1981 Seca 750 -- erratic idle

Hi everyone:

I'm posting on behalf of a friend who has a 1981 Seca 750. His bike has erratic idle. It will run fine while riding all day, but sometimes it idles at high RPM and at other times it will stall when stopped.

He says he cleaned up the carbs and he messes with some sort of screw at the centre of the carb assembly. He says he wants a new set of carbs but I'm not convinced another set that he knows nothing about would help him.

Anyhow, what could be the cause(s) of this problem so I can suggest them to my friend?

Any help is greatly appreciated.

Cheers,
Chris

 

New Carbs?
That's crazy!
The ones he has will work fine if they are Clean and Tuned-up right.

Offhand, I'd say he needs to have the Carbs Cleaned and set-up right.
Clean. Really and truly cleaned. Every component.
Float Heights set correctly.
Bench Synced.
Pilot Mixture Screws Pre-set at 3-turns Out from the Bottom.
Get the bike running.
Sync gauges for a Vacuum Sync.
Colortuned Pilot Mixtures for a steady Idle and off-idle performance.

The 750 Engine wants to be tuned a bit on the Lean side. Or at least the one I'm running does. (Maybe its because of the Oil Cooler ... it might need the additional heat in the Combustion Chambers.

Have him tell you when the last time the Carbs were off and cleaned.
If he doesn't know ...
It's time.

 

Can't just "mess with" carbs. Need to be "white room" clean and set up right.

The some screw in the centre is idle Speed, adjust only when bike is at normal operating temp (ride it for 20 min Don't idle to warm up). Check for free movement of throttle grip and cable/linkages and kinks in cable.

Sync butterflys, adjust mixtures. A new set of carbs will need to be cleaned and set up too! Might as well set up the ones he has.

Check for air leaks too (wd40 spreyed on intake boots and joints no change in idle = no leaks).

 

I had a poor idle because because of weak coils and one of them was cracked, of course if the carbs are messed up that will do it too. Have him clean and adjust the carbs according to the manual then check the coils and wires for cracking and replace as necessary. If the problem persists monitor the ignition pulses with a spark tester to make sure they are firing properly.

 

Chris, welcome to the house of fun! Those carbs should be scrubbed out and set up. You've gotten some great advice from the previous posters so I won't repeat it. If you could post your area, there might be some folks nearby to you who could help you out.