Re: 1979 CB750K wont start

Posted by LukeM on Jan 06, 2012; 1:15pm
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/1979-CB750K-wont-start-tp3636825p3637798.html

One of your earlier posts says it was running recently.  That means at some point this bike had everything in place to make noise.  Something happened.  Could have been any number of things.  So....

Let's back up a few steps:

The 4 elements are Suck, Squeeze, Bang, and Blow.  

The first two can be verified with a compression check. If the compression is low, then you'll need a WHOLE lot more spark for combustion to take place.  This will also vett out the valvetrain, piston rings, and camshaft timing.  If memory serves, the PSI reading for compression should be about 130-150.  All readings should be the same.

A fully charged battery will also help.  You can use a pair of jumper cables and connect to a car battery AS LONG AS THE ENGINE OF THE CAR IS NOT RUNNING.  Too much current from a running car will blow fuses and other electrical bits.

Next, we see you're getting spark.  If it's not bright blue like a welding flash, that's OK. If everything else is in place, that should be enough.  You may have fouled plugs, and you may want a new set, but I'd hold off until you check the other stuff. I'd also double check the stuff in the left side engine cover (hall sensor pickups, wiring, etc).  Be advised that the left engine cover will have a little oil in it.

Verify the main ground for your wiring harness is clean and tight.  I believe it's a green wire under the seat.

Then I'd look at the fuel in the float bowls.  Hook up a hose to the carb overflow tube, drain out some gas and take a good look at it.  Should be clear, with no little floaty bits in it.  Do the same for all 4 carbs. If you find stuff in there, clean the carbs again, then back up to the fuel line, then the petcock, then the tank. Check the manual and make sure the mixture setting on each carb is at factory spec (1.5 turns out, if I recall).

I'd start with these things, then let us know what you find.  If you don't have a manual, you can download one from the home page of this forum, under the Manuals section.

Here to help if I can.
Luke M