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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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 Posted: Thu May 12, 2011 7:21 pm Post subject: Carb sync issue, Another thought provoking problem |
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Hi All,
Another mind probing problem and I’ll give the answer in a day or two after a few tries at an answer.
You just pulled the engine for the Rotax 5 year rubber replacement maint. Everything is back on, oil purged and new plugs. It’s time for the carb sync. You do your mechanical sync and head out to run the engine for the pneumatic sync. You start up and everything looks good and the gauge needles at idle are only 1“ of vacuum off and the idle is 1750. You throttle up to 3500-3800 rpm to start the high rpm sync. The needles here are 4“ of vacuum off. So you adjust the bowden cables to sync the two carbs and you run out of adjustment. You shut down make a cable length adjustment on the throttle arm and restart. Now the gauge needles at idle are 7”-8” of vacuum off at idle. (Frack) You try and try to get them closer, but the left carb doesn’t seem to adjust. The engine is running rough and is now down to 1620 rpm. This particular plane has a Dynon D120 EMS with all the temps available to you. Oil temp, CHT and oil pressure are good and normal. The EGT temp span warning is flashing yellow which means your EGT’s have a span greater than 120F and that’s about where they are. One EGT is 1050F on #4 cyl while the other is 900F on #3 cyl. You just can’t seem to balance these carbs. “Frack”
NOW WHAT AND WHY?
By the way these thought provoking problems I give are real problems that had a particular solution. I post these in the hopes that some day if you have something similar you may remember and find a quick and painless solution.
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_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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williamtsullivan(at)att.n
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| Posted: Fri May 13, 2011 4:48 am Post subject: Carb sync issue, Another thought provoking problem |
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I would start with an air leak at the carb socket.
Bill Sullivan
Windsor Locks, Ct.
Kolb FS, 447
--- On Thu, 5/12/11, Roger Lee <ssadiver1(at)yahoo.com> wrote:
| Quote: |
From: Roger Lee <ssadiver1(at)yahoo.com>
Subject: Carb sync issue, Another thought provoking problem
To: rotaxengines-list(at)matronics.com
Date: Thursday, May 12, 2011, 11:21 PM
--> RotaxEngines-List message posted by: "Roger Lee" <ssadiver1(at)yahoo.com (ssadiver1(at)yahoo.com)>
Hi All,
Another mind probing problem and I’ll give the answer in a day or two after a few tries at an answer.
You just pulled the engine for the Rotax 5 year rubber replacement maint. Everything is back on, oil purged and new plugs. It’s time for the carb sync. You do your mechanical sync and head out to run the engine for the pneumatic sync. You start up and everything looks good and the gauge needles at idle are only 1“ of vacuum off and the idle is 1750. You throttle up to 3500-3800 rpm to start the high rpm sync. The needles here are 4“ of vacuum off. So you adjust the bowden cables to sync the two carbs and you run out of adjustment. You shut down make a cable length adjustment on the throttle arm and restart. Now the gauge needles at idle are 7”-8” of vacuum off at idle. (Frack) You try and try to get them closer, but the left carb doesn’t seem to adjust. The engine is running rough and is now down to 1620 rpm. This particular plane has a Dynon D120 EMS with all the temps available to you. Oil temp, CHT and oil pressure are good and normal. The EGT temp span warni!
ng is flashing yellow which means your EGT’s have a span greater than 120F and that’s about where they are. One EGT is 1050F on #4 cyl while the other is 900F on #3 cyl. You just can’t seem to balance these carbs. “Frack”
NOW WHAT AND WHY?
By the way these thought provoking problems I give are real problems that had a particular solution. I post these in the hopes that some day if you have something similar you may remember and find a quick and painless solution.
--------
Roger Lee
Tucson, Az.
Light Sport Repairman - Maintenance Rated
Rotax Repair Center
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056
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http://forums.matronics.com/viewtopic.php?p=339729ww.matronics.com/Navigator?RotaxEngines-List" target=_blank>http://www.ma - MATRONICS WEB FORUMS -
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Fri May 13, 2011 7:47 pm Post subject: Re: Carb sync issue, Another thought provoking problem |
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Hi All,
Here is the answer to this little problem. Here is a couple of tattletale signs that help point the way.
1. You just finished a full hose change which includes pushing new
rubber hose through the fire sleeve.
2. On startup it ran fine for about 30-seconds to a minute and the idle
was within 1” vacuum at first.
3. The high rpm adjustment at 3500-3800 rpm was no problem to
adjust.
4. When it was throttled back to idle again the needles were split 7”-8”
of vacuum.
5. The right carb would adjust with the idle stop screw and the gauge
needle would move and was about 13” of vacuum.
6. The left carb was at 20” of vacuum and the idle stop screw didn’t
seem to make any difference.
7. Applying the choke made the engine smooth again and the needles
move to within 1” of each other.
8. The rpm at idle is having a hard time getting above 1650 rpm.
9. EGT split is too far. The left carb is running 150F hotter (1050F) than
the right carb at 900F.
(p.s.** Next time someone says that EGT temps aren’t that helpful on a 912 you will know different in a few minutes)
So right off the bat you know it’s not electrical. You have an extremely high suspicion that it involves the left carb and something with the idle. The vacuum at 20” is way too high and the engine is rough because the carbs aren’t balanced. So now you need to stop the carb balance and follow a logical diagnostic pattern to a solution.
1. Pushing rubber hose through fire sleeve. Possible fuel line debris.
2. Left carb pulling way too much vacuum at idle only. High rpm is
smooth and can balance.
3. Left carb won’t adjust at idle.
4. Left carb EGT’s running hotter at lower rpms.
So everything points to the left carb. It also points to the low side of the rpms around idle and not the higher rpm settings.
So what is it that affects the low rpms at idle?
So we need to take a look at the left carb and if all you do is pull the bowl and look” it was” and could be nice and clean.
HMMMMM!
THE IDLE JET AND or CIRCUIT may be restricted.
The only way to do this right is pull the entire carb and pull all the jets. Use air pressure and carb cleaning spray with the small red tube on a can of carb cleaner. Clean and flush every orifice no matter what you think, but pay special attention to the idle circuit.
When this was done a small piece of red silicone ¼” long and about as thick as a thread comes out. It was tucked back up in the idle circuit and would not have come out without the flush or air pressure. The carb was re-assembled and the carb sync went well and was not an issue any longer.
I post this in the hopes that if you ever run across this scenario you will know what to do and save yourself all those extra hours of banging your head against the wall looking for the solution.
Good Luck!
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_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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lucien
Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM
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| Posted: Mon May 16, 2011 5:15 am Post subject: Re: Carb sync issue, Another thought provoking problem |
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| Roger Lee wrote: | Hi All,
HMMMMM!
THE IDLE JET AND or CIRCUIT may be restricted.
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lol... this is exactly what happened to the other titan on our field after he did his carb rebuild. When they were doing the balance, it was all over the place as they advanced and retarded the throttle. It was either way off at idle or way off at full power and they went back and forth adusting the cables pretty much the whole afternoon.
So they pulled the carbs and checked for debris - sure enough a small piece of rubber was found in the passages on one of the carbs. Once they were cleaned and reinstalled, the balance went normally.
One thing I do when changing the lines to insure that they stay clean during installation is I cap the ends of the lines with rubber stoppers from the autoparts place. You can get them large enough to fit over the ends of the lines. Basically I make the assemblies, clean them out with gas and high pressure air then cap them. Then I can push them through firewall holes and firesleeve without having to worry about stuff getting into the lines. I also cap the fittings on the carbs, fuel pumps and tank just to make sure....
May sound kind of anal but you can't be too clean when doing anything to the fuel system 
LS
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LS
Titan II SS |
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