Topic: CB 350 Factoids

[tbpmusic]

Gleaned from the Honda CB/CL 250/350 manual -

First, this page lists 95 octane as a minimum. It's safe to assume it applies to almost all Hondas of this era.

350 Octane - http://home.comcast.net/~tbpmusic3/350octane.pdf


Next, this page states 170 pounds compression is ok, if it's 150 or less, you need to tear down and fix.

350 Compression - http://home.comcast.net/~tbpmusic3/350compression.pdf

I know for a fact that 450's should aim for 180, but it's a slightly higher compression engine.

Hmmmm.... I'm no genius but seems like if you're using low octane gas and/or have compression 150 or lower, don't expect it to run like it's supposed to. You can mess with the carbs all you want.


bill

[Pip]

What formula was listed to give 95 octane?

RON, MON, or RON+MON/2?

[tbpmusic]

What formula was listed to give 95 octane?

RON, MON, or RON+MON/2?

Don't know- you can read the page as well as me.

I do know that 95 octane was almost "regular" back in The Day, at least in most places in the US.
It was not considered high octane, when 98 was commonly available.

bill2

[Green550F]

Per an artical I read, the RON and MON numbers may have a typical 10 point difference. So if the book calls for 95 and if they used the RON number, We riders of CB350s need to look for 100 Octane gas! hopefully they used just a MON which means 90 Octane by the normal pump designation.

Either way, I don't see how one could use too high an octane rating in these bikes!

[CBGhia]

You can easily run too high of an octane rating.  If you run premium and you don't need to you will get a lot of carbon buildup.  Ideally you should run just enough octane to keep it from pinging.  If you ping on 87 got to 89.  if you are running 89 now, try a tank of 87 and see if it still runs well.   

My last bike was a Ducati Monster and I ran premium all of the time until someone on our forum posted a link to a page that talked about this stuff.  He said that he ran regular in his Monster and it was fine.   i stepped down to 89 then down to 87 and the bike ran better I think (maybe it was just wishful thinking).  But the point is, you don't need to spend more on gas because higher octaine will not make it run better if it runs fine without it.

[LWATCDR]

Actually the lower the octane the more power you will get out of the motor. As long as it is not pinging.
So yea your Duck probably ran better on the lower grade fuel.

[Green550F]

Cool. never thought of the carbon issue...

[schwebel]

I wonder if in a the manual they were reffering to American or Japanese Octane ratings?, they are different. Also (please correct me if I'm wrong it was before I was born)  but wasn't the gas back in the 70's leaded? Didn't the leaded gas yeild higher octane ratings?

[markjenn]

Debated often on internet forums, I believe the problem of carbon-buildup with higher-octane gas to be urban legend.  Likewise, unless the engine has knock sensors that are engaging or has adaptive ECU's (neither of which our old bikes have) I don't think higher (or lower) octane makes one whit of difference in power or fuel mileage as the energy densities of regular and premium gas are about the same.

My opinion:  Spending more on higher-octane buys you more headroom for destructive engine pinging, nothing more and nothing less.

I'd bet the 95 octane requirement in older Japanese owner's manuals is RON which is about equivalent to US premium which is typically 90-92 pump octane as marked on the pump.  There's a good conversion chart between the three octane measurement systems (research octane number, motor octane number, and pump octane number which averages the other two):


RON MON PON
90 83 86.6
92 85 88.5
95 87 91
96 88 92
98 90 94
100 91.5 95.8
105 95 100
110 99 104.5

My CB750K1 says to use "91 research octane number or higher" which would be regular grade.  I use this and I've never noted any pinging.

- Mark