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Onan engine ran great now doesnt

5K views 16 replies 7 participants last post by  Lionel 
#1 ·
So last week I used my new to me tractor for 3 days doing work around my house. I spread 4 yards of loam, pushed back overgrown areas and hauled my trailer around.

Last week the tractor started right up. Sounded great and didn’t miss a beat.

Today I go outside to show my wife I finally came out ahead on a tractor deal and let my son take it for a spin.

Well I grab the key turn it on hit the start button and it fired right up. Puffed some smoke and died. Then it was very hard to get started. It did not sound right either like it was running on 1 1/2 cylinders.

Last week it was surging at anything less than 1/3 throttle but assumed it’s because of the electric fuel pump.

The only things I see that are not factory is the electric fuel pump.

I feel the pressure is just to high
 

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#2 ·
It probably is only running on 1 cylinder.. Onan's can do that ..

Make sure both cylinders are firing.. the exhaust temps right at the engine will tell if one cylinder is not running. Or I use my inductive pickup timing light to verify that both plugs are firing.. The Onan CCK in my 646 dropped another spark plug just yesterday.. Ran like a top, shut it down for a while and when I restarted I had only 1 cylinder firing.

This engine already has a new and correct Onan coil, new points and a Kirk engines Transdenser II electronic ignition. So no idea what killed the plug. I swapped in a spare and its back to running like a top.
 
#6 ·
So my tractor this weekend was doing the same thing.. hard to start the. Finally gets running then 5 minutes later dies.. I have determined the problem is fuel..

So fuel is getting to the card just is not going through it. It has a electric fuel pump and will squirt the fuel 10 feet. So I know it it pressurized . If I pour fuel down the carb the tractor will run continuously.
 
#8 ·
So I removed the manual fuel pump. I didn't see anything that would be "blocking " it.

These carbs are a little different than what I am used to.

I am going to take the top of the carb today to see if I can see something.

As for the surging at low RPM I assume it has a small air leak somewhere and the at higher RPM it becomes irrelevant.
 
#9 · (Edited)
"As for the surging at low RPM I assume it has a small air leak somewhere and the at higher RPM it becomes irrelevant."

Air leaks should be easy to find. With the engine idling, maybe a touch more, spray carb cleaner around the intake (split manifold, right?), carb seat and carb gaskets, without letting it get into the carb itself. If you hear the engine change, you found the leak. If not, probably not a leak to find.
 
#10 ·
Ok so I messed with the carb tonight. The carb is definitely not getting fuel.

So with the top of the carb off if I turn the key on. Fuel pumps up from the corner. But when I assemble the carb and turn the key on the tractor won’t fire. Then if I remove the carb top there is no gas in the bowl

Keep in mind I have an electric fuel pump. And I disassembled the top of the carb and cleaned it.
 
#12 ·
Do you have a brass float, if not get one! I would rebuild the carb and original fuel pump and get rid of the electric they can be headaches or cause big issues. I have a tractor I just got in that I was told ran real rough loading on the trailer and was leaking oil. I seen the electric pump and figured right away what was going one. Well none the less drained 1gal plus of oil/gas out of the crankcase (only holds a couple quarts.) The electric pump was flooding the carb and then flooded the crank case. I know your is opposite as its a float and needle valve issue but this could happen too. Luckily they only ran it 1 min to load it on a trailer but if ran longer could have wiped out a motor or cause a fire etc. Onans are awesome engines if set up correct.Where are you located?
 
#17 ·
If you require a carb kit, or rebuild, or float, I'd go with Boomer or one of our 3 dealers here on the site. I know I've purchased mine from Boomer, and he typically has exactly what you'll need, including the buoyancy spring that should be considered. Lots of knock offs sold on ebay, but I've found they aren't all made equal,,,,
 
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