Onan carb gurus- main jet/carb mods?

[Greentored 3,220]

I see a lot of threads about this 'Onan surge' and dirty carbs. My 91 520H has started to do it, although only on occasion and not bad- yet. Reading up on some of these threads I understand there are a few different carbs used, and that the main jet cannot be replaced?

Has anyone ever attempted using a pin vise drill bit of correct size and opening up the main jet a touch? The EPA has so many carbureted engines running on the ragged edge of 'too lean', back in the 90s/2000s, a simple main jet change/enlargement on a bone stock Harley, go kart, etc...used to make a day and night difference in how they performed. Cant help but think this is why the slightest little bit of buildup has our beloved Onans starving.

FWIW, I do not know anything about the carb thats on my 91 except it does have a low speed mixture screw, and I do not believe the carb has ever been swapped.

[JoeM 8,042]

I ruined one on a Honda engine trying to use a pin drill to open up the jet a touch. If you are going to try this, I would get the drill that matches the current jet and run it though a few times and giving it a try before going bigger. The small engines are pretty sensitive. Going a hair to big on a race engine will be hardy noticed cause they are dumping large amounts of fuel anyway. 

 

[lynnmor 7,650]

Your 1991 has a good carburetor, it was only the very last 520's made in 1997 and possibly some 1996 models that had the miserable accelerator pump carbs.  If you are having surge problems, there is nothing special about Onans, all carburetors need to be clean and in good repair.  One thing that is an Onan issue is the ball spacer in the governor cracking and allowing the engine to over speed, some might describe it as a surge.  If you are having an issue, describe exactly what is going on.

[peter lena 9,153]

lynnmor, having read many similar problems with the onan  engine , is there a  simple easier change out or swap of parts to eliminate this chronic problem  ?  i do not have an onan , but if i did , i would search out a  fix to eliminate the regular issue. just asking , pete

[peter lena 9,153]

just looking at part of the problem , there has to be a  fix based on this . pete

 

[lynnmor 7,650]

 

If you ever find a spacer with the extended tab, grab it.

Here is more information:

 

https://www.mytractorforum.com/threads/onan-governor-flyball-spacer-fix.260975/

 

I use super glue to retain the spacer instead of the weaker red loctite.  I also made an installation tool to quickly and accurately drive the spacer on before the super glue sets up.

[Bill D 2,144]

I recently saw a metal one shatter into about 5 pieces.  It was replaced with another metal spacer.  My local shop heated the replacement spacer in a toaster oven then installed it on the cam gear.  Came out fine.

 

 

[Greentored 3,220]

On 2/21/2020 at 7:50 AM, lynnmor said:

Your 1991 has a good carburetor, it was only the very last 520's made in 1997 and possibly some 1996 models that had the miserable accelerator pump carbs.  If you are having surge problems, there is nothing special about Onans, all carburetors need to be clean and in good repair.  One thing that is an Onan issue is the ball spacer in the governor cracking and allowing the engine to over speed, some might describe it as a surge.  If you are having an issue, describe exactly what is going on.

Wow, had no idea about these governor issues- probably overshadowed by the valve seat issues haha, and yes I had to replace an intake seat when I bought the tractor a couple months ago. The surge im getting is actually around 2500-2800 and it is mild and occasional, as if it is still cleaning itself out- I think this tractor sat a long time and was also not maintained very well- I went through it from one end to the other, cleaned the carb, replaced fuel lines, intake seat, valve job and lash, decarboned, etc.  The main jet idea was more along the lines of thinking these things are most likely calibrated extremely lean, which is why the slightest bit of varnish or speck of dirt starts them surging, not to mention the lean calibration doesnt exactly help the hot running. I was hoping enlarging the main jet a few thousandths may help prevent future issues, not necessarily to fix mine as it is  fine 99% of the time.

Whats interesting is that I HAVE noticed a 400-500rpm random 'jump' in rpms if I have the throttle pinned- most of the time it settles dead on 3500-3600, but every once in awhile I will open it up and it will go to 4000 and I have to back off the throttle a bit. Sure sounds like thats a governor issue, doesnt it?  Will it do any damage if I run it like that and use the throttle to keep it at 3600 max, or do I need to rip her down and fix it before the season starts?

[lynnmor 7,650]

3 hours ago, Greentored said:

 

Whats interesting is that I HAVE noticed a 400-500rpm random 'jump' in rpms if I have the throttle pinned- most of the time it settles dead on 3500-3600, but every once in awhile I will open it up and it will go to 4000 and I have to back off the throttle a bit. Sure sounds like thats a governor issue, doesnt it?  Will it do any damage if I run it like that and use the throttle to keep it at 3600 max, or do I need to rip her down and fix it before the season starts?

That is the symptom.  It usually happens after the engine warms up and will only get worse over time.  Allowing over-speed can cause engine failure, so do a few more trial runs and if it does get worse as it warms up, time to fix it.  No matter how bad a carburetor might be, the governor should limit RPM to the set speed.

 

The factory jet is rich enough, these Onans are plenty thirsty, except for the newer models with the accelerator pump.  Drilling jets is not recommended because a poor finish can actually cause less flow.  There is a welch plug near the idle mixture screw that can be pulled for a complete cleaning.  First find some soft 3/8" plugs, perhaps at NAPA.