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Old 07-26-2011, 01:05 AM   #1
Butler Cox
Join Date: Jul 2011
Posts: 7
Default Can Anyone Figure Out This FR125-SR Problem

The below has been sliced/diced/dissected and trisected by Rotax techs, savvy top racers, and others, but NOBODY can figure what's wrong...have you any ideas?

Problem: Mounted on a 2010 CRG BadBoy, with any gear ratio combination, this motor will accelerate and run cleanly UNTIL 10,500 - 10,700 RPM. Then it stumbles, sometimes pops a small amount, and just lays down, although it will rev up to 13,100 with certain ratios.

Here's what we've tried, none of which fixes the problem:

1) Every jet from 145 to 172 on any given test day (jet range depending upon conditions).

2) Swapping carbs for ones that run well on similar CRG race carts.

3) Changing plugs (two brands, three differing heat ranges)

4) Swapping *every* part of the entire electrical system for parts that come from well-performing FR125s, from pickup, to wires, coil, ground wires, switches, plug lead...all of it.

5) Swapping PVs for ones from well-running FR125s, cleaning and specing my own two PVs. Sometimes this will make a very slight difference, but no matter how many times we've cleaned, replaced bellows, speced measurements, etc. on the PV, the problem won't go away.

6) Cleaning and replacing fuel pickups, fuel lines, filter in carb,the entire fuel system except the tank which is clean and shows no sign of damage.

7) Cleaning the airbox several times, including several thorough cleaning and very light re-oiling of the filter.

8) Full engine rebuild (at 47 hours). The problem started before the rebuild and continued unabated after break-in following the rebuild.

9) Full cleaning and inspection of reeds which have looked fine, according to the techs and tuners whov'e worked on this problem.

This FR125 was purchased in June, 2010, likely built in February - March 2010. Until the problem started five months ago, it always ran well, provided I hit the tune on any given session.

This FR125 starts easy and pulls well from low RPM...until 10,500 - 10, 700.I keep the water temp between 145 and 155. Have tried a bit hotter to see if it influenced the problem, but it didn't.

The motor has never been overheated. At the 50-hour rebuild all wear parts looked fine and showed no anomalies.

So...what gives with this FR125??? Anybody got any original ideas? Or do I just junk the motor. I got back into kartng after 45 years off and never expected nobody would be able to fix a poor running Rotax. Or is this Rotax just a lemon and I should junk it for a Maxter? The Maxter I've driven sure ran better than this Rotax.

Really hope someone has some constructive suggestions for the issue.

Butler Cox is offline   Reply With Quote