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Old 09-17-2010, 08:27 PM   #1
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Hard starting Onan HGJAB - choke issue

My problem is that during cranking, the choke plate does not close completely - maybe 80%. That means I have to crank for about 30 seconds before it fires. If I hold the plate shut, it fires instantly. In either case, once it's running, it runs without skipping a beat.


According to the service manual, the vibration of the engine cranking should cause the choke plate to close completely. The vacuum break assembly does snap the choke open immediately once the engine is running.

I don't understand why the choke plate does not close completely during cranking. There doesn't seem to be any mechanism (i.e. spring or actuator) that pulls the choke closed. The choke break and the bi-metal heater are all working to OPEN it.

Anyone know how to get it to close? I'm trying this at 90 dF in FL during the summer...
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Old 09-18-2010, 09:47 AM   #2
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erkme73,

I had a similar problem with my Onan, it would crank for a long period, fire then belch black smoke and sputter. When it warmed up it would run fine for hours. If I manually operated the choke it would start immediately and not sputter or belch.

The problem turned out to be the choke membrane had a hole or tear in it. I replaced just the choke and it has worked fine since. Now the bad news, I went to my local Onan rep and he couldn't order just the choke, I had to buy the whole carburetor for something like $250.00! Well after I whined a bit he agreed to remove one off of an old carb he had and sold it to me for $25.00, I did the work myself. It's been running fine for more than a year.

Good luck.
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Old 09-18-2010, 09:48 AM   #3
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Oh yea, in your photo I replaced what's labeled "Vacuum Breaker Assembly"
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Old 09-18-2010, 10:18 AM   #4
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When you say you manually operated the choke, do you mean you had to hold it closed during cranking, or had to open it once it fired? I think the vacuum break opens the choke, so if it had a vacuum leak it would not open? I'm trying to figure out what makes the choke close, not open because mine slams open just fine when its running. Thanks for sharing your experience.
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Old 09-18-2010, 10:32 AM   #5
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I held it closed during cranking, it would run fine after running for several seconds when it warmed up.
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Old 09-18-2010, 11:59 AM   #6
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If the Vacuum breaker assembly is opening the choke properly ,you may be able to adjust the Bi-metal assembly by loosening the 3 hold down screws and turning the assembly slightly to allow the choke plate to close completely, tighten screws. Then watch that the Vacuum breaker opens the choke plate completely once it starts. If it doesn't then re loosen screws and readjust . You may have to compromise on the adjustment. This is what we did on older cars that had the same choke setup. Hope this helps. Joe
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Old 09-19-2010, 01:05 PM   #7
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JRNOLL - what a great idea. I didn't realize the heater could spin under the 3-screw clamp. I found the sweet spot that had the choke plate closed when cold/off/cranking, and fully open when running/hot. Works like a champ. I think there is little doubt that the vacuum breaker is working as it should. When the engine is on, it sucks in so hard I can barely pull the piston back out.

Thanks to all for the great advice and ideas. I managed to get the genset back up under the MH - no easy feat since the main engine exhaust pipe comes out directly under the unit. Got it up with help of a forklift at work. I look like I just climbed down a chimney, but oh how nice does it feel to know I didn't have to rely or pay anyone to make it work.
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