Sport Pilot Talk

roger lee · Posted: Fri May 13, 2011 10:49 pm Post subject: carb sync problem solution

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!

drseti · Joined: 28 Nov 2009 Posts: 1390 Location: Lock Haven PA

Roger, you've probably saved me hours of grief downstream. Thanks. As Butch Cassidy said to Mr. Harriman's loyal railroad employee, whatever they're paying you, it's not enough!