2002 Rubicon lost top MPH by 20%

[BC-Buck 11]

I use this for farm work with average 4 MPA.  Has 1600 hours and under 7K miles. Couple times a year we take them down to St Joe state park and run them hard. Could not get more than 36 to 41MPH out of atv. Years ago I know was able to get 51 mph before rev limiter would kick in. Had bike go into limp mode and had to replace speed sensor. Thought that would take care of speed issue but NOT. Bike starts and runs excellent. Rubimatic is shifting into all gears and seems to be working correct. Oil pressure at 1400RPM 43PSI and at 5K rpm 143 PSI. The only variable I can think of is new tires. Did not change tire size from stock 25x8 and 10 R12. These are heave duty Bighorn Maxxis and are run at 3.6PSI.

[01RUBY500 1,253]

The Bighorns are a much heavier tire than the stock tires and is most likely your cause.  It will take more power to turn them. 

Edited August 1, 2021 by 01RUBY500

[BC-Buck 11]

38 minutes ago, 01RUBY500 said:

The Bighorns are a much heavier tire than the stock tires and is most likely your cause.  It will take more power to turn them. 

Im just shocked it made that much difference. What is top end with these guys running big swamp tires and lift kits?

[Fishfiles 20,147]

1 hour ago, BC-Buck said:

Im just shocked it made that much difference. What is top end with these guys running big swamp tires and lift kits?

I would think there is another factor in a 20% deceased power from the same diameter tires just a bit heavier -----  I run two size up tires and wider on the wife's  300 with no gear reduction in her 300 and can barley notice it ,  but I have read that the Hondamatic don't like tire upgrades  

[56Sierra 1,845]

The weight of the tire given the diameter being the same would only affect acceleration.  Slightly.   Very slightly. 

[BC-Buck 11]

2 hours ago, 56Sierra said:

The weight of the tire given the diameter being the same would only affect acceleration.  Slightly.   Very slightly. 

Thats what I would have thought also. 

[56Sierra 1,845]

Think of tires as flywheels.   Some old aircraft engines in WWII were started by a man cranking up a flywheel to a high RPM and then with a clutch  transferring that stored energy into the engine's crankshaft. 

 

 

[jeepwm69 7,749]

I don't think something is right.  You shouldn't see that kind of difference in a Rubicon.

 

Was it running ok otherwise?

[BC-Buck 11]

3 hours ago, jeepwm69 said:

I don't think something is right.  You shouldn't see that kind of difference in a Rubicon.

 

Was it running ok otherwise?

Thing runs excellent, shifts good and oil pressure is like new bike.

[jeepwm69 7,749]

I have an 05 Rubicon with 27” Mudbugs on it and it would have no problem hitting 50+.

 

Did you check speed with GPS and see if it is accurate?

 

I’d be checking air filter, spark plug, and carb.  

[BC-Buck 11]

10 hours ago, jeepwm69 said:

I have an 05 Rubicon with 27” Mudbugs on it and it would have no problem hitting 50+.

 

Did you check speed with GPS and see if it is accurate?

 

I’d be checking air filter, spark plug, and carb.  

Everything on bike checks out. Ran GPS and speedometer almost perfect. Is your 05 running a hondamatic?

[Fishfiles 20,147]

Maybe it is on the way out !!! 

[jeepwm69 7,749]

3 hours ago, BC-Buck said:

Everything on bike checks out. Ran GPS and speedometer almost perfect. Is your 05 running a hondamatic?

 

Yep.  Several of my friends run the Hondamatic rubicons as well (01-14) and all are running 26-27's.  Something isn't right.

 

Have you done any work to it at all?  The early Rubicons had crappy oil pumps, and THAT is what kills the Hondamatics, not a flaw in the Hondamatic itself.

 

@Brian Bertram is the Hondamatic expert.  He's not on here often, but he does pop up every now and then.  If it's related to the oil pump or Hondamatic, he might have an idea. 

 

My 06 Foreman FM runs fine, but won't pull speeds that it used to.  Pretty sure the engine is just worn out.  I have an extra in the shop floor now to rebuild and swap in in the near future.