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Honda SL125 timing - SANITY CHECK.

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Alan Donovan12/04/2021 10:43:25
61 forum posts
29 photos

Hello all.

May I have a 'sanity check' please!

From my last post on the Honda (re electrics), I mentioned that my timing may be 'out'. I have since checked the timing by two methods (instructions in the manual using feeler gauges and the T and F timing marks on the rotor, and a method using a multimeter) and both say it is OK.

A subsequent check using a timing strobe 'suggests' it is way out - but I have now come to the conclusion that the instruction manual is a little ambiguous. The manual instruction (5) says when using a timing light ............ The two marks on the rotor should swing around and line up with the pointer when the engine is revved.

What the manual doesn't say is that it is specifically referring to the two marks WITHOUT the T and F annotation. If it is referring to these unannotated marks then my timing is correct using the strobe light.

Have I now interpreted that correctly?

Please find the relevant extracts from the manual below.



Best regards & many thanks.


David Noble12/04/2021 10:52:28
308 forum posts
15 photos

Hello Alan,

My reading of the instructions is, that the two un-annotated marks will swing around to the pointer when the engine is revved showing that the advance and retard is working correctly.


Hopper12/04/2021 10:52:50
6188 forum posts
319 photos

LOng time since I worked on a Honda single-banger but I believe the T mark is Top dead centre and the F mark is the Fully retarded timing mark that you use for static timing with the auto advance unit sitting in its natural fully retarded position. Then when you start it up and strobe it, the two marks without letters should swing around as you increase the revs and lie either side of the pointer. At idle I would expect the F mark to be in line with the pointer. You should see those two unlettered lines move back and forth as you rev the engine up and down.

Easy check would be poke a wire down the plug hole and set piston on top dead centre and see if the T mark lines up with the pointer.

But it sounds like you have it right.

SillyOldDuffer12/04/2021 11:19:05
8469 forum posts
1885 photos

I think so, if it's like other engines. T is top dead-centre and F is the 'Fire' point at idle. These marks are used to set-up the engine before it's actually run.

Ignition is advanced and retarded according to speed and load, and I think the Honda tune-up instructions are in two stages: first, with the engine off, set correctly for start and idle; second, with the engine running off-load at high rpm, that the timing has shifted correctly. When the engine is run fast the timing changes and the unmarked pair should align, as revealed by the strobe.

Ignition timing is changed to get the best burn in the time available during a stroke. This varies depending on load which changes up-hill, down-hill, on the flat, accelerating, etc. It can be altered to favour fuel economy, performance or to reduce pollution. Ignition control on a basic motorbike is rather simplistic compared with the sensor rich engine management unit on a modern car, which pulls every trick in the book to optimise engine effectiveness. Which is great until it goes wrong and not even the dealer understands how to fix it!


John Olsen12/04/2021 12:29:34
1240 forum posts
94 photos
1 articles

As you say, the unannotated marks are correct for when the engine is revving. It is more important for the ignition timing to be correct at high revs than at idle. If the advance retard unit is a bit worn it may be giving more advance than needed, you can tweak the stops in a little with brute force if this is required.

It might not be all that sophisticated compared to modern stuff, but over 100 bhp per litre is not too bad. You can also get well over 100 miles per gallon if you try. From what we could tell back in the day, the 100 cc version produced just as much power as the 125, maybe because the valves and the carb were the same size. Although my 100 ended up with a 125 barrel, a hot cam, and a 24mm carby instead of the 22mm. It went really well like that.


Ady112/04/2021 13:04:02
5065 forum posts
734 photos

Electronic timing running two sparks?

Alan Donovan12/04/2021 14:35:23
61 forum posts
29 photos

Hello again.

Thank you all for the responses to my question. It is very much appreciated.

Best regards to all and keep safe.


John Olsen12/04/2021 14:57:37
1240 forum posts
94 photos
1 articles
Posted by Ady1 on 12/04/2021 13:04:02:

Electronic timing running two sparks?

No, it is not that sophisticated. The two marks for the high speed are I think intended as a tolerance range for where the spark should occur when running fast. My experience was that there was a lot of jitter in the spark timing at speed, which given the mechanical advance retard is probably reasonable.

Apparently there were later versions of the CB125 with CDI, I dunno if that ever made it into the SL version


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