520-HC - Sputtering at higher RPM

[TC10284 109]

C120oe02 520H. This might be hard to follow, as I'm confused myself, but here goes...

 

Last season I worked on this one and got it working acceptably. It would at first want to be "poppy" with slight backfires but then clear up. But I could get to 3600RPM and use the 60" deck fine. I used it maybe a max of two hours last season. Maybe not even that. 

This season, if I get it to say, 3000RPM. It'll start getting very poppy and backfire. Trying to engage the 60" deck about killed it and backfires. PTO safety switch should not be an issue as it was worked on last season also. So I assumed I needed to clean the carb. I took the top of the carb off and blew out all orifices with a lower pressure nozzle the first time and then a higher pressure air nozzle the second time, ensured no trash in the bowl, etc. I did this twice. After the second time it seemed better, but as I try to get it 3600RPM, the whole engine acts like it wants to shudder like it is not firing properly. It makes me concerned that I'm going to break a rod or something so I throttle back. And then when I idle down, it surges around 2500. 

I've tried some NGK TR5GP (platinum) plugs vs the two NGK TR5 (copper) plugs that were in it last season. No real difference.

I do not see any white smoke that would indicate oil getting into the fuel mixture. 

My coil is new as of last season. 

I have tested the plug wires with an Ohm meter and I do not see any high resistance on the wires. 

I filled the tank with fresh gas from maybe 1/4 or less of last seasons gas. 

The carb I think is an aftermarket carb that I think I put on last season. 

 

Next I might try two NEW NGK TR5 plugs and maybe a coil if that doesn't help. Double check compression also. 

Any other suggestions would be helpful. Thanks!

Edited May 8, 2022 by TC10284

[WHX?? 46,773]

Sounds like still a carb issue. Does it run any better with part choke? Almost sounds like starving for fuel. Some guys report having to clean the carb many times.  Doubt spark plugs will help.

Check the fuel lines/filter/pump. Tank outlet not plugged? 

[TC10284 109]

1 minute ago, WHX?? said:

Sounds like still a carb issue. Does it run any better with part choke? Almost sounds like starving for fuel. Some guys report having to clean the carb many times.  Doubt spark plugs will help.

Check the fuel lines/filter/pump. Tank outlet not plugged? 

 

Choking makes it worse. Acts like it's getting too much fuel or not burning off what's there already and it builds up until it finally fires. 

I may try another genuine Nikki carb I have next. Just a lot of work to swap the carbs. 

 

I had to reprime the fuel pump, but after doing so, the fuel filter has a steady supply of fuel and never gets low. 

[WHX?? 46,773]

Ok shoots down my diagnosis. 

Timing issue? But I doubt it... just covering all the bases here. I had a 520 that kinda did the same thing and was ripping my hair out. Fed it a half can of seafoam and the problem mysteriously went

 away but can't say if the foam was the cure. 

15 minutes ago, TC10284 said:

 Just a lot of work to swap the carbs. 

I'll second that.... 

Manifold not leaking?

[TC10284 109]

I resealed the intake last year but I guess it could be. I can do a total swap of the intake and carb since I have a spare one from another p220g. 

 

It leaves me wondering also if it's an ignition control module going out. But I think it's new too...

Edited May 8, 2022 by TC10284

[8ntruck 6,376]

I agree with @WHX??.  It sounds like a fuel system issue.  When you pull the plugs, what does the cylinder end look like?  That might give you a clue as to rich or lean.

 

The surging suggests an air leak somewhere in the intake.

 

I don't have an Onan, but it is said they can be difficult to time at times.

 

Good luck and have fun.

Edited May 8, 2022 by 8ntruck

[TC10284 109]

2 minutes ago, 8ntruck said:

I agree with @WHX??.  It sounds like a fuel system issue.  When you pull the plugs, what does the cylinder end look like?  That might give you a clue as to rich or lean.

 

The surging suggests an air leak somewhere in the intake.

 

I don't have an Onan, but it is said they can be difficult to time at times.

 

Good luck and have fun.

 

The end of the plugs are black, but that's "normal" from the plugs I've looked at on literally every one of my WH with Onan's. 

 

[TC10284 109]

So an update:

Cleaned the aftermarket carb. Found that the bolt covering a jet on the side was loose enough where I could turn easily. Tightened it. 

Replaced an intake gasket. Checked intake for any obvious leaks - none that I found. 

Reinstalled. Replaced fuel line because I was concerned it was cracking. Replaced fuel pump because it was not pumping properly compared to a replacement.

New NGK TR5 plugs - gapped according to manual. 

Tried it briefly yesterday - engine shook at about 3000RPM - not like a broken connecting rod imbalance/shake. That isn't the issue. 

But then started smelling belt rubbing. Found that the belt was slipping off the idler pulley near the transmission. 

Going to try pulling the belt off the transmission later and rev'ing her up to see if it's just that idler shaking bad enough or something else. Maybe it's the drive belt/pulley shaking the whole thing?

 

This one has been one thing after another, and I didn't have these issues just last season...

Not sure what happened other than just time/sitting. Hardly even used it.

Edited May 18, 2022 by TC10284

[lynnmor 6,777]

Check both the idler pulley and the mounting of the idler arm where it is mounted, they often wear forming an oblong hole.

 

 

 

[kpinnc 10,732]

Looks like you're about 40 miles from me. 

 

Sounds like the issue is catastrophic, and you should cut your losses. I'll take one for the team and begrudgingly drag that HC home with me... 

 

In all seriousness, I'm inclined to agree with everyone else: swap that carb and see what happens. Probably a good time to check valve lash while you're in there too. 

 

Good luck! A 520-HC is a great machine!

 

 

[TC10284 109]

I found out why the belt is sometimes slipping half way off the idler...

I have no idea how this happened - must've been previous owner because I certainly haven't allowed anything like this to happen. 

 

How hard is it to replace the pulley on the trans? 

Hopefully you can see how bad it is in the video. 

 

https://photos.app.goo.gl/mJHTwAaaSyViBA4R9

[TC10284 109]

Also, I am going to try one or two more original (cleaned) carbs on this thing (as time permits) to see if that helps. Right now she's surging pretty bad at 3/4 throttle. 

[Handy Don 10,597]

1 minute ago, TC10284 said:

Hopefully you can see how bad it is in the video

 

Ayup. Gotchyerself a bent pulley there. 

 

Excellent sleuthing. The replacing isn't particularly hard but the finding can be. 

Get the outer and shaft diameters and start with an ad in this sites Classified/Wanted.

With that info you can also check with Classified/Vendors for used or NOS options.

[wallfish 15,919]

24 minutes ago, TC10284 said:

Hopefully you can see how bad it is in the video. 

It seems to vary how much it's moving. Also seems to be sliding in and out. If it was only a bent pulley it would stay more consistent so I'm guessing it's all boogered up at the shaft and possibly the shaft too. That pulley needs to come off for some inspection as to what's actually going on with it.

[kpinnc 10,732]

56 minutes ago, wallfish said:

It seems to vary how much it's moving. Also seems to be sliding in and out. If it was only a bent pulley it would stay more consistent so I'm guessing it's all boogered up at the shaft and possibly the shaft too. That pulley needs to come off for some inspection as to what's actually going on with it.

 

I definitely would not use it until you replace that sheave. If you wear the input shaft on the pump down significantly, you got an expensive problem unless you have a parts hydro.

[TC10284 109]

How hard is it to remove that pulley? Looks like I'll have to take the wheel off at least. Use impact to remove the bolt.

Pulley puller?

[TC10284 109]

Also, I have a newer parts-only 520H with a transmission that I don't know 100% works, but "should". How hard is it to swap the entire trans?

Edited May 26, 2022 by TC10284

[Handy Don 10,597]

3 hours ago, TC10284 said:

Also, I have a newer parts-only 520H with a transmission that I don't know 100% works, but "should". How hard is it to swap the entire trans?

Pump+transaxle swap is not complex but it’ll be time consuming. A bunch of stuff to remove to clear the way. A few linkages and the hydro plumbing to deal with disconnecting and reconnecting (and keep from having contaminants enter). And the weight, since together the combo probably weighs well over 100 lbs. Best to have a helper or lift. 

The transaxle in the HC has higher final gearing than the H, though, and is sought after for that. Top speed in the neighborhood of 9+ mph vs. 6+ mph. 

[TC10284 109]

Just an update on this. I finally had time to swap a cleaned carb. Same behavior. Sometimes when I throttle up, it takes a second to catch up and blows black smoke like it's got too much fuel. Acts like it's missing or not firing enough to burn fuel. Thus, it wants to pop or backfire at higher RPM. The coil is new as of last year. The only other thing I can think of is I might have the wrong P/N of Ignition Control Module because it is also new as of last year. It fit properly, but I am not 100% it is the correct P/N for a P220g. 

 

Bad coil or incorrect ICM??? 

 

Otherwise, I'm at a loss. It ran OK last year. The popping was there, but not as bad. And it cleared up after running a few minutes. 

[TC10284 109]

Bad condenser? 

[TC10284 109]

Hmmmm... The more I research on "symptoms of a bad condenser" the more similar they sound to my situation. 

I am not expecting that is the cause, because I've not had that be an issue for me before, but I will try testing it with a voltmeter and swapping it with a new one to see if that helps. 

Edited June 26, 2022 by TC10284

[lynnmor 6,777]

I believe you said in another thread that you have low compression in one cylinder.  Until you have corrected that, give up on the band-aid fixes.

[TC10284 109]

8 minutes ago, lynnmor said:

I believe you said in another thread that you have low compression in one cylinder.  Until you have corrected that, give up on the band-aid fixes.

 

That's actually a different tractor - a 520H. That one I'm eventually going to pull the engine and pull the heads in the engine and check the rear cyl. 

 

This one (520HC) is another tractor I own. 

 

I have too many 😊

[lynnmor 6,777]

10 minutes ago, TC10284 said:

 

That's actually a different tractor - a 520H. That one I'm eventually going to pull the engine and pull the heads in the engine and check the rear cyl. 

 

This one (520HC) is another tractor I own. 

 

I have too many 😊

Sorry.

 

Did you check the compression in this tractor?

[TC10284 109]

8 minutes ago, lynnmor said:

Sorry.

 

Did you check the compression in this tractor?

 

No worries. I'm hard to keep up with. 😁

 

Yes. 120 on the rear and the front I forget, but it wasn't much different. That was one of the first things I checked the other month and that was good.