Fuel Injection Servo Friction, Effort to Move?

Flyxaos

Flyxaos

Pre-takeoff checklist
Joined
May 16, 2020
Messages
104
Location
NW WA
Display Name
Display name:
Flyxaos
Hi,

This is a fuel injection servo on an IO390. I have had the plane since September. The throttle has always seemed too stiff to me. Meaning, I have to push and pull on it with more effort than it seems I should. But, I don't know what is 'right'.

The mechanic and broker weren't sure if it was wrong but both felt it was a little stiff. It seems as if it gets stiffer when it the motor gets hot. But when cold it feels fairly normal, but still stiff-ish. So I had the throttle cable replaced. It was clear that wasn't the issue as soon as it was disconnected from the servo.

Moving the servo arm without a throttle cable connected it seems a little stiff...So, for any that have experience with this type of fuel servo, how easy should this arm (with red arrow pointing at it) be to move? Like a carburetor throttle with basically no resistance or something with a little more? It is a friction lock cable, so this is done with that friction tension removed.

Precision Airmotive is on the field and I will go ask them, but just curious what your thoughts were.

FIServoArrow.png
 
Last edited:
In my experience there will be a slight bit more resistance to movement than a typical carburetor throttle but nothing terribly excessive. I’ve never seen one that had more resistance to movement than any of the others, they’ve all been pretty uniform.

I have however found plenty of gummed up throttle and mixture cables, and cable routings that caused a lot of resistance to movement.
 
Umless the cable bending radiuus exceeds the mfgr recommendations somewhere, it is likely time to change out the cable.
 
Flyxaos said:
Meaning, I have to push and pull on it with more effort than it seems I should. But, I don't know what is 'right'.
Click to expand...
E/AB aircraft?
Is there any info in the OEM drawings, etc. on throttle linkage rigging?
Is the mixture linkage easier to operate than the throttle?

Without any common reference, tough to give input on what would be the "right" amount of force to move the throttle linkage on the injector. But as mentioned above the linkage "feel" on an injector tends to be firmer than on a carb.
 
My API servo’s throttle arm moves smoothly with very little resistance/effort. I see a Silver Hawk servo in the pic. I have no hands-on with that one. Airflow Performance is a great resource for fuel injection systems, including RSA Bendix. It may be worth your time to ask an expert.
 
Stewartb said:
Is the 390 in the GlaStar in your avatar?
Click to expand...
Yes, that plane, E/AB Sportsman, But same thing pretty much. It seems to be a great motor. Easy starting and more power than is needed. (Is that possible?)

Good to hear yours doesn't require much effort to move, mine just can't be right. I will talk to Precision for sure. I just was curious what others experience where with the same type of set up.

When we disconnected the old cable it was butter smooth. We went ahead and installed the new, just because I bought it. The old one did have hard rubber from heat on the end, but that was it. We added heat protection tubing over the new cable. It was smooth before connecting to the servo. Probably none of changes where needed.
 
Last edited:
Flyxaos said:
Precision Airmotive is on the field and I will go ask them
Click to expand...
Do that. Some of the folks here are guessing. Those servos should not be stiff. I never found them much stiffer than a carb.

Here's the deal:

1704755248997.png
The cable is connected to the throttle lever, which moves the throttle valve. The idle mixture adjustment link moves the valve stuff in the servo. None of it is stiff. Yours must have a binding bearing in one of the shafts, either in the servo or the throttle shaft, or the valve in the servo is getting torn up.
 
Flyxaos said:
Yes, that plane, E/AB Sportsman, But same thing pretty much. It seems to be a great motor. Easy starting and more power than is needed. (Is that possible?)

Good to hear yours doesn't require much effort to move, mine just can't be right. I will talk to Precision for sure. I just was curious what others experience where with the same type of set up.

When we disconnected the old cable it was butter smooth. We went ahead and installed the new, just because I bought it. The old one did have hard rubber from heat on the end, but that was it. We added heat protection tubing over the new cable. It was smooth before connecting to the servo. Probably none of changes where needed.
Click to expand...
Please give us a follow-up when you get this resolved. I’m always interested in outcomes. That’s where we all get an opportunity to learn.

Sounds like a fun plane. What prop? My IO-390 spins a Whirl Wind constant speed STOL prop on my exp Supercub. Like you said, there’s no lack of power!
 
@Flyxaos can you send me a private message. I would like to ask you some questions about zenair ch650’s. Thank you very much. it won’t let me private message you and give you my number.
best regards
Alan
 
Just a follow up on this...

I had my A&P take off the servo. I brought it over to Precision Airmotive. It is just a couple hangars over. He took it into the back room and had it back to me in 20 minutes. No charge, (Thank you!). He said he took apart the mixture section, lapped and lubed it some. It felt better. Then the A&P also played with and lubed the other linkage a bit. He said that Lube isn't really supposed to be required, but a little isn't bad.

It does work pretty good now. It isn't as smooth as a simple throttle, or like the Rotax I had been flying. But it is consistent and doesn't get bound up feeling. There are a lot of linkages and such going on there so I suppose it isn't really un-expected to feel a little different friction going on.

In all I'm good and glad we took the time and $ to take it apart.
 
Last edited:
Flyxaos said:
Just a follow up on this...

I had my A&P take off the servo. I brought it over to Precision Airmotive. It is just a couple hangars over. He took it into the back room and had it back to my in 20 minutes. No charge, (Thank you!). He said he took apart the mixture section, lapped and lubed it some. It felt better. Then the A&P also played with and lubed the other linkage a bit. He said that Lube isn't really supposed to be required, but a little isn't bad.

It does work pretty good now. It isn't as smooth as a simple throttle, or like the Rotax I had been flying. But it is consistent and doesn't get bound up feeling. There are a lot of linkages and such going on there so I suppose it isn't really un-expected to feel a little different friction going on.

In all I'm good and glad we took the time and $ to take it apart.
Click to expand...
Thanks for the follow up as to your repair.
It would nice if more folks would follow up with us after getting advise here.
 
Gary Ward said:
Thanks for the follow up as to your repair.
It would nice if more folks would follow up with us after getting advise here.
Click to expand...
I know this is going to sound stupid. But it is human nature we are discussing. Saying thanks like you did, or "liking" the OPs response actually encourages not only the OP but others to do the same.

Tim
 
You must log in or register to reply here.
Back
Top