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RPM Increasing During Flight

MVPILOT

Well Known Member
I have a problem with my RPM increasing on my 2015 RV-12 S-LSA . For example say I set it to 4500 RPM. Over 3-5 minutes it will increase to 5000 RPM with it tightened all the way. If I loosen the nut then it will quickly accelerate to maximum RPM and I can feel the spring tension. Could it be that the springs are too powerful. Anyone else having this problem?
 
I made new friction lock (don't know actual name of part) for throttle. This is tapered piece that pinches the throttle when knurled knob is tightened.

Sorry for bad explanation...
 
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Thank you. My mechanic said to order a new McFarlane throttle but I hate spending money needlessly and this looks like a McFarlane anyway.
 
The current McFarlane throttles don't do that. They have a set friction and vernier control. Also, the current spring set up stops the accelerate to full throttle nonsense that Rotax previously had.
 
My mechanic said to order a new McFarlane throttle but I hate spending money needlessly and this looks like a McFarlane anyway.

It may be a McFarlane ... does the knob rotate left and right??? If so, you likely have the vernier McFarlane throttle.

If that is the case, there is a video somewhere (think at McFarlane) that shows how to reassemble the vernier throttle's inner parts correctly. Perhaps at some point the throttle was pulled out too far prior to instillation and the internal parts are not in the proper location now.

Also as previously mentioned by others, the carb springs also make a big difference. My original Rotax springs were removed and replaced with the slightly weaker McFarlane springs when installing the McFarlane vernier throttle. I had very little throttle creep with those springs ... but there still was some occasionally. Sometimes I can see the locking ring rotate a little if it is not tightened down enough and the throttle will begin to creep forward. I've recently installed the new Van's helical torsion spring and like them.

Even if your throttle is not the McFarlane vernier unit, you should seriously consider replacing the Rotax throttle lever springs with the new Van's helical torsion spring. The longer stretch springs have had a long history of breaking ... I don't see that happening easily with the new Van's spring.
 
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When installing the new springs there is a LH and RH side to them identified by Blue paint on one, if my bad memory is correct ...the Blue paint is for the LH spring?
 
When installing the new springs there is a LH and RH side to them identified by Blue paint on one, if my bad memory is correct ...the Blue paint is for the LH spring?

Follow the instructions that come with the springs to identify L & R.
 
if my bad memory is correct ...the Blue paint is for the LH spring?

That would be correct .... the pair I installed had the left spring denoted with a marking.

When switching over to the new springs, I discovered using a piece of waxed cord made the instillation a snap .... the cord was placed over the inboard portion of the spring and pulled to tension the spring as it was slipped in place. The link below has two photos near the end of the post showing how to use the waxed cord to easily tension the springs without the use of any tools. Clicking on the photos will make them larger and easier to view.
http://www.dogaviation.com/2020/05/service-bulletin-18-03-06-carburetor.html
 
It may be a McFarlane ... does the knob rotate left and right??? If so, you likely have the vernier McFarlane throttle.

Also as previously mentioned by others, the carb springs also make a big difference. My original Rotax springs were removed and replaced with the slightly weaker McFarlane springs when installing the McFarlane vernier throttle. I had very little throttle creep with those springs ... but there still was some occasionally. Sometimes I can see the locking ring rotate a little if it is not tightened down enough and the throttle will begin to creep forward. I've recently installed the new Van's helical torsion spring and like them.

Even if your throttle is not the McFarlane vernier unit, you should seriously consider replacing the Rotax throttle lever springs with the new Van's helical torsion spring. The longer stretch springs have had a long history of breaking ... I don't see that happening easily with the new Van's spring.

Yes mine has the vernier throttle and mechanic replaced with all new McFarlane. I purchased the shorter recommended Spring-00002 but my mechanic didn't want to use them and preferred the older style. Flying it yesterday seems to me to much force required on open throttle.
 
I purchased the shorter recommended Spring-00002 but my mechanic didn't want to use them and preferred the older style.

The older style springs are prone to busting .... and tend to creep the throttle.

Guessing your mechanic was busy and just wanted to take the easy route ... no tools required to change out the old style springs.

Switching to the new style spring is not hard to do ... all the parts are in the open. If your mechanic uses my waxed cord trick covered in the link of post 11 and some of the tips there, he can get the job done very quickly. Only down side is the carb sync should be rechecked and may need a slight tweak.
 
Yes mine has the vernier throttle and mechanic replaced with all new McFarlane. I purchased the shorter recommended Spring-00002 but my mechanic didn't want to use them and preferred the older style. Flying it yesterday seems to me to much force required on open throttle.

I believe you have misconception on how throttle springs work and their purpose.

Throttle springs assist in opening the throttle - not closing, therefore, any human assist against the springs is needed to close the throttle.

Purpose of springs is to open throttle in event of throttle cable breakage - which is pretty much unheard of...
 
Also to note that spiral wound cable is for pulling only so the springs allow the cable to be pulled to open the throttle...and when you retard the throttle you are pulling on the cable..I didn't know this until I looked at throttles Spruce was selling, they had a foot note saying that...the solid "Piano" wire cables are push/pull and spiral cable pull only...if you ever have the chance to see a Tecnam with a Rotax 912 ULS the system is set up where the springs are tensioned to pull to max power and they are strong springs, friend has one and if he is flaring out on landing and if his friction is too loose the engine accerates if he removes his hand from throttle, poor design IMHO.
 
Yes mine has the vernier throttle and mechanic replaced with all new McFarlane. I purchased the shorter recommended Spring-00002 but my mechanic didn't want to use them and preferred the older style. Flying it yesterday seems to me to much force required on open throttle.
Claude:

It looks like your RV-12 is an SLSA. That means your mechanic does not have the option of choosing not to comply with Van's SB's. Somebody correct me if I am wrong but I believe they are mandatory for SLSA's.
 
Also to note that spiral wound cable is for pulling only so the springs allow the cable to be pulled to open the throttle...and when you retard the throttle you are pulling on the cable..I didn't know this until I looked at throttles Spruce was selling, they had a foot note saying that...the solid "Piano" wire cables are push/pull and spiral cable pull only...if you ever have the chance to see a Tecnam with a Rotax 912 ULS the system is set up where the springs are tensioned to pull to max power and they are strong springs, friend has one and if he is flaring out on landing and if his friction is too loose the engine accerates if he removes his hand from throttle, poor design IMHO.

Not poor design, it is well thought-out safety consideration.

The Rotax 912 is really two separate engines that share a common crankshaft. What makes it two separate engines is that the left and right cylinder assemblies each have separate carburetors unlike a Continental or Lycoming horizontally opposed engine that have a single induction path.

The carburetor throttle plate springs are to prevent loss of power if a cable brakes. It would be pretty catastrophic if you lost power on one side of the engine. I suspect you would have less than 50% power available because the remaining side would be powering the dead half. Hard to put into words…

In addition to tremendous power loss you would have horrendous vibration - maybe bad enough to shake the engine loose from its mounting. That would end in a bad day because an airplane can’t remain in controlled flight without the weight of an engine for center of gravity concerns.

So, what Rotax decided is to install springs to advance throttle to full if a cable brakes or becomes disconnected. Instead of losing power on one side that side will go to full power which is an easy emergency to manage. To manage this emergency the pilot would advance the throttle to full so both halves of the engine would be producing full power and the engine would remain running smooth, albeit above redline RPM. This is far better than the scenario I outlined above. The pilot would continue to nearest airport at a safe altitude and landing would be accomplished by shutting off ignition and landing dead-stick.
 
It's my understanding that the throttle springs are there to allow for flexible-wire (pull-only) throttle cables, and to allow for better carb synchronization. The springs can be set for pull-to-idle, or pull-to-full throttle, depending on the installation. There are good reasons for each method. On the Searey they pull to idle because spring failures are more common than throttle cable failures.
 
You guys need to shop the Home Depot aviation section and pick a spring that works. I went through a couple of iterations before finding a set that didn’t break, kept enough tension for low RPM carb synching, and didn’t cause throttle creep. I’ve had them in for over 6 years, and I’m closing in on 1000 hours service time. Don’t ask me the specs, because I bought them a long time ago.
 
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