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Discussion Starter #1 (Edited)
Hello all.

After a longer period of time I have picked up restoring my 1973 CB125 Twin.

Yesterday I worked on it. I started the engine. It is only running on the rightside cylinder. I can hear some sputting once in a while comming from the leftside exhaust with the rpms slightly raised. I have re-adjusted the fuellevel within the bowl in the leftside carburettor (which was wrong).
I have only had it running on both cylinders for a few seconds.
I have tried swapping the ignition leads - same symptom so I don't think it is the ignition.


Later yesterday I came back to continue troubleshooting it.
Now it will not start at all.


I have an old Nibbi garden cultivator (500cc, 4stroke) and I use an additive in the fuel.
If not using this additive the sparkplugs simply go dead after the engine have been running.


Could this be the case with my Honda ? Do people in this forum use additive in the fuel on your old Honda Twin ?


It just came to my mind that this could be the case as the Honda was quite easy to start earlier yesterday.
Don't ask me why I didn't add this additive to the fuel on the Honda Twin. Guess it didn't struck my mind at the time.
But I will do that now.

Best regards
Carsten
 

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Discussion Starter #3 (Edited)
The engine should be able to run without additive. I'd make sure both carbs are clean, use fresh gas, and make sure the points are clean.
Strange thing this weekend...
During all the startups I have have had (using the old spark plugs), the engine always started running on the rightside cylinder.

Saturday morning I tried starting it again with both the new spark plugs in. I could not get the engine started. Not even a puff from the exhaust.

I knew one of the old spark plugs was working earlier.
I then replaced the leftside spark plug with this old spark plug (working).
I also took out the rightside spark plug to ease off the load on the left cylinder (if it should start..)
And it did start. After running it for a couple of minutes I the turned it off to insert a spark plug in the rightside cylinder.

I have not had it to start one time since. Once in a while I get a puff from the exhausts while kicking it, but no starting.
When I unscrew the spark plugs they both seem wet from gasoline, so the carburettors are likely to work.

I think the next step could be to replace the ignition coil (twin coil) for two new single coils.

Both the old and new spark plugs are NGK DR8HS. Would it be better to get NGK D8HS (without the noise supressing resistor) ?

TF
 

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The plug caps already have resistors, non-resistor plugs would be better. How much battery voltage do you have?
 

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Discussion Starter #5
The plug caps already have resistors, non-resistor plugs would be better. How much battery voltage do you have?
I have 6.3VDC (new fully charged battery). The battery is connected directly to the ignition coil (via fuse of course..) so I am not connected via the original wireharness (still to be installed)
The ignition coil draw approx 1.2 A when contact point is closed.
 

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The engine should be able to run without additive. I'd make sure both carbs are clean, use fresh gas, and make sure the points are clean.
Strange thing this weekend...
During all the startups I have have had (using the old spark plugs), the engine always started running on the rightside cylinder.

Saturday morning I tried starting it again with both the new spark plugs in. I could not get the engine started. Not even a puff from the exhaust.

I knew one of the old spark plugs was working earlier.
I then replaced the leftside spark plug with this old spark plug (working).
I also took out the rightside spark plug to ease off the load on the left cylinder (if it should start..)
And it did start. After running it for a couple of minutes I the turned it off to insert a spark plug in the rightside cylinder.

I have not had it to start one time since. Once in a while I get a puff from the exhausts while kicking it, but no starting.
When I unscrew the spark plugs they both seem wet from gasoline, so the carburettors are likely to work.

I think the next step could be to replace the ignition coil (twin coil) for two new single coils.

Both the old and new spark plugs are NGK DR8HS. Would it be better to get NGK D8HS (without the noise supressing resistor) ?

TF
Are you consistently getting spark on both sides now (when you hold the plugs against the block and kick it over)?

If you do have spark on both sides I'd check the carbs. Have you cleaned them thoroughly? Verified the bowls are getting fuel?
 

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Discussion Starter #7 (Edited)
Are you consistently getting spark on both sides now (when you hold the plugs against the block and kick it over)?

If you do have spark on both sides I'd check the carbs. Have you cleaned them thoroughly? Verified the bowls are getting fuel?
I have checked that the bowls are getting fuel. Although the leftside carb have had incorrect fuellevel in the bowl. That I have corrected now.
I get sparks on every side when I kick it over.
After every (numerous) start attempts (kickstarter) with no luck starting the engine, the spark plugs had clear signs of fuel.
 

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Discussion Starter #8 (Edited)
Tried starting "the beast" today..
Couldn't get much more than a few puffs, although I think I kicked it over many many times.
The puffs I got was with the old spark plugs that is. The new spar plugs are totally dead..

I checked the fuel level within the bowl of each carburettor - see pic :

20180630_143624.jpg

Is this level correct.??
It is some 3mm above the alu edge of the center overflow tube.

One thing that puzzles me is that on each carburettor, the main jet cannot be seated to the edge - see pic :

20180630_143929.jpg

Can someone verify if this is normal.?

Btw, as you can see the float on the pic have some dents. I have checked each floats in each carburettor and they are both floating evenly as they should.

Unless I have a wrong fuel level in the bowls, I think my problem is in the ignition area.
I noted that the spark plugs I have is NGK DR8HS.
On the David Silver Parts page I can see that the spark plugs they sell for the CB126K5 are D8HA. (https://www.davidsilverspares.co.uk/CB125K5/part_27987/)
Getting some spark plugs without the internal 5kOhm resistor (the "R" designation) could help getting a better spark. I have ignition caps with 1kOhm resistor.
The "A" designation I can only find as being "special design" of the "firing end construction" (https://www.sparkplugs.co.uk/spark-plug-symbol-codes/)

I might order an aftermarket ignition coil (https://www.davidsilverspares.co.uk/CB125K5/part_206269/)
It is only £25. Could be interesting to if this would help.
 

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Hi I just joined HondaTwins this is great site. I just restored same kind of bike. I had same kind of troubles with engine, still some. My biggest worry was to get ignition timing correct to get bike running. You must get both cylinders to ignite in "F" mark on rotor(First other cylinder, then 360 degree turn on rotor and check the other cylinder. Actually I advanced ignition (the points plate) so that points open even before F mark that`s how I got mine to idle and rev. If you like you can check with automotive advance lamp with engine running(spits some oil with cover open.., at 3000rpm you must have ignition pointer between II mark on rotor. (Check ignition advancer springs open/close freely to get results..) Make sure there is enough fuel in tank!, on normal fuel cock position the tank must be quite full or use reserve position.I am using D8HA plugs too.
 

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Discussion Starter #10
Hi I just joined HondaTwins this is great site. I just restored same kind of bike. I had same kind of troubles with engine, still some. My biggest worry was to get ignition timing correct to get bike running. You must get both cylinders to ignite in "F" mark on rotor(First other cylinder, then 360 degree turn on rotor and check the other cylinder. Actually I advanced ignition (the points plate) so that points open even before F mark that`s how I got mine to idle and rev. If you like you can check with automotive advance lamp with engine running(spits some oil with cover open.., at 3000rpm you must have ignition pointer between II mark on rotor. (Check ignition advancer springs open/close freely to get results..) Make sure there is enough fuel in tank!, on normal fuel cock position the tank must be quite full or use reserve position.I am using D8HA plugs too.
I have ordered an aftermarket ignition coil and some of the D8HA plugs you mention.
Also gonna replace the condenser and the breaker. I'll be back when I get the parts...

Then I would think the ignition dep should be covered.
 

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Discussion Starter #11
Well I got the parts in April 2018 but never got to move further..
Two problems solved :

1.
I seemed that the sides of the rubbergasket on the carb bowl was touching the floater which caused the floater inoperable.
After having the gasket set in a fix for some time, the incorrect fuel level in the left carburettor was solved.

2.
The ignition coil I got proved to be a 12V coil ;0(

But I then separated the circuit and connected a 12V battery to the ignition coil only.
Now the bike starts every time, so I guess the problem was a bad ignition coil.
I am now waiting to get a step-up circuit so that I can use the 12V coil on the 6V circuit.

Moving along with the project....
 
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