Hard starting 656 gas

New to me 656 gas always started good with high octane ethanol from the gas station. My fuel guy said he carries 89 octane with no alcohol for older tractors so I bought 150 gal. Man this thing starts hard now. Once it's started it runs great. I have to believe it's the gas...could it be old? Would adding some octane booster help? (I know this thing takes 93 octane). Or, has something else gone wrong coincidentally?
 
Octane is a measurement of resistance to detonation. The higher the number the more reluctant it is to ignite, and detonate. You may have gotten a bad batch of fuel, or you might have something wrong with the starting system or ignition on the 656. If the 89 octane fuel is very slow moving at the distribution terminal, it could be old. I believe that the highest power from any gasoline engine is the fuel with the least octane that will not spark knock when the engine is adjusted to specification. The original fuel specification was written in the era where Research Octane Numbers "RON" were specified (a specific test procedure). Motor Octane Numbers "MON" test procedures are more rigorous than the RON numbers resulting in from 8 to 12 number difference between them, with the MON numbers being lower. In the USA we combine (average the two methods) and the result is a number that is lower by 4 to 6 published octane numbers on the pump. I believe the 87 and 89 octane fuels we can buy (ethanol or conventional makes no difference) are suited to engines with stated RON numbers of 93. If it spark knocks and is adjusted correctly, the fuel may be bad, or carbon has built up raising compression or causing glowing preignition. Hot sparkplugs can also induce preignition from glowing electrodes. This is in contrast to some opinions, but I stand on it. Jim
 
Check if there is a resistor bypass wire from R or I terminal of starter solenoid to junction of resistor to side of distributor and be sure it is connected. It bypasses ignition resistor for hotter starting spark.
 
In your opinion, why is it only the engines in this family that are supposedly prone to damage from low octane fuel? Is it because of the pistons? If flat-tops were installed, would that help, even though then compression might be reduced? I have followed these discussions when they surface, and I always wondered why, when the engine was pinging on cheap gas, why can't the timing be retarded until the pinging stops?
 
The issue is complex. First is that the the six cylinder engines were transmogrified from trucks. The truck engine had 200% RPM operating range over the tractor application. The speed at which the flame front travels across the combustion chamber has far less time to do so when the speed is higher. The truck engine also had cam profiles, ignition, and fuel delivery that were tailored to higher speeds. The second is that the farm fuel in the early sixty was less than 80 R+M octane. The truck engine had (guessing) 7.5 to one compression. The much slower engine speed and the higher compression made substantial power, but required higher octane fuel than a 6 to one compression ratio M or JD 70. The fuel of today approximates the high test fuel of that period. Jim
 

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