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912 Running rough all of a sudden

 
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peteohms



Joined: 31 Oct 2007
Posts: 224
Location: Leander, TX

PostPosted: Fri Oct 20, 2017 2:00 pm    Post subject: 912 Running rough all of a sudden Reply with quote

Engine has been running great. Flew last weekend and then Wednesday was going to fly. Started up and it was sounding rough at idle. I warmed up a minute and advanced the throttle and it continued rough until it got to 3000 rpm. Let it warm up some more then ran it at 3000 rpm for a couple minutes hoping to clear up the problem. No joy. I ran it at high idle and turned off the fuel valve. After a while it smoothed out just before it was starved for fuel. Opened the fuel valve before it died and it ran smoothly for a few seconds before it began to run rough again. Sounds like a carb problem where maybe one carb is getting too much fuel. Mag check at 3k is normal. I dropped the float bowels and the floats looked to be floating normally. Fuel looked clean. I had run on this tank for a couple hours Saturday. Gascolator had clean fuel. Suggestions? Why would a good,running engine start running rough after just a few days?

Quote:
On Oct 20, 2017, at 4:06 PM, william sullivan <williamtsullivan(at)att.net> wrote:



Check the Viton seals in the enrichers. One could be deteriorating. Cheap and easy fix, if that's it.


--------------------------------------------
On Fri, 10/20/17, italianjon <jon.catilli(at)gmail.com> wrote:

Subject: 912 UL Intake Manifold Pressures
To: rotaxengines-list(at)matronics.com
Date: Friday, October 20, 2017, 3:28 PM


"italianjon" <jon.catilli(at)gmail.com>

Hi All,

I've been tracking a vibration for a
while, and it has been very difficult. Carburettors have
been sync'd more than I've had hot dinners, plus other areas
of the engine. So far I've eliminated the propeller,
gearbox

I noticed a strange effect today, and
I'm unsure about it. Today, as a result of a vibration plot
taken last weekend, decided to carefully check the
carburettor sync again. The vibration plot is showing the
vibration at half-crank and twice-crank frequency.

I connected two vacuum gauges, so that
I am reading absolute vacuum (i.e. 0" is ambient and 10",
for example, is 10" of vacuum).

> From about 10" up to 0", which
corresponds to rpms of about 3400 up to 5400, the
carburettors are in sync. Below that they aren't.

At idle of about 1750/1800 rpm i get
12.5-13" in the port carb, and 15.5" in the starboard carb.

I found Roger Lee's very good guide and
thought that I could take out this idle difference with the
idle mixture screw. But it didn't really change anything, no
matter where set on the port carb. On the starboard carb it
did have an effect.

The other strange symptom that I found
is that if I take the engine to a fair speed, say 4000 rpm
and then quickly idle the throttle, then both carbs come
down to 15" in sync before the port carb raises to
12.5/13".

Compressions were checked 12 months
ago, all ok. Carburettors we overhauled 12 months ago, all
ok. Carb mounts renewed 12 months ago.

Any one seen this effect before?

I'm thinking of taking off the manifold
and checking the O-rings between the engine and manifold for
leaks. And also stripping back the carbs, cleaning and
rebuilding them.

Anything else I should be thinking?

Best

Jon




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Pete
Leander, TX
Kitfox III, 912ul, Grove
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PostPosted: Fri Oct 20, 2017 4:05 pm    Post subject: 912 Running rough all of a sudden Reply with quote

Like I said, check the enricher seals. The one on my 447 had dissolved, making it run very rich. I have very little experience with this, but your symptoms gave me a good reminder. It will only take 5 minutes or so to pull each one out and check.
--------------------------------------------
On Fri, 10/20/17, Pete Christensen <pchristensen10(at)austin.rr.com> wrote:

Subject: 912 Running rough all of a sudden
To: rotaxengines-list(at)matronics.com
Date: Friday, October 20, 2017, 5:58 PM


Pete Christensen <pchristensen10(at)austin.rr.com>

Engine has been running great. Flew
last weekend and then Wednesday was going to fly. Started up
and it was sounding rough at idle. I warmed up a minute and
advanced the throttle and it continued rough until it got to
3000 rpm. Let it warm up some more then ran it at 3000 rpm
for a couple minutes hoping to clear up the problem. No joy.
I ran it at high idle and turned off the fuel valve. After a
while it smoothed out just before it was starved for fuel.
Opened the fuel valve before it died and it ran smoothly for
a few seconds before it began to run rough again. Sounds
like a carb problem where maybe one carb is getting too much
fuel. Mag check at 3k is normal. I dropped the float bowels
and the floats looked to be floating normally. Fuel looked
clean. I had run on this tank for a couple hours Saturday.
Gascolator had clean fuel. Suggestions? Why would a
good,running engine start running rough after just a few
days?


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John Hauck



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

PostPosted: Fri Oct 20, 2017 5:34 pm    Post subject: 912 Running rough all of a sudden Reply with quote

Make sure the idle jets and main jets are completely clean and clear when
you have the float bowls off.

Also, make sure the float bowl gaskets are sealing 100%. Best replace both
float bowl gaskets to be sure.

All the above can give you major headaches, although they are simple to
check and simple to cure.

john h
Kolb MKIII - 3,400.0+ hours
912ULS - About 850.0 hours on this one.

--


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MKIII/912ULS
hauck's holler
Titus, Alabama
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DHeal



Joined: 15 Feb 2017
Posts: 3

PostPosted: Fri Oct 20, 2017 5:47 pm    Post subject: Re: 912 Running rough all of a sudden Reply with quote

Sticky carb pistons? Remove air filters, reach inside carb throats, see if carb pistons move up and down smoothly without sticking.

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