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Onan 4.0 BFA idle surging


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  #1  
Old 09-25-2010, 01:09 PM
Ebony Mule Ebony Mule is offline
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Default Onan 4.0 BFA idle surging

I finally got my gauge panel hooked up and ran the generator today. With no load it surges a little, voltage goes from 118 to 132 and the HZ are about 56-62. I connected my shop's A/C unit to it (10 amp load) and the volts maintain 127 and the HZ 60.2. I've worked the idle and H/S jets both ways and changed the preload on the govenor spring and it still surges with no load. Is this normal, or anybody have an idea how to resolve it ? New tune up, valves cleaned, carb kit. Thanks, Tim
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Old 09-25-2010, 04:07 PM
armandh armandh is offline
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Default Re: 4.0 BFA idle surging

put about half load on it and try optimizing
setting the mixture, unload, hold the throttle shut and set the idle
return to speed plug the load in and set the mixture for smooth running again
[not too lean you will fry something]

the position of the governor spring related to the arm movement can create some inverse feed back to reduce hunting

a little surging with no load is annoying but not a problem
not usually run that way for back up
a bigger problem for construction use
here idle down features are desired.

Last edited by armandh; 09-26-2010 at 07:39 AM.
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  #3  
Old 09-25-2010, 06:49 PM
Ebony Mule Ebony Mule is offline
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Default Re: 4.0 BFA idle surging

Thanks, I'll try that in the am
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Old 09-26-2010, 12:22 PM
Ebony Mule Ebony Mule is offline
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Default Re: 4.0 BFA idle surging

Spent two hours on it today. Cannot stop it from surging without a load on it. Guess I'll go back through the carb again
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Old 09-26-2010, 12:27 PM
Roland Hayes Roland Hayes is offline
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Default Re: 4.0 BFA idle surging

Set a throttle stop on the carb so that the no load minimum speed is controlled by the stop, then adjust the governor speeder spring so that the governor is slightly closing the throttle at the no load speed, worked for me when nothing else would. Good luck
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Old 09-26-2010, 01:58 PM
armandh armandh is offline
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Default Re: 4.0 BFA idle surging

at how few watts of load does it start surging

here is how I got a 7200 watt K series 582 powered gen right from no loas through breaker opening overload

first you need a spring long enough to deliver near constant tension theough the full range of the throttle motion

start with the fixed end end of the spring in the same plane as the governor arm motion and perpendicular to the arm about mid position.

in the same plane as the arm motion moving the fixed end attachment away from the arm piviot will increase the inverse feedback and reduce surging
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Old 09-26-2010, 02:13 PM
Gunny Gunny is offline
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Default Re: 4.0 BFA idle surging

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Originally Posted by Roland Hayes View Post
Set a throttle stop on the carb so that the no load minimum speed is controlled by the stop, then adjust the governor speeder spring so that the governor is slightly closing the throttle at the no load speed, worked for me when nothing else would. Good luck

Do NOT do this!

Will cause an over speed when dropping off a large load.

Fix it right.

Tim:
Usually a sign of it running but rich can do it too.
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  #8  
Old 09-26-2010, 04:53 PM
Ebony Mule Ebony Mule is offline
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Default Re: 4.0 BFA idle surging

Thanks for all the replies. I've adjusted the high speed jet every way possible and it makes it worse if I make it over lean or rich. If I hold the throttle closed the engine quits. The carb was in good shape after I cleaned it, But I guess maybe I missed something
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Old 09-26-2010, 05:20 PM
Roland Hayes Roland Hayes is offline
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Default Re: 4.0 BFA idle surging

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Originally Posted by Gunny View Post
Do NOT do this!
Will cause an over speed when dropping off a large load.
Fix it right.
Tim:
Usually a sign of it running but rich can do it too.
My set does not overspeed at all with this mod?? I tried it from overload to zero load and it performs normally, can you give more information, rgds

Last edited by Roland Hayes; 09-26-2010 at 05:26 PM.
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Old 09-26-2010, 07:29 PM
K D Redd K D Redd is offline
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Default Re: Onan 4.0 BFA idle surging

If you hold the throttle close and it quit, this means you idle mixture is WRONG or you are not getting ANY idle mixture. Wrong or no idle mixture causes 90% of surging governors.

Kent
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  #11  
Old 09-26-2010, 07:53 PM
Gunny Gunny is offline
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Default Re: Onan 4.0 BFA idle surging

With further thought, another possible fix/solution. Governor linkages are linear or non-linear, geometrically speaking, depending if gas or diesel. A change in the governor to carburetor linkage geometry can cause governor instability, sometimes just at certain load levels, sometimes across the entire load range. Might be worthwhile to change the length of the linkage but be sure you still have a full range of control.
With an electronic governor, this would be the same as changing the gain setting.
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Old 09-27-2010, 09:42 AM
Dave Edmonds Dave Edmonds is offline
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Smile Re: Onan 4.0 BFA idle surging

The engine should continue to run at idle (considerably below 1800 rpm), even though it never runs this way normally. The carb does have idle jets, etc. in it, since this type engine is used for other purposes than generators. You should be able to hold the linkage and make the engine idle (with no load). If it won't idle, you probably missed something in the carb (been there, done that). The engine is probably surging because it's trying to use the carb idle circuitry with no load, but dying in the process; therefore, it opens the throttle and heads back toward the high-speed circuitry to get fuel.
Dave Edmonds
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Old 09-27-2010, 08:59 PM
Ebony Mule Ebony Mule is offline
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Default Re: Onan 4.0 BFA idle surging

I agree Dave, Even though I know better I didn't pull one of the little jets out of the carb as it felt like it was going to wring off. I just stuck the whole thing in the ultra sonic cleaner and then put it together. Bad thing is I bitch out techs for doing the same thing . Thanks again, sounds like a come back to me :8)
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Old 09-29-2010, 12:28 AM
bbuchorn bbuchorn is offline
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Default Re: Onan 4.0 BFA idle surging

Well? I guess I'll weigh in on this thread as well.... Being a jack of all trades and master of none, here goes. My experience with gasoline powered engines, linkage has nearly never been the problem. with todays fuels and just the nature of what we deal with over and over. Gas engines that have been run for a bit then sit up for months or sometimes years between uses. fuel breaks down, corrodes the carbs, varnish plugs the jets etc. Nobody makes a carb cleaner that will rinse this garbage out.
I have no doubt in my mind that your Idle jet is plugged .Idle jets on these size engines are near microscopic and take very little to plug. I have rodded out many jets and usually have to use a very fine wire . followed by compressed air and liberal flushing with a good solvent. fine tuning the jets is another story. If you can hook up a vacuum gauge to the vacuum side of the carb, you can tune to peak vacuum under load, then open the high speed jet about 1/8 turn to richen the mixture a bit. the low speed jet on the other hand should be tuned for good vacuum also but needs to be adjusted for good throttle transition from unloaded to loaded. and no hunting. If the engine bogs or stumbles some when the load comes on, open up on the idle jet a bit. this richens the mix giving you enough fuel with the initial shot of air when the throttle butterfly pops open to catch the load. I have successfully used this method on everything from my weedeaters to My R/C airplane engines and the many other projects I have agitated mama with over the years. the best thing I have found is to make sure the fuel system on anything I am gonna store for a month or more is totally empty Before I put it away. good luck getting it running good BB
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Old 09-29-2010, 08:45 PM
Ebony Mule Ebony Mule is offline
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Default Re: Onan 4.0 BFA idle surging

Thanks for the input. I did the carb in an ultrasonic cleaner. We use them on mid size outboards as they have very small idle jets in them and varnish easily. Carb has not set since being cleaned, but as I said I'm guilty of not removing a small fixed jet in it as it felt like it was going to wring off. On the road during the week, will try this over the weekend
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Old 09-29-2010, 11:46 PM
pegasuspinto pegasuspinto is offline
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Default Re: Onan 4.0 BFA idle surging

Get a screwdriver into the slots on the jet, it needs to be in good condition and well fitting screwdriver. then give the screwdriver a good whack with a hammer. generally most jets will twirl right out after that.

Robert
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Old 09-30-2010, 12:35 AM
rhkoch rhkoch is offline
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Default Re: Onan 4.0 BFA idle surging

bbuchorn has great ideas but I've had problems with the last one about draining fuel from stored units. Draining is the best way only if it's done perfectly. Any residual fuel in the small carb ports will dry out into varnish. I've had the best luck with leaving stored stuff with a full tank of good gas with Sta-Bil. That's good for a year. For longer periods, be sure to blow out the carb well after you drain the fuel.
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Old 09-30-2010, 06:55 PM
Ben Cowan Ben Cowan is offline
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Default Re: Onan 4.0 BFA idle surging

I run my engines after fuel is cut off or fuel line is disconnected. When engine begins to lean out I choke the engine and try to burn every drop of fuel in the carb. this has allowed me to get 10 years of service from used outboard engines without touching the carbs during those years. Other engines must remain with fuel in the system. I generally stabilize those for the one year period previously mentioned. Ben
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Old 10-01-2010, 07:49 PM
Gunny Gunny is offline
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Default Re: Onan 4.0 BFA idle surging

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Originally Posted by pegasuspinto View Post
Get a screwdriver into the slots on the jet, it needs to be in good condition and well fitting screwdriver. then give the screwdriver a good whack with a hammer. generally most jets will twirl right out after that.

Robert
Careful on this. Some of those jets are epoxied in, EPA mandate or something like that, for our own protection you know. Have ruined more than one carburetor trying to take out jets.
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Old 10-01-2010, 10:32 PM
pegasuspinto pegasuspinto is offline
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Default Re: Onan 4.0 BFA idle surging

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Originally Posted by Gunny View Post
Careful on this. Some of those jets are epoxied in, EPA mandate or something like that, for our own protection you know. Have ruined more than one carburetor trying to take out jets.
I've personally never had the whack-da-jet plan fail on me. It's even worked on jets that practically had no screw slot left. In my world, the jet is comin' out or it's getting a new carb. Just too much chance of having junk behind the jet. An old mechanic showed me that trick, and have built a few hundred carbs since...

Robert
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