Topic: Sticky Carbs: Accelerator pump problems?

[themagicalatsi]

Hello everybody,

My 1978 CB750F has been giving me some trouble.  It refused to start last week,  I took the carbs off, checked the jets and float seats and found nothing.  I put them back on and it started immediately.  Again this week it was refusing to start.  So I decided to just mess with them on the bike, check one thing and try to start it and so on.  As I was draining the gas from the no. 2 carburetor I heard a click so I hooked gas back up and it started.   I am guessing that something is going on with the accelerator pump. 

The symptoms when trying to start are obviously fuel, and vary a bit.  The first week it was firing occasionally and the temp of the pipes was telling me that 1 and 2 were firing best, 3 not at all and 4 better than 3.  Today it was rarely firing. 

Another symptom that might be related is a fast idle after reeving the engine.  Every time I would come to a stop the engine would be revving at ~4k rpm.  I could get it to idle slower if i let the clutch out a bit to load the engine for a short moment. 

I also believe it could be an accelerator pump problem because I broke the factory spring for the accelerator pump, I replaced with something of similar stiffness. I took this spring off this afternoon and the revs jumped to 4K when i started it.  I backed off the idle screw and it idles fine.  I took it for a little spin around the block and it runs pretty well with no high idle symptom.

What do yall think?

Thanks!

[shorterdanny]

sounds like you solved it yourself, if removing the non standard spring and backing off the idle screw has worked...

i would try and get yourself a stand spring for the accel pump, (specs must be out there some where) or a assortment and try them and see what happens.

there is always going to be a rpm spike on start as the fuel goes in, prop shouldn't be up to 4k but one problem at a time me thinks.

[eurban]

The accelerator pump is designed to shoot a stream of gas down the throat of each carb when you whack open the throttle rapidly.  It should not be affecting your bikes ability to start. However, its operation is critical to achieving good acceleration performance from these carbs. You can easily observe its function by removing the air box, rapidly whack open the throttle and look for a stream of fuel from each of the pumps brass nozzles located at the bottom of the carb bore next to the choke plates.  Do you have squirt or no squirt? You can download the factory manual here:  http://www.mediafire.com/?5ttzdg9yyuz   .Your issues could be any number of things but you probably would be well served by going back over the carbs and baselineing them.  Make sure that the idle circuit is completely clean.  This means pulling the pressed in idle jets and cleaning them and the associated passages meticulously.  Make sure that the float levels are set correctly (reality check this by observing the level of fuel in each carb bowl with a piece of clear tubing attached to each overflow nub{drain screw open} and held up high along the carb body)  Set the adjustments for the accelerator pump rod gap and for the fast idle cam that bumps the idle speed when the choke is applied.   Bench synchronize your carb's slides.  Examine the condition of your carb to intake rubbers and consider how well they are sealing.  Test for air leaks once things are running by spraying something like WD-40 on the rubbers while listening for obvious changes in idle speed.  . . .Are the rest of the bikes systems up to snuff?  Read the routine maintenance section in the factory manual . . .