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What's it trying to tell me ?

17 posts in this topic

Posted

I've been chasing a rough idling (2000-3000 rpm) on this 912 for a year.  It will run fine above 3000 and idle OK around 1800 but between its got a very annoying miss.  It also has a high mag. drop (around 150 rpm.)  I've done all the usual things, sync'ed the carbs (over and over) had the carbs rebuilt, by myself and two separate professional companies, replace the plugs, plug wires and leads etc. checked for manifold leaks etc.,etc. Nothing had any effect, nada zip.....  The only thing I've found that has made any change is to turn the idle mixture screws almost all the way in.  The manual and all the experts say turn them out 21/2 turns and leave them, but mine have to be about only 3/4 of a turn out or its rough as a cob between 2000 & 3000 rpm.  I'm thinking something is screwy in the idle circuit because it runs smooth above 3000.  But what else to try? Any ideas ?

Thanks

EG       

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Posted

I don't know if your 2 1/2 turns out is a typo but the manual states 1 1/2 turns but if it wants to be at 3/4 to run properly that is where I would run it.

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Posted

hummm, is says 21/2 turns, must be typo.  Any idea on the mag. drop ?  This is my third 912 and the other two were less than 75 rpm, this one is more like 150-200.  About the same on either ing. box.

Thanks

EG

 

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Posted

That is well within spec, @ 4000rpm it must not exceed 300 rpm and 115 max difference between the two.

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Posted

Tried a different brand or type of plug? I'm not a 912 guy but my 582 really likes the iridium plugs. 

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Posted

I had the same issue a while back with my old 912UL powered Kitfox.

It was carbs.  I had them apart so many times I lost count but they finally worked and the problem went away.  I believe I had one of the enrichener valves reversed.  The other issue is this vibration eventually damaged the gearbox.  I wound up rebuilding the box completely.

How many hours on the engine?  Is it a UL or ULS?

Did it just start doing the vibration suddenly or gradually over time?

Has the gearbox ever been re-shimmed or serviced?

What prop do you have?

What do you have your plugs gapped at?  These are not very hot ignitions.  I keep mine at .022 - .024".

Stay with the NGK plugs.  DCPR7E for the 80 and DCPR8E for the 100.  I run the 8Es in my low compression Zipper motor.

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Posted

Has the prop been used on another engine? Any chance that it's out of balance as well? I realize you're getting a mag drop below what you want but just trying to throw other ideas at you as well. 

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Posted

Gearbox has been rebuilt. Prop is a new IVO three blade. Adjusted the idle mixture screws to 3/4 out and it is definatly better but still some roughness around 2500 rpm. Thing is I can slightly pull on the choke(enricher) knob on and it will run very smooth at 2500. However when I push up the throttle to 2500 it's rough.  I'm thinking the problem is it's too lean at idle but I can't figure out why. I've checked for induction leaks, replaced the rubber carb mounts etc.  sprayed starter fluid around but nothing turned up.  Errrrrrrrrr

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Posted

Make sure your idle jets are clear. they are tiny and it doesn't take much to clog them and give poor idle.

John M

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Posted

Sounds like one of the carb springs might not be working correctly? Maybe when you advance the throttle one carb is "outrunning" the other causing an imbalance? Either way I would go thru the carbs and/or see if you could swap a known good working set of carbs with someone? 

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Posted

Hey Eg,

on my 912 I had the same and found the choke lever return springs too weak, the chokes were not resting on their "off" position properly. Also make sure your floats are good (guess you checked that). One more item is the ignition leads which might not be properly inserted/screwed in their respective coil and spark plug cap.

Check the float chamber vent tube, it should be installed so as to be at the same pressure than the intake (do you have an airbox) and if it's in the airstream (low or high press) you'll get rough running.

Another easy item to mount backwards in the carb is the diffuser..item#2     https://www.cps-parts.com/catalog/rtxpages/914bingconstant.php

 

Regarding the mag drop my 912 is the same and I would not worry about that.

My 2 cents

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Posted

I had rough idling and a big mag drop between the two.  Turned out to be the gapping on the trigger coils.  Once I got that right then no problems.  You could have a combo problem with different items like spark plugs and trigger coils combined.  The 912 has such high compression if anything lacks it will show up.

 

 

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Posted

If it's possible, borrow somebody else's properly running carbs to rule yours in or out. If it's still acting up, move on to ignition. Any of the above-mentioned carb problems could cause your problems even if it's only an issue on one carb. 

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Posted

As a Jabiru owner, I see no mag drops most likely because the plugs are both at the top of the cylinder.

Most of my wingmen have 912s and have all reported vibration at one time or other. Had to have the gear box worked on. 

Two friend with Zenith 750s have carb linkage issues which give some issues with both carb throttle levers not working in sync.

Question: Has anyone modified the 912 manifold by joining the two carb inputs so that the engine could run on one carb as the Jabiru does? Instead of the two smaller carbs used on the 912s, the larger carb, as used on the Jabiru engines coud be used. Should work on the 1600 cc Rotax since the jabiru Bing works on the 2200 cc engines. No carb syncing issues.

Doesn't seem that hard to do and would be worth it in my opinion.

That said, I realize there are thousands of 912s doing just fine as is.

John M

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Posted

Don't really have an answer for you John, but I suppose it has something to do with getting  good and equal fuel distribution between the cylinders.  We both know how with the Jabiru, EGTs can easily be 100 degrees different from one cylinder to the next.  Tilting the carb to try to even things out and such.  If they could have done it well enough with one carb, I'm sure Rotax would have.  I don't really have any experience with the 912s though.  I do have one on a test stand that I'm going to try to fire up today though.  If I can get my butt off this computer that is.... :-)  JIMChuk

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Posted

Has anyone modified the 912 manifold by joining the two carb inputs so that the engine could run on one carb as the Jabiru does?

Contact Rotec, I spoke to them about this at OSH this year. IIRC, they make a 40mm single TBI and flange that has been used successfully and can likely answer questions about the manifold mods. It was actually installed on a plane at a vendor 2 tents over but I can't recall their name now. If you google you should find some info. 

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Posted

Not trying to hijack the thread, but the 912 on the test stand runs!  At least on one ignition.  I knew there some an issue with the ignition when I got it, now to find out what it was.   I'm happy to hear it making noise anyway.  JImChuk

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