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Tooweel

Bert

TooWeel
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Cape Town South Africa
My 1981 CB750F develops a miss fire after riding for +- 20 minutes.
I have removed and cleaned carbs a few times, new fuel filters etc, no improvement. Tried hotter and colder plugs no change.
Plugs 3 and 4 always look as if they are running rick, ie black and sooty but not oily.
Some have suggested one or two carbs are flooding, but plastic floats are not adjustable.
On the other hand some advise me that the coils on the pulse generators are the problem and must be replaced together with the two spark units.
These electronic units are expensive and not that easy to obtain.
Is it a fact that the ignition items mentioned are prone to failure particularly once the engine has heated up ?
Any recommendations would be appreciated.
Thanks,
 
for one those rubber tipped float valves are awful. even new they will leak. I would be willing to bet your fuel cutoff valve on top of the carbs is leaking into your vacuum system, and or overfilling your floats through the vent. pretty common issue. are the rubber plugs in the bottom of the carbs sealing? failure of the electronics will result in loss of half the engine , and not a miss, on one. And failing electronics seldom return to life. the next culprit after fuel {which I am sure It is} that could cause this is a failing coil, however that's gonna be a 2 cyl drop also. try bypassing that fuel valve.
 
Thanks for info. Cut off valve has been by passed. I also replaced what you call the awful rubber tipped float valve needles. When the bike is left standing with the fuel tap open fuel does not tap out of the float chamber drains so I seem to be lucky that fuel is not leaking through the float valves. In fact when you start the bike all cylinders fire, this problem only develops as you go along, sometimes sooner than other times.
I am not sure which rubber plugs in the bottom of the carb you are talking about, obviously it is not something I can see externally is it?
Another question, do you think I should get metal floats, something that can be adjusted, not like the plastic ones?
All assistance appreciated.
 
adjustment has never been an issue for me. just those hideous rubber tip needles. what is the effect of 4 new plugs on this issue
 
As I can recall, with new plugs goes like a bomb, just when you are thinking the problem is solved it loses power, you slow down and there you have a serious miss fire.
At one time I would try closing the fuel tap, running awhile and then reopening it, but this did not always seem to work.
Thanks.
 
You have given me a few things to check through on the carbs again, so let me do that and take the compression on each cylinder, then I will get back to you.
Thanks anyway.
 
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