# Generac GP5500



## FOrdtrucksforever (Feb 23, 2020)

A friend of mine owns a small engine repair shop. Customer didnt want to spend the money to fix this Generac GP5500. So it was either going to the scrap yard or I could take it home with me.

It had absolutely no compression. Just like if a valve were stuck open or spark plug was out. I pulled it over and didnt get any reason to think there was something major wrong tho.

My first thought was the valves were not seating at all. Like too much carbon buildup. Valve cover had previously been removed and both valves were moving freely.

After pulling the head, it was then obvious there was way too much carbon buildup. Easy fix I thought. Removed valves, chucked up in lathe and indicated them true to less than 0.0002" tir. I was glad the stems were not worn and heads not out of round or distorted.

There was an obvious worn ring on sealing edge of both valves. It only took about 0.003" to get a complete clean up. Valve seat is 45 degrees. So I rotated the compound on lathe to 46 degrees. The valve was cut to a 44 degree angle. Then lapped them back in to seat. 

I was able to reuse the old head gasket. Gas tank and carb were cleaned out and filled with AV gas. It stated on the second pull and smoothed out quickly.

Everything checked out good until I loaded it down. Then engine surged, jumped around, bucked, and rpm dropped really low. When carb was torn apart, I noticed someone had jacked up the choke. The choke plate had been tampered with and put in upside down. It wouldnt open all the way or shut. 

The choke shaft is plastic and was broken on one side. This choke plate is not bolted in but pushed thru the slot and held in with two raised stops. I can see how easy the plastic shaft could break.

I swapped the plate back and choke would work, but there was no detent to keep it rigid when open. While running, choke plate would vibrate and flutter slightly. Now with it loaded down, the choke would vibrate severely, disrupting the air flow way too much. This making it run really rich. But no black smoke or rich smell.

The replacement parts were more a lot more expensive than a generic carb. So I took the gamble and spent $13 for a knockoff replacement. A week later it arrived. I installed the carb and generator now works better than I could have expected. I can load it down with several high amp draw tools at once. It does not even act like there is any load at all. No surge, studder, or sounding like it is going to self destruct like before.

I figure someone had tried to clean the carb or compensate for old gas/dirty carb running lean by jacking with the choke. This caused it to run too rich for too long and built up all the carbon. Then finally would not start or stay running very long.

Sorry for the long story. I am not familiar with this motor at all. I assume it is a Kawasaki version of the Honda GX 390. Very similar with exception to a hemispherical combustion chamber, domed piston and canted valves. The valve cover is much larger and rectangular. From appearances this looks to be a well built and solid engine package.


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## RedOctobyr (Aug 8, 2018)

Interesting, glad you got it fixed! It's always good to hear about less-common engine problems, it's a nice way to learn about things to watch for.


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## exmar (Jun 29, 2015)

Great story! I have the same generator and it's a nice machine. Curious about the "$13 knockoff replacement." Where did you get it and what part number? While I have no issues with this one, previously had "fun" trying to get parts for a Generac 3750 until I discovered it had a HF Predator engine. So, given the gas we have to buy would like to be prepared JIC.


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## FOrdtrucksforever (Feb 23, 2020)

I hope this link to the carburetor is still current. It is in the states. So you dont have to wait 2 months from over there.

HERE

AV gas is your friend.


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## HarryN (Jun 2, 2018)

Did you see any signs of polymer build up in the fuel tank or carb?

Just curious because a lot of the newer gasolines are not very stable and will gum up the fuel lines.

In the honda engines that I have used, it almost seems like there is a catalytic interaction with one of the metals in the system.

No fuel stabilizer of any kind has successfully stopped this effect. A common effect of this problem is attempting to run the carb richer to compensate.


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## drmerdp (Apr 10, 2018)

Nice job resurrecting that generac. Cheapest replacement head I’ve seen is about $100 bucks, that’s great you were able to repair the valve seats. 

I had a gp5500 for a little while, My only qualm was it’s relatively high Total Harmon Distortion. Always started easy and ran like a top though.


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## FOrdtrucksforever (Feb 23, 2020)

HarryN said:


> Did you see any signs of polymer build up in the fuel tank or carb?
> 
> No fuel stabilizer of any kind has successfully stopped this effect. A common effect of this problem is attempting to run the carb richer to compensate.


Do you mean varnish in the gas tank? When I hear polymer I think plastics.

I eliminated all fuel issues a long time ago. Went to 100LL AV fuel. If something runs on gas then it gets changed over.


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## exmar (Jun 29, 2015)

FOrdtrucksforever said:


> I hope this link to the carburetor is still current. It is in the states. So you dont have to wait 2 months from over there.
> 
> HERE
> 
> AV gas is your friend.


Thanks, link is still current, at least I ordered one. AV gas is not available down here in the sticks. Get by with Stabil, Seafoam, and much cursing at the Guv'ment who had to protect the family farms. a.k.a. Agri-Buisness. :-(


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## HarryN (Jun 2, 2018)

exmar said:


> Thanks, link is still current, at least I ordered one. AV gas is not available down here in the sticks. Get by with Stabil, Seafoam, and much cursing at the Guv'ment who had to protect the family farms. a.k.a. Agri-Buisness. :-(


At least the gasoline sold in this area (San Jose CA) is not stabilized at all using products like Stabil and retail competitors. Believe me I have tried.

It isn't the ethanol - it is other refining methods that are being used to narrow the molecule mix in fuel. I know that people like to blame ethanol and it isn't perfect, but it isn't the cause of your fuel stability problems.


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