1986 Onan BFA 4 - Starts every time, but runs just 5 sec.

[KendallP]

So today I finally got around to pulling the top off the carb, including float, needle and spring. But first I started er' up and ran her for a minute or so. But I didn't want to heat her up before taking it apart, so I shudder' down.

Got 'er all put back together after the clean. Verified everything in it's proper place with photos. Went to start her up and...

1. Ran for a few seconds. Probably running on carb and choke cleaner. Bowl was empty. Probably should have put some gas in the bowl before putting the lid back on.

2. Then wouldn't start forever.

3. Sprayed a little carb and choke cleaner in the air cleaner and got 'er to fire up.

4. Now time after time she will start... run beautifully for about 5 seconds and then shut down.

The weirdest part (to my anyway) is what's going on in the clear fuel filter (fairly new, by the way.) It's only about a third full of gas, which bothers me a bit, but what's weird is... when I hit the start switch, I can see gas flowing into it. But after she fires up, it seems to quit. Then 5 seconds later, she dies.

The fuel filter is sitting intake side up. So there isn't any air flowing down into the fuel pump.

Coach currently on shore power.

I need the generator by morning.

Any thoughts?
.

[KendallP]

Thanks j-d, but I don't think that's it in this case.

But yes. Hunting is probably the right term.

dougrainer,

I'm pressed for time and couldn't locate the oil sensor, though I have an idea of which wire it is that goes into the control box.

But I did have time to try that "B-12" idea... only with Gumout carb and choke. Here's the report...

Started 'er right up. "Hunted" like mad for like a minute. Then I took hand of the throttle and smoothed her out, myself. Only took a second. Then she ran perfectly smooth for about another minute. Then she began to do that little fade and back to full thing. Then a bit later a full, single drop and return to full. Did this about every 10 seconds. 1 drop... followed by full throttle... and then return to standard "idle."

It happens to quick to compensate with spray, so here's what I did instead. Knowing that she surges about every 10 seconds, I just gave the air cleaner a good spray. After this she would run much longer and then, eventually return to the 10 second surge and return to full idle routine. In the name of (pseudo) science, I repeated this process about a half dozen times. The results were the same. And after the last time, she would just go on her 10 second routine over and over. I am very satisfied with the results of my lab work here.

So according to your post, there is a fuel starvation problem, yes?

[dougrainer]

KendallP wrote:

dougrainer wrote:
1. FUEL in Bowl will NOT deplete in 10 to 15 seconds, so that is NOT your problem for running 5 seconds and stopping.
2. IF you still think it is, just spray B-12 into the intake when it starts to die. IF it IS fuel starvation then it will stay running. If it does NOT then fuel is not the problem
3. Did you replace the Condensor? A bad condenser will cause your type problem
4. Are you positive all 16 pinholes in the Needle fuel jet are completely open? Soaking will not get all completely open. You must used compressed air to clean them out after using carb cleaner
5. Have you bypassed the Oil pressure switch? A Defective pressure switch will also cause your type problem. Just ground it and see if it stays running. Doug

2. I'll Try that

3. Condensor is pretty recent, yes. 100 hours max. Probably 50.

4. Definitely not. I did a "quick" cleaning by popping the top, float, spring and needle. Then Gumout carb and choke into the visible orifices via the red straw in addition to spraying the heck out of the bowl and such. But nothing else disconnected, so spray had nowhere to go but in and back out. Then I let it sit and later wiped it out with some blue paper towel and then hit it with air from my compressor. I'm just surprised that it will run so smoothly and even run the A/C, but then pulse every 10 seconds or so and then continue on.

5. So it will pulse, but continue running? I'll try that.

Years ago when I did FULL service on Old style Onan's, I had a few Condensors right out of the box that were bad. So, I always kept an old condenser to quickly connect it to test if a new condenser was defective. To clean the needle jets requires you to unscrew it from the carb tower and clean and inspect. 1 orifice out of the sixteen that is plugged will cause hunt and surge. Doug

[j-d]

Is this "surging" the RPM rise and fall that ONAN calls "Hunting?"

When we had a BFA, I tried several times to clean the carb. I'm pretty good at small engines, but I could not get it to run right, and turning the High Speed adjustment didn't cause anything to change.

I was lucky enough to find one of the few-and-far-between shops who would clean small carbs electronically. It was about 90-pct better, but still had a "hunt" to it. The manual had troubleshooting for hunting that included cleaning the governor cup and a couple other invasive steps. Then, replace the Governor Spring. DW persuaded me to get a Spring. I was already going to ONAN for some gaskets.

The Spring replacement cured that last little bit of Hunting.

[KendallP]

dougrainer wrote:
1. FUEL in Bowl will NOT deplete in 10 to 15 seconds, so that is NOT your problem for running 5 seconds and stopping.
2. IF you still think it is, just spray B-12 into the intake when it starts to die. IF it IS fuel starvation then it will stay running. If it does NOT then fuel is not the problem
3. Did you replace the Condensor? A bad condenser will cause your type problem
4. Are you positive all 16 pinholes in the Needle fuel jet are completely open? Soaking will not get all completely open. You must used compressed air to clean them out after using carb cleaner
5. Have you bypassed the Oil pressure switch? A Defective pressure switch will also cause your type problem. Just ground it and see if it stays running. Doug

2. I'll Try that

3. Condensor is pretty recent, yes. 100 hours max. Probably 50.

4. Definitely not. I did a "quick" cleaning by popping the top, float, spring and needle. Then Gumout carb and choke into the visible orifices via the red straw in addition to spraying the heck out of the bowl and such. But nothing else disconnected, so spray had nowhere to go but in and back out. Then I let it sit and later wiped it out with some blue paper towel and then hit it with air from my compressor. I'm just surprised that it will run so smoothly and even run the A/C, but then pulse every 10 seconds or so and then continue on.

5. So it will pulse, but continue running? I'll try that.

[dougrainer]

MEXICOWANDERER wrote:
"Condensers" are in fact - capacitors. To end "condenser problems" forever I used to select apppropriate microfarad 450 vdc rated caps - seal the wires forever with JB Weld and mount the cap somewhere convenient. A good cap will minimize flicker point erosion.

A Onan condensor cost about $10 and has a specific mount on the point plate and has 1(ONE) wire which goes to the Point. What the hell are YOU talking about?????? Doug

[MEXICOWANDERER]

"Condensers" are in fact - capacitors. To end "condenser problems" forever I used to select apppropriate microfarad 450 vdc rated caps - seal the wires forever with JB Weld and mount the cap somewhere convenient. A good cap will minimize flicker point erosion.

[dougrainer]

1. FUEL in Bowl will NOT deplete in 10 to 15 seconds, so that is NOT your problem for running 5 seconds and stopping.
2. IF you still think it is, just spray B-12 into the intake when it starts to die. IF it IS fuel starvation then it will stay running. If it does NOT then fuel is not the problem
3. Did you replace the Condensor? A bad condenser will cause your type problem
4. Are you positive all 16 pinholes in the Needle fuel jet are completely open? Soaking will not get all completely open. You must used compressed air to clean them out after using carb cleaner
5. Have you bypassed the Oil pressure switch? A Defective pressure switch will also cause your type problem. Just ground it and see if it stays running. Doug

[KendallP]

MEXICOWANDERER wrote:
It ran OK before the "work"?

Hey Gramps! Great to hear from you again.

No.

It ran worse. Much worse.

But...

It's been something like a year since it started troubling me, but I hadn't had need for it 'til now. So it was time to do the work that TEXAS (old Onan tech on these boards) had thrown out. Just a basic clean from the top, as I mentioned.

Before I started the work, I started it, just to see how it would do. It ran ok-ish, but I shut it down because I needed to start and didn't want to work on it hot and just figured it needed it, regardless. Never put a load on it during that short run either.

[MEXICOWANDERER]

It ran OK before the "work"?

[KendallP]

***UPDATE***

Pulled the output line from the pump, placed some blue paper towels in front of the nozzle and hit start... several times. Plenty of gas flow.

Time has run out on me. Looks like I'm off to Harbor Freight for genny. Always wanted a dedicated one for the homestead anyway.

Unless you have any other thoughts that would take less than 5 minutes' more worth of diagnostics...

Float and needle?

I have photos of those in place... with the set screw holding the horizontal fulcrum pin... and I tested the freedom of movement of the float and needle before a buttoned 'er up, so...

What the heck could cause it to run so smoothly and then surge every 10 seconds or so.

If it were a constant surge, that I could understand. It just seems like it has to be fuel!

[1995brave]

A test you can try for the fuel pump is take a gas can put a hose into it connected to the fuel pump of the generator. Start the generator and if the filter fills up with gas and the generator keeps running then you have a problem along the fuel line to the tank. By the way most gas tanks have a hose between the tank and the hard line along the framerail. That way the tank can be dropped some to disconnect the lines.

[KendallP]

1995brave wrote:
You said you removed the tank lid? Is that the gas cap? If it is and the generator kept running, then I suspect at sometime you replaced the gas cap with a non vented cap and it should be a vented cap.

Yes. Gas cap. Guess I was tired.

Turns out that wasn't likely the problem. I even added 9 gallons of fresh gas I had in the yard, thinking, perhaps the slight angle I was parked on made half a tank look like low fuel to the genny. But now it's got 3/4 of a tank and still a problem.

I don't know the expected service life of a fuel pump, but I don't know that this one has ever been replaced. I would have to check the log. But if it has, it would have been at least 15 years ago.

[KendallP]

j-d wrote:
A bad fuel pump is always a possibility. There's probably a generic replacement that bolts right in. We had a BFA and I replaced ours with a little Purolator generic. There are "Standard (sometimes called High) Pressure" ones 5-7 PSI and "Low Pressure" more like 3 PSI. BFA calls for "Low" and it worked fine.

Do you have a Service Manual for BFA? I kept mine, and can scan the troubleshooting pages for you if you like. PM me with your email ID.

I do have a service manual, but thank you much for the kind offer.

Now to see if anyone local actually has that replacement in stock! Wish me luck.

[KendallP]

Well so much for that. The morning after... same, basic trouble.

When she's got fuel, she runs smoothly. But with no load, she hunts every now and then. It seems clear that the cause is fuel supply.

I added the A/C load and... same thing. She took on the load, but surges every 10 seconds or so. I watched the fuel filter and it looks like the same thing. When she's surging, you can see the gas pour in. But when she goes back to smooth... the fuel seems to stop... and... if I recall correctly... even bubble backwards... as the filter is still only about a third full.

I have not run down all the lines, but I'm guessing they are steel. So the likelihood of an issue there, I believe, is low. And there don't appear to be any kinks between the filter and the steel, which is only about a foot-worth of rubber hose.

Pump seems the most likely candidate at this point.