Small Engine Issue

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walhondingnashua

walhondingnashua

Minister of Fire
Jul 23, 2016
644
ohio
So my wheelhorse has been running fine until tonight. Jumped off with engine running just above idle. Heard a “puff” and “bang” sound. Continued to run very rough and vibrated hard. Shut it off. Started twice with choke on but only ran for a few seconds. Won’t start at all now.
Amy small engine advice here?
 
I ran it yesterday and once a week to mow. I use ethanol free gas. I pulled the plugs. The front cylinder about sucked my finger in but the rear one only had a little air. Should I assume I have lost compression and if so, what is the most likely cause of that?
 
You could run a leakdown test, that will reveal a lot of problems. You need a compressor and a leakdown testing set. They are not very expensive (the kits). It can save you lots of time and effort, and maybe money.
 
Is it possible that it could be the head gasket around the bad cylinder? I did hear that “puff” sound before it started… like suddenly the gasket let loss and air puffed out?
 
Quite possible. Not to belabor the point, but a leakdown test would show it up right away. You could put some low pressure air in through the plug hole, with both the cylinder's valves closed, and listen to see if any air is escaping, through the carb (intake valve), muffler (exhaust valve) in the head gasket area, or through the oil fill (holed piston, really bad rings, etc.) You'll want the cylinder to be at top dead center. Since it occurred suddenly, I'd suspect the head gasket first.
Depending on how noisy the starter is, you could crank the engine with the plug wires disconnected and grounded, and listen near the head gasket, good cylinder plug out, bad cylinder plug in.
 
heat seeker said:
Depending on how noisy the starter is, you could crank the engine with the plug wires disconnected and grounded, and listen near the head gasket, good cylinder plug out, bad cylinder plug in.
Click to expand...
I will give that a try this morning before I go get a leakdown kit.

It is a 520h and the rear cylinder is the issue. If it is a valve seat, how difficult is that to fix? I am capable of taking it all apart and putting it back together. I am mechanically inclined enough but maybe its beyond my abilities.
 
walhondingnashua said:
I will give that a try this morning before I go get a leakdown kit.

It is a 520h and the rear cylinder is the issue. If it is a valve seat, how difficult is that to fix? I am capable of taking it all apart and putting it back together. I am mechanically inclined enough but maybe its beyond my abilities.
Click to expand...

The fix unfortunately is removing the whole engine to get it to a machine shop to get an oversize seat installed. Some folks have had luck with removing the head and peening around the seat and putting it back together.

There's just not a whole lot of airflow around the rear cylinder and keeping it clean is a PITA. I think I spend more time cleaning my 520 than actually mowing with it.

Other fix is much more expensive short term but probably saves cash on fuel considering how much those 20hp Onans drink.;lol


(broken link removed to https://www.smallenginewarehouse.com/386447-WH520H-R3)

www.wheelhorseforum.com

520H / Harbor Freight

Did the HF Predator repower. Pretty easy. Sleeve for shaft, Spacers under motor plate, modify PTO mount, modify exhaust.Ignition switch, circuit breaker, throttle, fuel lines, all easy to hook up.(eliminated all gauges and crazy old wiring) Best thing $600. Would have had much more than that in m...
www.wheelhorseforum.com www.wheelhorseforum.com
 
Replacing the offending cylinder head is probably the best plan, although it may be expensive. Peening the seat back in, from what I've read, is hit or miss. If the head weren't too expensive, I'd just replace it. Your money, your call.
 
heat seeker said:
Replacing the offending cylinder head is probably the best plan, although it may be expensive. Peening the seat back in, from what I've read, is hit or miss. If the head weren't too expensive, I'd just replace it. Your money, your call.
Click to expand...

Valve seats are in the blocks on these Onans. New blocks are about $1200.
 
I took the intake, exhaust and cylinder head off. The valve seats look fine and the valve don't look like there is any damage to them. When I turn the belt pulley to move the valves, the valves move, but the piston does not. Does this mean rod?
 
walhondingnashua said:
I took the intake, exhaust and cylinder head off. The valve seats look fine and the valve don't look like there is any damage to them. When I turn the belt pulley to move the valves, the valves move, but the piston does not. Does this mean rod?
Click to expand...

Sounds like it. That's some crappy news.

Out of curiosity how many hours on it?
 
It has 866 hours. I blow the grass out of it every time I mow and there wasn't any grass behind the shields. It also has oil in it. What would cause a rod to go? From what I am seeing, Onans are known for having rods go.
 
Can't answer that question for you. I've never been deep enough in one to have to figure it out.

If your piston isn't in pieces and the cylinder walls look good I'd probably just throw a new setup in and run it.
 
The big question in my mind would be - what caused the failed rod and is it likely to be affecting the other rod?

If the rod itself broke, or there was a bearing issue, or lack of lubrication causing bearing destruction, etc - do you have confidence the other rod is any better? This is probably the root of the 'replace both' advice. Though if something like a lose bolt on the rod cap, that might be localized to only one rod and the other is fine...assuming the engine didn't pump a bunch of crap through the oil after the initial failure.
 
I have done some calling around. It's sounding pretty expensive to rebuild period. I can try it myself but I would just be tearing it down and replacing the rods as long as everything looked good. I am looking for a replacement engine at some junk yards or maybe put a predator engine from harbor freight on it that converts well.
 
My Onan is getting close to 1800 hours. My plan if it blows is to get one of the repower kits up above. More expensive than the Predator but it's plug and play.

Figure you're going to spend about $500 on your current engine for rods/pistons/bearings.

You can also probably sell your current engine for a couple hundred just because of the expense of available parts.

Other option instead of $900 on the Predator, try to track down a whole different tractor and use one for parts..
 
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