I have a 2002 harley fat boy that I have installed a Frankenstien kit on it. Right now I am getting 40 to 42 MPG burning Premium ethonal gas. Would I hurt the motor running regular gas in it? The motor is completly stock exept for vance and hines short shots on it. I have set my carb to 2 1/2 turns out.

 

"regular" as in 87 octane?

If 91 octane wasn't available, I wouldn't be all that concerned about running 87 once in awhile.

or are you considering 87 due to price difference?
if so...
Donno how many miles a year you ride, nor the price difference in Canada...butt..
here premium is 70 cents to a buck more.
I look at it this way, 5000 miles a year, @42 MPG = $120 additional a year.
$10 dollars a month additional won't make or break me to use the octane the manufacture recommends.

I'm hopeful your financial situation isn't so dire that you're unable to afford the gas the manufacture recommends

..KTF

 

I ride about 15 to 20 thousand miles a year. The price difference in Canada is about $1.00 diference. The compression ratio on the fat boy is 8.9:1. The manual says to use premium gas. But a 2005 Sportster has a compression ratio of 9.01 and the manual says to use reg gas. I am wondering whats the difference between the two ratios? Its not that I cant afford the cost of premium but why waste money on something that is not needed.

 

Basically the tune... Well, and obviously the 0.11:1 compression difference

Run the bike on the gas it was tuned with/for will produce the best performance and safety (less spark knock) for the engine....

A very simplistic explanation:

Higher Octane fuel is more stable, thus resistant to spark knock than lower octane fuels. That allows an engine to burn gasoline at higher pressures and temperatures.

Higher performance engines usually have higher compression, higher engine temps, and higher performance, so they come tuned for Premium gas. If you are using lower compression, you can tune the engine to run safely on Regular gas, albeit with possibly less performance.

 

bump bump bump

 

Run the 87 octane and if no detonation is experienced and the butt dyno doesn't register any difference; keep running it. I am betting that CCP is not anywhere near being detonation sensitive. I ran an all bore 107" motor with CCP at 190psi that ran on 87 octane all year long in south Texas where summer temps get hot enough that I don't want to ride. Motor made 110HP/124TQ on 87 octane fuel with no ping.

 

You could try the low octane and see if you get the ping. Pick up an octane boost additive from a auto parts store so if you get the ping, you could pour some in to finish the tank, then go back to prenium.