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.