520H Surges with choke off

[CRE1992 135]

So the 520H I am going to pick up this weekend surges when the choke is not on. It is fine when the choke is half on, what would cause this and what would be the remedy? Should I try spraying some carb cleaner in when it is running? Maybe replace the fuel filter? Any help would be appreciated!

Thanks,

Charles

[puddlejumper 67]

There is just to many possibilities, simple dirty carb/fuel filter, bad carb mounting gasket, Bad throttle shaft in carb. these would be my first checks but with just that info the list is a long one.

[Shuboxlover 474]

I would start by taking the carb off and cleaning the snot out of it....I've heard that the Onan's are picky when it comes to carb cleanliness....although I've never experienced this problem....thankfully :hide:

[farmer 1,075]

Welcome to one of the most frustrating pleasures of Onan ownership, my 518 behaves like it every so often. I think mine must be very fussy about its fuel, I filter from the can into the tank, and change the inline twice yearly, yet it still goes hunting now and again. A squirt of carb cleaner settles it down...till the next time. From reading RS we aren't alone :hide:

[CRE1992 135]

Thanks so far guys, what I think I'll do after I get home is let it run, spray some carb cleaner, see if they helps. If it doesn't I'll seafoam the engine, let it soak, start it up let it smoke itself away and then drain the gas, change the fuel filter and hopefully it will be a ok! ... To be continued Sunday afternoon/night... <_<

Thanks again,

Charles

[mr.pipes 4]

I would pull the tank first and clean it out real good. Just draining it will leave debris in the tank that will most likely break down in the future.

Install a new filter and run a good amount of seafoam in the gas at first. You could spray the carb too.

If that doesn't do it you're going to have to pull the carb. If you have the time and know how I would do it first anyways.

Congratulations on the 520! They are nice machines.

You might want to give it a good cleaning also. I believe some Onans have failed prematurely on due to grass clipping build up in the cooling fins on one of the cylinders.

[bigd 0]

spraying carb cleaner in the carb wont help at all,if you want to really clean it you must take it off(a real pita)get a can of berrymans chem dip at autozone and soak it a couple days blow it out good.but thats assuming you carb is the cause.

[CRE1992 135]

Thanks Mr Pipes, however I don't think I have to worry about the fins, as the entire machine is spotless. Btw.. I am sure you have seen the 520H on this website before... ;]

If you look around for it you might find it.

Thanks also bigd, I really don't want to pull it apart just yet. If what I said doesnt work, looks like I will be, as you said.

-Charles

p.s. It's a surprise, and I will post pictures on Sunday!

[canam1991 2]

sounds just like mine simple stuck needle and float

[mr.pipes 4]

I am sure you have seen the 520H on this website before... ;]

If you look around for it you might find it.

p.s. It's a surprise, and I will post pictures on Sunday!

I guess I am not that clever

I'll be waiting for you to post the pics then :)

[dennist 139]

Charles, I purchased a 520-H a month or so ago and it was surging quite bad also. I decided to tackle it yesterday-I started by reading this thread and a couple others regarding this issue. I pulled the tank and cleaned it out, replaced the fuel lines and filter, new gas with Seafoam(never used it before, but read it was great),then took the top of the carb off, removed the float and sprayed with carb cleaner then put the air to it- put it back together and started it up....Still surging bad! I let it run for a while, shut it off and said a few words..... I remember reading "if needed do the carb clean thing again", but thought this will be a waist of time. I took it apart again, cleaned and put back together. I started it up and NO surging!

Thanks to all you guys who post the ways to fix problems like these-to say try it twice was something simple, yet I would not have done it again if I had not read it first. I hope you get yours under control as it gets frustrating not knowing if you are going in the right direction, but I think with all the info regarding this issue you will find the answer. :hide:

[bigd 0]

one of the reasons its hard to get the carbs clean with out takin them off is the whole for the high speed if theres no mixture screw it has a meter hole you cant get to soaking is the best.onan has the fussiest carb i've seen they even beat out techys.

[tlewis 0]

My 1996 520H had this same problem. Re-building and cleaning carb did not

change/fix problem.

The problem was completely fix by removing and cleaning the idle mixture

needle screw.

The idle mixture needle screw is covered by a TAMPER-RESISTANT PLUG

cover.

The fix is to remove idle needle value, clean, re-assemble and adjust.

Remove the muffler to gain access to the right side of carburetor.

Looking at right side of carb on lower left is accelerator pump. Pump is round

and held on with 3 screws. To right of pump there is 2 temper resistant soft metal

round plugs. A larger one above a smaller one. The lower smaller plug is the

one we want to remove.

I used a small nail. Tap the nail into center of small plug making hole. Using nail,

bend to side and pull plug out. In my case plug was very soft and came right out.

Remove idle needle valve with small flat screwdriver. Using small spray tube on can of

carburetor cleaned spray cleaner into needle valve hole. Give valve hole several

good shots of cleaner.

Clean idle needle valve and re-install. Very lightly turn needle until it bottoms

in hole. Now turn back out (counter-clockwise) 2 turns.

The muffler has a bracket on bottom. The bolts for bracket go through part number

113874 Engine bracket (3318-524 Parts Catalog sheet 24). Install muffler, leaving off the 113874 engine bracket.

With engine bracket removed we can run engine and still get to idle needle value

and make idle adjustment.

Start engine. In my case idle was about 2200 RPM with NO hunting.

Let engine warm up. Using a long small blade flat screwdriver turn needle valve clockwise

slowly until idle is about 1400/1600 RPM.

Re-install 113874 engine bracket.

[Raider12 13]

When taking these carb's off the engine. Does this distroy the base gasket and do I need an over haul kit? Or can it be reused? When I pull my carb off I want to have the needed parts all at the same time. :thumbs2:

[Kelly 1,028]

It's best to use a new base gasket, and you will need 2 intake gaskets, as you have to pull the intake to get to the bolts under it that hold the carb on.

[Raider12 13]

It's best to use a new base gasket, and you will need 2 intake gaskets, as you have to pull the intake to get to the bolts under it that hold the carb on.

Cool, thanks Kelly. I will get these parts tomorrow if there in stock.

[stevebo-(Moderator) 8,041]

I have been there and after failed attempts to clean it properly because I could not get the needle out and ended up breaking it. I purchased a new carb for short money and that was it, runs perfect now. Save yourself the hassle and find a nos carb and be done. As stated earlier in other posts here the onan is a very fussy carb and there really are no adjustments.

[Raider12 13]

Now that you brought the idea of a "new" carb up. I do have a 340hr engine sitting in the garage I could steel the carb off of? Any difference between the years? :thumbs2:

[RicherGabe 0]

I had the exact same issue with my 520H. The reason it'll run good with the choke half on is because it's not getting enough fuel so you have to lessen the amount of air to even out the mixture. I'd take the air cleaner off then you can see the top of the carb. Take the 4 screws off and pull up the top part as far as possible and then turn it to the side. you may have to take off the one linkage rod but that should be it. On that top piece there will be a tube that is for the idle mixture, make sure all of those holes are completely clean and clear. Then also check the hole that tube sits in and blow air and cleaner through that and then reassemble. I did that with mine and it runs awesome now.

Give it a try, as long as you don't rip the gasket it's pretty easy to do.