My 71' 883 ironhead will sneeze once and seem sluggish while I am slowly accelerating (less than 1/4 throttle) when the bike is cold and the temperature is below 65 degrees F. . If I jump on it (anything over 1/2 throttle) , no problems . After it warms up (both engine and outside), it's fine, in all ranges , for the rest of the day; in fact , it's incredibly fast. The carb is a mikuni VM 38 ,the air adjusting screw is set at 1 3/4 turns open.
Up 'til now ,I just avoid that range when it's cold , but I've been thinking that this could be a symptom of a more serious problem and should be addressed before any permanent damage can be done.

 

This really points to your intake gaskets to the heads. If they are not positioned jsut right, or if (it is a 71) there is a little warpage, then you sneeze because you get air sucking through instead of enough air/fuel from the carb...that causes a missed ignition due to too much air and not enough fuel. When you hit the throttle at 1/2 you actuate the Throttle pump that squirts a bit of extra gas into the carb and thus there is enough fuel in the mix to keep the cylinder firing on that stroke.

With the bike cold and running, carefully spray just a little brake cleaner around the intake gaskets, and cautiously smell the exhaust. If you smell a really caustic odor (it will burn your nostrils and make you gasp so don't just stuff your face behind the pipes) then your intake is leakeing and change the gaskets.

 

I'ts lean, turn your idle screw out a1/4 see if that helps.

 

msulli what did you end up doing? Was it the intake gaskets or something else like vacuum leak, better tune?

 

adjust the accelerator pump on the carb to squirt fuel a little sooner when you twist the throttle. I chased that problem for years, and that fixed it.

 

Thanks for the fast reply. Glad it got fixed.

 

What was the fix? The idle screw?