Idles great but...

[Frank Sorbello]

Yesterday, I noticed something might be wrong with one of the cylinders. It appeared to be a loose plug wire.
Today, I noticed I don't have much power. She idles nice. Sounds very strong. However when I step on the treadle she doesn't move too fast and the engine bogs down.

 

[MossRoad]

So it idles nice.
Then you throttle up. Does the engine run good at high throttle also?

Then you try to move the tractor and it bogs the engine down?

I would try raising all four tires off the ground and then operating the treadle to see if it moves the wheels with no load on them or if that bogs the engine as well.

 

[BobRip]

Could be ignition. Pull off one ignition wire and see if that makes a difference. If it does not then the coil or spark plug on that side is bad. Then try the other side.

 

[Frank Sorbello]

I just put it up on blocks. Didn't notice the engine bogging down. There was a lot of backfiring when I sped up the idle.

 

[MossRoad]

Get the engine running good while its up on blocks first.
Once you can attain high throttle with no problems, put it back down and see if the engine still functions good under load of driving.

 

[MossRoad]

Yesterday, I noticed something might be wrong with one of the cylinders. It appeared to be a loose plug wire.
Today, I noticed I don't have much power. She idles nice. Sounds very strong. However when I step on the treadle she doesn't move too fast and the engine bogs down.

O.K. Forgive me for asking, but I re-read your post.... you aren't expecting the engine speed to increase when you step on the treadle are you?

 

[Frank Sorbello]

No, when driving at full throttle she bogs down, treadle down all the way maybe going 3 mph. Something is wrong. Just can't figure what it is. A couple hours later I got on it and she acted normal for a few minutes. I thinking maybe something with the carburetor. What would cause it to repeatedly backfire while running?

 

[Marks481]

What would cause it to repeatedly backfire while running?

My guess is that you need to adjust your intake valve clearances (required every 500 hours of operation).

Easy to do, will take about 15-30 minutes tops.

Look at the service manual on page 38:
http://robinamerica.com/media/manuals/128988858968757500.pdf

The only trick is how to get the engine to Top Dead Center (TDC), as the timing marks on the flywheel aren't viewable. Take out the spark plugs, and put a straw in the spark plug hole. Rotate the engine by brief bursts of the starter key until the straw shows that cylinder is at the top. That means you are either at TDC of the compression stroke (which is what you want), or the cylinder is at the top following the exhaust stroke (the exhaust valve will be open as the cylinder comes up).

Could also be because your carburator is providing too lean of a mixture. Could be the carburator needs to be cleaned. I don't know how to adjust the mixture on the EH72, haven't had to do that yet.

 

[Clumber]

No, when driving at full throttle she bogs down, treadle down all the way maybe going 3 mph. Something is wrong. Just can't figure what it is. A couple hours later I got on it and she acted normal for a few minutes. I thinking maybe something with the carburetor. What would cause it to repeatedly backfire while running?

My PT-425 started doing that on a heavy work load day. It turned out to be bad spark plugs. When the engine became hot the plugs would not fire correctly causing backfire. Letting the engine cool down for 20 mins produced a normal run for about 1.5 hours until it heated up again. The electrodes on the plugs were whitish when hot. Installing new plugs solved the issue -- running normal now.

Note that my Robin user's manual says the correct replacement plugs are NGK-BPR5ES. Jack's Small Engines site says NGK-BPR6ES. Manuals on the Robin support suggest different plugs. The plugs that came with the new tractor are NGK BPR4EY. A Robin tech support guy said I could use anyone of those. I did not like that answer. The Robin Director of Tech Support suggested the following:

You need the Production number (PN) for you engine, located just above the serial number. For example for my PT-425 it is EH720DS00117112.
Then go to the Robin Parts Catalog (ordered by Specification, that is, the Production number):

https://euc.robin-fhi.co.jp/parts_ca...el.aspx?lang=e
Look up the 1st 5 digits of your PN to get a list of productions for your engine. You will see blow up parts diagrams for that specific engine production.

You can also Google the PN leaving out the last 4 digits. I found a site that has better prices than Jack's on several parts:
Subaru / Robin EH720DS0011 Parts List and Diagram : eReplacementParts.com

The correct spark plug for my engine is NGK-BRP4EY.

 

[murphy1244]

try running some Sea foam in it before you change the plugs.

 

[woodlandfarms]

Beyond the engine issue, I wanted to mention treadle control. Remember the more you mash the treadle the faster you will go but with less torque. You want to pull that stump or go up the steep hill the less treadle you should use. I forget this constantly, mashing the pedal to achieve great force only to realize I hid to lighten up to get moving.

 

[MossRoad]

No, when driving at full throttle she bogs down, treadle down all the way maybe going 3 mph. Something is wrong. Just can't figure what it is. A couple hours later I got on it and she acted normal for a few minutes. I thinking maybe something with the carburetor. What would cause it to repeatedly backfire while running?

I'm going to agree with Tim_in_CT and suggest you check the valve adjustment.

 

[Frank Sorbello]

I had the valves adjusted by a very reputable small engine mechanic less than two months ago. If I can make an attachment to help me lay wire with it, I think I will put a new engine in it. Somedays it runs strong, doesn't backfire, starts right up, runs smooth. The next day, the complete opposite. I've had quite a few people look at it and try to figure out what is wrong with it. It is very inconsistent. The fact that it was just rebuilt in the fall and has these issues has me wondering if it is even worth putting another nickel in the motor.

 

[BobRip]

I had the valves adjusted by a very reputable small engine mechanic less than two months ago. If I can make an attachment to help me lay wire with it, I think I will put a new engine in it. Somedays it runs strong, doesn't backfire, starts right up, runs smooth. The next day, the complete opposite. I've had quite a few people look at it and try to figure out what is wrong with it. It is very inconsistent. The fact that it was just rebuilt in the fall and has these issues has me wondering if it is even worth putting another nickel in the motor.

I may have missed it, did you check the ignition coils?

 

[Frank Sorbello]

try running some Sea foam in it before you change the plugs.

Do you think Sea foam will help? How much should I use?
Thanks

 

[MossRoad]

Do you think Sea foam will help? How much should I use?
Thanks

Sea foam is a fuel treatment. If your tank, filter, lines and carb are all clean, sea foam will not help. It will not hurt, but it probably won't help.

Backfiring is usually timing related, valves not opening and closing properly, etc...
Have the compression checked.
Have the valves adjusted again.
Have the timing checked.
Change both spark plugs, both plug wires and check the coils.
Fresh fuel, fuel filter and clean out the bowl in the carb.

 

[washita]

sparkplugs can be cantankerous, I have had plugs misfire under a load only, misfire because of oil/grease on the ceramic under the high tension wire boot, a hairline crack in the boot will cause a misfire also, abrasion on the high tension wire will short out the spark also especially under load, start your engine at night while dark and watch for a blue spark to ground or a halo effect around your high tension wires it will indicate a short to ground and the biggest mistake I have seen is trying to fix high tension wires with electrical tape it does not isolate high voltage