governor adjusting engine speed...too lean?

[SunDown 0]

Well my 20 horse onan is all stock and hasnt been apart in 1200 hours or so. I've had it since new and everyonce in awhile (years ago) the engine RPM would fluctuate up and down 400 rpm or so like I was cycling the throttle lever in some idiotic display of coolness. This condition would resolve itself and I wouldnt think about it till it happened again in a year or so. Fast forward 18 years, I now have the tractor, 520h - 87 vintage, over at my house (it was my parents and I grew up with it) and it does this engine fluctuation all the time now. I am able to stabilize the engine rpm by adjusting the choke along with the engine rpm, essentially richening the mixture. Well, i have rolled like this now for two seasons and im tired of it. it has new plugs, new wires, new fuel filter, new air filter, I've seafoamed it. Whats going on here?

a friend (not familiar with these engines) tells me that there should be a idle adjustment screw that changes the air fuel mixture...he says its just a matter of turning it a few degrees and listening to the engine...now mind you, im pretty handy around the garage but carbs make me nervous, im affraid im going to jack this thing up.

In any case, anyone care to take a stab at this and tell me what I might check or mess with on this enigne?thanks for the help,

jason

[PetesPonies 0]

it could be a governor problem or a mixture problem. The gov only controls the throttle speed. The mixture screw will effect idle at low speeds. An idle going up and down is a sign of the mixture being off too. You should not be afraid of adjusting the mixture. You can always put it back to where it was. Make sure your engine is warmed up before yuou make any adjustments. Go for it.

[SunDown 0]

that brings the next question then...anyone done this before and can tell me where I should start on getting the adjustment right? If I had a service manual on the tractor Id be dangerous. But i just need a break down of what screws on the carb do and adjust what....

[520HC 208]

There is only one adjusting screw on a Onan carb. On your 520 it will be on the exhaust side at the base of the carb.

[Buzz 502]

You can download an Onan engine manual from Toro. It has a section on carburetors. Go to this link:

https://lookup3.toro.com/request/request.cfm

If you know your model number put it in. If you don't know it, this will get you there. Put # 31-20OE01 for model number and "riding mower" for product type.

[Rideawaysenior 25]

Jason,

It may be time to clean the carb out too. I had the same problem with my 516 H so I pulled off the carb, and soaked it in a carb cleaning solution over night. Put it back together, and problem was resolved.

Jack

[SunDown 0]

all great ideas guys.... time to just DO IT and stop procrastinating.

Bought some 80 grit flap discs and some primer/paint...so I have my hands full there too.

[SunDown 0]

Help an idiot out here please!

on the idle adjustment at the base of the carb. I have adjusted this screw all the way in while the tractor was running and there was no change at all in its behavior as mentioned above. It still hunts at high rpm when the choke is letout. I will adjust it the other way of course but i dont have a 90 degree flat head so doing what I have so far has been a chore. I will run out and get one this weekend.

But if I dial this idle adjustment screw in all the way, what exactly is supposed to happen? Same way if I back it out a lot, whats the engine reaction supposed to be?

i just phoned in for a rebuild kit for the carb thinking its due anyway...sheeesh they are sure proud of that kit, $123.27!!! I will be removing the carb with full dissembly and cleaning before I pay the above!

[Curmudgeon 28]

I could be wrong, there is always a thousandth time, but I don't think you can adjust these carbs any significant amount. Limits are built in due to meeting emissions laws.

Any part for an Onan is seriously expense. I did spring for the entire carb rebuild kit for one I had. Made no difference at all in the way it run.

Do EVERYTHING else possible before making that kind of investment.

[TT-(Moderator) 1,162]

Adjusting the idle mixture screw has no bearing on the high RPM mixture. I also assume the 1/8 turn limiter cap has been removed from the adjustment screw? If so, turn the screw in until it lightly seats and back it out 1 1/4 turns. That's the factory setting and should get you back in the ballpark.

The carb has a fixed main jet, so you can't make any high-speed mixture adjustments.

Have you tried resetting the governor linkage / sensitivity?

[SunDown 0]

yeah the idle cap I removed. I will back it off 1.25 turns as per the manual and adjust from there. But this thing just running too lean, thus my moving the choke to stabilize the hunting...But this thing wont idle at all before any of the adjustments...so Im not so sure its a setting in the governor linkage but I will certainly check it as Im down for any suggestions at this point.

after setting the idle mixture back to factory spec I will check the gov linkage...if its still doing it, off the carb comes and apart it goes for a cleaning...then I will post back with what the results are...if there are any other suggestions I am happy to entertain them.

thanks again for the help!

[TT-(Moderator) 1,162]

Did you pull the carb top and check the float level? How about the main jet? Is it clean? Fuel filter?

Did you run any carb cleaner or Sea-Foam treatment through with your gas?

I've even mixed lacquer thinner with gas to help clean out the carburetors on some old Kohlers.

[SunDown 0]

I havent checked the float level or main jet...im not sure if they have been looked at before to be honest. The tractor has been in the repair shop when i was a kid but that was a long time ago.

Is the jet something I can unscrew and clean put back without messing with adjustments and what not?

I did run seafoam through this thing as a first line of offense, but sadly it didnt seem to do much.

The fuel filter was replaced last year, when this was happening, but as I tore the air cleaner assembly down I noticed that it looks like there is a second filter right before the line goes into the carb. Its solid red so I need to change that and see what happens.

[nylyon-(Admin) 7,878]

IF you're careful, you can remove the main jet without taking the carb apart. First, close the gas petcock, then run the tractor out of gas. At the front of the carb (closest to the hood) bottom of the bowl, there is a nut-plug. It may be easiest to remove the hood as you'll be able to see much better. Remove the plug look inside, you should see the main jet. You can also shoot some carb cleaner directly in without removing the main jet, which I would recommend. Then shoot some around the bowl and all the old crud should come out.

[linen beige 15]

Before I open up a carb without feeling comfortable with it I think I would check to see if you have a vacuum leak anywhere in the intake system. A cracked hose or a loose mounting screw can cause a fluctuating vacuum supply that can cause surging by allowing air in when vacuum gets to a high enough point. That changes the mixture toward lean, and causes the engine to slow, then the leak closes, the mixture gets rich again, engine speeds up, vacuum gets higher, leak opens, mixture leans, engine slows, Before you loosen any screws, make sure they are all still tight ( I don't mean crank them down, that might break something.). Walk around the intake and carb with a screw/nut driver and then maybe even wet down the edges of the gaskets with engine oil while it's running. Why oil and not wtaer? A vacuum leak will pull oil right through an opening you probably can't see. You don't want to pee on a piston. Some times they will alternately pull in and blow bubbles! I may be way off, and I agree that it probably is a dirty or shaken out of adjustment carb issue, but it may keep you from damaging a perfectly good carb. You said it has new plugs, are the old plugs burned like the mixture was lean making the combustion chamber too hot? A lean mixture will overheat the cylinders. A fluctuating mixture normally won't.

[TT-(Moderator) 1,162]

Good points, Jim....

except that manifold vacuum decreases as the throttle is opened. (Venturi vacuum does increase with engine speed though.)

I'd guess that the problem lies in the internals of the carb - or perhaps in somewhere in the supply parts of the fuel system. (shut-off valve, pump, etc.) :whistle:

[SunDown 0]

well its been a long weekend, but this is what I did. I changed both fuel filters, pulled the carb and cleaned it. removed the main jet and cleaned it...it was almost cloged shut. I readjusted the idle mixture screw with the carb off and clean out the fuel bowl. there was some grit in there but nothing to terribly bad given the circumstances. I figured the main jet being cleaned up and unplugged would fix the problem...nope. Seems to be a bit more touchy now, like there is a small window of allowable movement to correct the hunting. but she still hunts. I got it all back together and put the mower deck on it and mowed the grass....but to say I was dissapointed this didnt correct the issue is an understatment.

as for the vacuum leak idea....I didnt see too many hoses running around under the hood so where or what could be leaking? Im game for replacing what needs to be replaced for sure.

Puzzled :whistle:

[linen beige 15]

I was talking about carb to manifold, manifold to engine mounting surfaces, anywhere there is a gasket is a source of a potential leak.

A hairline crack in the manifold could also do it but is not likely with a metal intake. I used to monitor service of a 600+ vehicle police fleet. Some Crown Vic's developed a habit of surging, and I'm talking jump across an intersection surges, that we finally traced to cracks in the undersides of the intake manifolds. Most of these were fuel injected though, and they only did the surging at idle. Carb. models did it at road speed.

T.T. I realized that this afternoon, good catch! I should have said airflow increases with speed instead of vacuum, manifold vacuum that is, Ventura vacuum increases with speed due to increased airflow. The sizes of the intake opening, ventura opening and amount of it's restriction, and the jets are calculated to give a proper fuel/air mixture. Anything that changes these values, such as a leaking gasket or cracked manifold will cause a change to the fuel/air mixture. At lower engine speeds a leak will change the air/fuel mixture and slow the idle speed slightly, maybe cause a slightly rough idle. At higher speeds this air will enter a leak at a higher speed and not only change the air/fuel mixture, but also cause a turbulence that can breifly interupt the air/fuel mixture to the cylinder ( sort of sets up a "wave" in the mixture) and cause the engine to slow. That reduces the turbulence, restores the mixture and increases speed. This presents as surging. Try this, remove the mounting nuts from the carb of a K series Kohler and break the seal of the gasket. Push it up tight against the gasket and start the engine. with it at idle, slowly pull the carb away from the block until you notice a change. With some engines you can get an eighth inch away! Now push it up tight, open the throttle and slowly pull it away. What happens? The engine starts to surge! Hold it in this position and the engine will keep running, surge and all, till it runs out of gas. ( or the carb gets so hot you can't hold it any more!) I've also done this with an HH-100, and got a good three eights inch away, but it backfires too easily. Jason, Were you're plugs burned like the lean mixture had heated things up? If so you may now have a slightly burned valve in there. That type of miss SHOULD be rythmic, but not always. Although, from your description of the gunk you found inside the carb I'd say your getting warmer.

[linen beige 15]

TT also touched on another good possibility. The float level may be just low enough that it is almost closing the needle valve, causing her to breifly run out of gas, slow down and catch up, etc. but I'm not sure if tweaking the choke would compensate for that. :whistle:

[SunDown 0]

thats what I thought too about the float thing...not sure thats the prob (but Im NO expert) since the choke adjustment fixes the issue.

As for the plugs, I havent had them out in a couple of years...I know, Im bad..When iget back home I will pull the plugs and check them to see what kind of shape they are in.

As to the vaccume leak, changing the gaskets is SOOOOOOOOOOOOO easy I cant NOT do this to see if it fixes it! Great info Jim!

Ohhh and you struck a cord with me....growing up mom had a 87 town car with the fuel injected 5.0. That thing would surge like no ones bussiness and every repair shop swore that nothing was wrong and that we were mis-applying the gas pedal and not the brake...Our point was, mom had to have both feet on the brake pedal to keep it out of the intersection as it was surging and quickly reach for the ignition switch to kill the car. As a matter of fact, my family will never let me live down the fact that the lincoln went through the garage wall and into the house while I was at the helm...age 13. two skid marks from the front tires being locked up all the way through the garage was all the evidence I had to defend myself! LOL

I will try the gaskets and also report back the condition of the plugs.

thanks again!

jason

[linen beige 15]

I beleive the '87 police package was the first model we had this problem with! Although there was a mix of carb. and injected ones that year (Ford let us "test" some pretty hot 351s. Remember now that most of the guys were used to the old 460s so a 351 didn't thrill them, until they stepped down on one! We ran them to 50,000 miles and Ford swapped them for new engines so they could examine them. Our mechanics were NOT allowed to open them up but we did know they had "O-ringed" heads and the front seals looked like they were hiding roller bearings) We even had at least two officers I can recall who didn't quiet get their cruiser in park before getting out :imstupid: and ,you guessed it, the cruisers slammed into the back of the car they had stopped. Can you imagine being stopped by the police only to have them ram the back end of your car?:wtf: Needless to say, the city's law department wanted an answer from Ford, that would allow them to transfer blame, REAL FAST! :whistle: