Syncing cards..

by Ryan Goss in 912 / 914 Technical Questions 11 years ago

Doing a condition insp and I am syncing the carbs. When I started it was off just a little. At 3500 rpm it is dead nuts. As I pulled power to idle the engine began shacking really bad. I was able to advance the right carb with the screw that moves the arm to see how far that carb was off. It only took a few turns and the idle was smooth again. My problem is now the right cable is pretty tight at idle.

Any ideas where I should start. I adjusted because I think the cables have stretched alittle.

Re: Syncing cards..

by Roger Lee » 11 years ago

Roger Lee

125 5.12k 4.53k

Hi Ryan,

First are you using an electric sync tool or a set of gauges? Which engine are we talking about the 912UL or ULS?
A mechanical sync should get you in the ballpark for the idle. Ideally you want the throttle in the cockpit to hit its idle stop at the same time as the carb idle stop screw hits its stop plate. It's good you synced to 3500 and you should be able to get the carbs to sync at let's say 1700--1800 without any issues. You should not have to turn one of the idle stop screws a lot of turns verses the other idle stop screw. They should be within 1-2 turns at the most. The cable on the throttle arm on the carb should be snug and you should not have any free play in one throttle arm verses the other. You said then engine ran smooth after you turned the stop screw, does that mean both carbs are in sync at idle now? (and at 3500)
Cables on new engines will slightly relax and parts break in after a few ours so it's always good to sync the carbs at the first engine start then the 25 hr warranty inspection and then every 100 hr and or annual condition inspection. Once done properly it should only take 10-30 minutes to check them from there out unless you totally remove the cables or do some major carb work. Gauge are much easier to use and you can diagnose with them where the electronic devices take longer and you can't diagnose with them. Understanding a carb sync isn't hard if someone with some experience will do it with you once then they can explain what they are seeing and what should be done to bring both carbs to sync.

Your welcome to call me with any questions and we can go over you the readings mean and a few time saving tips.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Syncing cards..

by Ryan Goss » 11 years ago

Ryan Goss

6 17 0

Thanks Roger for the quick reply. I am using vac. Gauges. This engine is a 912ULS in a Rans S6. The left hand carb cable has a little slack. The right is tighter then I like. Right now the engine idles at 1700. One of the problems I have run into before was with the idle around 1700 I have had a problems trying to get into short runways because on final I can't get the RPM low enough until on roll out the prop unloads. I have had it at 1600 but I never ran it there.

At 3500 the carbs are synced. At idle 1700 they are good too. My concern was the right carb needed a little adjustment to smooth it out. I think at this point I need to run the left carb idle screw just so it touches the arm.

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Re: Syncing cards..

by Roger Lee » 11 years ago

Roger Lee

125 5.12k 4.53k

Hi Ryan,

The word tight may be a little subjective. It should be snug or taught so the cable or throttle arm has no free play when the throttle in the cockpit is pulled all the way back to idle. Both carb idle stop screws should touch the stop plate at the same time.
1700 rpm is okay, but remember the stick controls your speed and not the throttle. Pulling the stick back more will slow you down. If both carbs are at sync then the idle stop screw on both carbs should be touching.So long as they are at sync at idle now turn the idle stop screw inward until it touches the stop plate.

The best way would be to start over and do a mechanical sync using a .004 feeler gauge and then I usually turn the idle stop screw in 1 full turn. This way you should be able to increase or decrease either carb slightly to bring both into sync. A mechanical sync only gets you in the ballpark, but is usually reasonably close. (reasonably - now there's a subjective word :) )

The issue you may have latter on is that the one with a little slack may not be able to be decreased if need be in the future without an adjustment on the bowden cable adjustment to shorten the cable a little more, but if they are synced now it should be fine..

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Syncing cards..

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