without load.Good Luck

I will try to get hold of a manometer or similar to check the carb balance. I really hate twin carbs for this reason but the manifold is designed for twins. I just can't pin down what is going on, hence checking prop balance as I can't think of anything else at present.

 

Paul

 

 

[techie49]

do you have a damper between the engine and the gearbox?

To be truthful, I've no idea. The Rotax box came attached to the EA81 on a purpose made adapter plate in thick aluminium. The engine came off a Cobra Arrow as the owner apparently wanted something bigger. It's got low hours since original installation so I've no reason to suggest it was having problems initially.

 

Paul

 

 

[fly_tornado]

it really sounds like your carbs are out of tune, could be that the throttle cables are stretching. the rubber damper would smooth out a lot of that shaking. without some sort of device to absorb torsional vibration your gearbox will wear out from that shaking pretty fast.

 

 

[facthunter]

Putting the load on the engine will change the vaccuum. I think your mixtures are out just at or off idle. Keep clear of the prop and play with the idle mixture screws. They don't have to end up an equal number of turns out. Your throttle stops should be set equally to flow the same amount each carb. ( balance) Then ensure the cables actuate together for when you come off idle. I don't think it is the prop. Nev

 

 

[peter]

I believe this may have been my engine! If so, it was from my Cobra Arrow initially. It was professionally rebuilt(at a performance workshop in Adelaide, and would have had around 100 hours on it) and was very smooth at everything from 1500rpm (that's where my idle was set) up to 6500rpm.It was running through an AMAX belt redrive when I had it.

 

Peter

 

 

[M61A1]

Putting the load on the engine will change the vaccuum. I think your mixtures are out just at or off idle. Keep clear of the prop and play with the idle mixture screws. They don't have to end up an equal number of turns out. Your throttle stops should be set equally to flow the same amount each carb. ( balance) Then ensure the cables actuate together for when you come off idle. I don't think it is the prop. Nev

Just to clarify- The mixtures screws should be adjusted to achieve best idle, then the balancing is carried out by adjusting the idle stops ( make sure your cable adjusters have a small amount of free play so as not to interfere with the idle stops) to achieve equal vacuum, and then adjust the cables to make sure there is equal vacuum ( or as close as you can get) as they open.

[techie49]

I believe this may have been my engine! If so, it was from my Cobra Arrow initially. It was professionally rebuilt(at a performance workshop in Adelaide, and would have had around 100 hours on it) and was very smooth at everything from 1500rpm (that's where my idle was set) up to 6500rpm.It was running through an AMAX belt redrive when I had it.Peter

Hi Peter,

 

Interesting.

 

When I got it from Tom about three years ago, it had two small radiators and a home made air box. It came with a Rotax gearbox too. I have replaced the rad with a conventional one slung underneath. Can't understand what's going on at present. At 1500 it shakes the whole aircraft when the prop is on ...........

 

Paul

 

 

[bushcaddy105]

Just a thought - what ignition system are you using? Is it over-advancing in this rev range? Some years ago I experimemted with timing on my EA81 and found that it needed both centrifugal and vacuum advance control, just like in a car. It runs a Bolly 3 blade through a belt redrive and is silky smooth from 1000 to 5000 engine rpm. It runs a single big SU carby, torquey cam and 1mm off the heads. Now done 550 trouble-free hours.

 

Another thought - what flywheel do you use? In my experience, EA81's need a bit of weight in the flywheel to remain smooth. I use a lightened steel one - are you using a superlight aluminium one?

 

Otherwise, it sounds like you may have a torsional vibration issue brought about by the relative rigidity of the Rotax gearbox and what sounds to be a very modified engine. Torsional vibration will quickly break critical parts - beware! I wouldn't dare think of running up to 6500 rpm, even if I could! 1500 rpm idle sounds very high for an EA81, even a modified one.

 

I trust these comments will give you some ideas.

 

 

[techie49]

Just a thought - what ignition system are you using? Is it over-advancing in this rev range? Some years ago I experimemted with timing on my EA81 and found that it needed both centrifugal and vacuum advance control, just like in a car. It runs a Bolly 3 blade through a belt redrive and is silky smooth from 1000 to 5000 engine rpm. It runs a single big SU carby, torquey cam and 1mm off the heads. Now done 550 trouble-free hours.Another thought - what flywheel do you use? In my experience, EA81's need a bit of weight in the flywheel to remain smooth. I use a lightened steel one - are you using a superlight aluminium one?

Otherwise, it sounds like you may have a torsional vibration issue brought about by the relative rigidity of the Rotax gearbox and what sounds to be a very modified engine. Torsional vibration will quickly break critical parts - beware! I wouldn't dare think of running up to 6500 rpm, even if I could! 1500 rpm idle sounds very high for an EA81, even a modified one.

 

I trust these comments will give you some ideas.

Ignitions are two home brew electronic units. Both totally different deliberately. One is cap discharge and the other a transistor coil driver. Using two independent coils and a MSI coil combiner.

 

The engine is straight from a Cobra Arrow, from Tom Wickers, who designed it. The flywheel is standard and everything is EA81 apart from the twin Bing carbs on a specially made manifold. There are supposedly at least 60 of these set ups out there.

 

I'll have a play with the idle adjust on each carb as suggested in #15 above. I've already tried to match the throttle cable actions as closely as possible but trying to do any adjustment with the engine trying to shake itself off the aircraft is very difficult. The idle adjust is under the carb which doesn't help.

 

Paul.

 

 

[facthunter]

m61A1 . I did not intend to imply doing it in that order. You need to balance the two carbs then adjust the mixture. The mixture is sensitive to the throttle butterfly position so if it is altered the mixture screw may need an extra adjustment subsequently. You should probably start by making sure the butterfly is closing fully with the idle stop screw backed off sufficiently. You may have to "centre" it by loosening the screws and retighten them with the butterfly fully closed. The carb will have to be taken off to access these screws. A lot of people don't check them carefully, but it does affect idle. Nev

 

 

[bushcaddy105]

Another thought! Carefully check the diaphragms in the carbies - even a slight split in one could produce similar symptoms. Hours of use is not an indication of their condition, but age and particularly lack of use causes them to deteriorate.

 

Hope you soon get to the bottom of your problem

 

 

[M61A1]

m61A1 . I did not intend to imply doing it in that order. You need to balance the two carbs then adjust the mixture. The mixture is sensitive to the throttle butterfly position so if it is altered the mixture screw may need an extra adjustment subsequently. You should probably start by making sure the butterfly is closing fully with the idle stop screw backed off sufficiently. You may have to "centre" it by loosening the screws and retighten them with the butterfly fully closed. The carb will have to be taken off to access these screws. A lot of people don't check them carefully, but it does affect idle. Nev

No, but I was implying that, that is the best way. I do agree though that it pays to check your throttle butterflies, and often, yes adjust the idle mixture again after balancing. Keep tweaking back and forth until you get no more improvement.