Engine rough at 2400 RPM

[N6018Q]

A little background: I bought a 1966 M20E Super 21 last December. From the complete log books, I know that the engine was replaced in 1978 with a new IO- 360A1B6 and now has ~1050 hrs, with 2222 hours on the airframe. It was flown regularly until 2005 and then appears to have not even had an annual done until it was sold in 2013 to the guy I bought it from. He flew it an average of 10 to 15 hours/year. I know now (from about 30 hours of following this forum) that one of the worst things for these planes is to sit and not fly. My mechanic just completed the first annual which started as soon as I purchased the plane. I spent ~$5000 (which I expected as the last few annuals were basically rubber stamped). Mags rebuilt, new plugs, wires, hoses, etc. My CFI and I have flown a total of 6 hours over the last 2 weeks with no problems. Today we were in the pattern after a 2 hour flight at 2400 rpm when the engine suddenly started backfiring and running very rough. The runway was assured, and as soon as I touched the throttle, it immediately smoothed out. We landed and spent the next 20 minutes running up the engine on the ramp. At least 3 more times, when we hit 2400 rpm, it did the same thing. Both increasing and decreasing rpm immediately solved the issue. We let the engine cool completely and could not duplicate the anomaly again. Any ideas as to what could cause this? 

[kmyfm20s]

Looking at the maintenance you listed I would suspect a bad magneto. Did you do a mag check on the ground? If you take it back up in the air do an in Air mag check. 

[carusoam]

We have a lot of ideas...

It would help if you can supply some engine details from an engine monitor... Got one?

Expect to answer a lot of questions...

IO360s hate sitting.  Get familiar with cam lobes and cam followers corrosion...

Start out with Spark plugs, move on to injectors, finish the studies with magneto age/hours...

The usual discussions...

Best regards,

-a-

[N6018Q]

Just had the mags rebuilt last month in Tulsa.  We checked those first and they both dropped the expected 75-100 rpm. Brand new spark plugs also. I do have an older JPI with both CHT and EGT monitoring and nothing was out of the ordinary. We will do an air mag check tomorrow. My CFI is also a mechanic and his initial suspect was the fuel servo. We considered injectors but it seems odd that it only happens at 2400 rpm

[carusoam]

Speaking of fuel distribution challenges...

Got a fuel flow gauge?

Is it acting strangely?

A jpi700 is a great basic tool.  See if you can down load the data and find the flight segments of the quirky flight.

Whenever you have engine trouble, A JPI can usually catch it.

 

Wait til you see how many people can pick out the problem you may be having...

Are you familiar with sticky valves?  A JPI can find them for you...

 

Just some ideas that people have experienced around here.

 

post some data when able...

 

PP thoughts only, not a mechanic...

Best regards,

-a-

[N6018Q]

I do have a fuel flow JPI and a separate JPI engine monitor but they are much older than the 700 model.  and yes the fuel flow one is acting weird but I haven’t had time to learn exactly how to program it. I was planning on spending more time with it before flying xcountry. I’m not familiar with sticky valves but will mention it to my mechanic tomorrow. I really appreciate all the ideas

[carusoam]

Keep in mind the things I listed are not related to your specific situation...

Without some data or specific information, the best I can do is guess, and give things to look for.

I hope your mechanic can do better than this.

If your mechanic can help you get data out of your JPI, that would be helpful...

The engine monitor can

• identify if one of your eight plugs is misbehaving...
• identify if a mag isn't behaving the way you expect...
• Air leaks into an intake will also show...

You are indicating something is happening at 2400rpm...

Something  is making this happen.  Data usually points the way...

Best regards,

-a-

 

[carusoam]

Summary...

1) Plane is new to you...

2) engine doesn’t run properly at 2400rpm after a 2hour flight.

3) Problem fixes itself by changing rpm...

4) Raw Fuel is getting to the muffler, (typical description of a back fire)

5) technicality...  a backfire is fuel igniting in the intake...  a frontfire would be fuel burning in the exhaust

6) Sounds like something is causing a rich condition at 2400rpm and dumping unburned fuel out of a cylinder...or something is not igniting it properly...

7) What MP setting were you using when this occurred?  Low power nearing the traffic pattern?

Some engines have been known to dump excess fuel to the muffler generating popping sounds as the excess fuel burns..  there are settings to be adjusted for this as well...

Hope that gets you closer...

Best regards,

-a-

[Guest]

5 hours ago, N6018Q said:

Just had the mags rebuilt last month in Tulsa.  We checked those first and they both dropped the expected 75-100 rpm. Brand new spark plugs also. I do have an older JPI with both CHT and EGT monitoring and nothing was out of the ordinary. We will do an air mag check tomorrow. My CFI is also a mechanic and his initial suspect was the fuel servo. We considered injectors but it seems odd that it only happens at 2400 rpm

I would not recommend flying the plane until you can make it run smoothly at all RPM’s.

Clarence

[Yetti]

What Clarence said.

To be more specific.   Let's rephrase the problem.   At a high RPM high fuel flow situation your engine stumbled.  Where else do we do that? At Takeoff, so don't try one of those till you know.

Problem solving:

Fuel not flowing at high rate -  Check 2 pumps, 4 screens,  Tank drains and fuel selector.  Check for water in tanks.  Did roughness happen after turns that would move water around in the tanks?

Stuck valves usually happen when the engine is cold.   Cam failing is usually slow loss of power over weeks or months.

Spark plugs. fouled and cleared-  Do you lean aggressively on the ground?, Could one of your magnetos grounded out (turned off) - key switch test.  Check pleads to make sure they are tight.  There is a safety to ground if they were to come out.  Check wire harness

Fuel servo- do a search here for malfunction.  Also could have injested something.  Pull air filter and check box and expensive rubber boot.

 

 

[Greg Ellis]

N6018Q — Read through the topic on this forum entitled Fuel Servo Replaced—Everthing Changed.

some of the responses sound a little like what is happening to your airplane.

 

[jetdriven]

If you have a spot in the throttle travel where it runs poorly like you said, i'd suspect a bad fuel servo or divider. In many  of these RSA-5 injection units, you may notice a little of a dead spot when you bring the throttle up for runup. For me its typically around 1400-1500 RPM.  You push up the throttle and the engine accelerates, then pauses at 1500, then comes on strong at 1700 RPM and operates normally everywhere else. I think its leaning out when the fuel injection switching from the low speed circuit to the high speed circuit.   Also,  the flow divider has a diaphragm, a needle, seat, and spring.. At low demand (idle power or near), this spring overcomes the fuel pressure on the diaphragm and the needle is forced into the seat, this mechanically forces the fuel to divide evenly across 4 injectors. The fuel pressure is too low to do this evenly.. Again, at higher power settings, the fuel pressure overcomes the spring, and the needle retracts and allows straight fuel pressure to go to the injectors, no interference. If this needle were to stick, you may have real fuel distribution problems in flight.

 

Edited April 8, 2018 by jetdriven

[N201MKTurbo]

The fuel servo can only affect fuel flow to the injector nozzles. Anything it does will affect all cylinders equally. It can only provide too little fuel, too much fuel or a fluctating fuel flow. All of these would be obvious by changing EGTs and fuel flows. Fuel pump performance can be verified with the fuel pressure gage. As long as the fuel pressure is in the green the servo will have enough fuel to operate.

Did you notice the fuel pressure or EGTs while the engine was running rough?

Rough running is almost always ignition related, mostly spark plugs. Did you switch mags when it got rough? 

If it is an ignition problem it mill most likely change character when you change mags. If it is a stuck valve your CHTs and EGTs will quickly drop on the bad cylinder.

[Browncbr1]

If 1978 (and a lot of sitting) was last time flow divider and servo had been overhauled, I’d just send them out for overhaul.  It’s not too expensive.   I would also pull your injectors off and do the Dixie cup test and toggle your fuel selector while pumping to ensure no contamination.  Soak injectors in hopps as well.   

[N201MKTurbo]

Just to clarify how the flow divider works:

I had this discussion with the Precision Airmotive engineer at the recient Southwest Aviation Maintenance  Symposium. The Symposium is mostly an IA refresher course and trade show.

The flow divider has no effect on flow distribution except at idle. Above ~2 GPH it is just a manifold. Nothing in it is calibrated or adjusted in any way. 

With no pressure at the divider the plunger fully covers the outlet ports. At idle fuel flows, the plunger will rise enough so the pressure on the diaphragm equals spring pressure. The higher the fuel pressure the higher the plunger has to rise to keep the diaphragm and spring in balance. Once the plunger rises high enough to fully expose the outlet ports it is totally out of the circuit and the flow distribution is controlled by the orifices in the nozzles. 

[Dream to fly]

12 hours ago, N6018Q said:

A little background: I bought a 1966 M20E Super 21 last December. From the complete log books, I know that the engine was replaced in 1978 with a new IO- 360A1B6 and now has ~1050 hrs, with 2222 hours on the airframe. It was flown regularly until 2005 and then appears to have not even had an annual done until it was sold in 2013 to the guy I bought it from. He flew it an average of 10 to 15 hours/year. I know now (from about 30 hours of following this forum) that one of the worst things for these planes is to sit and not fly. My mechanic just completed the first annual which started as soon as I purchased the plane. I spent ~$5000 (which I expected as the last few annuals were basically rubber stamped). Mags rebuilt, new plugs, wires, hoses, etc. My CFI and I have flown a total of 6 hours over the last 2 weeks with no problems. Today we were in the pattern after a 2 hour flight at 2400 rpm when the engine suddenly started backfiring and running very rough. The runway was assured, and as soon as I touched the throttle, it immediately smoothed out. We landed and spent the next 20 minutes running up the engine on the ramp. At least 3 more times, when we hit 2400 rpm, it did the same thing. Both increasing and decreasing rpm immediately solved the issue. We let the engine cool completely and could not duplicate the anomaly again. Any ideas as to what could cause this? 

I posted pictures of my fuel servo and what it looked like when I was having the same issue as you but with the backfiring I had black smoke and increments of throttle movement that allowed smooth engine operation.  What turned out to be the problem was that the servo which for simplified terms is a throttle plate had seen better days and the plates pivots had literally worn the bushings oval shaped at certain positions of throttle control.  Moving the throttle caused the incoming air flow to position the plate centered and things would work normally, move it to a bad spot and the plate would settle into the wear and screw up air flow and cause crazy unexpected performance issues.  Mine was evident when I posted the picture and @Yetti and several others noted my enrichment thumb wheel was totally adjusted to one side.  Right before I took it off to replace I actually had fuel running out of it.  For years the previous owner was compensating for the wear by adjusting the enrichment knob.

- I would look there since you already have had the ignition system rebuilt and throttle movement "fixes" the condition.

- If you had valve train issues "like sticky valves"  or worn lobes it would not come and go immediately with throttle input.  There would be delay or continued issue.

- just a guess and .5 cent. 

 

 

[N6018Q]

Many thanks to all who took the time to share your collective expertise. My CFI/mechanic and I  tried ,and failed, again this morning to find any sign of roughness or other anomalies. Some asked if fuel pressure, egt, cht changed during the event. It only lasted about 15 seconds but seemed an eternity and we were both scanning every gauge at least twice and could not find anything out of the ordinary. My old JPI only displays 1 temp at a time and the CFI stepped through all 8 temps twice, all were good. Looks like a servo overhaul is in my immediate future. Will report for historical purposes when complete

[RustyNance]

May sound odd, but I've had something similar in a Franklin.  Check valve adjustment..

[N6018Q]

One thing I forgot: the fuel pressure dropped very slowly after engine shutdown. It took about 10 minutes to bleed completely down. Later starts and run ups had normal pressure bleed offs. Don’t know if that means anything....

[ArtVandelay]

One thing I forgot: the fuel pressure dropped very slowly after engine shutdown. It took about 10 minutes to bleed completely down. Later starts and run ups had normal pressure bleed offs. Don’t know if that means anything....

10 min is normal, at least for me?

[Guest]

11 hours ago, RustyNance said:

May sound odd, but I've had something similar in a Franklin.  Check valve adjustment..

Engines installed in Mooney’s use hydraulic valve lifters, nothing to adjust.

Clarence

[N6018Q]

6 hours ago, teejayevans said:

10 min is normal, at least for me?

I haven't timed it, but it seems it only takes a minute or so for most of the pressure to bleed off normally.

[Brian Scranton]

On 4/8/2018 at 10:05 AM, N201MKTurbo said:

The fuel servo can only affect fuel flow to the injector nozzles. Anything it does will affect all cylinders equally. It can only provide too little fuel, too much fuel or a fluctating fuel flow. All of these would be obvious by changing EGTs and fuel flows.

This didn't happen to me...just saw a burp/backfire at 2300 on run up...no fluctuating EGTs or FFs. But the servo was junk. 

 

[N201MKTurbo]

3 minutes ago, Brian Scranton said:

This didn't happen to me...just saw a burp/backfire at 2300 on run up...no fluctuating EGTs or FFs. But the servo was junk. 

 

Huh, at runup you are still at a low enough fuel flow that it could be a stuck or plugged divider. All it can do is change how much fuel is delivered to the cylinders. It has no idea what RPM you are running just what the airflow through the throat of the injector is. The idle circuit could be coming apart and dumping excess fuel through the system. Once again all it can do is put out to much fuel or to little fuel. Both of which can make the engine stumble.

[Brian Scranton]

2 hours ago, N201MKTurbo said:

Huh, at runup you are still at a low enough fuel flow that it could be a stuck or plugged divider. All it can do is change how much fuel is delivered to the cylinders. It has no idea what RPM you are running just what the airflow through the throat of the injector is. The idle circuit could be coming apart and dumping excess fuel through the system. Once again all it can do is put out to much fuel or to little fuel. Both of which can make the engine stumble.

This was high power, 2300 rpm, and it would throw a shit ton of fuel into the cylinders in a burst that would result in a backfire and 12 inches flames coming out the exhaust. Replaced the servo. No more funny car flames or backfire.