Total Motorcycle Community Forums

Well I have been doing some research, not always a good thing, and I think I may have stumbled upon something. I recently did an ignition upgrade(kzrider.com) on my '73 350, this has helped with fouling sparks plugs. After the uprgade I went to stock jets 60 and 115. The engine just wouldn't pull past 7,000 rpms. This bike is supposed to have peak power 36hp@10,000rpms. So i changed the jets to 60 and 105s and has definetely been running stronger. However I still don't believe i should be that far off on carb settings. I have been using 93 octane gasoline. I then read something interesting, that low octane gas burns faster and leaves fewer deposits, and that low octane is good for low compression engines, my bike 9.5:1 not sure how high that rate is? What do you think about the idea?

Thanks, Nate

Sorry, BTW manual reccomends at least 90

I don't know if the octane makes that much of a difference.

I don't know if it's because I'm cheap or what but I've always ran 87 octane in my 350 and they seem to like it. They also pull all the way to 10K with no problems.

The ignition upgrade you did was that the one running power from the key source to the coils? Just remember not to leave your key on to long with out the bike running. Constant power to the coils is known for burning them up. Done this a few times on cars.

Good Luck!!!

and post back!

Thank god you didn't give up!

I hate to here about a 350 be sent out to pasture.

Unless it's retired to my stable!

Mike Haverhill, MA

I wouldn't mind someone more knowledgable then myself speaking to this questions, but I do know that I just started testing lower octane (regular) in my ZRX after reading some posts on the ZRXoa board that suggested that the values listed in the manual are not the same values you see at the pump. They indicated that an 89 pump number is not the same as the number indicated in manual. Something to the effect that 89 was really 91 or something along those lines.

Octane is used to halt the pre-ignition problems that high RPM smaller engines sometimes encounter. The hotspots in an engine can cause the gas to ignite before the spark plug gets its chance. Higher octane allows the gas to be a bit more stable.

I think if your bike runs fine, without any pre-ignition problems (knocking and so on) you should be able to use the cheaper gas. But in the end it is your money and your engine. Good luck deciding

Octane is a measure of a gasoline's resistance to autoignition. Really kind of the opposite of cetane, the measure of diesel fuel's ability to autoignite. The octane number does not affect the combustion process.

Higher octane prevents engine knock. Engine knock occurs when autoignition occurs in front of the flame front in the cylinder. It produces pressure waves that you hear as a pinging and will destroy pistons or valves if left unchecked. Limiting the tendency of the fuel to autoignite lets the engine designer raise the compression ratio, lean out the mixture, and advance spark in order to get more power and efficiency.

However, it does squat to affect combustion. The flame front proceeds at about 30 m/s +/- no matter what once it's lit.

Many modern automotive engines sense knock and mentally challenged spark accordingly to eliminate it. Using a lower octane fuel can lower power output (probably not much though) if it senses knock and adjusts. Don't know the state of the art in cycles, so I can't address that.

Basically, if your engine is knocking, raise the octane of your fuel. If it's not, using high test is really a waste of money (provided bike engines don't automatically compensate for knock).

Oh, and there are a number of ways of measuring octane, usually with some kind of variable compression ratio engine. They have a variety of designations that escape me now (it's been a couple years). I do remember RON and MON. Fundamentally, if the manual recommends 90 RON and the pump says 90 RON, that's the ticket, although I think 90 RON is somewhat less MON.

Hope this helps.

RON and MON test different things. I believe one is testing with a cold intake on an engine with no load and the other, a hot intake with an engine under load.

Spoken like a true engineer

I don't think any of the latest bikes have knock sensors tho. At any rate shoey's old honda certainly doesn't.

Regular unleaded should be fine in his bike, but I don't think it will improve performance. Ince the leaner jet hepled things out, there might be a problem with the float heights, but that would probably affect the bike through the rev ranges. Maybe some air passages in the carb are still clogged or the timing is slightly off?

Well your right, the octane level had no effect on performance, and didn't cause it to knock. The bike runs well with the smaller jets, so as long as I'm not running to lean that it damages the engine I'm going to stop worrying about it until I can find the real cause. Thanks for the explainaiton on octane!!!