Cannot sync with new carbs

by Kenneth Brown in 912 / 914 Technical Questions 5 years ago

I have now tried three different sets of carbs including a new set on this under 100hr 912 and The same behavior with all of them when trying to pneumatic sync.

Left side makes good vac at idle but right side is will not. To get them close, throttle positions are very far apart. (3/16”+) And egt’s are then very far apart as well so that feels like a bandaid.

Direct Compression test shows 135psi on cyl 1 and 2
155psi on cyl 3 and 4
Diff compression test shows all within 2 psi. 82/85 etc.

I have done the intake leak test spraying starting fluid while at idle and no change in rpm.

New plugs, Also dee no change in right/left relationship during mag check.

I have replaced fuel lines and blown out banjo Tee fitting.

I have removed valve covers to confirm that all valve are opening completely and no rocker play is present at any point.

I am now 2 months in trying to find the issue with no resolve.

Please help.

Is there any possible ignition issue that could effect right/left?

They will not sync and the egt’s are 350 set apart in cruise rpm.

Engine does not run rough though.

Re: Cannot sync with new carbs

by Roger Lee » 5 years ago

Roger Lee

126 5.14k 4.58k

Since you have tried 3 different sets of carbs it is doubtful it is a carb issue. I would look hard at cable opening and closing smoothness and accuracy and at fuel delivery to the offending carb. Even though you checked the hose and cables you may have missed something. There are only so many things it can be and you've pretty much ruled out an individual carb. I know it's frustrating, but try to be even more detailed on this next check. Are you using a set on analog gauges or an electronic device for the sync?

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

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Re: Cannot sync with new carbs

by Garrett Wysocki » 5 years ago

Garrett Wysocki

1 572 1.45k

Good point Roger. What aircraft do you have? I had a situation on my Remos where the shaft for common to both carburetors which ran across the span of the cockpit behind the firewall, one end had a bearing that fell out of place and it changed the mechanical synch depending whether you were pushing or pulling on the throttle.

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Re: Cannot sync with new carbs

by Kenneth Brown » 5 years ago

Kenneth Brown

3 7 0

Thank you, it is a Highlander with a vernier 2 into 1 cable. Very simple, mechanical sync yields dead on both ends of the stroke.

I am using analog gauges.
When advancing the throttle, they seem like they are going to move in sync but then the left side builds vacuum (needle drops back)

The left side is also the side that maintains proper egt in cruise flight while the right side is much cooler.

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Re: Cannot sync with new carbs

by Garrett Wysocki » 5 years ago

Garrett Wysocki

1 572 1.45k

Is there a chance of an exhaust restriction on one side? When I used to use 100LL I noticed a lot of leakage of exhaust gases showing up in lead deposit where the down pipe connected to the muffler and the pipe inside the muffler full of small holes which were clogged with lead build up causing excess back pressure forcing the leakage thus increasing back pressure.

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Re: Cannot sync with new carbs

by Kenneth Brown » 5 years ago

Kenneth Brown

3 7 0

I will definitely add that to the list of things to check but the engine/fwf only has 90 hours on it.

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Cannot sync with new carbs

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