'85 250R Tuning

[Turbeau]

Just a question. What would low octane levels do to a engine?

[barnett468]

Just a question. What would low octane levels do to a engine?

In short, it can damage a piston, but if the octane level is definitely too low for the amount of compression, the engine will make a pinging or rattling sound upon acceleration, however, it can take a trained ear to hear this, plus, if your bike is loud, the exhaust can be louder than the pinging so you might not hear it anyway, therefore, one can use a basic guide such as the following that I just made which is a conservative estimate.

COMPRESSION VS OCTANE

. .0 - 134 psi - Repair engine
135 - 135 psi - 87 octane
136 - 155 psi - 89 octane
156 - 170 psi - 91 octane
171 - 180 psi - 93 octane or E85 or 104 race gas or half 91 octane and half 110 - 112 race gas
181 - 200 psi - E85 or 112 race gas

[Red Rider]

What would low octane levels do to a engine?

As the air/fuel mixture is compressed by the rising piston, it heats up. If the compressed mixture gets hot enough, it will ignite prior to the desired ignition time (Pre-ignition). This is bad because the piston is still traveling upwards as the mixture ignites & expands, ing the upward moving piston back down. As barnett said, this can damage the piston, which is taking the brunt of the impact, but it can also damage the rod & crank as well. The lower the octane of the fuel, the more suceptible it is to pre-ignition. Also, using a high octane fuel, beyond what your compression requires, will not give you more power, but it will waste more of your money.

[Turbeau]

Here is an update: The motor is being rebuilt. The crank was bad,(replaced with a Hot Rods crank and rod assembly) as well as the main bearings, found a bent center shift fork, and the cylinder was bored .80 over and the piston was replaced with a weisco. I will be running race fuel and will be running a brand new 38 mm airstriker carb.(may not be needed but I had new one laying around) I am having a squish test done before I pick it up from the shop and will be running a richer fuel mixture. I took the time to tear down the trike completely. I sent the frame and swing arm out to be powder coated. The frame is back to a beautiful honda red and the swing arm has been returned to gold. I wanted the stock colors because I wanted my R to look like a 1987 model. Only need the white rear plastics to complete the build. Later I will install a 310 or 330 kit from ESR with a +8 swing arm. Again I will update everyone on the progress of the break in and tuning so people have some results.

[Mattematt]

85 ATC 250r
I'm having trouble with jetting i think.
It's breaking up on acceleration.
This is what i have...
Bored 50 over
Main jet 142
Pilot jet 52
Needle 3rd notch
Fuel/Air 2 turns
Gold series exhaust
NEW items
Stator
Cdi box
Voltage reg.
Coil

Compression test 182 psi
I don't know whats next?