Need Some Onan Help - Starting

[leeave96 490]

I've got two 520H tractors, both low hours, both with the Onan 20 hp engine in them.

The 1992 Onan powered 520H has about 400 hrs on it and is VERY easy to start - almost to the point that you don't need the choke, else you'll flood it.

The 19888 Onan powered 520H has about 60 hrs on it and has ALWAYS been HARD to start. I have cleaned the carb, changed plugs, fresh gas, etc., but it just seems to be very difficult to start - even from day to day when used as a worker.

Today, I thought I'd get it out and run it a bit, but no start. It will fire if I hit it with some starting fluid or carb cleaner, but won't run - very frustrating. I've got gas to the carb, the carb is clean, but for whatever reason, I don't think gas is getting to the spark plugs. I don't think the carb on these Onan engines have a fuel shut-off solenoid, but that is what the symptom is - stuck fuel shut-off needle via solenoid.

My question is - where am I going wrong with trying to get this tractor started?

I'm going out here in a bit and buy some new gas to rule that out again and letting the battery recharge.

I'm getting to the breaking point with this #$%@# tractor and may save a bit of gas for a fire - if it won't fire...

Any tips advice, like experiences with the Onans would be much appreciated.

Thanks!!!!!

Bill

[Sparky-(Admin) 23,120]

How much trouble is it to swap the "good" carb for the "bad" carb? See if that makes the issue go away.

Just a thought ....probably a pain to do.

Mike.......

[TheReDStAnG 4]

Is this only on cold starts?

[Callen 64]

Try putting a little pressure in the tank before you try to start it.

[maxed500 10]

If the issue is at initial cold start, then maybe check to make sure choke cable is pulling choke fully closed. If you still need to burn it, i'll offer to bring you a dead mtd in it's place and bring that "shamefull" horse back to a Missouri stable for rehab.

What about a valve that is not closing fully

[JackC 617]

It could be a number of things.

Based on the age of the engine (1988) it would not be surprising that the diaphragm in the fuel pump is weak or shot. If it still has the typical vacuum fuel pump on the engine tin, I would install clear fuel line from the pump to the carburetor to visually be sure the line is full of fuel. The pump could still be weak and need replacing with a new vacuum pump or better yet with an electric pump. The vacuum line could also be providing weak vacuum to the pump. You could also set up a temporary gravity feed tank to try to eliminate fuel delivery issues (such as weak fuel pump, plugged lines and filters, or tank vent issues). I have also run into partially corroded plug wires so the plugs don't fire properly so you may want to take the plugs out and ground them or use a plug tester to verify good spark when you crank it.

A 1988 with only 60 hours has been sitting around a lot and Mr. Corrosion (oxidation) does not sleep.

I find that a strong fresh battery also helps when dealing with a hard start situation.

[roscoemi 245]

:text-yeahthat:

[Cable 180]

I recently had the exact same problem with a 1990. After doing most of what you did I did a visual inspection of the choke. What I saw was that the chock cable was loose and the chock was not closing when the control was moved.

[leeave96 490]

How much trouble is it to swap the "good" carb for the "bad" carb? See if that makes the issue go away.

Just a thought ....probably a pain to do.

Mike.......

Swapping the carb would be a pain - but I have thought about replacing the carb with a new one - thanks!

Is this only on cold starts?

Yes - only on cold starts - thanks!

Try putting a little pressure in the tank before you try to start it.

I've done that - no help. I even pulled the fuel line and made sure gas was up at the carb too. Thanks!

If the issue is at initial cold start, then maybe check to make sure choke cable is pulling choke fully closed. If you still need to burn it, i'll offer to bring you a dead mtd in it's place and bring that "shamefull" horse back to a Missouri stable for rehab.

What about a valve that is not closing fully

Choke cable is working - thanks!

It could be a number of things.

Based on the age of the engine (1988) it would not be surprising that the diaphragm in the fuel pump is weak or shot. If it still has the typical vacuum fuel pump on the engine tin, I would install clear fuel line from the pump to the carburetor to visually be sure the line is full of fuel. The pump could still be weak and need replacing with a new vacuum pump or better yet with an electric pump. The vacuum line could also be providing weak vacuum to the pump. You could also set up a temporary gravity feed tank to try to eliminate fuel delivery issues (such as weak fuel pump, plugged lines and filters, or tank vent issues). I have also run into partially corroded plug wires so the plugs don't fire properly so you may want to take the plugs out and ground them or use a plug tester to verify good spark when you crank it.

A 1988 with only 60 hours has been sitting around a lot and Mr. Corrosion (oxidation) does not sleep.

I find that a strong fresh battery also helps when dealing with a hard start situation.

I have thought about the diaphragm in the fuel pump being bad. I may replace it and/or the pump. Plugs are firing nicely. Fairly new battery and on a charge tonight. Thanks!

I recently had the exact same problem with a 1990. After doing most of what you did I did a visual inspection of the choke. What I saw was that the chock cable was loose and the chock was not closing when the control was moved.

Choke is working - Thanks!

Thanks everyone for your replies - now I've got some observations and another question, now that I've calmed down a bit...

The last time this happened, which lead me to tearing into the carb - only to find it in brand new shape, I noticed that no matter what, gas wasn't getting into the carb. I could spray carb cleaner or starter fluid into the carb and the engine would fire, but not run. I blew on the fuel line going to the carb and it was blocked. I think what was happening was the needle was stuck in the seat and keeping the carb dry. At some point, the needle came loose and the engine started. It might have been after I put a column of carb cleaner in the fuel line and let it set for a day or two.

Here's my question: If I give the fuel inlet of the carb more air pressure via the fuel line, can I force the needle off the seat without causing any damage? Once I know air can pass through the seat, I might be good to go.

Thanks!

Bill

[TheReDStAnG 4]

I don't think adding "low" air pressure will hurt the needle/seat/float (like 10 psi, not 150 psi) but if this problem is persistent, it won't be a long term fix. I would verify that the float isn't binding in the closed position. Try cleaning the needle and seat, if that doesn't work, sound's like it's time to replace them.

[JackC 617]

Not sure. If you have to do that there is still a problem somewhere that may or may not get resolved like deposits or a bad seat for the float needle. I have seen deposits from Ethanol block the input to a carburetor at the needle. Ethanol is an alcohol that cleans crud and varnish out of the fuel system and deposits it into your carburetor. It also absorbs water. If there are Ethanol deposits causing a blockage, applying air pressure will not get rid of the deposits but will force the deposits further into the carburetor.

When cleaning a carburetor it is good to soak it in carburetor cleaner overnight and then use compressed air to blow out all the passages. Before final assembly is is good to visually inspect the float seat and needle and check that the needle is responding to the float position properly. It is best to replace them even if they look good. While all of these steps are not always necessary, If you skip any you may still have a carburetor problem. Also the O rings on the needles can dry up and cause air leaks that can cause problems.

One shop I talked to does not even bother cleaning carburetors they just replace them. By the time you add up the cost of an overhaul kit and the time for the labor involved with less than 100% guarantee of success, they feel it is better to just replace with new. I picked up a new carburetor to use as a spare but I did have success cleaning when I had a problem. The ONANs start very quickly and run beautifully when the carburetors and fuel pumps are working properly and just the opposite when they don't.

[ericj 1,579]

i had a P216 one time that had hard time starting, it had sat for a long time so i cleaned the carb, before even attempting to start, finally figured out that it would start some were above half throttle and run up to wot

i cleaned and recleaned that carb finally pulled carb and replaced with another one i had off another motor, and ran great. i have wanted to take that carb and get put in a tank and soaked but never have always worried that if it didn't work it is such a pain to take back off. someday i will get around to rebuilding it. so my point is did you try taking it off idle up to wot and try to start it, just asking from experience.

eric

[Callen 64]

Pull the fuse and replace it several times and do the same to the 9 pin connecter. Have you replaced the whole fuel line?

[leeave96 490]

Update:

I got the tractor started today - yippie!!!! Of course after running around for a while, I ran it out of gas...

So I did a couple of things.

First, I blew on the gas line until I could feel the needle move off the seat. I sprayed some carb cleaner into the carb and only got it to fire on what was sprayed in to the carb. (Don't know if anyone else does this - but for whatever reason, a lot of times I use carb cleaner rather than starter fluid. Don't know if there is any harm in doing this - but that's something I do) Still, the tractor wouldn't start.

Along about this time, my Brother came by and gave me another set of hands.

So - the second thing I did was have my second set of hands via my Brother, spray some carb cleaner into the carb when I would crank the engine. As it tried to fire, he would give it a squirt and keep it going. Within a few seconds of doing this, the engine cranked and ran on it's own - double yippie!!! Engine started and no one got blowed-up!

Not really comfortable from a safety standpoint spraying into the carb while it's trying to fire - but question is - any danger/harm in doing this?

I am beginning to think the problem is more fuel pump, so I'm going to try to find one for a replacement. Anyone know the part number for a replacement on these 20 hp Onan's and any tips for replacing it? I know the vacuum line comes into the back side of the pump, perpendicular to the inlet/outlet lines. is there enough slack in the vacuum hose to replace the fuel pump without having to pull the engine tin on the flywheel side?

All I know is one I get the tractor started, this Onan is a fine running engine. If I can get this tractor to be an easier starter, that would be super-duper!

Next stop is one of Matt's foot control kits.

Thanks!

Bill

[Kelly 1,033]

I never use starting fluid on small engines that is not good for them, I use brake clean or carb cleaner, just cheap stuff with all the tractors I get here I buy it by the case when it's on sale, in fact I get cases of the stuff for Christmas, as old as the tractor is I would pull the tin and replace the vac. line heck that may be the whole problem, fuel pumps are not cheap for the Onans last one I priced was over $50

[TheReDStAnG 4]

:text-yeahthat:

Before you start throwing parts at it though, you should probably pin point the problem.

To verify that it isn't carb related.

On cold engine

1. Remove air cleaner assembly

2. Disconnect choke linkage from top of carb

3. Remove top 4 screw and pull top of carb off.

4. Top off the bowl with gasoline.

5. Reassemble and try to start.

If it starts, then it more than likely is fuel delivery/float and seat related.

The fuel pump is semi serviceable (can clean out but haven't been able to find internal parts). I found out that in one of my 88's, dirt was getting in there and blocking the reed valves from closing properly. I cleaned mine out thoroughly using air to clean the reeds. Reassembled and used a mighty vac vacuum to recreate crankcase vacuum as the test. Once that worked, hooked everything back up, and worked good as new.

[Jefflb 0]

I have a 89 520h mower that has 375 hours and when I try to crank it it will not start without a shot of carb cleaner . It seems to run fine for about 10 -15 mins then engine shuts down . As it starts to die I  can wiggle the pto or motion bar and it will backfire and start running again . I replaced the micro switched on everything that has one . I have all new filters . Have cleaned carb . It runs great but will not cold start . It will crank if I turn it off while mowing but just won't cold crank . I have a clear filter I see gas pumping in filter . Ready to light it up .