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Onan 4.0BFA-3CR Hunting/Surging under 160V

NickMcBride

Registered
Hi, I just replaced my starter solenoid and such from following everyones tips from this thread:
http://www.smokstak.com/forum/showthread.php?t=132998

Now, I'm working on getting everything tuned right. The problem that I'm having is with the unit hunting really bad under 160vac. I have a good DMM that I have wired in, so I can watch the voltage response as I tune everything.

So it runs real nice and smooth at about 180VAC, and I can creep it down to 160VAC by adjusting the tension on the governor spring, but if I get anywhere below that, it just surges like crazy.

My initial tuning was per the manual... 1/4 - 3/4 turn on the idle carb adjustment and about 1-1/4 turns on the main adjustment. Then I worked with the governor stop and spring tension to get it where I'm at now.

The only thing I find a little odd is that when it's running without load, I only see any difference in the surging when I mess with the main fuel adjustment screw. I can pretty much bottom out the idle adjustment, and it'll run exactly the same. Seems that it should be the idle adjustment and not the main fuel adjustment that should be doing something?


Earlier today, I pulled the top of the carb off, and it's really clean and everything moves fine without issue.

I just can't get it tuned to idle really nice and low. Any tips or thoughts?
 
K

Kpack

Guest
Do have the service manual ? Sounds like your rpms are way to high .:wave::D
 

YellowLister

Subscriber
Last Subscription Date
o9/30/2019
should be putting out 132 volts max unloaded at 1890 rpm..and 120 119 volts loaded at 1790 rpm.. whatever you have done is a good way to burn out your windings leaving you with a nice paper weight.. unless of course your setting it up for 220 volts.. if 120 you have the rpms way to high and are most likely running lean being there not made to run that fast...
 

NickMcBride

Registered
I do have a service manual, and I've followed that as for setting the carb, and governor, but like I said, I just can't get it any slower without it surging or
.

Also, if I remove the idle screw completely on the carb, no gas comes out of that hole (while the unit is off) Is that normal?
 
K

Kpack

Guest
I would have to look in the manual at your carb what you are calling the idle screw. What number manual do you have and what is the spec letter on your gen ? It's the last letter after the model number. :wave::D
 

Kevin K

Subscriber
Last Subscription Date
07/12/2019
That's a very common problem with the Walbro carburetor used on the BFA's. The idle passage becomes clogged, the engine surges with no load, and someone "fixes it" by advancing the throttle stop until the surging stops. Of course, now the frequency and voltage are too high.

Remove the high speed needle and the brass piece the needle screws into. Behind this inside the carburetor is the high speed jet. I always remove this to clean it, but you have to be careful. Use a screwdriver that exactly matches the slot in the jet. Or use a small piece of wire to clean it.

Behind the high speed jet are two fuel passages leading up to the air break in the carburetor top. The idle passage has a small jet (in the idle passage) that often becomes clogged. A piece of solid 22 gauge wire (0.0253") will just fit through this idle jet. If you remove the high speed jet, you will be able to see the wire when it is pushed down through both passages.

The initial needle settings are very close to optimum. You might have to go out to 1 or 1-1/4 turns on the idle needle.
 

NickMcBride

Registered
This is a diagram of the carb showing the needles:


When you say high speed needle, are we talking about the main fuel adjustment needle?
 

Kevin K

Subscriber
Last Subscription Date
07/12/2019
When you say high speed needle, are we talking about the main fuel adjustment needle?
Yes.

You will need to remove the carburetor top to see the passages drilled down to behind the main fuel adjustment needle.
 

len k

Subscriber
Last Subscription Date
12/12/2018
Other thing is once you clean carb, you generally do final idle adjustment at low rpm (read manual you may have to disable gov to do it) and main jet adjustment with engine fully loaded. Hairdriers and space heaters work nice for that.
 

NickMcBride

Registered
Thanks! You guys don't know how good you really are. Kevin_K your advice was perfect. I removed everything just like you said, and I also took the top of the carb off again and removed the float. Once I had the float removed, I realized how dirty it actually was! I removed everything and cleaned all the passages and nooks and crannies.

FYI- If you strip the plastic off of a twist tie (like for bread), that's a perfect size to clean all those little holes.

I got everything back together, and got it perfectly tuned in at 110 at the lowest and 120 at the highest. On my meter I even had it running smoothly down to 88vac (when trying to set the governor).

Thank you, thank you, thank you. I just joined these forums a few days ago, and I've gotten nothing but excellent advice every time.
 

Kevin K

Subscriber
Last Subscription Date
07/12/2019
Glad it worked out for you, and thanks for the report.

These old Onans are really marvels of simplicity and durability. Think of it... thirty years old and able to be simply and economically repaired. How many of today's hi tech products will be around in thirty years?

Now one more thing to do... remove the generator band and check the push on spade terminals. They get loose and brittle with age. Also check the brushes to make sure they move freely in their holders.
 

NickMcBride

Registered
Thanks again. Will do. I do love the simplicity of these. I got 2 Onans at once, and one had a burned out starter coil supposedly, so it became a donor to this one. When I started on the known good Onan, all the wiring was disconnected and the control box was hanging from it. Thanks to some good service manuals, wiring diagrams, and the folks on here, it's running as it should. Now I just have to put the hydraulic pump on the end of it and throw it back onto the bucket truck and we should be good!
 

Billy J Shafer

Subscriber
Age
69
Last Subscription Date
09/03/2019
While you have it out. Replace the wire from the battery terminal to the fuse. Some got out with broken wires. Looks good on the outside but broken on the inside. I have found several with that problem.
 

NickMcBride

Registered
OK, I've had the genset running really nice recently, and got it all mounted back onto the truck. Yesterday I was working on getting the remote connection made. All the remote buttons and such worked fine, but at one point something happened and I can't get the genset to turn over anymore.

The genset just cranks and cranks, but won't start. The fuel pump isn't running either when I press the start button.

Here's where I'm at in the trouble shooting process:

-I disconnected all my remote stuff to eliminate that as an issue
-I checked both fuses and they're good
-I got another small 12v battery and I was able to run the fuel pump by itself
-I checked both relays by putting 12v across the coil, and it activates, and the associated terminals open/close fine
-I get alternating 0 and 10vdc on the negative terminal of the ignition coil as the points open/close

The only thing that doesn't look right is that when the engine is cranking, the points contactor doesn't look like it sparks at all as it opens and closes. Before, it had a little bit of a spark/flash before when it would open and close. But I don't understand if that would have anything to do with the fuel pump.

The genset is also sitting on a slight slope too, but it's not that much. The oil level at that angle is just hitting the L line on the dip stick.

Anyone have any ideas? I'm out so far. Can anyone tell me where I can check the oil pressure switch or where it's located?
 

lokay5

Registered
Last Subscription Date
01/11/2015
The Oil Pressure switch is under the tin on the oil filter mount.
 

NickMcBride

Registered
Thanks for the tip. The problem ended up being a loose screw on the terminal strip that should have been holding a bridged connection down. After about 5 hours and tracing everything, I finally found that issue when testing continuity.

My wife told me I had a screw loose, but I just thought that was a metaphor ;-)
 
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