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912 80hp violent vibration on cold start
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John Hauck



Joined: 09 Jan 2006
Posts: 4497
Location: Titus, Alabama (hauck's holler)

PostPosted: Tue Aug 25, 2015 9:19 pm    Post subject: 912 80hp violent vibration on cold start Reply with quote

My own experience with my second 912, an early 912S (before the ULS designation).  No clutch and HD starter.

Engine started violently shaking both carbs enough to aeriate the fuel in the float bowls.  This is why throttle above idle only caused more shake, aggravated the situation, and no increase in rpm.  Before Rotax came out with the clutch/HD starter upgrade (at owners expense) my start procedure was by the book with the exception of cracking the throttle to about where 2,000 rpm would be.  This helped some, but the startups were still violent and put a lot of stress on the engine and the airframe.

When the update became available for the 912S, I had Ronnie Smith of South Mississippi Light Aircraft, Lucedale, MS, install.  From that point on, the 912S started like a baby when book procedures were followed, summer and winter.  Idled nice and smooth.

My current 912ULS had the clutch and HD starter from the factory.  Never had a start problem with it as long as I kept the spark plug gaps closed up to about .020".

john h
Kolb MKIII, N101AB
Hyder, Alaska/Stewart, BC

From: owner-rotaxengines-list-server(at)matronics.com [mailto:owner-rotaxengines-list-server(at)matronics.com] On Behalf Of Paul Kuntz
Sent: Tuesday, August 25, 2015 9:07 PM
To: rotaxengines-list(at)matronics.com
Subject: Re: Re: 912 80hp violent vibration on cold start


Oops, sorry, I missed the part about no throttle response, which is obviously the real problem. Yes, it sounds like a fuel delivery issue, but I don't have any good ideas to offer.

Paul Kuntz


On Tue, Aug 25, 2015 at 9:03 PM, Paul Kuntz <paul.r.kuntz(at)gmail.com (paul.r.kuntz(at)gmail.com)> wrote:
Michael,

1000 RPM is way too low for idle on the Rotax 80 hp engine. It will always run rough below about 1800 RPM, and idle speeds lower than that are very hard on the gear box. I am not at all surprised to hear that your idle is quite rough at 1,000 RPM. Your idle should be 1800-2000 RPM, minimum. If it runs smooth at that speed, it's fine.

Regards,

Paul Kuntz


On Tue, Aug 25, 2015 at 7:31 PM, michaelrosenberg <mickyrosenberg(at)icloud.com (mickyrosenberg(at)icloud.com)> wrote:
--> RotaxEngines-List message posted by: "michaelrosenberg" <mickyrosenberg(at)icloud.com (mickyrosenberg(at)icloud.com)>

I have tried many different options for starting from a lot of different advice. All options end with the same issue which is perfectly describe from Froghair on the initial post of this thread.

Basically, cold weather starry is:

Fuel pump on (5secs or leave on)
Enriched on
Throttle closed
Start

Engine cranks over well and immediately fires and starts, resonance or shacking happens for 10-30 seconds. Slowely removing enriched or adding throttle has no effect. I can pith the throttle to the firewall and has zero response. Engine probably resonated at around 1000RPM(ish)...... No throttle response means I can not effect this.

Once everything smoothed out I warm it up at approx 2000RPM and it will idle around 16-1800RPM.

Once it moved out of this resonant time, it purrs.

I can only put it down to a fuel delivery or enriched issue. Once I get the engine running smooth at idle, I added some enriched to see what would happen and exactly the same symptoms occurred. Engine wound back to low RPM and resonated again. I kind of expected it to happen in this scenario, how ever the starting issue I have no control over.


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John Hauck
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AmphibFlyer



Joined: 29 Dec 2009
Posts: 24

PostPosted: Wed Aug 26, 2015 4:43 am    Post subject: Re: 912 80hp violent vibration on cold start Reply with quote

froghair wrote:
Wondered if anyone has experienced this:

On cold start, anytime OAT is below ~60F, with full choke (enricher), engine will start but shake violently as though not running on all cylinders. I have to coax it to take any throttle increase for up to ~30 seconds. As soon as it will take throttle, it breaks out of this 'harmonic' and runs normally and will do so until allowed to cool back down to ambient temperature.


Mick, another possibility is that the gearbox needs shimming. There's a mandatory 600 hour inspection of the gearbox, but it can need this work even at 300 hours if the engine has often been allowed to idle below about 1800 rpm, especially with a heavy prop.

The gearbox contains a dog clutch that's supposed to accommodate the strong pulsing when a cylinder fires at low rpm, but what wears first is the Belleville washers (which are springs that look like washers). When they get loose, or when the prop shaft gets excessively worn, the cylinders will fire, but the rpm won't increase to the point where the spark timing can advance from the retarded start timing to the advanced run timing. (I forget the specific degrees.) When this happens the engine is likely to rock violently. If it does start, it will then run smoothly and will restart when still warm. For a while, until the wear gets worse; then it won't start, period.

One sign of this condition is that the engine might make a chicka-chicka-chicka sound when it's trying to start.

The remedy is to remove the gearbox and add shims until the Belleville washers are tight enough. Gearbox removal is easy, but you'll need a good Rotax shop to do the shimming.

A somewhat similar condition is when the sprag clutch (the starter drive) gets worn. When that happens the engine will fire once and the starter will disengage immediately--too soon to keep the crankshaft turning until the next cylinder can fire. That's quite violent on the 100 hp 912S, but I've not seen it on the 80 hp 912 or the 914. (The gearbox repair is much easier to do, by the way.)


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KeysFox



Joined: 05 Jan 2009
Posts: 137

PostPosted: Wed Aug 26, 2015 9:38 am    Post subject: 912 80hp violent vibration on cold start Reply with quote

I recently had carbs gone thru by aa experienced Rotax shop.
Float bowl gasket did not FULLY seat, allowing fuel dampness under bowl.
912 80 hp Engine ran very rough up to maybe 2500 rpm, probably suckling air and running lean with popping/backfire. A Tap with small ball pan hammer upward under float bowl solved "leak" AND rough low rpm running.
Yes, I know a particle could have been dislodged or float may have been bumped free BUT leak stopped and lean running back fire dissappeared together so I bet the two were caused by float bowl not being seated allowing fuel out and air in.
BJ Kitfox IV 912 Warp Drive.


Sent from my Verizon Wireless 4G LTE smartphone

-------- Original message --------
From: Roger Lee <ssadiver1(at)yahoo.com>
Date: 08/25/2015 20:46 (GMT-05:00)
To: rotaxengines-list(at)matronics.com
Subject: RotaxEngines-List: Re: 912 80hp violent vibration on cold start

--> RotaxEngines-List message posted by: "Roger Lee" <ssadiver1(at)yahoo.com>

Most of the time these starting issues come from only a couple of things.

Weak battery and or too small a size wire for the starting system and or battery.

Carbs not synced,

Idle rpm too low or too high,

Old black colored starter. The newer gold colored high torque starter is better. It doesn't turn faster just harder.

Using a primer system instead of the regular choke setup. (This is a big one.)

--------
Roger Lee
Tucson, Az.
Rotax Instructor &amp; Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056


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BARRY CHECK 6



Joined: 15 Mar 2011
Posts: 653

PostPosted: Wed Aug 26, 2015 10:34 am    Post subject: 912 80hp violent vibration on cold start Reply with quote

What do you expect!

On Tuesday, August 25, 2015, michaelrosenberg <mickyrosenberg(at)icloud.com (mickyrosenberg(at)icloud.com)> wrote:
Quote:
--> RotaxEngines-List message posted by: "michaelrosenberg" <[url=javascript:;]mickyrosenberg(at)icloud.com[/url]>

I have tried many different options for starting from a lot of different advice. All options end with the same issue which is perfectly describe from Froghair on the initial post of this thread.

Basically, cold weather starry is:

Fuel pump on (5secs or leave on)
Enriched on
Throttle closed
Start

Engine cranks over well and immediately fires and starts, resonance or shacking happens for 10-30 seconds. Slowely removing enriched or adding throttle has no effect. I can pith the throttle to the firewall and has zero response. Engine probably resonated at around 1000RPM(ish)...... No throttle response means I can not effect this.

Once everything smoothed out I warm it up at approx 2000RPM and it will idle around 16-1800RPM.

Once it moved out of this resonant time, it purrs.

I can only put it down to a fuel delivery or enriched issue. Once I get the engine running smooth at idle, I added some enriched to see what would happen and exactly the same symptoms occurred. Engine wound back to low RPM and resonated again. I kind of expected it to happen in this scenario, how ever the starting issue I have no control over.




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http://forums.matronics.com/viewtopic.php?p=446424#446424







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kenw



Joined: 01 Oct 2015
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Location: Oklahoma City

PostPosted: Thu Sep 21, 2017 5:57 pm    Post subject: Re: 912 80hp violent vibration on cold start Reply with quote

mick are you still in the Tulsa, hows the Kolb?

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