Topic: 3 exhaust pipes get hot but one runs noticeably cooler

[cosmo20]

if this has come up before and has been addressed i apologize, but after looking for an hour and a half thru the faq's for something on this, i gave up and decided to start a new thread.  does anybody know why 1 pipe would run noticeably cooler than the other 3 while the bike is otherwise fine?

for background, my bike is a 78 cb750k w/41k miles--supposably rebuilt at some time, but cannot be verified (although it does have stainless allen bolts thruout.)  the engine is stock, but it has a mac 4:1 chrome pipe w/baffle, a hi-flow air filter and i drilled the bottom of the airbox out.  i opened the stock 110 jets out from .0395" to .0410" and it runs clean and sounds fine to me and others.  all cylinders have 145 psi compression and the plugs show it is getting a decent fuel mixture (given todays' unleaded fuels.)  pipes 1-3 all show blue thru the first bend, but pipe #4 has not changed color yet--although it does get hot enough to melt the sleeve of my summer mesh jacket.  i checked/adjusted the valves, and cleaned out the carbs (but i'm afraid to pull the idle jets out to see if they're open since they're pressed in.)

it starts in hot temps, below freezing w/the choke, and pulls well from idle to redline. it gets decent fuel mileage and runs heads up w/my buddies 2007 triumph 855 bonneville from 70-100 mph. 

thanks in advance for any ideas,

jt

p.s. if it helps i could post a pic or video--if i can figure how to.

[Inigo Montoya]

Generally this is either caused by a dirty pilot circuit, the mix from that carb being too rich of lazy spark on that cylinder.
Dirty pilots are a common issue in the 77/78 K bikes. the idle jets need to be pulled out and manually cleaned, spray alone does not work usually.
Check your plug on that cylinder and see what color it is compared to the others.
Once you clean the carb, if it still does it, then swap the plug AND wire with another one I.E. 1 and 4 or 2 and 3.
If the problem stays, it is something with the carb. if it moves, then it is either the plug, cap, or wire that is not working right.

[TwoTired]

Have the carbs been vacuum synced to ensure all the cylinders pull evenly?

You really have to pull the pilot jets out to ensure they are clean.  Wrap with leather, grab with pliers (parallel type is best) and pull straight out.

Drilled jets leave jagged edges and make fuel flow unpredictable.
Kinda like if there is a 1 foot rise in the road with long ramps leading to it, you can easily go over it at many speeds.  If it is a 1 ft curb, you will greatly disturbed going over it at most any speed.
In a fuel orifice, those entry and exit ramps are significant.