520-HC surging at idle

[cafoose 3,355]

I just got done mowing and when I parked my 1988 520-HC with the 20hp Onan I noticed the engine was surging at idle between 1500 to 1700 rpm and the vacuum gauge fluctuated between 4 to 13hg. At full throttle it seemed to straighten out and run smoothly. Does anyone have any ideas what I should check? It ran fine while mowing at full throttle.

[daveoman1966 3,752]

I ahd a similar issue with my 520H.  the cure:  Take the top off the carb and clean it out....careful not to loose the needle seat valve under the float tab.   The is an instruction link in the forum somewhere on how to to this.

Edited May 7, 2016 by daveoman1966

[Kurt-NEPA 755]

I agree with Dave.  My 88 520H was doing the same thing.  I pulled to top off the carb and cleaned everything with carb cleaner and the problem went away.  You can pull one of the jets out from the top and clean that too.  Watch out for that needle valve and the link that connects to the choke, both go missing easily.

 

Kurt

[Ed Kennell 38,411]

    Surging = dirty carb

[cafoose 3,355]

OK I will clean the carb later and let everyone know what happens thanks for all the quick responses I really appreciate this forum and I'm glad to be a part of it.

[cafoose 3,355]

I took the top off the carb yesterday and blew out all the passages and cleaned the fins too while I was at it. I also removed the muffler to get the mixture screw out and blew through that passage as well. I used this post which helped a lot:

 

 

When I put everything back together it ran great with no surging and steady vacuum.   I would like to thank everyone for your help

[Horse Newbie 7,105]

Why is a steady vacuum important ?

[cleat 6,191]

A steady low vacuum would indicate a vacuum leak.

 

A surging vacuum that moves in relation to the engine surging is just responding to the throttle plates opening and closing. That is just the governor trying to maintain engine speed.

 

A steady high vacuum under no-load conditions is a good thing.

 

Back in the days when cars had carbs, you would adjust the mixture screws to get the highest vacuum.

 

[KC9KAS 4,741]

On ‎5‎/‎13‎/‎2016 at 1:18 PM, cafoose said:

I took the top off the carb yesterday and blew out all the passages and cleaned the fins too while I was at it. I also removed the muffler to get the mixture screw out and blew through that passage as well. I used this post which helped a lot:

 

 

When I put everything back together it ran great with no surging and steady vacuum.   I would like to thank everyone for your help

It is always nice to hear "The rest of the story"!   I think it helps me when someone posts what they did and what worked when they had a problem!

[Duramax7man7 506]

Don't forget to ensure that your engine governor linkages are nice and tight. 

 You can do this by popping of the clips at each end of the governor linkage and lightly pinching them with some pliers to tighten them up. Sometimes the heat and rattling can loosen them up and cause surging as well... 

[bo dawg 516]

Whats the easy way to unhook choke link to just pull top of carb ?

[Duramax7man7 506]

you can either unscrew the choke cable lock screw or unscrew the entire link from the carb. But it does wiggle free without those.

Edited September 11, 2017 by Mastiffman

[cleat 6,191]

You can just carefully lift carb top up and while being careful not to bend brass tube coming down from top into a hole in the main body, carefully tilt top and the small link will unhook.

 

Carefully remove this link and just reverse for reassembly. Do not drop or lose this link.

 

Once you have done this once or twice it gets easy.

[Duramax7man7 506]

Yep!