Topic: advance timing mechanism - weak springs??

[Hodgeman82]

Hi,

Engine idles fine at startup and until warm but hangs at 3,000-4,000 rpms when throttling up after hot.  It then hangs there and no amount of messing with the throttle brings it back down.  Killing the engine and restarting it puts it right back where it should be.  I pulled the advance timing mechanism and there is no sticking and the weights snap back fine. Would weak springs be the problem?  My guess is engine heat creates more slop in the springs that is unnoticeable otherwise. Shutting the engine off would send the weights back to where they should be and drop the idle back down - correct?  Here is what I checked beforehand:

Throttle cables - fine
No air leaks
Timing set
Carb synch
Choke cable adjustment

Another symptom is the choke seems touchy at startup/warmup; slight variation in how far the knob is pulled out will cause rpms to vary widely and jump to 4,000 rpms, then almost have to push the knob all the way in to bring it back down, almost killing the engine unless pulled back out again.

Any help is very much appreciated - Thanks!
 

[Don R]

I'm thinking vacuum leak.

[dave500]

lean idle circuit,you might have partially blocked slow jets?i doubt your advance mechanism is the problem if it operates nicely,a frozen advance might cause it.

[Spanner 1]

Need to re-do the carb sync IMO. You did do a vacuum sync., right ? Also could one of your slides be installed back-to-front ??

[HondaMan]

Have you checked the timing with a timing light? If there is too much static advance, these engines will do exactly what you're describing.

[dave500]

it would have to be way over advanced to the point of kicking back on starting,i run these a few degrees over factory anyway.

[Hodgeman82]

OK, I just completed a dynamic timing check with a timing light as suggested and it is only very slightly advanced, which seems to be normal from some of the posts I've read.  I watched the advance mechanism as I twisted the throttle and it is at full advance at 2500 rpms.  Is this normal?  The manual says it should be at full advance at around 6,000 rpms. It is a CB750. I also resynched the carbs (vacuum sync}.  The bike is still doing the same thing, only after it warms up and the throttle is held over 3000 rpms for a short time - hangs there but goes away if I shut the engine down and restart immediately.  I sprayed starting fluid all over the carbs while running and did not see an increase in rpms when at idle - no leaks. Thoughts?
 
Thanks!

[Spanner 1]

Full advance @ 2,500 rpm is correct ( not 6,000 rpm  ).

[HondaMan]

OK, I just completed a dynamic timing check with a timing light as suggested and it is only very slightly advanced, which seems to be normal from some of the posts I've read.  I watched the advance mechanism as I twisted the throttle and it is at full advance at 2500 rpms.  Is this normal?  The manual says it should be at full advance at around 6,000 rpms. It is a CB750. I also resynched the carbs (vacuum sync}.  The bike is still doing the same thing, only after it warms up and the throttle is held over 3000 rpms for a short time - hangs there but goes away if I shut the engine down and restart immediately.  I sprayed starting fluid all over the carbs while running and did not see an increase in rpms when at idle - no leaks. Thoughts?
 
Thanks!

So long as it does not reach full dvance BEFORE 2500 RPM, it is OK. Most of these bikes have advancer springs that have become slack and/or annealed from the heat over time, and they advance too quickly. Very common.

Check your air screw settings. If it is a 1975 or eralier CB750 with non-PD carbs, these should be set at 7/8 to 1 turn out from (a relatively snug) closed position. The springs on these screws can make you think they are bottomed when turnig them in, so turn them in until they stop, back up slightly, turn in again. You might find that they are not yet bottomed. Then turn out 7/8 to 1 turn, no more. If you go further, it makes them too rich (yes, rich...) and the idle will be erratic and hard to control.

All this presumes the carb bowls are nice and clean: if the idle emulsifier tubes on either  the mainjets or the idle jets are plugged or even partially plugged, the idle will be tough to get to work at all.

[dave500]

pull the idle screws and run a cotton bud and compressed air gently in the hole to clear grime that has built up,,it might be stopping the screw from closing enough?