IO-320 won't go over 2500 rpm

[Wyo_flyer]

First time poster here on vans forums but I've been studying for a long time. I also don't have an RV but someday....hope to grab a -7.

I currently own a rans s-16 shekari with an io-320 that I have owned for a few months. I have put over 50 hours on it since March. I purchased it from a guy that lived at sea level. I live at 7200 feet in Wyoming. I knew going into it that I would have some tweaking to do based on my elevation. The plane at sea level was only getting 2500 rpm at wot. I figured it was just pitched too coarse (68-74) and ordered a new ed sterba prop (68-70) as soon as I got it home. I put the new prop on an barely gained 50 rpm....so I sent it back to Ed, he re pitched it to a 68-64. I put it on and had zero rpm change. What's going on here? It's like it's stuck at 2500 rpm wot straight and level. I can get it to go to 2700 in a 2000 ft/min dive but pulling out to straight it goes back to 2500. No change in speed between 70 pitch and 64 pitch.

I have checked all the simple stuff, compressions all 76/80, cleaned injectors, checked spider, blew out all injector lines, plugs look good (slightly lean), cleaned air filter, exhaust is clear, throttle cable adjusted correctly, mixture cable good.

My a&p thinks it's the servo, I tend to agree but just wanted some other opinions. I am leaning toward the fuel servo because I have very little lean adjustment at cruise and wot. I can only lean 1/2-3/4" from full rich (cable is adjusted correctly).

Egt's
1 - 1325
2 - 1400
3 - 1325
4 - 1300
Cht's
1 - 300
2 - 300
3 - 300
4 - 350

Any ideas/opinions why I can't get over 2500 rpm?

[Neal@F14]

Might want to also check the valve lift to determine the health of the camshaft lobes. If the lobes are worn, that could also explain low rpms.

[skylor]

Originally Posted by Wyo_flyer

First time poster here on vans forums but I've been studying for a long time. I also don't have an RV but someday....hope to grab a -7.

I currently own a rans s-16 shekari with an io-320 that I have owned for a few months. I have put over 50 hours on it since March. I purchased it from a guy that lived at sea level. I live at 7200 feet in Wyoming. I knew going into it that I would have some tweaking to do based on my elevation. The plane at sea level was only getting 2500 rpm at wot. I figured it was just pitched too coarse (68-74) and ordered a new ed sterba prop (68-70) as soon as I got it home. I put the new prop on an barely gained 50 rpm....so I sent it back to Ed, he re pitched it to a 68-64. I put it on and had zero rpm change. What's going on here? It's like it's stuck at 2500 rpm wot straight and level. I can get it to go to 2700 in a 2000 ft/min dive but pulling out to straight it goes back to 2500. No change in speed between 70 pitch and 64 pitch.

I have checked all the simple stuff, compressions all 76/80, cleaned injectors, checked spider, blew out all injector lines, plugs look good (slightly lean), cleaned air filter, exhaust is clear, throttle cable adjusted correctly, mixture cable good.

My a&p thinks it's the servo, I tend to agree but just wanted some other opinions. I am leaning toward the fuel servo because I have very little lean adjustment at cruise and wot. I can only lean 1/2-3/4" from full rich (cable is adjusted correctly).

Egt's
1 - 1325
2 - 1400
3 - 1325
4 - 1300
Cht's
1 - 300
2 - 300
3 - 300
4 - 350

Any ideas/opinions why I can't get over 2500 rpm?

What are you using for a tachometer? Could it be that your tach isn't indicating over 2500 for some reason?

Skylor

[tim2542]

What's the manifold pressure and fuel flow?
You should see manifold pressure up near what it is static on a low approach at WOT.
Also the fuel flow needs to meet spec for that engine.
Tim

[gasman]

Manifold pressure and rpm will give you power in % output. A compression test..... (reading the compression of the motor as it is cranked with the starter) can help you find a bad cam, and even help you to find an out of time cam that has been installed a tooth off.

[Wyo_flyer]

I have an analogue tach, but I double checked it with two different hand held digital tachs in flight. One of them you put a little reflective target on the prop. Both tachs match my analogue.

I don't have a fuel flow gauge...wish I did but sounds like I may need to invest in one.

Is it somewhat common for the cam lobes to wear down? The engine came out of a twin piper Comanche, and has roughly 850 hours since new.

[GeneL]

Check valve spring tension, also slightly sticking exhaust valves (wobble test). This is one of the few things that won't be affected by density altitude.

[Wyo_flyer]

Manifold pressure always seems low but sitting at rest in the hangar at my airport it's around 22-23 most the time. That's the tough part about living 7000 feet above sea level...I'll never see 29" of manifold.

[Wyo_flyer]

Originally Posted by GeneL

Check valve spring tension, also slightly sticking exhaust valves (wobble test). This is one of the few things that won't be affected by density altitude.

Can you briefly explain how to test this? Is it just to remove valve covers and "hand wobble" the springs to check for play?

[tim2542]

A worn cam is a possibility and you can cut the filter open and look for ferrous metal in the element by washing it in solvent and then running a magnet around in the solvent. If one or more is worn enough to cause a lack of power there will be a lot of steel in there.
But there are many other possibilities. Just off the cuff, ignition timing, throttle/mixture cables not giving WOT or full rich, Air filter issues, fuel flow insufficient, perhaps contaminated flow divider or injectors, low compression, insuffienct valve clearance, etc.
One suggestion, see if you can is borrow a prop from another 320 of the same rated power, check it's static rpm before you swap it out. Yours should turn it the same rpm.
Tim