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IO-320 won't go over 2500 rpm

W

Wyo_flyer

Member
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?
 
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.
 
Tach?

Wyo_flyer said:
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?
Click to expand...

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

Skylor
 
Manifold pressure

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
 
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.
 
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.
 
Low RPM

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.
 
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.
 
GeneL said:
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.
Click to expand...

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

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
 
What kind of exhaust system does this engine have, straight through? or muffler?

If muffler, you also might check to see if it is restricted.
 
An engine, any engine, is nothing more than an air pump. Restrict either the intake or exhaust and you harm efficiency.

Add to that the timing and carburetor jetting.

Assuming the hardware is good, it is either going to be restricted intake/exhaust or timing or fueling.

Timing is the easiest to check, so start there.

Good luck and report what you find.
 
Thanks for the replies!! You guys are awesome.

It has vetterman straight pipes without mufflers so I think I can rule the exhaust restriction out.

I am leaning heavily towards fuel and timing. Wish I had a buzz box to do my own timing check but my a&p can help me out on that.

I'm going to do a fuel flow at the nozzle test tomorrow with baby jars and check for even fuel distribution at the cylinders. Also going to check all my fuel lines from the tank to the pump to make sure there are no leaks.

I think I'll also pull the valve covers and see if the valves and springs are in good shape.

If none of that changes anything...I think I'll pull the fuel servo and have it flow tested.
 
You might also check your fuel pressure. I have seen an IO -360 with .150" worn off the intake lobes(yes that is .150" missing from a .350" new/proper spec) and it would still make 2750 RPM on take off. However, the peanut can gas filter just ahead of the engine driven fuel pump would get a slight restriction at about 40 hours and you would only get 2600 RPM with an indicated fuel pressure of about 16 psi instead of the normal 20-26 psi at take off power. As soon as it was serviced, we would get 2750 RPM on the take off run.

There was a thread on here a couple of months ago regarding a new lycoming in France with a similar issue that you describe.
 
Well I got a couple hours to work on it after work today and I think I found two possible causes.

First: did a nozzle flow test and found one injector nozzle that has a really poor fuel stream. The other 3 nozzles have a clean straight flow. The bad injector flowed about 1/4" more fuel than the other 3 for a 30 second flow test. This tells me that I've had 3 lean cylinders and one rich one. So I'm going to try to find a used nozzle.

Second: fuel filter had some gunk in it and found a possible leak in an -an fuel fitting. It's possible I was sucking air. I cleaned and tightened up all the -an fittings.
 
This does sound like exactly what my Carb'd 320 did when the intake lobe started to go on the back to cylinders. Low CHT's and Low power.
 
Assuming a standard nozzle, you can clean the nozzle in Hoppes No 9 gun cleaning fluid and an ultrasonic jewelry cleaner for about 1 hr. If it is dirt this will take care of it. If not the nozzle can be disassembled and checked for obstructions.
 
Well I spent the afternoon tinkering some more and tried cleaning the bad injector again. Soaked it and blew it out with compressed air. Did the flow test again and it's definitely not flowing right. The other three have a nice tight straight stream and this one has a very erratic flow. It is flowing about 1/4" more fuel than the other 3. The 3 good ones flow the exact same amount. I found a used injector that I'm going to try. I'm crossing my fingers that it's really this simple.
 
Well I found a good used injector and got it installed over the weekend. I did a flow test and all 4 injectors are now flowing perfectly.

So I put everything back together and started it up...really no change at idle. Always has idled a little rough. So I did a run up and tested mags. Everything seemed ok. Lined her up and full throttle and it coughed once and took off normal. I stayed tight in the pattern (density altitude of 10,200 feet) and it just felt rough. Tried a few different throttle/mixture settings, and got no real change. Max rpm I could get was 2300...so I set her down and taxied off the runway and did some more throttle/mixture adjustments. It's running really rough at idle, mag drops are normal (125 @ 1700 rpm).

My a&p was standing there the whole time and walked up after I shut it down and said it doesn't sound right. He is firmly convinced that the servo is bad. I asked him about a worn cam lobe or stuck valve and he said it doesn't sound like that. He feels it all fuel. I'm gonna break down and send the servo off to airflow for a flow test and then check intake valve opening.

Could air in the upstream side of the boost/engine pump cause similar rough running and rpm issues?
 
Wyo_flyer said:
Well I found a good used injector and got it installed over the weekend. I did a flow test and all 4 injectors are now flowing perfectly.

So I put everything back together and started it up...really no change at idle. Always has idled a little rough. So I did a run up and tested mags. Everything seemed ok. Lined her up and full throttle and it coughed once and took off normal. I stayed tight in the pattern (density altitude of 10,200 feet) and it just felt rough. Tried a few different throttle/mixture settings, and got no real change. Max rpm I could get was 2300...so I set her down and taxied off the runway and did some more throttle/mixture adjustments. It's running really rough at idle, mag drops are normal (125 @ 1700 rpm).

My a&p was standing there the whole time and walked up after I shut it down and said it doesn't sound right. He is firmly convinced that the servo is bad. I asked him about a worn cam lobe or stuck valve and he said it doesn't sound like that. He feels it all fuel. I'm gonna break down and send the servo off to airflow for a flow test and then check intake valve opening.

Could air in the upstream side of the boost/engine pump cause similar rough running and rpm issues?
Click to expand...

I am not new to engines , but specific diagnostics for bendix FI,yes. SO - - - since the servo just manages fuel pressure to the fuel distributor and thus to the simple orificed injection nozzles, it would seem that there is a pressure specification at the spider. I know there is a 1/8 NPT location that could be used for pressure. After all, doesn't pressure across the nozzle determine fuel flow??

Here is a publication found - that may help. It indicates that the pressure does indeed indicate flow, but you need to pressure vs flow curve to check against.
http://www.precisionairmotive.com/Publications/15-812_b.pdf

Maybe this is redundant, but if you are using mogas, is it possible it is old and degraded? Old gas runs like ****. OK, ratty.

Good luck and keep us informed on your success.
 
Wyo_flyer said:
Could air in the upstream side of the boost/engine pump cause similar rough running and rpm issues?
Click to expand...

A couple folks on the forums have had exactly this issue in the past. Loose fittings in the fuel line let air in but are not loose enough or under enough pressure to leak.

Checking that every fuel fitting is tightened to spec wouldn't hurt.
 
Well servo is off and headed to airflow performance tomorrow. They told me it would be about a week turn around. Sounds pretty good to me. I checked the intake tubes again while I had everything off, pulled the bottom plugs, and changed the oil. A&p cut the filter open for me and it looks good. No metal. I used my bore scope and looked at the cylinders and pistons and all appears normal.

I plan on pulling all the valve covers and checking the valves again. Ran out of time tonight but it's worth look.

I really have been pondering the fuel system and think I may have an air leak somewhere. I remember while flow testing the nozzles there was some air coming out of the nozzles for a while. I'm sure if there is air with the fuel in the metering chamber of the servo it's gotta mess up the flow. I think I better dissect the entire fuel system from tanks to pump to see where I could be getting air.

I do run mogas but I've been flying a couple times a week so I don't believe the fuel is bad. I ran av gas when I first purchased it and it made no difference. But I'm not above draining the tanks and trying a fresh batch of fuel...but the fuel in it should be fresh.

C'mon I just want to try to keep up with an RV
 
Last edited:
Have you done a flow (volume test) at the end of injector line and compared cc's? (no injector installed) Possible fuel distributor/spyder issue.
 
Spider

Bills post above got me thinking, I wonder if you have an injector plumbed into the fuel pressure (fuel flow gauge) port on the spider? Also, I hope you sent the spider in with the servo.
Tim
 
tim2542 said:
Bills post above got me thinking, I wonder if you have an injector plumbed into the fuel pressure (fuel flow gauge) port on the spider? Also, I hope you sent the spider in with the servo.
Tim
Click to expand...

I have checked that and it is all plumbed correctly. This engine came out of a piper apache, and nothing has been significantly changed as far as I can tell.

My a&p and I disassembled the spider 2 weeks ago and he said it looked perfect. No scoring, moves freely. The spring and diaphragm look new. We blew out the injector lines and have cleaned and flow tested all the injectors.

I'm not saying it's not a possibility, I am open to all suggestions but I don't believe that is the problem...I hope I don't eat my words.
 
I spent a number of hours working yesterday. Pulled bottom plugs and they look a fine, possibly a little rich (but this could be due to a long taxi back to the hangar).

Pulled rocker covers and checked valves and springs. I did a poor man's check of the intake and exhaust valves and measured them each at full open. Nothing out of the ordinary.

Disassembled spider and all ss injector lines. Blew everything out. It all looked clean and the piston moves freely up and down.

I found one possiblity while blowing the injector lines. I pulled all the injectors out of the heads and started blowing down the injectors with the lines still hooked up with an aerosol type cleaner. One of the injectors was blowing all of the cleaner out of the air bleed holes on the side of the injector. It's got me wondering if I may need to replace another injector. I had previously flow tested all of the injectors and found no leaks. The injector is clean with no obstructions. I can't test it with fuel because the servo is out. Any ideas?

Found another possibility. My fuel system consists of 2-16 gallon tanks that are behind the seat. The boost pump is behind the seat with a small amount of hose connecting the two tanks then to the pump. There is an clear automotive type fuel filter before the boost pump. The filter always has air in it. I took this all apart and found that I have good fuel supply before the filter (gravity). But when I put the filter back in it only fills about half way with a large air bubble. So I'm positive the pumps are getting air. I have been thinking that I should put a tee in the line before the boost pump a the highest point and run it to the top of the tank to let air out of the system. What do you think?
 
Wyo_flyer said:
It's running really rough at idle, mag drops are normal (125 @ 1700 rpm).
Click to expand...

I'm never happy with more than a 50 rpm drop at 1700. Forgive me if I missed a post but when a I get an aircraft with a rough idle, low rpms, and a mag drop of 125. I'm going to check timing and clean plugs.
 
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