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Carb balancing device?


FastEddieB

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Rotax years ago posted in a bulletin that 80-87 psi for a compression test was fine. I'm still old school at 87 psi. 25% is the standard, but you normally only see a 2-4 psi drop. If you saw 10 psi in a Rotax I would want to know why because that is way past normal. Even Rotax says you need to find out why. 25% in a Rotax would be very serious. Higher psi loss in an air cooled Cont. or Lycoming are far more acceptable and even normal because of the sloppier tolerances, but not in a water cooled tight tolerance Rotax. Once in a while I see a compression test come from someone that has the test pressure and residual the same. Can't be, there has to be a leak of air some where or it would be a differential test. Most likely they need a new set of gauges. You should hear air in the oil tank when it is done properly. Air out the exhaust or intake means you need to rotate the prop 180 degrees. If you set the TDC point exact you don't need to hold a prop either. It will just sit there.

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Rotax years ago posted in a bulletin that 80-87 psi for a compression test was fine. I'm still old school at 87 psi.

I'll be going to rotax school in sebring coming up in june, so I am sure I will be brought current :P

 

As for Tom: that's kinda typical for any engine. Sometimes the rings got a little unseated, a quick run reseats.

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Rotax years ago posted in a bulletin that 80-87 psi for a compression test was fine. I'm still old school at 87 psi. 

 

I do not have my logbook here, but when I last had my compressions checked (by you when you did my hose change) I saw you used 87 psi and I was going to ask about that.

 

I had checked to see if that cylinder had been low then - it had not been.

 

Next time I have the cowling off for any reason I will recheck.

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We have always been taught in schools to warm the loose tolerance air cooled engines and that was the way I always did it until 2 years ago. I did a test 2 years ago with 4 Rotax 912's.  I checked compression cold and I checked the same engines hot. The difference was............................................................... wait for it...................................................................nothing. They were the same. So now i don't worry about it any more. Rotax is a very tight tolerance engine. You'll find more errors in many gauge sets for the testing. If a gauge was off 2-4 psi how would you know without putting it on another test gauge. So people just use their gauges  without a thought to any minor errors.

 

When I teach this stuff in class I usually say a warm engine. It's just more PC.

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How well do you trust those O-O-O-O'Reilly gauges to be accurate and consistent?  Most of that autopart store stuff is made in China and pretty low quality.  One could show 7 and other could show 9 and the real pressure might be 11.  Dialing them into "sync" by substandard gauges could get you farther away from your goal.

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Hi Andy,

 

You're right. Places like Lockwood and a few others buy groups of gauges and test them. Then pair up the ones that read the same. One easy test is to take a pair of gauges and just tee them together and pull a vacuum and see if they read the same. the actual true number isn't as important as the gauges reading equal.

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How well do you trust those O-O-O-O'Reilly gauges to be accurate and consistent?

Well, the one CPS sent seemed equally, if not more, cheap. As indicated before, they did not indicate the same at zero, and the fellow at CPS stated they were looking for a better supplier. The O'Reilly ones do appear more solid.

 

I will try a "Calibrated Fast Eddie Suck Test" when next at the hangar.

 

For now, "close enough for gov't work" comes to mind.

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Well, the one CPS sent seemed equally, if not more, cheap. As indicated before, they did not indicate the same at zero, and the fellow at CPS stated they were looking for a better supplier. The O'Reilly ones do appear more solid.

 

I will try a "Calibrated Fast Eddie Suck Test" when next at the hangar.

 

For now, "close enough for gov't work" comes to mind.

 

The proof is in the pudding, and you have flown the airplane and are happy with it, so I'm sure it's pretty close.  I was just wondering how the quality of the gauges could have an impact.  Not that we are going to go out and buy $300 each calibrated scientific-quality precision gauges to balance the carbs on our 912s.  :)   Good enough is always, well...good enough!  

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We have always been taught in schools to warm the loose tolerance air cooled engines and that was the way I always did it until 2 years ago. I did a test 2 years ago with 4 Rotax 912's.  I checked compression cold and I checked the same engines hot. The difference was............................................................... wait for it...................................................................nothing. They were the same. So now i don't worry about it any more. Rotax is a very tight tolerance engine. You'll find more errors in many gauge sets for the testing. If a gauge was off 2-4 psi how would you know without putting it on another test gauge. So people just use their gauges  without a thought to any minor errors.

 

When I teach this stuff in class I usually say a warm engine. It's just more PC.

Hi Roger!

 

While I agree, it doesn't seem as big of a deal with rotax, we want to make sure we assess the condition of the engine at running tolerances and clearances. Case in point on air cooled engines before hydraulic tappets: if you tested while cold, it might not show a problem, but as an engine heated up and the rods expanded, it would close the rocker arm to valve clearance. If the clearance was not adjusted properly, the clearances would go negative in running condition, killing performance of the engine and greatly accelerating wear. You wouldn't detect this on a cold compression test, but it would show on a hot one.

 

There's just so many unknowns.

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Hi Corey,

 

We get to throw out air cooled engines and not even have to consider them. Hurray! Think motorcycle and you'll be closer. You're only checking the rings and valves more or less and if the value is let's say 87/85 cold then it won't be any different hot to tell you any thing else from warm. Most of the pressures I see are 87 test with 83-85 differential.

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Hi Corey,

 

We get to throw out air cooled engines and not even have to consider them. Hurray! Think motorcycle and you'll be closer. You're only checking the rings and valves more or less and if the value is let's say 87/85 cold then it won't be any different hot to tell you any thing else from warm. Most of the pressures I see are 87 test with 83-85 differential.

 

Yep, hence my part about "While I agree, it doesn't seem as big of a deal with rotax". I would still prefer to know my compression at running temp than cold though, since cold means nothing :P

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The results of yesterday's "Fast Eddie's Precision Calibrated Suck Test":

 

13718331324_7665628f28.jpg

 

I think its even closer than it looks, there being some parallax involved in the photo.

 

Conclusions:

 

1) Close enough for government work. When you're actually doing the test the suction varies a bit, and vibration from the motor comes into play. I don't think you can get it exact exact regardless of the exact accuracy of the gauges.

 

2) Next time someone says, "Fast Eddie sucks!" you can agree!  :P

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  • 4 months later...

Hi Corey,

 

We get to throw out air cooled engines and not even have to consider them. Hurray! Think motorcycle and you'll be closer. You're only checking the rings and valves more or less and if the value is let's say 87/85 cold then it won't be any different hot to tell you any thing else from warm. Most of the pressures I see are 87 test with 83-85 differential.

 

I just had a failed cold compression check on one of our engines. Cylinder 4, 35/80 cold. Tried 3 times, and I could even pull the prop through with moderate effort with the 80 PSI on it. I could not do that to any of the other cylinders.

 

After warming up to spec, cyl 4 picked up to 80/80 and I can't pull it through.

 

It's a brand new engine, and this is the first time it was cold checked, and while running I can't hear or see any problems. I'm not worried, I just figured I would bring that up.

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I have seen a cold and warm compression test show a bad cylinder before, but usually after turning the prop a few times or a quick run everything is normal. I have never had one stay low and I have never seen one that low.

 

I had also scoped it while it was cold, didn't see anything abnormal. I just couldn't get the rings to seat for the test. I could hear the oil tank bubbling like crazy, and could feel the air rushing out of cylinder 2 if I put the back of my hand to it (that's what led me to scoping the bore, I was expecting to see something ugly). Decided I had better warm up the engine before calling it a dud cylinder just to see what happens, and saw the 80/80 with a much less bubbling noise in the oil tank :)

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