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Discussion Starter · #1 · (Edited)
Trying to trace my "running rich" issue on my CB250 and one of the things that I've looked at are the air filters.

Does anybody reckon that these are in need of replacement? I don't feel that they are dirty enough to affect running. The two pics are of the l/h filter.

Thanks

IMG_20181222_094031251.jpg IMG_20181222_094058601.jpg
 

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Doesn't look that bad to me but pic's can be deceving.

Post pi'cs of your plugs.

And describe any troubleshhooting you have done
 
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Discussion Starter · #4 ·
Doesn't look that bad to me but pic's can be deceving.

Post pi'cs of your plugs.

And describe any troubleshooting you have done
Oh, my plugs are very sooty, nice dry black soot! Actually, there's a bit of nice brown color, but not that much.

To be honest, I've played around with so many things that my head hurts. I had the carbs out and checked over by an experienced classic bike builder. That was a few months ago. Recently, the bike has been running mostly fine although I always sensed a smell of fuel every time I started her up and struggled to get past 70mph. At that speed, the remaining play in the throttle wouldn't produce any more power. I've swapped plugs twice, fiddled around with the fuel screws, checked the carb floats (one of the needles was a bit sticky), changed plug caps, checked the valves and timing, checked the clutch plates, changed the oil and cleaned the centrifugal filter. I haven't done a compression test yet and I have a spare coil and condenser which I'm thinking of trying out. Obviously, I've had a look at the air filter elements and I've got a good battery.

Sadly, this is not a very methodical approach but I just do what I can when the weather permits. I am really quite an amateur at this kind of stuff and whilst the bike obviously has some sort of fueling issue I can still get out for rides, and I'm learning quite a bit about basic mechanics, even if I haven't succeeded just yet.

To paraphrase the above, it feels like there's too much fuel going through the carbs. The bike ticks over too easily from cold (although I do need the choke to start it) and it just smells rich. I guess that I should start from scratch, hence my post about the color of the filters. Hopefully I can cross that one off the list.
 

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Original exhaust on the bike, or aftermarket mufflers (silencers)? Modifications there to make it quieter (opposite of what most of us do) would have an impact on jetting, but the mixture screws won't have an effect on anything much above idle
 

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Discussion Starter · #7 ·
Original exhaust on the bike, or aftermarket mufflers (silencers)? Modifications there to make it quieter (opposite of what most of us do) would have an impact on jetting, but the mixture screws won't have an effect on anything much above idle
They look like aftermarket mufflers (the bike was restored about 8 years and 1000 miles ago).

My current thinking, in no particular order, is....

Need to check the carb floats again - possibly too much fuel being used.

Compression check. By looking at how quickly the new plugs soot up I guess that the cylinders must also be fairly sooty so I've ordered some Sea Foam Deep Creep.

Weak spark - to my eye, the sparks look ok but I'll think about fitting my spare coil/condenser to the "most sooty" side.

Maybe I'm over-thinking this whole thing and I should be happy pootling along at 60 and forget about the other issues :) Unfortunately, that's not going to get the fuel burnt properly and give me another headache further down the line.
 

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Discussion Starter · #8 ·
Things seem to be improving. I fitted a new condenser and re-set the timing. The bike starts, revs and idles really well although I haven't had a chance to take it for a run yet. I did notice that when I opened the choke after starting that it wouldn't reach the fully open position and it turns out that the bracket connecting the two choke plates is catching on the oil breather pipe, so that has to be adjusted out of the way. The rich running smell is still evident so I might check the float operation although it could be that the choke operation is causing this. I'll take the bike for a run after the Christmas parties are over and re-diagnose at that stage.

Thanks again for the advice received.
 

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Discussion Starter · #9 ·
On closer inspection this morning it appears that the choke bracket was being restricted by both the oil breather tube and the main earth cable which runs parallel to it. Both showed small gouges where the bracket has been repeatedly in contact with them (surely incorrect placement during the previous rebuild?) and not allowing the choke to fully open. I now have a fully opening choke which should allow me a better diagnosis of the fuel supply / running rich situation. I might get a run later today and have a look at the plugs again.
 

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Discussion Starter · #10 ·
I had my test run today (about 30minutes) and the bike felt much better, but not quite perfect. I ran it quite hard and managed to see 75+ mph but still felt that there was a fueling issue. I pulled in near the end for a walk round and immediately noticed vapor coming from the bottom of the oil breather pipe. It condensed on my fingers and there was a definite smell of gas in there. I pulled the plugs and the l/h side was a nice brown color, which looked good to me. The r/h plug was black, with a trace of wetness around the outer rim.


So, I'm working on the theory that too much gas is entering the r/h cylinder causing it to run rich and also seep into the oil system. I'll take the carbs out and re-check the r/h float.


My Sea Foam has arrived and my plan was to soak the cylinders for a week or two then drop the oil and see where we get to. I still have to split the cases to fix my bearing problem but, as a temporary fix, I simply pressed it in flush with the case to at least allow me to ride the bike.


Whilst the bike rides really well, I'm obviously concerned about the vapor from the breather tube so I'd better start searching for similar posts on here.
 

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All the older air-cooled twins have a little wetness in the blowby when they get hot, unless they're new and the engine is in perfect mechanical shape. Fuel smell in it is a concern and you shouldn't really run it with any potential fuel infusion and diluted oil can cause serious damage to the oil-fed areas of the top end like cam, cam bearings and rocker arms
 

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Discussion Starter · #12 ·
I checked the breather again and I'm now not convinced that it's fuel that I'm smelling, more an oily combustion kind of thing. I checked the dipstick and the oil is fresh with no smell of fuel there. maybe all the cold starts and short running over the past few weeks have caused some condensation build up.

I think that I may also have sorted the carb float issue this afternoon although when I re-assembled the carbs I managed to get a slight weeping due to bad mating of the two halves - another job for later! There certainly didn't seem to be that familiar "running rich" smell from the exhausts and the engine purrs beautifully at a steady 1250 rpm. So, some progress mixed with a couple of backward steps but I'm getting to understand how it all works.

Thanks again for keeping an eye on my posts. Knowing that others are out there makes the whole thing less daunting!
 

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I've been through the same thing with my 350. My air filters were dirtier than yours, and new ones didn't help. I've adjusted the float levels, but at this point, another look at them seems to be about my last option. Good luck with yours.
 

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Discussion Starter · #14 · (Edited)
Success !

Both cylinders are running sweetly - no more sooty plugs :) and the bike really flies like she was intended.

I can't say for sure which of the various changes produced this result but, among others, I used new plugs, one new plug cap, trimmed and re-attached both HT leads, replaced the condenser, improved compression with a quick Sea Foam treatment, stripped the carbs and replaced a leaking float needle housing, re-set the floats and replaced a leaking main gasket. Cleaned out some crud whilst I was there. Previously, I had checked the cam chain, re-set the timing, adjusted the tappets and moved the main earth wire and oil breather hose (which were interfering with the choke mechanism).

When I have time, I'll re-check the cam chain adjustment as I can hear it clicking away but that can wait until after my bearing issue is properly fixed (engine out).

Disappointing end to my test run today - I lost one of my side covers somewhere and they're not easy to find these days.

Anyway, not to worry, my bike is running properly so thanks again to those who helped me stick at it :cool:

IMG_20181231_150845044.jpg
 

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