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Author Topic: Onan engine help  (Read 3669 times)
JimW7
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« on: June 14, 2009, 12:56:53 PM »

I have an Onan 5.5 gas, model HGJAB. Bought new in 2005. Ran about 1 hour. Left old gas in it. Sat for 3 years. This spring I flushed out fuel tank, took off carb, cleaned, and started. Only runs on one cylinder. Installed new plugs. Still misses on same cylinder. No spark.
Question is: Where can i find a service manual, or a diagram of the engine. I have lots of info on the generator, but nothing on the engine. I would kind of like to know where I should be looking before I tear everything apart. Thanks, Jim
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Len Silva
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« Reply #1 on: June 14, 2009, 02:07:39 PM »

Sounds like it might be a stuck valve.
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gumpy
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« Reply #2 on: June 14, 2009, 02:15:39 PM »

There's an Operator's manual here: http://www.cumminsonan.com/rv/support/manuals.  Don't know if that will help, though.
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« Reply #3 on: June 14, 2009, 02:17:12 PM »

Time for a compression check of all cylinders.>>>Dan
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« Reply #4 on: June 14, 2009, 04:44:08 PM »

Sorry, I didn't read your post completely, the part about no spark. Probably not a stuck valve  Embarrassed
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« Reply #5 on: June 14, 2009, 05:01:23 PM »

Jim, try replacing the spark plug on the bad cylinder...it's the one that has the cool exhaust pipe coming out of it after running for a few minutes.

Hope this helps.

NCbob
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« Reply #6 on: June 14, 2009, 05:03:00 PM »

Not sure how these work but might look at flywheel and if it has magnetic pickups make sure corrosion is sanded away. Might be dumb advise but haven't had one apart.

Good luck John
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John Riddle
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« Reply #7 on: June 14, 2009, 05:15:07 PM »

   Unless it a bad plug or wire.
If it run even just a few minutes with old gas, odds are the old gas builted varnish on the intake valve sterm & they or it stuck open. This is a V 90 degree on the side engine.
  To check that out: take the valve cover off on the missing cylinder(if it the inside cylinder will have to pull the genset out), odd the push rod is laying on the block. If so, then it can be save without lifting the head but it's  meticulous work, if so post back & I try to find a way to lead you in it.
  Clean the carb?HuhHuhHuh?? how did you clean it??  it need to be taken apart, 2 jet s out the hole, float, needle, adjusting screw, get a gallon of gunk at part house(there is a small basket in it) put all parts in the basket, put the lid on snug, put the  jug on the grill for the AC fan outside so it shake when the fan run, leave it alone 24 hrs.
 Next spray all parts with brake or carb cleaner, one at time & blow each part dry, & be careful not to drop one or loose one. put back together, re-adjust carb & governor.
                 wrench
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JimW7
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« Reply #8 on: June 16, 2009, 10:40:02 AM »

Thanks for the help guys. I did a compression check, 150lbs each cylinder. Traced the plug wires back and found a worn spot on one, that didn't look that bad, but was stuck under the body of the cylinder. I pulled it out, and kept it from touching any ground. It has fire to both sides. Now I just need to find the proper carb adjustments. Again, thanks to all for your help,Jim.
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gus
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« Reply #9 on: June 16, 2009, 02:35:07 PM »

Jim,

That seems to be the same series as my old one. It is an odd duck in that both cyl fire at the same time, there is only one cam lobe??

Also, make sure the points are set properly.

The automatic choke has very delicate wiring.

The AC output frequency is adjusted at the engine governor.

It is noisy as h...... and vibrates more than any engine I ever saw.
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« Reply #10 on: June 17, 2009, 06:37:32 PM »

Thanks for the help guys. I did a compression check, 150lbs each cylinder. Traced the plug wires back and found a worn spot on one, that didn't look that bad, but was stuck under the body of the cylinder. I pulled it out, and kept it from touching any ground. It has fire to both sides. Now I just need to find the proper carb adjustments. Again, thanks to all for your help,Jim.

  They're pretty standard, start with disconnecting the governor linkage, screw the low mixture all the way in without forcing it, just snug & out 1 turn. then same with the power mixture(under the float chamber) back out 1.250 turn. Start the engine & let it warm up 5- 10 minutes. Here you may have to unscrew the idle stop to bring it to idle & go back to low mixture & adjust for smooth running +1/8 turn out.  Hook the governor, adjust the idle stop screw to set the speed to 55Khz, now adjust the no load speed to 62KHz,with the speed nut on the governor linkage.  Put a load on it & re-adjust the power mixture for a smooth running + 1/8 turn out.  Adjust governor speed to 60KHz.
    This is valid as long you got a clean carburetor, good plug & wire, I f you try to overcome a bad plug with the mixture screw or governor speed you just wasting time.
     wrench
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Hartley
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« Reply #11 on: June 17, 2009, 06:47:28 PM »

Jim,

That seems to be the same series as my old one. It is an odd duck in that both cyl fire at the same time, there is only one cam lobe??

Also, make sure the points are set properly.

The automatic choke has very delicate wiring.

The AC output frequency is adjusted at the engine governor.

It is noisy as h...... and vibrates more than any engine I ever saw.


The old Onan's 2-cyl horizontal opposed use one set of points and a single coil
with 2 HV terminals. Both plugs fire at the same time, One is on the compression stroke while the other cyclinder is on the exhaust stroke. ( 4-cycle ).

It was a cheap and reliable way to fire a 2-cylinder engine. some others may also do that. ( Like motorcycles??? ) I had an outboard motor that did that too...

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« Reply #12 on: June 18, 2009, 05:53:30 PM »

DD,

I thought they were opposed too but when I posted this a while back it was posted that the two pistons actually travel together and fire together?

Now I really don't know which is correct?
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TomC
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« Reply #13 on: June 18, 2009, 10:30:47 PM »

The opposed 2 cylinder Onan has a crank that the two pistons work opposing each other-both up and down in their cylinders to top dead center at the same time.  As stated, they use one set of points, coil, that fires both spark plugs at the same time.  If one is firing, the other should also.  If not-maybe a defective coil, or spark plug, or spark plug wire.
I have an inline 6kw Onan Diesel DJB air cooled that both pistons go up and down the same.  In any 2 cylinder 4 stroke engine, both on the opposed engine and the fore described inline two cylinder, this makes for an even firing engine.  As compared to the inline two cylinder Kubota, Yanmar, etc small Diesels that sound like they are misfiring and run rough.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #14 on: June 19, 2009, 03:44:14 AM »

Thanks for the help guys. I did a compression check, 150lbs each cylinder. Traced the plug wires back and found a worn spot on one, that didn't look that bad, but was stuck under the body of the cylinder. I pulled it out, and kept it from touching any ground. It has fire to both sides. Now I just need to find the proper carb adjustments. Again, thanks to all for your help,Jim.

He had just found the problem.

Sojourn for Christ, Gerald
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« Reply #15 on: June 19, 2009, 07:30:11 AM »

The carb adjustment is easy.  Get the gen started, warmed up and put about a half load on it.  Start screwing the carb bottom bowl screw slowly in until the unit starts to labor, then reverse rotation on the screw 1/4 turn.  If you go into altitude, you're going to have to readjust it everytime.  I know this since I had a 6.5 Onan Commercial on my big rig-it was a pain in the you know what.  Now with my Diesel genset in my bus, I do zero to it.  In fact in 1200 hours of use (like 48,000miles), the only thing I've done is oil change and filters-can't beat the reliability of Diesel. Good Luck, TomC
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« Reply #16 on: June 19, 2009, 07:00:22 PM »

TomC,

Mine was a 7.5JB gas, air cooled.
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