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Discussion Starter · #1 ·
Hello everybody, thanks for letting me join this forum. I have been working on getting my CB125 B6 running better with no luck!
It fouls the plugs. I had it rebored with new pistons but no improvement.
I bought another set of used carbs and a used 6volts ignition coil. I can not keep it running on both cylinders.
If it runs on one cylinder and the other starts to fire the first will stop firing. Both plugs look equally black. I have checked spark with a nail in the plug cap and it will spark about 5mm. I have only tested one side at a time.
Today I tried lifting the cap from the left plug a bit and it started to fire on that cylinder. If the left cap was lifted more it would continue running on the right cylinder. Shouldn't it had stopped? Could it be crossfiring inside the coil or outside somewhere? Should I suspect both coils to be faulty?
 

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Hi Hans,

Rebored no compression now good ? How good please.

It was not running well before hence the rebore ??

Please post a picture of the bike.

Get some new plugs. Lifting the ht lead can make a "sooty black" plug "fire", but why are they getting black.

How is the float height ?

When the engine is running, how is the exhaust smoke, black, or white, or blue like a 2 stroke ??? The engine has the correct amount of oil in it ??


Twin carb, single set cb points, so a wasted spark "twin lead" ht coil.

Make a tester for the ht for your coil, so both ht leads and outputs can be tested at the same time.

Do the ht leads gobble and seem loose where they are fixed into the ht coil, the leads are not removable !

I may have some test data but not with me today.
 

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Discussion Starter · #3 ·
Thank you drydreamer for your reply. I had it rebored due to pittings in one cylinder from rust and suspected bad compression due to poor running. The cylinder head was also rebuilt with new guides, valves and seat job. But this work has not made a big change in the running. I cannot give any compression readings at the moment because I am away for 2 weeks work.
I have changed the spark plugs so many times now that I am worried the threads will soon wear out! There are pictures in the project logs department, but I can post newer pictures when I'm back at home.
Float height is 21mm as I found the numbers on this forum, that should be correct. I read the shop manual has a misprint.
Some white smoke is coming from exhaust but it has been cold outside. Exhaust smells of unburnt fuel I would say.
I have lowered the needle 1 notch now. And while holding the plug cap at a distance for some time I managed to get the left cylinder to start firing without removing the plug again. Before, when right cylinder did not fire I was able to get it to start firing by screwing out the air bleed antil it almost fell out. I had it running yesterday for several minutes on both cylinders but the idle was around 3000rpm and if I lower it it will start cutting out.
The leads looked fine on the ignition coil.
Where can I find how to make a tester for the coil?
I think I should try some smaller jets, both sets of carbs have the same, 38 slow jet and 96 main.
 

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Discussion Starter · #4 ·
Hi! I Think I have found the problem to be the float height! I lowered it and it started idling much better. Unfortunately the manual was incorrect and both set of carbs I have gave the same poor result. Still it is puffing white smoke but at least it stays running for longer times. I had to screw in the mixture screws more than the instruction book says but maybe it was not completely warmed up. So I still have some tuning to do.
 

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Don't worry about where the book says to set the screws (what book are you using?), that's just a starting point from which to adjust them.

White smoke from a cold engine may just be water vapor in the exhaust and/or moisture which collects in cold mufflers.
 

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Discussion Starter · #7 ·
Hans78, glad you are solving the problem.
what is the "correct" float height you used and where did you find it?
I don't know the correct float height ethano. I just lowered it little by little. Then I checked the that the level in both carb float bowls were the same by remowing the bowls and putting them side by side on a table. I have not measured it.
Unfortunately I had some rust get to one needle jet so I had to start over as that carb started overflowing after a test run.

Still, I think the exhaust fumes smells like there is unburnt fuel. But when leaning out on the mixture screws, it won't stay running.
 

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Discussion Starter · #8 ·
Don't worry about where the book says to set the screws (what book are you using?), that's just a starting point from which to adjust them.

White smoke from a cold engine may just be water vapor in the exhaust and/or moisture which collects in cold mufflers.
J-T, I am using the manuals for the CB125 B6 I found on this forums.

I hope the white smoke will dissappear when I get to run it for longer periods, so it will be completely warm.
 

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Hans, the 125 and 160 in '65 were basically the same engine except for cylinder dimensions. I have used 19mm off the gasket seat for the float height without problems. Plugs have a nice coffee & cream tan core with a slightly sooty ring on middle needle setting with first level overbore cylinders/pistons.
 
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