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What Would/could Cause


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#1 BairleaFarm OFFLINE  

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Posted February 27, 2012 - 08:18 PM

An 18hp onan... what would cause it to start hard when cold and run fine once its warmed up.

#2 olcowhand ONLINE  

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Posted February 27, 2012 - 08:38 PM

Sorry David, your email landed in my server email filter & I didn't find it till a while ago, and been working getting my daughter's school laptop to link to my wireless printer.
Ok, to your question. These engines require a FULL choke to fire when cold, so be SURE the choke butterfly is closing completely. In cool weather they won't hit a lick without full choke. But once they hit, let off the choke or they will flood.
Have you ran a compression check cold, then hot? You need at least 90psi, and better yet up to 115psi. If compression is too low when cold, that would do it too.
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#3 sieborg400 OFFLINE  

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Posted February 27, 2012 - 08:40 PM

Hello! I have worked on a few onan engines at work mainly generators engines. I would make sure the choke is working properly. The engine needes a rich mixture on start up. If you can get to the carb make sure the the choke is closed on a cold start up. As the engine warms up the choke should open fully when the engine warms up. I hope this helps. Steve....

#4 sieborg400 OFFLINE  

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Posted February 27, 2012 - 08:59 PM

Red tractor your right a compression check 90 psi no less. Also make sure you do not have a vacuum leak when cold she will run lean but as it warms up may run a little better. So many things can affect the engine. Good luck! Steve...

#5 BairleaFarm OFFLINE  

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Posted February 27, 2012 - 09:17 PM

Thanks guys. Youve givin me a couple easy things to check. A starting point is always good.

#6 Cvans ONLINE  

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Posted February 27, 2012 - 09:22 PM

Another thing to check is the carburetor mounting bolts. They need to be tight or you will be running a lean mixture which will make cold starting hard to do. Otherwise the above suggest would be the first place to look.

#7 olcowhand ONLINE  

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Posted February 27, 2012 - 09:28 PM

Once running, you can spray WD40 around the intake areas to check for leaks. If there is one, the engine will stumble a little when it sucks in the WD.

#8 TUDOR OFFLINE  

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Posted February 29, 2012 - 01:42 PM

Depending on your definition of cold, heavy oil will slow the starter enough that it won't fire. I've found that mine absolutely needs a multi-grade oil below freezing temps and a hairdryer to warm it up a little when it's below -20 F.

The choke thing is the first lesson that my tractor gave me. Full choke until it fires and immediately back to half choke or less. It will let you know how much. The choke comes all the way off in short order, even at -30 F.




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