Topic: 1977 CB 750 accelerator pump question, again.

[jcmatin]

Just ordered new rebuild kits from vintage cb 750.  I agree that the float needles must be off, not doing their job.  I will change the main jets back to stock as well.  Getting closer gang.  I feel that when the rebuild is done, and the vapor lock is fixed, I will be there.  Question: How can I relieve the vapor lock issue with the fuel cap?  Should I just drill a hole in the cap?  There is a long hose coming from the tank, that feeds out under the frame.  Should I just replace this?  As always, I appreciate the help.  These Mikuni carbs are a real learning experience for me.

[flybox1]

Just ordered new rebuild kits from vintage cb 750.  I agree that the float needles must be off, not doing their job.  I will change the main jets back to stock as well.  Getting closer gang.  I feel that when the rebuild is done, and the vapor lock is fixed, I will be there.  Question: How can I relieve the vapor lock issue with the fuel cap?  Should I just drill a hole in the cap?  There is a long hose coming from the tank, that feeds out under the frame.  Should I just replace this?  No, this hose is the tank overflow tube.  There is a hole on the top of the tank, under the filler recess flap.  If you were to accidentally overfill the tank at the filling station, the gas will funnel to the ground, instead of onto a hot engine.  Replace it if you want, but its probably fine. As always, I appreciate the help.  These Mikuni carbs are a real learning experience for me.

Mikunis?

You should have Keihin PD carbs 

[jcmatin]

They are Keihin, my bad

[jcmatin]

Update.  I have rebuilt the float bowl area of the carbs.  No more leaky fuel, so thats good.  Still runs poorly, so I need to do more testing on the carb bank.  It is blowing a lot of white smoke right now, but I think it is due to the one cylinder not running for awhile, and there being a buildup in the pipe that needs burnt off.  I replaced the main jets with stock.  I unscrewed the air mixture screw 1.5 turns.  The bike starts on full choke, revs very high, then as the choke is lowered the revs lower, but it cuts out and will not rev well at all, and will die at no choke.  And smoke like a Cheech and Chong concert.  I will re check the float levels.  When I did before, they were fine, but maybe they have been altered from taking it apart so much.  Right now, I don't have air filters on it, as I am just trying to get it to run average at best.  I may have to send the bike to a guy, but I don't want to.  I really want to solve the riddle, but there are so many variables.  Tomorrow I will take the upper end of the carbs apart and see what is what.  I have the full rebuild kit, so I might as well use it.  I appreciate all of the tips and feedback.  This bike is beating me up pretty badly, mentally.  Thanks, as always.
                                                                                      Joe

[TwoTired]

Your symptoms say blocked pilot jets.  Have you yanked those out to cleen them?

[jcmatin]

Update:  She runs......better.  I cleaned everything again, made sure the accelerator pump was working, put all back together and bingo, she works.  There was a vacuum leak where the carbs meat the rubber mounts, so that took some fiddling to make sure they were all air tight.  Idles well, but under power, lugs a bit and surges.  I think I need to get them synced up.  I also need some type of air filter.  I put the baffle back in the pipe, as it was too loud.  While they were apart, I noticed that the float levels were all off, so I fixed those.  re cleaned everything in the bottom part of the carb.  I think I will check the timing next, just to be sure.  Things are looking up, finally.  Anyone have a link to the carb parts diagram or know where to find a good one?  Thanks again for the help.