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simps2002uk
L Plate Warrior



Joined: 03 Feb 2008
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PostPosted: 09:09 - 04 Apr 2011    Post subject: Honda CB125tde Reply with quote
Hi,

I recently did a full rebuild on my little bike but seems its not running right its got all new pistons rings crank bearings etc and has good compression on both cyliniders but seems to only run on 1 cylinder (the right) and while on tickover she pops back like she is trying to kick in and kicks in and runs at about 2500 revs. i thoughth it could be that the carbs needed tweaking but no amounts of playing make teh left kick in while ticking over, if i take the plug lead off and it sparks across to the plug it runs on tickover. I have swapped coil from 1 side to the other and cdi and no change. Does anybody have any ideas on what this could be.

Many thanks
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Teflon-Mike
tl;dr



Joined: 01 Jun 2010
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PostPosted: 10:13 - 04 Apr 2011    Post subject: Re: Honda CB125tde Reply with quote
simps2002uk wrote:
Hi,
I recently did a full rebuild on my little bike but seems its not running right its got all new pistons rings crank bearings etc and has good compression on both cyliniders but seems to only run on 1 cylinder (the right) and while on tickover she pops back like she is trying to kick in and kicks in and runs at about 2500 revs. i thoughth it could be that the carbs needed tweaking but no amounts of playing make teh left kick in while ticking over, if i take the plug lead off and it sparks across to the plug it runs on tickover. I have swapped coil from 1 side to the other and cdi and no change. Does anybody have any ideas on what this could be.
Many thanks

I need you to be a bit clearer on the symptoms, & we need to chase the logic from scratch. Your applying a bit of 'fuzzy' logic here based on what you have done, not system logic based on how engines work.
We need compression, ignition & charge for 'life'.
You reckon good compression on both pots, so onto sparks.
Both plugs out, do you have sparks on both plugs, in free air?
(Use Jump Lead to get good earth. Use Crock Clip one end to hold Sparker by the thread, and clip other end direct to battery)
Are the plugs good / new?
Plug Caps? HT Leads?
You said swapping coils & CDI made no difference.... but could you repeat plug in free air, if LH is not there or weak.
Next up chase the wiring.
Have you painted the engine and or frame?
Noted minir design flaw in the electrics on these things, the battery Negative has only one atachement point, via engine mounting bolt, causing a pinch point in the circuits and many electrical maledies.
Make sure that Battery neg strap is clean both ends, and clean paint / rust of frame attachement plate around bolt, plus bolt, plus engine.
Also check the Coils earths via the metal core of the coil and the plate they bolt to and the pegs they bolt onto the frame with... loads more oportunities for bad earth in rust / paint, poor joints.
They can benefit from a small 'earth loop'. Four ring terminals, one on for each coil mounting bolt, to a common junction, then a wire from there all the way back to the battery negative, with two 'spurs' one dropping down to the Cylinder Head / Frame mounting plate bolt, and one back to the frame / Engine / Battery main Earth strap bolt. (Probably over kill, BUT... the main loom also has no other eart node but the bolt under the seat, there is a spare 'earth point' by the flasher relay, can be worth belt and braces-ing the earth loop, and adding a female lucar to couple into the loom earth and the looms earth point via ring terminal)
Next up, check the wires coming out the mag cover.
Three wires Blue, Three Wires Yellow coming into a single bullet connector, should be behind LH side panel by the air box.
First check the actual bullets, they can be a bit 'iffy'.
Next check the wires going into bullet, its a 'heavy' junction.
Trace wires back... they are prone to fretting and earthing out on the engine cases or frame, either where they are routed through frame clip, or through Sproket cover, or into generator chamber.
There is supposed to be a rubber grommet for them behind the sprocket cover, but often goes AWOL, then they fret there, or worse get nibbled by the chain.
Cover those 'basics' then report back.
Carbs are shall we say 'wear sensitive' on these things. But lets make sure on decent sparks before looking at them.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'
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prawny1
World Chat Champion



Joined: 28 Jun 2005
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PostPosted: 10:27 - 04 Apr 2011    Post subject: Reply with quote
Did you refit the screws or bungs that block off the holes used for vacuum guages after balancing your carbs?
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simps2002uk
L Plate Warrior



Joined: 03 Feb 2008
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PostPosted: 14:22 - 04 Apr 2011    Post subject: Reply with quote
Thanks for the reply.

Should have proberly been a little clearer rather than rush through the basics.

both plugs are new and giving a nice spark out of pot plug caps were replaced not long ago and dosent look like any signs of problems no rust or cracks ( not been ridden since replacement) ht leads were done at the same time. i have painted the engine but was aware of single earth points so after painting took a wire brush to the engine mounts to clean them up the frame is still in need of painting but not as much as I am in need of riding, so cleaned the earth strap and bolt and frame when I reassembled did not play with any other wiring but i dont think it used to run as well as now so means nothing. I will check your other suggestions when I next get to the bike hopefully I can take a look tonight.

Many Thanks again fo the info 1 thing still puzzling is that it runs when I partially remove the plug cap and put an extra spark between the lead and plug. possibly just the extra resistance in the coil enhancing the spark at the end.
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Teflon-Mike
tl;dr



Joined: 01 Jun 2010
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PostPosted: 17:45 - 04 Apr 2011    Post subject: Reply with quote
simps2002uk wrote:
Many Thanks again fo the info 1 thing still puzzling is that it runs when I partially remove the plug cap and put an extra spark between the lead and plug. possibly just the extra resistance in the coil enhancing the spark at the end.

Novel!!?!?

Right, so its running up on one pot, but not the other. Compression, Sparks.... leaves charge (& an anomolouse spark plug cap! Will have to ponder that one)

How long are you letting it run for? They do often start on one cylinder, until warm when teh second pot will kick in.

Next up, odd starting on Right Hand pot, not left... that is RH pot as you are sat the bike, isn't it?

Choke cable is on LH pot, then theres wampy flex link to RH..... often choke on RH doesn't fully 'open' when you turn teh choke off, due to wear or misalignment in the flex link, hence that one floods until hot, and runs rich after.......

Worth investigating....

Float needles are another.... we seem to have a tenancy to run ritch, often down to worn float needles.

And finally how far are you from Coventry / Nuneaton? If not far off, your welcome to bring it over, and we can have a look and a fiddle.

Have Carb balencers too, and plenty of ready spared we can try substitution diagnostics with!
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'
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simps2002uk
L Plate Warrior



Joined: 03 Feb 2008
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PostPosted: 00:06 - 05 Apr 2011    Post subject: Reply with quote
FIXED IT Very Happy turns out I managed to get 3 dud plugs out of 4 new ones. I swapped the left cylinder plug with a new one 3 times (all dud) but didnt think to see if the misfire moved by swapping the plugs round (well who would when you keep swapping for shiny new plugs) put an old 1 in from prior to the rebuild and pop pop pop runs on 2 as sweet as a nut. Rolling Eyes still all sorted as the plug as only done about 100 miles since it was new it can stay. if it aint broke dont fix it. Thanks for the advice teflon-mike I checked all of what you mentioned and then had a thought of swapping plugs again so great advice also checked all my earths with multimeter and no resistance loss at all as standard so think I will leave them as is for now.

Thanks again

Also found the reason for running with plug cap slightly removed is it creates a higher voltage spark means the plug insulation is shot and is starting to short out. used to be able to buy little gadgets called spark arresters a long tome ago that gave the same effect, apparently they fitted them to land rovers to stop them running like a pile of poo (dont think they ever worked).
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