1989 Onan Emerald III Starts, then BANG! Backfires and Stalls!

[ClydesdaleKevin]

I looked at all the posts and even the Onan manuals, and didn't quite find a solution to my problem:

Our "new" generator is an Onan Emerald III 6500 watt, circa 1989.  It only has 108 hours on it.

When we bought our 89 Holiday Rambler Imperial, the previous owner said the genny just quit on him one day, and he had a mechanic friend check it out who told him the float level was messed up...and the PO never bothered fixing it.  It sat not running for 4 years.

When we brought it home, with fresh gas in the tank, I tried to start it.  It will start right up when you hold the button in...and runs as long as the button is in.  And it will run for a few seconds when the button is released...then BANG!!!  Like a shotgun blast!  It backfires out the muffler tailpipe in a big black cloud of soot...scared the crap out of me and made my sock turn black....lol...and stalls out.

So what do you think?  Sounds to me like his mechanic friend might be right...that the float isn't closing the needle all the way and way too much gas is getting into exhaust system and causing a massive backfire condition.  Does that sound right to anyone?

Should I just rebuild the carburator and see what happens?  It sat for 4 years with old gas sitting in the bowl. 

The manuals also suggest that it could be the oil pressure switch wiring or even a relay, since it runs with the button pushed in but stalls and backfires when released...but it also backfired on me once with the button depressed.  But the manuals don't mention anything at all in the troubleshooting section about backfiring and a big cloud of black sooty smoke.

Again, it only has 108 hours on it and does fire right up.

Thanks for any help!

Kev

[tiinytina]

sounds like a carb issue running to rich.... but thats me. is there a "winter" summer" switch on yours. I know ours in the wrong position for temp causes it to run rough. air filter clogged up? 
T

[ibdilbert01]

My generator is a lot older than yours, but in mine there is a relay that powers the fuel pump and ignition.  When you hold the switch to the right to start the Onan, it temporally bypasses the relay to power the fuel pump and ignition.   Once the onan gets started, it self powers the relay and keeps running, feeding back the extra amps through a diode to the battery to keep it charged.

So when my relay went out, I could hold the switch over and it would run, but when I let off it would diesel and backfire a lick or two then stop.

But again, mine is a lot older (76), but I'd start with a meter and see if the ignition is getting power when you let off the start button.  As the years went by, the gensets got more complicated. 

The PO of mine had a switch wired to the ignition and fuel pump so I had a clue as to what was wrong.  I pulled the old relay out and put 12v to the coil, oddly it clicked yet had no continuity.   A quick search online showed the relay to be rather expensive, so I just used a 3 dollar headlight relay that was rated at 30 amps, way more than the ignition and fuel pump would ever use. 

The oil pressure switch usually grounds the coil to prevent the engine from firing.  Because it runs when you hold the switch over, I don't think that would be your issue, I'd lean towards testing the relay.

[RV Mech Tech]

Kevin - what is your Model and Spec numbers? - very important info for Onans as every model (Emerald. KY,  Marquis etc.)  has been installed in many different applications and if you order a manual from Onan they will not sell you one  without this info (very specific wiring info for each generator) - its very possible that the manual for your generator is already in the member area resource that Mark just posted - you should have a decal somewhere at the front of the RV with this info- one model of generator such as the Emerald may have one very specific 'pin' assignment for the electrical wiring for an RV  and may be different for other applications (such as a service vehicle ) - taking for granted that this is the original RV generator in your Holiday Rambler most of the Onans  of this vintage have control boards and also separate 'START' and 'RUN' relays and this is the most probable cause (defective relay- but certanly not the only cause) )  since you have to hold the start button down to keep it running ( a very common cause on Onans)  - with the correct diagram for your genset you can troubleshoot it yourself so let us know what the model and spec numbers are- thanks!

[tiinytina]

Also need to add that after we did a full tune up on our Onan Emerald I (points, plugs, wires, oil, filters etc) it was still running badly... we pulled off the control panel cover and found that whoever had worked on it last (paperwork showed a full tune up etc by a dealer) but who knows... had used all household level wire nuts and connectors behind it, not marine grade water proof ones... All connections were redone and it ran smooth (when I wasn't at speed it was conking out so think air filter or carb needs a nudge....

To add to the issues it had.. meece had built a nest in the middle of it where the air blows through the unit... so air flow was impaired... that was a mess to clean out...

Tina

[RV Mech Tech]

There is a  troubleshooting guide on the Flight Systems website that includes Onan Emerald series - www.flightsystems.com  - look under the  'RV Controls'  link.

[ClydesdaleKevin]

We had a mobile Onan service tech come out and found out what was wrong...and fortunately it wasn't major!  First, the PO had messed with the carb adjustment so badly that it was flooding...so the tech readjusted it to spec, and it still wouldn't stay running, but at least it wasn't backfiring.  Then he determined that the brushes were stuck, so he freed those up.  Lastly, he found that the control circuit board was bad...you could see the brown burn mark going right through the board.  He didn't have the right board in stock, so he temporarily bypassed the board with a toggle switch, and vroom!  Starts right up and runs like a champ, perfect voltage, and runs smooth and quiet, and the automatic transfer switch works flawlessly.  The only real issue now is that you have to start it and stop it from the toggle in the compartment and can't shut it down or start it up from inside.  He ordered the board and when it comes in he's coming back out to put in the new board and make sure it starts and stops from inside.  So far he only charged us 220.00 bucks, not bad for a service call that actually got it running!  He's really a nice guy and schedules small service calls with bigger ones to save his customers money.  It will probably cost about the same amount when he comes back with the expensive part, but its still a LOT cheaper than a new genny!

Kev

[ClydesdaleKevin]

Another 120 bucks later, and the genny runs like new...yay!  That genny guy is awesome, and very reasonable.  He left the toggle in place just in case the board ever goes bad again...sort of like a manual override to the starting side of the circuit board.

Kev