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Idle mixture adjustment

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Re: Idle mixture adjustment

Postby Steevo » Mon Nov 12, 2018 10:06 pm

Well the idle jet is easy to change so worth a try. The original Jab specification was 35 idle jet and then they changed it to a 45.

The needle itself is the only tricky bit and needs the top off the carby. Much easier with it on the bench.
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Re: Idle mixture adjustment

Postby MrBean » Tue Nov 13, 2018 9:52 pm

Thanks Steevo. Yes, I thought the needle might be a pain. But it might be worth taking the carb off and the top off to examine the diaphram too.

But first I'll try the easy option - the idle jet. ;)
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Re: Idle mixture adjustment

Postby LSAGuy 104 » Sun Nov 25, 2018 2:22 am

Steveo, all, Well RATS! my problem is exactly the opposite. I cannot get the engine to idle with the enrichener (choke, starting carburetor, call it what you will) off. The engine is running very lean. I've replaced the carb coupler with a new one from Jabiru US (Arion) because the old one was rotted. Expected it to solve the problem, didn't. I've replaced the intake pipe couplers for the same reason. No Joy. Rebuilt the carb with all new o-rings (yes, even the one on the throttle shaft), new float bowl gasket, new needle valve and enrichener body gasket and verified that starting carb is positioned properly (dimple on shaft toward carb intake or slot up however you want to look at it). Cleaned and blew out all orifices. Nada. Took carburetor body completely apart again and soaked in carb cleaner then cleaned out everything again. Zippo! I've rebuilt the entire intake track TWICE. I don't believe I have a leak there. I used all the recommended sealers and cleaned every part until it looked like new. Rebuilt per overhaul manual, didn't freelance anything. NOPE, still won't idle. I even made a test stand that would let me mount the carburetor and use my shop vac to draw vacuum. Hooked a vacuum gauge to the appropriately name vacuum port. Removed the idle mixture screw and hooked up a vacuum gauge to the boss so I could see if there was any difference between the two gauges as the throttle is worked. I adjusted the idle speed screw full out so the throttle plate would close completely and block off the idle bypass orifice. Both gauges read the same. I blocked off the idle air passage at the carb intake. The guage on the idle mix boss went up about 3/4 in Hg. Opened the idle air passage and both gauges equalized. Slowly opened the throttle and idle mix gauge dropped a small amount which I took to the idle bypass being opened. From there to full throttle the gauges kept the same difference until the idle circuit shut off at about 1/3 open throttle (at this point both gauges had dropped to zero). Looked in the carb intake and watched the diaphragm raise and lower the piston as the throttle was opened and closed. At that point I thought the carb was working properly. When I finished the second rebuild of the intake tract this afternoon we were able to get the engine to idle briefly with the enrichener closed. I ran the idle mixture out 3 full turns to get that. Tomorrow, assuming it doesn't snow, we're going to put a vacuum gauge on the carb and see what we're getting with the carb on the engine (my shop vac was only able to pull about 4" in Hg compared to 19 to 21 in Hg for a Rotax 912 when synching carbs) I'll also run some phosphorescent dye in the gas and see if I can determine that both the idle bypass and idle mixture ports are working.
Open to any and all suggestions as i'm just about stumped.

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Re: Idle mixture adjustment

Postby diablo » Sun Nov 25, 2018 11:14 am

Have you done a compression and more importantly leak-down test ? If the carb has been rebuilt and it all checks out (float level, Jets, settings etc), air leaks due to root or cracks on the coupler and so forth, I would start looking at other possible causes which are likely to show up in a leak-down ...
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Re: Idle mixture adjustment

Postby LSAGuy 104 » Sat Dec 01, 2018 2:30 am

Diablo, Another fellow and I did the annual on the airplane just before my friend bought it. I did a dynamic compression test and a bore scope of the cylinders. Valves are perfect and compression is even. I've gone through the Bing manual again and even it doesn't explain the purpose of the idle bypass orifice. At this point I'm going to increase the idle jet to a 45 and I've got a 55 ready to try if the 45 doesn't do it. My theory is that before I rebuilt the carb there was a bit of crud in the idle air tract and when it came out the idle mixture went lean. Well, that's my latest idea. I'll know on Sunday.

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Re: Idle mixture adjustment

Postby hobson » Thu Dec 20, 2018 10:31 am

Just a Q - was this a 32 or 40mm carb?
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Re: Idle mixture adjustment

Postby Steevo » Thu Dec 20, 2018 10:53 am

Mine is a 40mm
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