Strange starting behavior 1988 520H

[NOSValves 0]

I acquired a fairly nice 520H this last summer and it's been very dependable after I repaired a few electrical gremlins. The one thing I found strange all summer is the only way I can seem to get the unit to start is with the choke all the way off and the throttle all the way down at idle. I also have to let it warm up for at least a couple minutes before I can touch the throttle when I do go to raise it I almost always have to raise the throttle and the choke at the same time than when she takes off quickly turn the choke back off. If I don't follow this procedure I might end up not getting it started until I leave it for a few hours or get out the starting fluid. Once the engine is through the couple minute warm up and achieve the throttle increase as described the engine runs like a top and restarts without issue. As cold weather has set in I tried to fire it up the other day and never did get it going even with a few shots of starting fluid. Today I remove the baffle over the carb so I could squirt starting fluid directly into the carb and she fired up but took a few tries to keep it running. Again had to let it dead idle until it got some heat in it then both levers up and then choke quickly off again....any clues what I'm up against here? I seafoamed it twice this summer. It really seems strange that once you get it running it literally runs perfect. 

 

    Thanks for any tips Craig

 

PS I have 2 other Onan powered horses and they both like half throttle and full choke when cold....

Edited December 7, 2020 by NOSValves

[lynnmor 7,503]

Remove the carburetor and give it a thorough cleaning, and lose the starting fluid and snake oil.

[Back on a horse 55]

Really strange ... it sounds like the carburetor is too rich but that should make it start easier in the cold.  Yes, a good cleaning for the carb f you haven't already.  You are sure the choke is not somehow backwards?  Perhaps the float is stuck or the the float needle is not sealing?  Maybe check the plugs and see if they indicate too rich or too lean.

[lynnmor 7,503]

I should mention that the 1996 and newer have an accelerator pump in the carb and it may be bad.  pump

 

Be very careful if you take this apart while on the engine, small parts will fall.

[NOSValves 0]

3 hours ago, Back on a horse said:

Really strange ... it sounds like the carburetor is too rich but that should make it start easier in the cold.  Yes, a good cleaning for the carb f you haven't already.  You are sure the choke is not somehow backwards?  Perhaps the float is stuck or the the float needle is not sealing?  Maybe check the plugs and see if they indicate too rich or too lean.

Like I said the tractor runs better than just about any of the other 6 I have once it warms up. I mean no surging, plenty of power and so on. It idles better than any tractor I've owned so far. The hard cold start is real strange...oh and yes the plugs look great clean with just a tad bit of caramel color. 

 

Edited December 7, 2020 by NOSValves
fixing auto correct errors

[NOSValves 0]

17 minutes ago, lynnmor said:

I should mention that the 1996 and newer have an accelerator pump in the carb and it may be bad.  pump

 

Be very careful if you take this apart while on the engine, small parts will fall.

This one is an 1988 so I guess no accelerator pump. 

Edited December 7, 2020 by NOSValves
Correction

[WVHillbilly520H 10,376]

23 minutes ago, NOSValves said:

This one is an 1988 so I guess no accelerator pump. 

No sir, and sometimes people's brains only read the 520 part not the 1988 part, and some people believe that all or certain fuel related products are "snake oil" when in fact it could be beneficial if were simply a varnish/gum build up vs complete solid debris said systems personally I will give the additives a try before a tear down it does harm it any worse than where you started and may actually help... BTW it prefer choke and carb cleaner over "starting fluid" ether.

[oilwell1415 563]

My neighbor growing up had a Suzuki 125 dirt bike that did that exact thing.  They fought it for a season or two and then had me look at it.  It took less than half an hour to pull the carb, identify the plugged pilot jet, and put it back together.  Started and ran great from then on.  Sounds like you've got a similar issue with something plugging the low speed fuel circuit.  A good cleaning may be all it needs.  It's amazing how many poor running issues on old small engines are fixed by taking the carb apart, cleaning it, and putting it back together again.

[Back on a horse 55]

I can vouch for Sea Foam mixed in the gas … the stuff is really good and works fast if you want to try that before removing the carb.

[NOSValves 0]

1 hour ago, Back on a horse said:

I can vouch for Sea Foam mixed in the gas … the stuff is really good and works fast if you want to try that before removing the carb.

Thanks I've been using sea foam for years both for periodic maintenance and after sourcing a new vintage piece that has sat unused. I figure for a few bucks if it saves tearing down a carburator why not! It's not like the stuff is known to cause damage or anything. Kind of like a couple squirt of starting fluid in a gas engine never hurts one to get it running to help diagnose the problem....

 

     Cheers Craig

[wallfish 17,231]

My guess is the gas is draining/siphoning back to the tank like many under the seat tank tractors. An electric fuel pump or hand primer bulb will fill the carb bowl for easier starting.

If you have ever run an engine to drain the fuel from a carb for storage you can get it to run longer by choking it and run at idle speed. It will continue until there's not enough fuel to run. Adjust either controls and it will quickly stop running. So I think the same thing is happening here like your carb bowl is drained a bit from sitting. Here's an inexpensive way to prime the fuel back up to the carb. A couple squeezes and it's ready to go

https://www.ebay.com/itm/Fuel-Line-Pump-Primer-Bulb-Hand-Primer-Gas-Petrol-Pump-Rubber-Aluminum-Sales-US/253734053378?hash=item3b13ba6602:g:~e8AAOSwodlasfzD

[NOSValves 0]

55 minutes ago, wallfish said:

My guess is the gas is draining/siphoning back to the tank like many under the seat tank tractors. An electric fuel pump or hand primer bulb will fill the carb bowl for easier starting.

If you have ever run an engine to drain the fuel from a carb for storage you can get it to run longer by choking it and run at idle speed. It will continue until there's not enough fuel to run. Adjust either controls and it will quickly stop running. So I think the same thing is happening here like your carb bowl is drained a bit from sitting. Here's an inexpensive way to prime the fuel back up to the carb. A couple squeezes and it's ready to go

https://www.ebay.com/itm/Fuel-Line-Pump-Primer-Bulb-Hand-Primer-Gas-Petrol-Pump-Rubber-Aluminum-Sales-US/253734053378?hash=item3b13ba6602:g:~e8AAOSwodlasfzD

 

     That sounds like a plan and really could possibly fits the situation. Since it runs so good other than the cold start the bowl draining of gas makes some sense. I still find it strange that choking it doesn't seem to help to get it fired up and that when just the motor cold but puts de temps are not with throttle at idle and choke open it starts right up and slow idles fine. Than if you want to raise the rpm you have to quickly apply throttle and choke or it dies.... really strange.

 

   Thanks for your input

                           Craig

PS any idea the best metris inlet and outlet size will fit the bill?

 

 

 

 

 

Edited December 9, 2020 by NOSValves

[wallfish 17,231]

8 hours ago, NOSValves said:

PS any idea the best metris inlet and outlet size will fit the bill?

6 mm is 15/64" close to 1/4"

That was just a quick link so you knew what I was referring to. You might be able to find a SAE sized or one with 1/4" fuel lines already on it or one less expensive.

Check the float adjustment too. You'd probably have to refer to the Onan manual for that procedure.