OS 52 4-Stroke priming problem...

Have an unusal problem that has just surfaced with my near new OS 52 4 Stroke.... Never had a problem priming it until recently it would not prime at all.
I have always primed by having the throttle wide open than placing my finger over the exhaust. Then I turn the prop through a couple of strokes and the fuel sucks up through the fuel line and into the carby. Works every time and starts first flip.
Now, it won't prime at all unless I flip the prop really fast over and over heaps of times even then it sometimes won't prime unless I use an electric starter...
I have tried swapping fuel tanks and fuel tubing to no avail. I have checked the carby for leaks and also the backplate. No leaks found. I have readjusted the valves and checked for leaks around the exhaust. No leaks found. The engine has plenty of compression but it just won't suck the fuel up. Even when I finally get it to start it idles fine but full throttle running is very wierd. It goes rich,lean,rich,lean, rich,lean every second or so. It definitely sounds like an air leak somewhere but its got me stumped where it could be???
Could a sticky or bent valve cause this problem?? There just doesnt seem to be enough exhaust pressure getting to the fuel tank and enough suction from the cylinder. Yet the compression feels fine.
Appreciate any help here guys.
Cheers,
Troy.
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Check to see if your exhaust pressure tap is blocked.
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Yep, I checked that and its clear.

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one thing I did notice just now is, if I swap the fuel and muffler lines and try to prime, it pumps fuel very quickly up into the muffler. Its like as though the compression is blowing out the carb instead of the muffler. Its got me confused....

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Troy, Try using a primer bulb connected directly to the fuel nipple, an see if that will allow priming. It sounds as if you may have a plug between the fuel inlet and the tip of the needle valve...
Bill http://www.customcutgrafix.com /

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Hi Bill,
I tried that and it still doesnt prime. The carby is clean and leak free... I pressure tested it and no bubbles...
Not sure what to try next?
Troy.

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lines
Troy,
If you cannot force fuel into the carb with a primer bulb hooked directly to the fuel nipple, something HAS to be plugging the fuel inlet path. I'd disassemble the carb and backflush the main needle valve seat.....
Bill
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Have you checked to make sure the the pressure line and carb line are not reversed. If so it would take a good bit of spinning to build up enough air pressure in the tank to force the fuel into the engine ......if your vent line is not in the fuel , and it and it shouldn't be.

Only one thing that will cause it to exhaust thru the carb , ... running backwards. It happens a lot on 4 strokes.
One will sound very 'weird' when you throttle up if it's running backwards..
Don't put your finger over the card to kill the engine because it will burn you . as the carb is now the exhaust. Pinch the pressure line from the muffler to kill it.
Running backwards can be caused by a number of things , but the most common is a glow plug too hot for that engine. Also can be caused by too much compression which will require a shim under the head to lower it. My guess would be the glow plug , unless you're spinning it backwards with the starter.
Not trying to insult your intelligence , but are the leads on your starter by any chance reversed ? It can happen very easily.
Sometimes I flip the prop backwards to start mine when they are starting backwards. Most times . it will hit compression , bounce back and start in the proper direction.
Ken

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Sounds like the needle is blocked or you have the engine tuned way too lean. If you can't force fuel up to the carb by blowing in the pressure line you most definitely have a blockage, most likely at the needle.
Pull the needle out, flush the carb, then reassemble and try again. I'm pretty certain it will start then.
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Troy wrote:

I just remembered something that happened to me once. I had an OS 10 that had similar problems a few years ago. I would start it and tune it, and then it would go lean in the air. Then I would richen it up and take off again, but then it would get too rich, and so on. I finally figured out that there was some kind of particle of crud in the fuel tank that was intermittently getting sucked onto the fuel pickup.
If I were you, I would remove the needle and backflush the valve, and remove the tank and clean it thoroughly.
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Thanks for your help, but unfortunately I have already explored the things you mentioned to no avail. I have never encountered such a problem before and it has me baffled.
Replaced the fuel tank with a new one and also cleaned and pressure tested the carby.
Its starting to give me a headache now and I have posted it back to OS Engines for a warranty repair.
Cheers,
Troy.

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I'm going to guess that they'll replace the bearings and all will then be well.
I'll put 5 mental dollars on this bet.
No matter how it turns out, please let us know what they think. I'm sorry to hear about your problems, but I sure want to know how the story ends.
                Marty -- The Big-8 hierarchies (comp, humanities, misc, news, rec, sci, soc, talk) are under new management. See http://www.big-8.org for details.
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Troy -- Please post what OS had to say about your problem. Lyman
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Yes I will let you all know. Im guessing the motor will be gone for quite sometime because I live in Australia and these things take time (a lot of time!)
Cheers,
Troy.

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