493 Issue - Help with diagnosis

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Greg Zimmer
Posts: 22
Joined: Mon Aug 27, 2012 3:10 pm

493 Issue - Help with diagnosis

Postby Greg Zimmer » Sat Apr 19, 2014 12:37 pm

I am trying to figure out what caused the motor issues in my F-Mod car. At the last event (Wilmington Match Tour), the motor clearly had issues (down on power and a knock at idle (you can hear the motor in this video of our last run of the day http://youtu.be/8uYV08iva-0). Compression test showed 60 psi for the clutch side and 120 psi for the flywheel side.

I have since pulled the motor and started disassembly. So far, I haven't found the level of damage I was expecting. I've uploaded pictures here: https://plus.google.com/photos/111873912689560290179/albums/6004036953208765169?authkey=CJnStsSt8t2YPQ

A few notes:
- the ring on the cylinder with low compression was lightly stuck on the exhaust side of the piston
- the ring end gap looks to be pretty different across the two cylinders
- I pressure tested the motor before disassembly, and it held 6 PSI for 6 minutes without any issues. I realized after starting to disassemble the motor that the test setup I put together wouldn't have shown a leaking center crank seal.

I haven't measured the pistons or cylinders, because I don't have the proper tools. I plan to either find someone local that will let me borrow the tools, or I'll be looking for some used micrometers and a bore gauge.

A little back story on the car / motor. I acquired the car last summer with a motor that needed a top end (mag side piston was burned). I had the top end rebuilt on this motor (2 new pistons + cylinder hone + bottom end cleaned out) and the spare 493 that came as part of the package by a well respected F-500 engine builder. I have put roughly 20 autocross runs on the motor, but it never seemed quite right (EGTs were significantly different across the 2 cylinders). I had flood the clutch-side cylinder on the initial startup. Not having any experience with the 493, I adjusted the carbs based on spark plug readings, EGTs, and what I could see of the piston wash through the plug hole. It was running 40-pilot / 360-main jet at the last event. Having talked to a few people, the main jet seems significantly larger than is typically needed for autocross with a 493.

Any thoughts after seeing the pictures? This is the first time I've been into one of these motors. So, I'm not sure what looks good / bad for the piston and cylinder.

Bill Schmidt
Posts: 1271
Joined: Sat Oct 13, 2007 1:17 pm
Location: United States, Kansas, Kansas City

Postby Bill Schmidt » Sun Apr 20, 2014 9:06 am

My guess would be that one of your carb jets is partially blocked. Inspect and clean everything carefully, put new rings on both pistons, do a light hone and go play. Also, take a good look at the rubber carb mounts for cracks/air leaks.
1988 Red Devil 494/4 link
1995 Trackmagic 125 shifter/ KX125
Kansas City

Greg Zimmer
Posts: 22
Joined: Mon Aug 27, 2012 3:10 pm

Postby Greg Zimmer » Sun Apr 20, 2014 10:08 am

Thanks, Bill. I definitely will be rechecking the carbs and intake boots. Once I acquire the necessary tools, I'll also check the piston and cylinder to see if they are still in spec. I hope you are right that new rings and honing the cylinders is all it needs.


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