Idle/Mixture Problem

Capflyer · #1 12-20-2008, 03:50 PM

I have a Silverhawk fuel injection servo with about 170hrs on it. For the first 6 months or so it idled fine around 950 rpm's. Over the last few months it continued to creep up and now idles around 1200. I tried adjusting the idle and mixture but it has not improved the idle speed. The idle set screw does not hit the stop and the arm cannot go any further. The cable adjustment is fine as well. Anyone have any experience with this that can help me out?

Thanks

AlexPeterson · #2 12-20-2008, 06:31 PM

Have you tightened the hose clamps on the induction tubes? Sounds like you have an induction leak or the throttle is not being closed by the cable. Is the throttle cable pulling as far as it should? The throttle arm is what should be limiting, not the control cable.

BTW, how do you land it with such an idle speed???

Geico266 · #3 12-20-2008, 06:34 PM

My idle is around 650 RPM.

cyrilmclavin · #4 12-20-2008, 09:23 PM

temperature affects mixture ratios summer idle fine same setting too fast in winter. btw you should idle around 600 rpm.

cyril
ps nad the same issue on injected 8a.

Capflyer · #5 12-21-2008, 01:23 AM

Quote:

Originally Posted by AlexPeterson

Have you tightened the hose clamps on the induction tubes? Sounds like you have an induction leak or the throttle is not being closed by the cable. Is the throttle cable pulling as far as it should? The throttle arm is what should be limiting, not the control cable.

BTW, how do you land it with such an idle speed???

I'll check the induction tube clamps but could a small leak really cause such an increase and why the higher rpm's to begin with? The throttle cable is adjusted properly as the throttle arm on the servo has full travel to both stops.

I get the idle is too high thing and would like to get it down to around 650. I tolerated the 900 rpm but it's way too high now.

As for landing, it lands just fine and touches down 55-60 kts without any problem.

So back to the original question of what could be causing this.

Mel · #6 12-21-2008, 07:06 AM

In the first post, you say that the throttle arm does not go to the idle set screw. In a later post you say that it goes to both stops. The idle screw "IS" the idle "stop". It has to be a cable adjustment if the throttle is not going to the idle set screw. The idle set screw is there to adjust the idle speed. Idle mixture is a totally different thing. Idle should be in the 700-750 rpm range.

Capflyer · #7 12-21-2008, 07:40 AM

Mel, I guess how I described this was confusing. The idle set screw is backed all the way out so it does not contact the stop. The throttle linkage arm of the servo is making full travel in both directions.

Alex may have the answer. After doing a lot of internet research. It is possible that I have an induction leak causing me to overcompensate the mixture adjustment causing the higher idle. I'm off to the airport today to check all my connections to the induction system.

On the Precision Airmotive site I found the Installation Troubleshooting Chart Form 15-901C. On page two it says "Large air leaks into induction system such as missing pipe plugs, etc. usually unable to throttle engine down below 800-900 RPM"

One thing I didn't mention in the initial post and not sure if this might be a symptom as well is that over the time that the idle has been creeping up my cruise speeds have gone down a few knots at the same engine settings.

I hope this is it and will update my findings later today.

Capflyer · #8 12-22-2008, 05:23 AM

Well, I met up with a friend at the airport cafe who is an A&P and he sldo suggested checking the induction tubes and hose connections for cracks. To do this I removed each and inspected but found no problems. Some of the bolts were not loose but definitely not tightened to their correct torque. The clamps around the rubber tubes that connect them to the sump were also not loose but certainly not tight enough. The bolts that hold the servo to the sump were also looser than their proper torque. Other than this I found nothing major. Went out for a run up and the idle rpm had dropped down to around 1000, a 200 rpm drop! My guess is that a rich mixture was needed to compensate for the induction leak causing the rpm to be higher.

I then decided to re-evaluate my linkage. After I removed the cable rod end from the servo arm there was...and I kid you not... only 1mm of play between that to its stop. I turned the rod end in one half turn to take out that tiny bit. Restarted and idle dropped into the low 500's. I found it hard to believe that this tiny bit of movement accounted for this huge drop.

Re-set the idle and mixture and all is good. Idle set to 650 and mixture is perfect.

Two lessons learned that hopefully helps others is to first to double check the tightness of all these bolts and clamps, they did make a difference. Second is that on these Silverhawk servos, they are extremely sensitive to small movements.

Thanks Alex for cluing me in on the induction system and Mel for putting the seed into my head that I should probably check the linkage again.