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1978 Hondamatic 750 runs poorly in cold weather... runs great on warm/hot summer days

vasi0011

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I've been trying to chase down a problem with my 1978 Hondamatic 750A running very poorly in cold weather, in the 40's or 50's F degrees. It runs great in the summer, but predictably runs terrible when it cools off in the fall. I completely gone through the carbs, with no change in the situation. I've been told that cold weather causes a mc to run lean. I've got to fully choke it to get it started, and have to leave the choke on till it FULLY warms up. It was originally from CA. Wondering if anyone has had the same problem, and if re-jetting helps. Thanks!
 
'I've got to fully choke it to get it started, and have to leave the choke on till it FULLY warms up.'

If that is what one considers poor running, well every motorcycle I've ever had did that to varying degrees, it's normal. I've had almost all brands and well over 30 bikes. Normal cold engine behavior, they will likely not need nearly so much choke at all in summer.
 
Welcome to carburetors. These are jetted on the lean side. First make sure you have no vacuum leaks then play with jetting if you feel up to it.
 
Welcome to carburetors. These are jetted on the lean side. First make sure you have no vacuum leaks then play with jetting if you feel up to it.

True. Cold weather may create gaps on intakes which would mean vacuum leaks. Good thought. Have to check that.
 
Cold air really has very little to do with vacuum leaks as far as the creation of them. What he was actually hinting at was that a vacuum leak will act up worse in cold air.
 
I didn't mention that in really cold weather it runs terrible, even after fully warming up. It cannot accelerate without sputtering, and stalls at times when trying to accelerate from a full start.
 
Pull your air filters off in the winter time, cover the ends of the air horns with panty hose until you’re ready to reject correctly.


 
Post #6 changes everything.

Need to look at the basic tuneup now, including a compression check to make sure all effort not wasted.
 
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