Black Smoke and missfire!

P

pzarro21

New Member
HI everyone as you can tell im new here and im having some issues. I bought a 78 cb550 with open headers (i know they suck) and pod filters. well she starts up fine but at idle any rpm its blowing some black smoke and covering the right side of my bike in soot. I did also notice that one of the head pipes up close to cylinder is not as hot at the other three,even after replacing the spark plugs. Well i took the carbs off the seem pretty clean but i will clean them. The current settings are 110 main stock press in pilot, clip on the needle at the highest position(Furthest away from needle tip), and idle screws at 2 turns. FYI i have 90 mains and 120's that came in a box with the bike. ill also check the float levels, if i can remember correctly you measure from the end of the float opposite from the piviot and up the bottom of the carb body and set 12.5mm, correct me if im wrong. So if anyone had suggestions on this jetting set up right now that would be greatly appreciated.

Thanks Pat
 
look in your factory service manual for settings if you dont have one >stop<

you may have a choke plate stuck closed,low speed mixture screw way outta whack,high float or leaking float needle those are some easy checks to get you started
but firstly you MUST make sure you have a 100% perfect ignition tune ...including new sparkplug caps and plugs points condesors timing set etc etc ,you can't tune carburetion with less than perfect ignition,dont even try
i am assumong the valves are clearnced and timing chain adjusted as well
carburetion pretty much comes last in tuning is what it bowls down to
 
xb33bsa said:
look in your factory service manual for settings if you dont have one >stop<

you may have a choke plate stuck closed,low speed mixture screw way outta whack,high float or leaking float needle those are some easy checks to get you started
but firstly you MUST make sure you have a 100% perfect ignition tune ...including new sparkplug caps and plugs points condesors timing set etc etc ,you can't tune carburetion with less than perfect ignition,dont even try
i am assumong the valves are clearnced and timing chain adjusted as well
carburetion pretty much comes last in tuning is what it bowls down to
Click to expand...

thanks for the reply! The valves and and timing adjusted and checked already, i know that for a fact. It has an electric ignition on it.
 
Float level is supposed to be 22mm I think. Please check that in a Service Manual and it will never run right with short headers and no muffler
 
teazer said:
Float level is supposed to be 22mm I think. Please check that in a Service Manual and it will never run right with short headers and no muffler
Click to expand...

the open headers are not my ideal situation, im searching for a good used exhaust. i have to find a manual somewhere so i can check float level and timing.
 
Set float level to 23mm and drop needle clip to #2.
Main jets are probably way too big because of open pipes.
Start around 115~120
 
crazypj said:
Set float level to 23mm and drop needle clip to #2.
Main jets are probably way too big because of open pipes.
Start around 115~120
Click to expand...

main jets are at 110 so i probably wont go bigger and make it richer. i will move the needle and adjust float height
 
you need to educate yourself on the specific functions of this type carb, slide valve carb(or variable venturi)... the main jet has not a single thing to do with black smoke at idle and neither does the slide needle. until you learn more about the actual function of the carb you are just wasting time and effort
 
xb33bsa said:
you need to educate yourself on the specific functions of this type carb, slide valve carb(or variable venturi)... the main jet has not a single thing to do with black smoke at idle and neither does the slide needle. until you learn more about the actual function of the carb you are just wasting time and effort
Click to expand...
i already know that i need to educate myself more thats why im here.......
 
pzarro21 said:
i already know that i need to educate myself more thats why im here.......
Click to expand...
yes i know and i sound harsh, and you can learn plenty here ,but get some reading material as well
 
In fact if the needle jet and needle are the right sizes, you can get over half throttle without a main jet fitted!!!!

Search for the HONDA Carburetion manual http://minus.com/lbrGNSbzU9N4UN and the Mikuni VM manual http://www.mikuni.com/pdf/vmmanual.pdf for light reading.
 
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