Simple Carb Mixture Screw and Sync Adjustments

[5twins]

The mix screws are mainly a low RPM and speed adjustment. Granted, they do keep flowing throughout the RPM range but I doubt they would stop the motor from revving past 4K. Dirty carbs with plugged up jets and/or passageways might, or bad diaphragms on the slides (holes in them).

If you haven't cleaned the carbs, the thing to do as a first step would be to drop a float bowl, carbs still mounted and gas still in the bowl. Have a look inside and see what you see. If the bowls have gunk in them or bits of dirt floating around, chances are the rest of the carbs are gummed up too.

 

[leodr]

The mix screws are mainly a low RPM and speed adjustment. Granted, they do keep flowing throughout the RPM range but I doubt they would stop the motor from revving past 4K. Dirty carbs with plugged up jets and/or passageways might, or bad diaphragms on the slides (holes in them).

If you haven't cleaned the carbs, the thing to do as a first step would be to drop a float bowl, carbs still mounted and gas still in the bowl. Have a look inside and see what you see. If the bowls have gunk in them or bits of dirt floating around, chances are the rest of the carbs are gummed up too.

OK. thanks for your quick reply.
I just took tank off and put where seat was just resting there.and I started up bike. It started OK with choke part way up and idled at 2000 for a while then lowered choke and it went down but not too much above 1500. Left it running for awhile so I could do adjustments to mixing screws and then synchronous . I am lucky that before these I can get at and screws are functional.. they move. I was about to start and then the bike started to rev by itself to 3000 rpm and bike started to shake on centre stand and was continuing to rev and I was afraid it would shake loose gas tank resting where seat was and bike was moving so I shut it down. This is something seriously wrong here. I will look at floats but I think I should remove the entire carburettors and do a visual examination to cover all bases. Want do you think?
Also what are those hollow tubes on both sides of the carburettors just above float bowls? Are there opening there with screws ... seems there is some dirt inside there . Is there something there that I should know about?
Leo

 

[pjc750]

Great post!
Once set and running right, I like to turn me head a bit while cruising along and listen to the pulse and compare the left and right sides to each other.......yeah, still good!

I like this ol' thread. Mine bike is an 81 xj750. That said...my bike's exhaust on right produces a quiet smooth tone,even pressure( no puff puff)...but left exhaust has always run diff. The tone is lower, louder and somewhat uneven . Is it a bal or mixture setting or timing. Valves were done 2 yrs ago with no real effect.
Thanks guys
Patrick

 

[DogBunny]

I like this ol' thread. Mine bike is an 81 xj750. That said...my bike's exhaust on right produces a quiet smooth tone,even pressure( no puff puff)...but left exhaust has always run diff. The tone is lower, louder and somewhat uneven . Is it a bal or mixture setting or timing. Valves were done 2 yrs ago with no real effect.
Thanks guys
Patrick

First, I know nothing about XJ750s. Second, there are dozens of things that could be wrong, for example poor compression on the left. But, if the mix screws and sync screw are easily accessible, experimenting with them is easy, so I'd start there. With bike idling, try turning the left side mix screw a half turn out, and then a half turn in, and try turning the sync screw a little bit in each direction.

 

[jpdevol]

Is it a bal or mixture setting or timing.

It was not unusual for those bikes to be extremely lean from the factory. If the blind plugs haven't been removed from the fuel screws, it makes that more likely. They can generally be helped by a richer adjustment - then put a set of vacuum gauges on 'em and synch them again. But, as Dogbunny says: could be lots of things at this point