Small Engine Issue

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walhondingnashua

Feeling the Heat
Jul 23, 2016
391
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?
 

walhondingnashua

Feeling the Heat
Jul 23, 2016
391
ohio
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?
 

heat seeker

Minister of Fire
Feb 25, 2011
3,211
Northern CT
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.
 

walhondingnashua

Feeling the Heat
Jul 23, 2016
391
ohio
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?
 

heat seeker

Minister of Fire
Feb 25, 2011
3,211
Northern CT
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.
 

walhondingnashua

Feeling the Heat
Jul 23, 2016
391
ohio
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.
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.
 

festerw

Minister of Fire
Nov 16, 2009
509
Cambridge Springs, PA
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.

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



 

heat seeker

Minister of Fire
Feb 25, 2011
3,211
Northern CT
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.
 

festerw

Minister of Fire
Nov 16, 2009
509
Cambridge Springs, PA
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.

Valve seats are in the blocks on these Onans. New blocks are about $1200.
 

walhondingnashua

Feeling the Heat
Jul 23, 2016
391
ohio
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

Feeling the Heat
Jul 23, 2016
391
ohio
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.
 

festerw

Minister of Fire
Nov 16, 2009
509
Cambridge Springs, PA
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.
 

walhondingnashua

Feeling the Heat
Jul 23, 2016
391
ohio
Some places are saying that I should replace both rods. The front cylinder had great compression so I am afraid to mess with it.
 

Corey

Minister of Fire
Nov 19, 2005
2,588
Midwest
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.
 

walhondingnashua

Feeling the Heat
Jul 23, 2016
391
ohio
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.
 

festerw

Minister of Fire
Nov 16, 2009
509
Cambridge Springs, PA
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..