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Discussion Starter · #1 ·
Helo again, fellow "hobbiers"!!

I come here today before you lot to tell my new story. Rebuilding my cb500t I've gone through the electrical problems,I am now moving on to the gas mixture.

Left hand carb/cylinder runs like a dream, right hand carb leaves plugs totally black and runs very rough when it gets hot.

They've both been stripped, cleaned, rebuild with keyster kits. Eveything's new including the bolts and diaphragms. Floats have been adjusted , the right hand carb has been cleaned over and over again and I've even replaced it with another one but the problem remains.

The best way it'll idle is by turning the pilot screw all the way in...

Could this be electric? I've got big strong constant spark on this plug.
 

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Have you confirmed that everything else is within spec? Things like compression, timing, valve clearances, etc.

Also, regarding the blackened spark plug, what type of black is it? Sooty? Oily? Have any pics? Does putting in a new plug make it run good for a short while? Just wondering if oil is somehow getting into the right cylinder and fouling the plug.
 

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If you can't adjust your A/F ratio screw you're out a jet somewhere.
 

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It wouldnt be unheard of for the keyster jets to be iffy.....maybe try putting the old jets back in and compare.
 

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Discussion Starter · #6 ·
I appreciate your good will and all your inputs guys!

I just finished rebuilding the engine with new rings, valves, valve seats, valve seals and I have no way to test compression, but its very close to new.

The plug doesnt show any grease or exhaust smokes. It's just black dry carbon from too much gas in the mixture or not enough air.

I've changed carb body (both have been ultrasonic cleaned and blasted), all jets, main jet needle with "piston", pilot needle, taken the air filter off. I have gone through all the little holes with a very fine copper wire (copper smoother than brass).

These carbs are so clean you can use them as a spoon to eat (if they didnt have so many holes)

The only thing I didnt change yet is the air cut valve. It's got new diaphragm and o'rings but...

I have also timed the points and adjusted gaps

I dare you to point out something I havent changed yet!!!
 

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Well it sounds like your either running rich or the mixture is not being properly ignited. You adjusted the floats but wondering if its leaky or sticking allowing too much fuel? Have you checked the timing with a strobe? Also, any difference if you run it without the air filter?

I recently had a similar problem with my KZ and it turned out the float needle was worn and wasn't seating correctly causing one cylinder to run rich and foul the plug constantly.
 

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Another thing to check is your coil leads and caps.....could be developing resistance causing weak spark....
 

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Discussion Starter · #9 ·
".....could be developing resistance causing weak spark...."

The spark on this side is stronger than on the left hand side and I've also tried switching coils, caps and plugs. Still wont do it.


"wondering if its leaky or sticking allowing too much fuel?"

Unfortunately, I've rebuilt the carbs with keyster kits, so everything is new. I'm assembling another carb body with old bits to see if that does it :?

The only way to improve the lumpy way the cylinder runs is to adjust the pilot screw all the way in. Revs go up a bit.
 

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dude, you really need to check compression....even if your rings are new, the gaps may be alligned.....
 

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Discussion Starter · #11 ·
... I've tried the left hand carb on the right hand cylinder and it runs smooth :? :? :shock: :? :?

Heeeeeeeeeeeeeeeeeeeeeeeeeeeeeeelp!

Edit- Just thinking out loud but excess petrol is going trough either the air cut off valve or the main jet/needle, because its should stall when the pilot screw hits the bottom, right...?
 

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Here is what I found on a CL450 that was giving me the same problems.

A float that was sinking, filling up with fuel
Jets that were cleaned too many times and with something that opened them up
Old paper filters that wouldnt pass enough air and made the bike run rich

My opinion on your bike is the sinking float since it seems to be one carb,

Hope this helps.
Don
 

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Discussion Starter · #13 ·
dlslick said:
My opinion on your bike is the sinking float since it seems to be one carb,
Don
Yep, I've tried tested this one one as well.

I got together an old carb body with old jets, needles and old air cut diaphragm. Basically I used the old bits from the carb and just gave them a quick clean and that seemed to have solved it.

Now I've got the bike running to what I would call an acceptable level, maintaining idle at 1000/1200 and not choking.

Still doesnt answer my question: Why wont a totally rebuilt carb run ok??

In the name of science, I'll dig deeper into this.
 

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Simply because carb parts are new does NOT necessarily mean they are good or the proper sizes........Jets have been known to be mis-stamped, shut-off needles cut at the wrong angle, etc......
 

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I've had similar results with keyster carb kits...I won't buy them any more.

I ended up replacing all the keyster stuff with NOS honda bits and all was right with the world
 

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Hi
If you want to test the compression and fancy a trip to Southampton I have a Compression tester here you can use.
PM me
 

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Discussion Starter · #19 ·
I cant source the problem that caused all this but I can clearly state that I hate Keihin. The carbs in these bikes (CB450, CB500T) are basically the same and I dont know how on earth they managed to tune them during the assembly stage in the factory.
I've got an Africa Twin back home , in Portugal and thei're no good either.
I've always prefered Honda to all the other makes but as I see it, their carbs have always been their weak spots.
I now work with Harleys and guess what they bring as stock carburettors...?
 
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