GT750 weak idle related to headlight draw?

[Madbuffalo]

I've been working to solve a weak idle issue for some time that has developed slowly over several years. After sustained open throttle (can be 1/4 throttle on local roads or 1/2 throttle on expressway), and only when the GT is thoroughly warmed up, after I coast to a stoplight the idle will hold around 2500rpm's for about 10 seconds, then drop suddenly to 1300 and shortly after will begin wavering up and down +/-200rpm's each way. After a few low cycles the bike will stall. Partly to eliminate some possibilities I reinstalled the stock pipes and jetting and thoroughly cleaned the carbs... no change. Checked points and changed condensers... no change.

Yesterday I noticed with the headlight turned off the bike idles steady at 2,500rpm and when I turn the lights on idle drops immediately to 1300rpm's and wavers. Could it be a headlight draw issue causing low current through the ignition? The lights are generally always on which is why I've never noticed it before. Is the rpm change normal?

I'm using what I believe is a very good solid state battery that holds a strong charge and easily spins the starter. The GT is all stock right now and there are no other apparent issues.

Josh

[jabcb]

One of my T350s does something similar.
Don’t remember the exact numbers, but high idle is ~ 2.7k rpm & low idle is ~ 1.4k rpm.

Pull up to a stop with the clutch in & you get high idle.
Load the motor a smidgen by briefly letting out the clutch just a bit & you get low idle.
But it may eventually climb back up to high idle.

Its in my project queue & I don’t have an explanation.

Your problem may be similar in that the lights just add a bit of load.
Have no advice, just don’t assume its electrical.

[Alan H]

Check the battery voltage at tick over lights on and off. If it's a bit low fit some led running lights - on ebay very cheap - instead of using the headlight and it's likely sorted also check what rpm the battery/charging voltage goes higher, ie when the regulator operates. It may need tweaking a tad.

[Madbuffalo]

Thanks guys for the quick comments.

One of my T350s does something similar.
Pull up to a stop with the clutch in & you get high idle.
Load the motor a smidgen by briefly letting out the clutch just a bit & you get low idle.

Yes, that exactly. More frustrating is it also seems to waver then stall.

Alan, I will check battery voltage with lights on vs. off. Recently you guys helped me verify & improve operation of the charging system which was documented here. viewtopic.php?f=29&t=12235" onclick="window.open(this.href);return false;

Jabcb, Is there a way I can test head/tail light current draw vs. what it should be? I have a multi-meter with 'current' function but my experience using it has only been to check voltage & resistance.

just don’t assume its electrical.

I've been doing my best to reserve judgement but am running out of suspects. I suppose compression hasn't been checked in a few thousand miles so I could do that again. I did notice some carbon buildup on the exhaust ports and piston domes while changing the pipes so that could be contributing. I removed all the carbon 6 years/10K miles ago during the engine rebuild so with the idling issue slowly developing over the last few years that may seem to support carbon as a possible cause.

Then again maybe I didn't clean one of the idle air passageways thoroughly enough so carburetors should still be on the list. This one's been tough to diagnose guys.

[jabcb]

This is an interesting problem.

How long have you had the solid state battery? Do you have a link for it?
I’m wondering if the solid state battery is fully charged & is isolating itself from the electrical system.

A good place to start is a voltage check for both scenarios.
What happens to the voltage when it drops from high idle to low idle & then wavers.

Also wonder about a vacuum leak.

[akendall1966]

Don't forget turning the lights on loads the alternator So loads the engine through a magnetic field coupling between rotor a stator. For a given fuel input the engine is doing more work balance between torque developed and load has changed and engine rotates slower.

Modern engines have clever electronics to maintain steady idle rpm as conditions vary, our old bikes have fixed systems so will vary idle rpm under different operating conditions.

Weaker battery will put more load on the alternator to run lights too.

[tz375]

I'd wire in a relay to feed the coils to reduce the voltage drop across the switches and connectors. That way you can have a short wire from battery to relay to coils with reduced voltage drop.

[Madbuffalo]

Thanks all for the responses. Lot of things to check now so I will work through them when/as time allows.

Josh