Topic: '77 CB750 Power sags when throttle opens

[Vintagelover]

Well, I spent a little while searching previous posts, so if someone knows where this has been discussed before just lemme know.

I recently bought a '77 CB750.
Problem: It wants to die every time I want to take off from a stop. I crack the throttle and the power dips way down, so I have to let off the throttle and ease up to speed. End result: I can't accelerate quickly!
Any ideas?

I've recently cleaned and synched the carbs, leveled the floats, checked the timing, and added new pod air filters (b/c the old one wasn't seated correctly and the boots were worn, I thought maybe the irregular flow of air had something to do with it). This problem existed before all that. In fact, not much has changed since I've started tuning the bike

[Spanner 1]

The carbs on a 77 750 have an accelerator pump which 'squirts' fuel into each carb immediately above idle, if it's not working the bike will 'fall flat on it's face' when you go to move-off from stopped.... hello, and welcome to SOHC heaven and hell ... probably 1,2,3 or 4 of the nozzles that deliver this 'burst' of fuel are blocked.... easy check is to remove airbox and observe each carb throat while twisting the throttle ( fuel on, bike does not have to be running ).... the 'squirt' of fuel should be easily visible from each brass post in the carb throat..... check that out and report  
Whoops... you've got pods, even easier to check pump operation...

[HondaMan]

Vacuum leaks.
On those hard carb hoses.
At the carbs and at the head.

[beaniam]

I took apart my accelerator pump today since my 78 is torn down for tranny work.  Turns out the accelerator pump diaphram was all rotted and cracked, as well as the o-ring in there and all the ports were clogged with rotted gas and crap.  $37 http://www.motorcyclecarbs.com/ACCELERATOR_PUMP_DIAPHRAGM_KIT_P9641C261.cfm

[bender01]

Do you have enough gas in the tank? Try reserve.

[gnarlycharlie4u]

I hope that you didn't already pay more than you had to:

http://www.siriusconinc.com/pro-detail.php?pid=&product_id=1094

http://www.carbkitscapital.com/carb_kit_list_part.php?make=Honda&cat=Pump_Diaphragm


http://www.cb750supply.com/products/6/fuel-system (@ bottom of page)

http://www.vintagecb750.com/products/6/fuel-system (near bottom of page)


also, check this page out for Acc pump repairs. it might require that you do more than just change the diaphragm.

http://www.hondachopper.com/garage/carb_info/accelpump_valve_repair/page1.html

[HondaMan]

I hope that you didn't already pay more than you had to:

http://www.siriusconinc.com/pro-detail.php?pid=&product_id=1094

http://www.carbkitscapital.com/carb_kit_list_part.php?make=Honda&cat=Pump_Diaphragm


http://www.cb750supply.com/products/6/fuel-system (@ bottom of page)

http://www.vintagecb750.com/products/6/fuel-system (near bottom of page)


also, check this page out for Acc pump repairs. it might require that you do more than just change the diaphragm.

http://www.hondachopper.com/garage/carb_info/accelpump_valve_repair/page1.html

Great post, Charlie! 

[Vintagelover]

Thanks very much guys! I took a look at that accelerator pump (AP) and it looks pretty good (not obviously dry-rotted, no holes, etc) but I haven't gone through that 'hondachopper' AP valve fix that Gnarliecharlie posted (thanks, btw) so I'm gonna do that next (maybe that tiny ball-bearing IS the problem).
Also, HondaMan, I'm going to replace the intake manifold boots b/c I sprayed them with carb cleaner (w/ engine running) and watched/heard the tach rev up so I think you're onto something, but would that really effect acceleration from 0mph? Seems like that will help my engine overall, but I'm not seeing the connection to my problem. 
Thanks again everyone!
so thankful I found this awesome resource! 

[KingCustomCycles.com]

+1 on the vacuum leak.  Everything will change when that is fixed, you will swear someone installed a turbocharger when you weren't looking.  FYI, these bikes, when tuned properly, have no trouble spinning the tire or raising the frontend.

[Spanner 1]

Did you check for acc. pump 'squirt' in each carb throat.??.. takes 2 minutes, or you have ruled that out already?

[Vintagelover]

@spanner1: You mean just taking the air filter off and twisting the throttle? The 'throat' of the carb being where the air filter attaches? I guess if there was ever a doubt in your mind, I should reveal myself as a newbie to engines, b/c I don't see/hear any squirt from any carb. Am I checking the right thing? If all four of those are clogged, I could see that being a big problem 

Otherwise I'm just waiting on my local motorcycle shop to get those intake manifolds in so I can replace them. Any other places I should check for a vacuum leak?
Also, I synched the carbs and leveled the floats before I checked the timing... did I do that in the wrong order? (Easier to have faith in this dialogue than the random things I read on the internet)

thanks for your help!

[Spanner 1]

Yep,just take off each pod, look in each carb throat and you will see a small brass post @ 5 o'clock. This is the accelerator pump nozzle and has a tiny opening on the side facing into the carb. with the carb bowls full ( gas on ) each twist of the throttle must produce a spray of fuel into the carb from each nozzle.... my bet is at least 2 are not working ( blocked ). Bike does not have to be running to test...... try that.