I recently acquired an MPC-era GP9. Very nice condition and runs great except for a noticeable front-end wobble. You can really see the top of the cab shake. It's the cab end with the motor and assuming there is something loose in the motor/truck arrangement. Was going to pull the shell and take a look but thought I'd check with any of the MPC experts here. Any insights/suggestions greatly appreciated. Thanks. 
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I believe due to bad wheels. Same with my M&StL GP9 powered and dummy units. I meant to ask if there are a way to replace it with good wheels? Maybe someone will chime in.
CGWforever,
Mine is the M&StL Geep, too. Go figure. 
The combination of casting/parting lines on the wheels, with rubber traction tires and overall light weight, adds up to the observations you are making.
You can fine-tune the wheels by truing them with a file/tool bit(as in a lathe - loco upside down in cradle - engine running) and replacing the tires with fresh, even ones, or the Stewart/Calumet 2505/192-2505 "Super Traction Tires".
The combination of casting/parting lines on the wheels, with rubber traction tires and overall light weight, adds up to the observations you are making.
You can fine-tune the wheels by truing them with a file/tool bit(as in a lathe - loco upside down in cradle - engine running) and replacing the tires with fresh, even ones, or the Stewart/Calumet 2505/192-2505 "Super Traction Tires".
Would it better if it replace truck assembly with good wheels?
The MPC trucks(wheels) are all the same. They aren't defective, just poorly designed/executed. Some are worse than others depending on when & where they were made - the same basic wheels are still used on the starter set RS-3 diesels with the truck mounted DC motors.
You could replace the wheels with sintered powdered iron wheels from a postwar 200/600 series truck with or without the traction tire slots, or replace the whole power truck with a 600 series MagneTraction truck(the GE 44 ton trucks are a near identical cosmetic match).
Put the engine on the track at eye level, run it very slowly, and I'll bet your going to see one or more wheels of the power truck are pressed on the axle off center. The only cure for that is to replace them. Sadly this is a common problem with those engines.
Jim 1939, Yes, I did that and noticed them. That too bad!
hello johnstrains..
well you could buy a williams GP-9 and switch the M&Stl GP-9 shell on the williams classis and you will gain better wheels set, metal gears, double can motors. That what i would do if i were you.
the woman who loves toy trains
Tiffany
Thanks to all for the suggestions and ideas so far. Sounds like this was a known issue. I'll take a closer look at it this weekend and see what I find.
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