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Roughness at low RPMs

1flyingyogi

Well Known Member
I'm getting a lot of roughness/ vibration between 1100-1400rpms. It seems to be worse after the engine is fully warmed up and in hot weather. And worse in the air than on the ground. At start up (while engine is still cold/ cool), it's not bad at all. I feel it most after a flight and I'm coming in to land and need to throttle back. It's so rough at these RPMs that I have gotten in the habit of slowing down quickly so that I can get it below 1000RPMs so I don't have to experience this roughness/ vibration. I have tried playing with the mixture while on the ground at these RPM's to see if it makes a difference and it does not.

I have the Airflow Performance FM-150L Fuel Injection and EFII electronic ignition+1 mag. The electronic ignition was installed shortly after I bought the plane 2 years ago and I just finished installing the FI recently and have about 16hrs on it.

It runs great at idle and top end. Just very rough at these lower RPMs. Feels almost like a very out of balance prop. I saw a bit of fuel stain on one of the intake tubes near the gasket, so I thought an intake leak was causing this. I installed new gaskets on all tubes and while I was at it, I also replaced all the hoses that couple the intake tubes to the bottom of the sump. It did not help.

If I remember correctly, my plane always felt a *little* rough coming in to land (at low RPMs), but never this bad. So it probably is not a problem with the fuel injection. But I'm not sure... Any ideas or how to trouble shoot this??

Thanks.
 
That area of RPM is about when the flow divider starts to open up to provide more fuel. I'd suggest giving Don a call at AFP prior to flying again.
 
I saw a bit of fuel stain on one of the intake tubes near the gasket, so I thought an intake leak was causing this. I installed new gaskets on all tubes and while I was at it, I also replaced all the hoses that couple the intake tubes to the bottom of the sump. It did not help.

please provide a bit more detail here. Where exactly was the stain. You should not be seeing blue staining here with a bad gasket under normal conditions.

Larry
 
please provide a bit more detail here. Where exactly was the stain. You should not be seeing blue staining here with a bad gasket under normal conditions.

Larry

The stain was on the outside of the intake tube near the gasket. It was a *very* small amount, but I thought even a small air leak can cause problems. I tightened up the bolts and that seemed to have stopped the leak (I flew it for about 2-3hrs and checked again and saw no staining). But I decided to replace all the gaskets anyway.

Where I am getting a lot of staining is inside the throttle body. Photo here:
https://drive.google.com/file/d/1Q8mf6eZGC71k42iYYDLasDYnt7Mp-KVO/view?usp=sharing

Don said that it was from too much priming. I didn't think so b/c I've always been able to start very easily. No symptoms of flooding. But I took his advice and tried priming even less. I took off the cowl yesterday and still saw lots of staining. Could this be related to the roughness?? Don said to not worry about the staining inside the throttle body. As far as the roughness, he told me to check for air leaks and replace the gaskets, which I did - but problem still not solved.
 
Unless you have the purge valve, you'll see fuel come back to the servo after shutdown. It drains back from the injector lines and there isn't much you can do about, in my experience (mine does the same thing).
 
Unless you have the purge valve, you'll see fuel come back to the servo after shutdown. It drains back from the injector lines and there isn't much you can do about, in my experience (mine does the same thing).

Ok, so I shouldn't worry about the fuel staining then as Don said. I don't have the purge valve, so it totally makes sense that fuel would drip down into the fuel servo on shut-down, even if I don't over prime. Do you have the FM-150L also? And yours runs smoothly throughout the entire RPM range?

The roughness isn't so bad that I would want to ground the plane, but it sure would be nice to find the problem and fix it.
 
Do you have the FM-150L also? And yours runs smoothly throughout the entire RPM range?

The roughness isn't so bad that I would want to ground the plane, but it sure would be nice to find the problem and fix it.

Yes, I have the FM-150 mounted on a 90 degree elbow for horizontal orientation.

Consider reading the stuck valve threads, which had the same or similar symptom.
 
could it be a problem with the ignition?

My problem of engine roughness at low RPMs seems worse some times than others. For a while, it wasn't too bad, but recently, it's felt pretty bad and I thought to do a mag check at the low RPMs. What I found was that when I switched it to run on the electronic ignition only (no mag), it was MUCH worse. RPMs dropped by about 200 and it was very rough. When it's on mag only, or both, RPMs came back up and it's not as rough.

I thought maybe it's a problem with the ignition timing and called up Robert at EFII. He had me fly there and he reprogrammed my ECU to change the ignition advance curve, but it didn't seem to help. He suggested that I might have a problem with the grounding of the ignition coil or some other electrical issues. I'll check the grounding this weekend. I'll also check the resistance on the ignition wires. Is there anything else it could be? Any ideas on what else I should do to figure this out? I checked my plugs yesterday and they all looked good.

I am using an EarthX battery grounded to the firewall, but no grounding bus. Everything is just grounded to the airframe. Robert said that's not a good idea and I should have a ground bus.
 
Grounding will kick your butt. Not the least of which is engine electronics. I had a customer for an engine conversion in a vehicle call with engine running problems. He had grounded 2 huge amplifiers, and 2 gigantic capacitors to the same engine ground as the engine, sensor, and computer grounds. I would follow the recommendation of the EFI manufacterer.
 
My problem of engine roughness at low RPMs seems worse some times than others. For a while, it wasn't too bad, but recently, it's felt pretty bad and I thought to do a mag check at the low RPMs. What I found was that when I switched it to run on the electronic ignition only (no mag), it was MUCH worse. RPMs dropped by about 200 and it was very rough. When it's on mag only, or both, RPMs came back up and it's not as rough.

I thought maybe it's a problem with the ignition timing and called up Robert at EFII. He had me fly there and he reprogrammed my ECU to change the ignition advance curve, but it didn't seem to help. He suggested that I might have a problem with the grounding of the ignition coil or some other electrical issues. I'll check the grounding this weekend. I'll also check the resistance on the ignition wires. Is there anything else it could be? Any ideas on what else I should do to figure this out? I checked my plugs yesterday and they all looked good.

I am using an EarthX battery grounded to the firewall, but no grounding bus. Everything is just grounded to the airframe. Robert said that's not a good idea and I should have a ground bus.

Can I presume that mag only and both ignition yields the same RPM (i.e. no RPM drop when EFII is shut off) If that is the case, your EFII timing is likely way retarded. I would put a timing gun on it and check your base timing for the EFII.

Grounding doesn't seem like an issue here, given that it runs very well at idle and WOT. Ground problems don't typically appear in RPM dependant situations. intermittent yes, but only at certain RPMs seems like a stretch. Coil problems often appear only under high load or high RPM, but it would be odd for a coil to ONLY give problems at 1100-1400 RPM. I would also suspect problems with the timing curve or it's application via the brain box, causing only retarded timing at your problem RPM range. You can also do mag checks at cruise power to see if the same RPM behavior exists. It is possible that the retarded timing is only giving noticeable symptoms in the 1100-1400 range.

It is also possible that you have a bad plug or wire on the EFII side. If you have one plug not working on the EFII side, when you drop the mag, you become a 3 cylinder engine and hence the roughness. This would be unnoticeable when running on the mag only, though you should still be seeing some type of RPM drop when you shut down the EFII, if it is performing correctly, even with a dead plug. The fact that you don't makes me suspect base timing on the EFII.

Larry
 
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When I do my normal mag check at 1700rpm, the rpm does drop a little with EI off and running on mag only. (Runs best with both on) Then when I switch to EI only and mag off, it drops a little more (EI usually about 10rpms lower than mag). I would have expected it to be the other way around (more rpms from the EI compared to mag), but nothing alarming.

However, when I do the mag check at the lower rpms (around the 800-1200rpm range), it drops a lot more (EI is about 200rpms lower than mag when I switch from one to the other). The engine becomes extremely rough if I run it on the EI only and mag off at these lower rpms. With mag on and EI off, there's not a very big difference compared to having both on.

While I'm flying and with both EI and mag on, it's not so bad below 1000rpms. Between about 1000-1300, it's pretty rough. Above 1400, it's quite normal, although maybe not super smooth.

So the problem is at lower rpms, and the EI seems definitely involved.

The interesting thing is that when Robert reprogrammed the EI to have the exact same timing as the mag (25 degrees flat throughout the rpm range), it ran horribly. So we went back to the standard curve that these units come with (progressively advanced timing as the rpms go up). I'll post the exact numbers later. The data is on a sheet of paper in my hangar.

I'll check the plugs, wires, ignition coil and timing this weekend and see what what's going on. I'll also check to make sure I have a good ground.



Can I presume that mag only and both ignition yields the same RPM (i.e. no RPM drop when EFII is shut off) If that is the case, your EFII timing is likely way retarded. I would put a timing gun on it and check your base timing for the EFII.

Grounding doesn't seem like an issue here, given that it runs very well at idle and WOT. Ground problems don't typically appear in RPM dependant situations. intermittent yes, but only at certain RPMs seems like a stretch. Coil problems often appear only under high load or high RPM, but it would be odd for a coil to ONLY give problems at 1100-1400 RPM. I would also suspect problems with the timing curve or it's application via the brain box, causing only retarded timing at your problem RPM range. You can also do mag checks at cruise power to see if the same RPM behavior exists. It is possible that the retarded timing is only giving noticeable symptoms in the 1100-1400 range.

It is also possible that you have a bad plug or wire on the EFII side. If you have one plug not working on the EFII side, when you drop the mag, you become a 3 cylinder engine and hence the roughness. This would be unnoticeable when running on the mag only, though you should still be seeing some type of RPM drop when you shut down the EFII, if it is performing correctly, even with a dead plug. The fact that you don't makes me suspect base timing on the EFII.

Larry
 
The interesting thing is that when Robert reprogrammed the EI to have the exact same timing as the mag (25 degrees flat throughout the rpm range), it ran horribly. So we went back to the standard curve that these units come with (progressively advanced timing as the rpms go up). I'll post the exact numbers later. The data is on a sheet of paper in my hangar.

This is more evidence that your base timing may be off.. You can buy a timing gun for about $30 and I would advise verifying the timing for the EFII. It only takes about an hour (need to make a pointer for the TDC mark). It could also point to plug / wire problems that are being masked by the advanced timing.

Larry
 
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ignition

This aircraft has wiring issues.
There is no ground system installed.
All systems are grounded directly to the airframe in multiple locations.

The electrical grounding system is one half of the power distribution system of any vehicle. This is not something that can be simply left out. Without a proper ground system, there is no guarantee of the proper operation of any electrical or electronic equipment in the vehicle.

This aircraft needs immediate attention from a qualified avionics or electrical installer to bring it up to at least basic electrical standards.

Robert
 
I re-grounded the ignition and ecu directly the the battery ground. The guy who installed it for me grounded it to the engine case. I started it up and the problem was still there. So I and a friend went through and verified that the plug wires were within specs for the resistance, checked the plugs (they looked great), made sure the vacuum line going to the MAP sensor had no leaks or kinks, pulled all the plugs and cranked the engine and confirmed they all made sparks, then verified the timing with a timing light. We looked at both the mag timing and the timing on the EI. Everything looked good.

Any ideas on where to go from here? I'm thinking if we played with the ignition timing curve a bit, especially at the suspect rpms, we can find something that might get it to run more smoothly?

I think the improper grounding may have contributed to the problem, but obviously there is something else going on b/c the problem is still there. When I tested it again after properly grounding it, it *seemed* to run a little more smoothly. It's hard to tell, b/c how rough it is varies a lot. I'll test it again tomorrow and see.
 
The electrical grounding system is one half of the power distribution system of any vehicle. This is not something that can be simply left out. Without a proper ground system, there is no guarantee of the proper operation of any electrical or electronic equipment in the vehicle.

Robert

And since when is the airframe not an actual ground / ground system? Assuming the battery negative is attached to the airframe, the airframe is a completely adequate "ground system" for power distribution. One could argue it has the potential for noise, but it will certainly allow for the distribution of power. I have numerous devices grounded to the airframe that work fine.

Larry
 
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I re-grounded the ignition and ecu directly the the battery ground. The guy who installed it for me grounded it to the engine case. I started it up and the problem was still there. So I and a friend went through and verified that the plug wires were within specs for the resistance, checked the plugs (they looked great), made sure the vacuum line going to the MAP sensor had no leaks or kinks, pulled all the plugs and cranked the engine and confirmed they all made sparks, then verified the timing with a timing light. We looked at both the mag timing and the timing on the EI. Everything looked good.

Any ideas on where to go from here? I'm thinking if we played with the ignition timing curve a bit, especially at the suspect rpms, we can find something that might get it to run more smoothly?

I think the improper grounding may have contributed to the problem, but obviously there is something else going on b/c the problem is still there. When I tested it again after properly grounding it, it *seemed* to run a little more smoothly. It's hard to tell, b/c how rough it is varies a lot. I'll test it again tomorrow and see.

I would definately play with the curve. See if you can find timing that eliminates your symptoms. That would at least give you more data to help solve this. If you can't find a timing solution, I would start playing with mixture at the troubled RPMs to see if this could be a fuel issue. Have you checked for intake leaks yet?

Larry
 
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