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Discussion Starter #1
Hi friends, after some major cleaning, running seafoam and flushing the gas tank my '91 318 with an onan P218 runs extremely rich at start up even when warm. I have to take the air cleaner cover off and let it run for about 30 seconds and then I can put it back together. I tried the needle valve on the front of the carb and put it in as far as it would go. I took the top part of the carb off and noticed that the plumb-bob fuel meter is not the normal plumb-bob shape on the tip.

When I take the air filter cover off I can see fuel spitting from the carb.

Prior to my cleaning efforts it was hard to start but once started would be just fine now it starts almost instantly but is WAY rich.
 

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Discussion Starter #3
I installed a new carb kit that was the float, gasket, fuel meter. Same problem, the engine is running extremely rich at startup. If I take the air cleaner cover off and let it warm up everything is perfect. The float adjustment seems right on for down and up positions. I did not install the lower gasket just the upper gasket.
 

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does it do it only during startup from cold? i wonder if something isn't expanding once hot, sealing, and then you don't have the problem. did you check the intake manifold? there are some threads on here explaining how to tell if you have a leak from the manifold and how to re-seal it better (drill out the rivets, install better gasket, put bolts where rivets were).
 

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My thought is to re use the OLD Plumbob thingie (the fuel shut off needle) and see if anything changes,sounds like you might have gotten the wrong one letting too much fuel in the carb.
 

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Discussion Starter #6
The problem is identical before and after putting the carb kit on. My hope was that it was the upper gasket or plumbob was the problem since the old one is definitely warn. The engine is just choking out finally quits. Major excess fuel getting to the carb. Any chance this is a fuel line issue or would a leaking fuel line lead to leaning out rather than rich?
 

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Discussion Starter #8
Thanks boomer for the idea but the C clip is in place both before and after the trouble started. This is so odd that the engine was a bear to start (couldn't get enough gas to it) and then I used seafoam and the problem has snowballed into the air filter pan being coated with gas from the carb spitting.
 

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Wondering if the timing isn't a bit off. Gas spewing into the air cleaner sounds like backpressure up the throat.
 

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I am guessing the float level is set properly.DO you have an electric fuel pump?
 

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Discussion Starter #11
The float is new as of yesterday. 3/8" for low and level to maybe 1/16" below the rim of the carb high. Last night I cleaned the small hole that is inside the carb vertically on the muffler side, this leads to the long brass tube with multiple holes in it. I put the carb back together and this morning I started the engine and it ran absolutely flawless. Ran it for about 4 minutes and shut it off. I left it for about 20 minutes and came back and it started but ran rough- very rich but did not die. Sounded like it was running good on one cylinder but not the other. Increase the throttle and it slowly smoothed out after about a minute and ran perfect again. Prior to shutting it off I turned the fuel selector to off and let it run for a bit and then shut it off.

I believe I have an electric fuel pump. The fuel line input is connected to a small box on the back right side of the engine and the hose out of the pump is connected to the carb.

Questions:
-Where exactly is the idle jet? Is it the hole vertically in the top half of the carb on the muffler side? This is a P218 if that makes a difference
-The mixture needle valve in the front of the carb is that turn in for lean and out for rich? I had it all the way in for testing and that did not help. Last night I set it for 1 1/4 turns out prior to this mornings start.
-How does the onan act when there is a fuel line leak? Rich or Lean?
 

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The idle jet is in the front of the carb,(# 7 on the picture)the motor usally idles rough (hunts) rpm if this is plugged,it has to have a hole completly through the bottom so if you can't see light throughh it it is plugged.If the leak is detectable it won't get any fuel because the pump is vacuum operated and there is not enough vacuum created by the motor to overcome the leak.You have a vacuum operated pump not electric.I use a welders tip clleaner to clear the passage.
 

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Discussion Starter #13
A huge thank you to Biker to putting down the correct path. My 318 is running great now (still seems a bit rich). Thanks for posting the picture but for those that words are easier than a picture: The idle is not something you readily see when you take the half of the carb off. The top half of the carb has a brass tube with 3 big holes and 2 small holes vertically ever 90 degree rotation of the tube. These must be clear but is not likely the source of the problem. The idle jet is next to the hole that receives the brass tube. It looks like a screw with a hole in it. The IMPORTANT thing about taking this out is to find a screw driver that is just as wide as the hole but not any bigger and also fits the slot of the bolt. If you don't have a driver that matches these two very important criteria than adjust a screw driver that is bit to big for the hole and fatten the edge. The idle jet is very soft brass and it is easy to strip off the top that allows you to remove the jet. If the driver is too wide you can strip the treads that allow the jet to come out.

Once you have the jet out the problem is likely the bottom hole in the jet. Yes, there is a hole in there, it is probably so plugged with dark material you can see it. It is not a big hole but an important one. I started with using a strand of braided wire. Along with the bottom hole there are two holes on the side at the bottom and two at the top (along the side). Make sure all of these are clear.

The above was my big problem but my nemesis was bad plug wire connections to the spark plugs and the coil. I used a pliers to make the metal loop securely connect to the spark plug and a screw driver to expand the metal connector that goes into the coil. Now the engine pops right off and runs great if just a bit warm. Cold starts are still a delicate balance of choke. Too much and the air cleaner cover has to come off until the excess fuel is burned off. HAPPY DAYS!!

The first picture is the idle jet removed. The second is point to it's location, the third is NOT the idle jet but the lead to the choke function (I believe) the key is that it is not the idle jet.
 

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Discussion Starter #14
Ok, I wanted to cover the solution. Now I would like to provide a walk through of how to get to the idle jet:
1) Remove the air cleaner
2) Remove the two rear bolts (3/8 socket) that hold the air cleaner pan to the tractor. PUT THESE SOMEWHERE IMMEDIATELY, do not leave them in the pan, they could fall down into the start ring shroud and waste a bunch of your time (painful lesson learned)
3) Remove the three bolts that hold the plastic cover over the carb intake (5/16 socket)
4) Remove the four screws (not bolts) that hold the top of the carb on.
5) Carefully lift the top of the carb up. You will need to both lift and twist to get the brass tube out and disconnect the choke. If you want the safe approach remove the bolt that holds the choke cable so you have some slack.
6) Calculate the size of the flat screwdriver you need to take the idle jet out. Make sure it is not too wide.
7) When the jet is out thoroughly clean it make sure all the holes are clear but do not elongate or enlarge the holes.
8) Reinstall the idle jet. Don't go nuts with tightening it, just enough.
9) Reassemble the top of the carb. Don't bother putting the air filter box back on until you test run. If the engine does not run good adjust the set screw on the front of the carb (faces the muffler) This takes a very stubby screwdriver. I did not find much luck with getting a response to making changes with this setting.

If you still have problems you probably have a spark plug wire issue.

Don't give up, this carb is very simple. Taking it off the manifold is most likely not going to help the problem, you can see all the parts from the top half.

Try everything before you invest in a carb kit. I purchased one and it did nothing to resolve the issue. I purchased mine from Onanparts.com
 

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Discussion Starter #15
An important update to my saga of getting my onan to run well. The above carburetor maintenance seemed to have corrected the problem but things still did not seem right. After some more google searches I found a post about an onan generator suggesting to squeeze the fuel line so that the carb would run out of fuel. I first tried using the fuel shutoff switch, that didn't work so I squeezed the line. The engine ran the fuel out with no change in performance, it was way too rich right up to the point of it quitting. If it would have leaned out then a continued focus on the carb would have been justified.

So, the same post recommended checking for carbon on the valves. I took the left cylinder head off and HOLY COW! The intake valve looked like a science experiment. There was so much snow-white build-up on the intake valve that it could not seat thus the carb was getting back pressure and making the engine run rough and loud. After a couple of hours of cleaning all 4 valves and the cylinder heads (the heads were just normal black soot/oil residue) I put it back together and this is a completely different engine. Quiet, smooth and not pumping out a stinky gas smell.

To clean the valves I did not have a valve spring compression tool so I just left the valves in and worked slowly and carefully to remove the build-up making sure not to scratch the metal. Slowly turning the fly wheel (easily available behind, not in front of the firewall) to move the valves in and out for different cleaning areas. A tooth brush and gasoline were used for the final polishing.

As a quick gauge of whether the valve spring gap is in the ballpark or definitely causing problems turn the engine (by hand) so you know the valve is completely down. Put your thumb on the valve and try to spin it. If it spins you either have carbon build up between the valve and the seat or your springs need to be gapped. I do not mean for this to be an end-all but it is nice to know that the effort to get into the valve spring cover off is worth it.

I then gapped the valve springs (.13mm intake .33mm exhaust note metric not US gap measurements). The problem child gapped correctly so I'm not sure what started the problem but I doubt that the valves were ever cleaned and my use of seafoam cleaned the carb to the point that enough fuel could get in the carb and finally got my attention.
I managed to not have to take the intake manifold off to get to the valve spring adjustments. The right cylinder was a challenge with the manifold on but I was able to do it. I did have to loosen the right-side bolts on the manifold in order to get the bolt out of the valve spring cover plate.

Prior to putting the heads back on I put a light covering of engine oil on the cylinder walls so that the first start would not be bare metal since I used gas to clean the parts.

Numerous clean towels were used to clean the cylinder wall prior to moving the fly wheel so the piston would not pick up the carbon chunks that landed in the cylinder.

To clarify my issue, with the air cleaner cover off it was easy to see gas vapor coming out of the carb. A new carb kit was a complete waste of money. If you have similar issues start with pinching off the gas line (after the fuel filter) try the fuel shut-off valve first, this should work to stop the fuel to the carb. If the engine clears and runs good just prior to quitting focus on the carb, if not you are probably in carbonville and time to clean and gap the valves.

I reused my head gaskets, it is recommended to replace them. I will the next time I have it apart.
 

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When you have the top of the carb off take the compressor and blow the tube with all those little holes in out and blow down in the card. as there is usuly some dirt that settles down in the bottom.... My feelings about putting and additive in the system. is not good. I know many will dessagree . But every time i've used that crap it allways muptiplyed the problem. On a side note Where is that crap gonig to. OH I forgot It is going trough the engine;; How sick is that;; I have some real good stuff I'll sell you; Use it foranything,, No garentee;
 
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