Honda CB125

A Few Tips and Tricks

During the course of my ownership of my CB125 I learnt a few things along the way regarding the operation of the motorbike that I would like to pass along to others. Hopefully these few small tips will help you to get the most out of your Honda and enhance your personal experience with this great little bike.

Warm it Before You Ride It!

Honda CB125ESeriously folks, do you really expect to press the starter and ride off into the sunset when your motorbike barely has oil pressure? I have seen people literally push the starter button with one hand while releasing the clutch with the other. When you do this and it bunny hops then stalls then you look somewhat foolish. And quite rightly so.

When my CB125E was new riding it before it was warm was simply not an option, it would stall. When the engine loosened up somewhat after about the first 5000km or so, it began cope a lot better when being ridden cold but even now is still sluggish until it gets warm. The best and easiest solution is to let it warm up a little. Just let it idle for five minutes, it will make all the difference.

How to Make It Start First Time Every Time

How do you make your Honda CB125E start first time every time? Its easy. Regardless of what the temperature is the ultra lean fuel/air mixture imposed by the Euro 3 standard means that the choke is usually required to get the bike started when its cold.

Start by ensuring that the fuel tap is on and, if the bike has been sitting unused for a while, give it about 10 seconds for fuel to fill the carburettor. Turn the choke to the on position and turn the ignition on. Make sure that the motorbike is in neutral and that the neutral light is on. Do not even touch the throttle, press the starter firmly and hold it on until the engine starts. This should take no more than a couple of seconds. Do not operate the starter motor for more than ten seconds at a time or it will melt.

Once the engine has started let it run for about ten or fifteen seconds with the choke on. After that time it should be warm enough to turn the choke off and let it idle. The specified idle speed for the Honda CB125E is 1400rpm plus or minus 100rpm. I have mine idling at 1500rpm so that while it is warming up it is still spinning at a reasonable speed. Let the bike idle for five minutes or so, after that it should be fine to ride.

Use 95 Octane Fuel

An alternate title for this section could have been- Why does my CB125E backfire? I suppose that now Ive raised the question I should answer it first and then explain why the octane rating of the fuel you use, although it doesnt directly cause your CB125E to backfire, can have a bearing on much it actually does pop off through the exhaust.

The Honda CB125E has a secondary air supply that provides a pulse of fresh air to the area behind the exhaust valve. The purpose behind this secondary air supply is to ensure that no unburnt fuel exits the exhaust pipe, the additional oxygen this provides promotes any unburnt fuel to burn in the exhaust pipe instead. If there is a lot of unburnt fuel it goes off with a bang and that is your backfire. If youve had your CB125 for a while you probably have worked out that you can make it backfire reasonably impressively using a combination of throttle positions and engine rpm. The most common way to promote a backfire is to be off and on again with the throttle while going down a hill in a lower gear. POP!

In Australia and many other parts of the world petrol (gasoline) is available in three widely distributed octane ratings. 91, 95 and 98 octane. The lower the number the faster the fuel burns when ignited inside your engine. Engines with higher compression ratios require a higher octane rated fuel to avoid detonation as high compression engines burn their fuel faster than low compression engines. A high compression engine will make more power than a low compression engine.

The compression ratio of the Honda CB125E is a very moderate 9.2:1 which allows it to run on a very wide range of fuels. It will run on any of the commonly available fuels just fine. In fact the Honda manual even states that it will run on E10 unleaded as well.

If you use 98 octane fuel in your Honda CB125E it may not burn all of the fuel due to its low compression ratio (plus a few other factors) and the unburnt portion will exit through the exhaust valve into the exhaust pipe. Now remember a few paragraphs ago when we were talking about the secondary air supply? You guessed it, using 98 octane fuel or higher will likely result in more unburnt fuel in the exhaust system and more backfiring.

When running on 98 octane fuel the Honda will feel smoother however I have found in practical terms there is a trade off at higher engine speeds in the form of a measurable drop in power and top speed. Not much but it is there, quantifiable and probably caused by the engine expelling a portion of the fuel out of the exhaust rather than burning it. I have also run my motorcycle for several tanks on 91 octane fuel, not E10 but good old fashioned regular unleaded. The little Honda ran just fine, no pinging or issues but a slight and noticable drop in power, slowing down more on hills and once again a lower top speed. I refuse to run anything I own on E10 unleaded, which is petrol with up to 10% ethanol added so I cant say how it runs, I would speculate that it would perform about the same as 91 octane but possibly produce less power again as ethanol does not release as much energy as petrol.

So that leaves 95 octane. My honda has spent the majority of its operating life on 95 octane, specifically BP 95. I experimented with different fuels after running it in and have found that 95 octane provides the most power accross the rev range, a measurable increase in top speed and better hill climbing. As a bonus my little Honda rarely if ever backfires because the fuel is burnt efficiently. Astoundingly the fuel economy is noticably better with my bike getting another 20km or so per tank before hitting reserve.

So to summarize, use 95 octane fuel.