Topic: 78 cb750 Choke Plates

[workinprogress]

Hi there!  First I'd like to say that this forum is awesome.  What a tremendous amount of knowledge and passion....  This is my first bike and I love tinkering with the old girl even if I don't know what I'm doing half the time...
so, I'm trying to figure out a hard start issue.  It's not starting without starting fluid.  I put in new plugs and they all spark, but not a "fat" spark like I've read they should.  Also, the choke plates don't close all the way.  Should they?  I'd say they turn about 80% (100% being a whole quarter turn that would plug the intakes).  I'm guessing it's maybe my coils that's the real issue.  Can I test the output of these with a multimeter?  Whats this I hear about an accelerator pump?  Does my model have that? can't find it in the manual... and there's certainly no electrical going to the carbs to power a pump of some kind...  Once I get here going and warmed up she's a real beauty....  Any help would be greatly appreciated!  Thanks.

[flybox1]

on the right end of your carbs, there should be a stamp PD####
this will tell us the model number.
if you have a smaller bowl on the bottom of your #2 carb bowl, then yes, you have an accellerator pump.
when properly working, this can aid in startup and quicker acceleration.
If you have these PD model carbs, you'll also have a fast idle cam.  this is the system you'll need operating well for easy starts.

there have been many threads on these two subjects.  type in accel pump  and   fast idle cam  into the google custom search at the top of the page....and read a bunch of what has been posted.  these will give you a good knowledge base to start with

[martin99]

Hi and welcome to the forum.

Just to add to Flybox's comments, in the FAQ you will find a way to check your coils. These seldom give problems by the way, the majority of weak spark problems are usually down to points (worn or set incorrectly), condensers and thirty odd year old plug wires.

You're jumping about a bit, be methodical. 3,000 mile service is a good place to start. Get your ignition system in good order before you do any carb work.

Good luck, most (all) of what you are faced with has been covered many times before and the forum is a wealth of info. As said, the search function will bring up loads of useful stuff.

[workinprogress]

thanks for the response. still wondering if the plates are supposed to close fully...

[flybox1]

yes, they should
do you have a correctly fitted and working choke cable?

[workinprogress]

the choke cable appears to be in good shape.  I pull the choke and it just stops moving the plates about 3/4 of the way up.  I can push them closed with my fingers but resistance is felt and then it sticks when I let down the choke...  I'm guessing theres a spring in there that's worn or rubbing somehow. probably from messing something up the last time I took it apart...  I've located the accelerator pump and it's not squirting at all.  I need to take it apart and inspect it tonight.  it was leaking gas from all four float bowls the last time. When I broke it down i realized that almost every screw was stripped (the threads on the bowl) so I retapped all the screws to the next size up.... wondering if this has somehow caused the problem.  Thanks for the input.

[flybox1]

the choke cable appears to be in good shape.  I pull the choke and it just stops moving the plates about 3/4 of the way up.  I can push them closed with my fingers but resistance is felt and then it sticks when I let down the choke...  I'm guessing theres a spring in there that's worn or rubbing somehow. probably from messing something up the last time I took it apart...  I've located the accelerator pump and it's not squirting at all.  I need to take it apart and inspect it tonight.  it was leaking gas from all four float bowls the last time. When I broke it down i realized that almost every screw was stripped (the threads on the bowl) so I retapped all the screws to the next size up.... wondering if this has somehow caused the problem.  Thanks for the input.

that's your fast idle cam engaging poorly.
while your carbs are off, look deep between #2 & 3 carbs and watch as you lift your choke linkage.  you'll see a small screw with a set nut on top.  the tip of the screw is a rounded post, which will begin to ride up on a cam as you lift the choke linkage.
as the screw post rides up the cam, your slides will lift. 
now, step back and look at the big picture....
When you are starting your bike cold, your choke cable pull closes the butterflies, AND, because of the fast idle cam engaging, your slides are lifted.
a quick twist or two of the throttle here, and your working accel pump squirts gas deep into the carb throats.  a kick or touch of the starter and you're fired and running.
now, you have areas to adjust idle. 
the idle set screw on the R side of the carbs... idle should be set with this once the bike hot, and left alone.  wont need to touch it again.
your choke cable actuates the fast idle cam and lifts the slides as described above. this will also increase startup RPM, and can be adjusted at the fast idle cam set screw lock nut. with your choke cable fully pulled, you'll want your idle to jump to 2500rpm or so, and as your bike warms down the road, you can press your choke in all the way, and your bike will settle at idle based on how you set your idle screw....

[workinprogress]

Success! It was indeed clogged ball valves on the accelerator pump!  AAAAAnd, the number 1 choke plate was rubbing the side of the carb body and providing resistance at the last quarter turn.  re adjusted the screws on the choke plate, a little carb cleaner on the accel pump and I'm back in action! Thanks for all the help!