Topic: 78 cb750 having problems - idles but stalls with throttle

[markreimer]

Thanks guys! I'll have a look at the pump first and share the results in a day or two.

[bollingball]

Hey Mark I have the same bike. First off that little fork boot looking thing  is a dust/dirt boot same as the fork boot on the older bikes it slides down the acc. shaft on the outside and keeps dirt from going into the acc pump shaft I have not had one on mine for over a year but will put it on next time I have the carbs apart. Did you actually pull the slow jets out? Did you set the screws to 1 1/2 turns out from a gentle seat the ones at the bottom outside I forget what they are called air/fuel or fuel/air mixture screws? You need to check the mickey mouse ears on the new pump some times one is not open from the factory. You should see a good squirt of gas from each nozzle they are right by the choke butter fly air box has to be off to see them. All this can be done on the bench before you put them on the bike. I don't know how clear this is but just ask I and others will help.

                Ken
 
 sorry I got called away looks like others have posted with good advice get back with what ever you need

[markreimer]

So i took of the air box to see if the acc. pump was working and low and behold, only one of the four acc 'jets' were squirting gas into the carb throats. I blasted them open with some cleaner and compressed air until all four were spraying an even amount of fuel, and also installed an o-ring I forgot about between the acc pump and the main carb body, which was leaking as a result.

I also turned my air/fuel mix screws out another half-turn which made the engine run smoother at mid-RPMs. It used to back-fire frequently but that extra half turn smoothed it out quite a lot. 

All this was a definite help, but didn't completely fix my problem. The engine still dies if I rip the throttle back hard from idle or just above an idle. It sounds like the engine is sucking hard and then shuts down. If I'm above 3k, no problems. I'm suspecting an air leak or something with my slow jets, but I know for a fact that I cleaned them out. I could pass a wire through the jets easily. What are your thoughts? Test for air leaks first, then pull the slow jets out and double check?




And sadly, I finished the day off by changing the oil for the first time, only to find out that the oil pan drain is stripped to %$#@, thanks P.O. Couldn't get the plug even close to tight. So I have to drill out the drain hole and thread it for a 14mm plug instead, or find myself a new oil pan. That's job #1 at the moment.

[markreimer]

Double checked the slow jets, definitely clean. Is it possible that they are too Large? Almost seems like the engine gets too much gas with a sharp twist of the throttle and chokes out. Everything is stock except for the exhaust, which was replaced with a 4-2 before I bought it. I'm not sure how re-jetting works in this situation, but could I need a smaller slow jet for a 4-2?

[TwoTired]

Almost seems like the engine gets too much gas with a sharp twist of the throttle and chokes out.

The needed clue is how the engine recovers from the stumble.
If it burbles back to life, then it was too rich.  If it simply comes back, like suddenly switched on, cleanly then it was too lean to operate.

Cheers,

[markreimer]

Hm I'm not sure that I would say it burbles back to life. I tried breaking open the airbox thinking that maybe it was dying because when I blip the throttle it can't suck enough air in at that rate, so opening the box up would reduce the air flow resistance but it didn't seem to make any difference. Only thing I noticed was it was louder when I blipped.

You can hear it on this video I made, at the 6 second mark:

youtube.com/watch?v=wBNecJ0lMA0