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Not for the faint of heart carb sync

Roger Lee

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Hi All,




Another mind probing problem and I’ll give the answer in aday or two after a few tries at an answer.


You just pulled the engine for the Rotax 5 year rubberreplacement maint. Everything is back on, oil purged and new plugs. It’s timefor the carb sync. You do your mechanical sync and head out to run the enginefor the pneumatic sync. You start up and everything looks good and the gaugeneedles at idle are only 1“ of vacuum off and the idle is 1750. You throttle upto 3500-3800 rpm to start the high rpm sync. The needles here are 4“ of vacuumoff. So you adjust the bowden cables to sync the two carbs and you run out ofadjustment. You shut down make a cable length adjustment on the throttle arm and restart. Now the gaugeneedles at idle are 7”-8” of vacuum off at idle. (Frack) You try and try to getthem closer, but the left carb doesn’t seem to adjust. The engine is runningrough and is now down to 1620 rpm. This particular plane has a Dynon D120 EMSwith all the temps available to you. Oil temp, CHT and oil pressure are goodand normal. The EGT temp span warning is flashing yellow which means your EGT’shave a span greater than 120F and that’s about where they are. One EGT is 1050Fon #4 cyl while the other is 900F on #3 cyl. You just can’t seem to balancethese carbs. “Frack”










By the way these thought provoking problems I give are realproblems that had a particular solution. I post these in the hopes that someday if you have something similar you may remember and find a quick and painless solution.



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Outer cables hanging up on something?


"Choke" cables out of adjustment?


Were the carbs overhauled? If so, its possible to install the "choke" disc improperly?


The EGT difference would point to different mixture for the cylinders in question - that would also have me thinking "choke".

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Hi All,




Here is the answer to this little problem. Here is a coupleof tattletale signs that help point the way.


1. You just finished a full hose change whichincludes pushing new rubber hose through the fire sleeve.


2. On startupit ran fine for about 30-seconds to a minute and the idle was within 1” vacuum atfirst.


3. The high rpm adjustment at 3500-3800 rpm was noproblem to adjust.


4. When it was throttled back to idle again theneedles were split 7”-8” of vacuum.


5. The right carb would adjust with the idle stopscrew and the gauge needle would move and was about 13” of vacuum.


6. The left carb was at 20” of vacuum and the idlestop screw didn’t seem to make any difference.


7. Applying the choke made the engine smooth againand the needles move to within 1” of each other.


8. The rpm at idle is having a hard time gettingabove 1650 rpm.


9. EGT split is too far. The left carb is running 150Fhotter (1050F) than the right carb at 900F.


(p.s.** Next time someone says that EGT tempsaren’t that helpful on a 912 you will know different in a few minutes)


So right off the bat you know it’snot electrical. You have an extremely high suspicion that it involves the leftcarb and something with the idle. The vacuum at 20” is way too high and theengine is rough because the carbs aren’t balanced. So now you need to stop the carb balance andfollow 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 idleonly. 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 ofthe 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 thebowl and look” it was” and could be nice and clean.




THE IDLE JET AND or CIRCUIT maybe restricted.



The only way to do this right is pull the entire carb and pull all thejets. Use air pressure and carb cleaning spray with the small red tube on a canof carb cleaner. Clean and flush every orifice no matter what you think, butpay special attention to the idle circuit.


When this was done a small piece of red silicone ¼” long and about asthick as a thread comes out. It was tucked back up in the idle circuit andwould not have come out without the flush or air pressure. The carb wasre-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 youwill know what to do and save yourself all those extra hours of banging yourhead against the wall looking for the solution.


Good Luck!


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They are not usually a problem unless you have disturbed them or fowled it up putting it back together. Our engines are very tight and reliable. The only time these things you mentioned are an issue is usually after maint and someone didn't pay close enough attention as to how it went back together.

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These carbs are very cantakerous, and I have had problems with them too. A tiny piece of dirt from deterioration of the factory air hoses between the air intake and the carbs had me on a near emergency landing once. My mechanic and I had three sessions to clean it all up. I sure wish we could get a fuel injected rotax with a mixture control! WF

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These carbs are very cantakerous, and I have had problems with them too.




My BING experience goes back many years and involves a lot of "cantankerousness" on BMW motorcycles.


A suggestion and a question:


Suggestion: Maybe after any fuel line work, the first run-up should be done with transparent filters as the vey last item before the carb. This would catch any crap or detritus (silicone, flashing, just plain dirt) that might have made it into the system. As we've seen, it takes only a tiny bit of anything to block an idle jet, and I know from motorcycle experience the same thing applies to the float needle.


On that note, I'm surprised my Sky Arrow doesn't have a separate fuel filter anywhere in the system. There's a screen in the fuel tank pickup and another at the gascolator, but that's it. As I said in another thread, I purchased an inline filter that I plan on installing to filter out finer particles.


Question: In another thread, I posted about my issues with silicone getting into my idle jet. I recently pulled both float bowls, thoroughly cleaned and inspected the main and idle jets and installed new float bowl gaskets. I also pulled the top off of each carb, and pulled and inspected the slides and diaphragms, which both looked spotless. I only had to loosen one of the "choke" cables to get enough play to remove the carb tops.


Now, like Roger, something changed a tiny bit. I went on a one-hour test flight at all power settings and everything operated perfectly. But on landing I noticed the idle had gone from about 1,600 to about 1,900. I was careful to align the little "tabs" on the diaphragms, and all the cables seeme to operate through their full range, just like before. Nothing else should have changed. Before I start messing with idle speed setting, I'd like some idea of what might have changed. Anyone?

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  • 1 year later...

Wow! A zombie thread!


I think one of my "choke" cables had gotten into a position where the choke on one side was not closing all the way. The end was hitting the carb body, IIRC.


Just had to reposition it a bit and the idle went right back down.


I may have a photo somewhere - I'll see if I can find it tomorrow.

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