Charging system problem?

[JB Weld]

Almost two weeks ago I was riding my RD350 home from work when it starting running rough and backfiring. Had to rev the piss out of it to keep running as it wouldn't idle. Glad I made it home.
When home I noticed the left side exhaust was cold as the engine was backfiring.

Finally looked into it today. I have Dyna S ignition and thought maybe a screw vibrated loose and timing was bouncing all over on the left cylinder. Pulled the cover and discovered that's not the case. Tried starting after and would only "half" fire. Checked battery voltage and was just above 8 volts. Charged the battery and it fired up and ran great. Noticed my headlamp bulb is fried though. Battery voltage when fires was 12.83 volts and continued to drop to around 12.3 volts even while revved to 2500-3000 (with and without lights on).

Where do I start diagnosing this issue? How do I correctly verify it is charging properly? I know I've successfully checked the charging system prior to installing the Dyna IGN, but manuals I have in hand are not very helpful at the moment.

Thanks all

[1976RD400C]

   It sounds like you have tested and figured out the charging system is not working, low voltage and dropping while running. Check for voltage across the 2 brushes on the alternator with it running. If there is, then the regulator is at least on so make sure the brushes are making good contact on the rotor rings. If that seems ok, look at the diodes on the rectifier and see if anything looks burnt or bad. 
    The service manual should show how to test all the parts of the charging system with a meter, the rotor, stator, and rectifier. There is a good chance that if you still have the original mechanical type voltage regulator, that is not working right, but you must check the rectifier, rotor, stator, brushes too to be sure.

[JB Weld]

I have a Haynes which seems like it's not specific and also recommends having an "expert" evaluate the rectifier and other components.

I don't understand how I'm supposed to check the voltage regulator without the batter in to start the engine as stated in the attached photo.

Checked voltage at the brushes @3000 and had 2.8 volts.

[teazer]

There are a number of possibilities. 

The alternator generates Alternating Current which rises with engine speed.
The Rectifier changes that A/C into DC which is what our bikes use.The Regulator is there to keep voltage down to say 14.5 volts max.  Any more than that will fry the battery and bulbs.

There are two types of alternators on bikes - Permanent magnet and electro-magnet.  The RD are the latter.  That means that battery voltage is used to turn the rotor into a magnet.  The regulator switches the power to the rotor as required.

So, you need to determine which of those is faulty.  With electromagnet systems the usual problems are the Regulator/rectifier unit or bad connectors.  Sometimes a rotor will be bad or there's a break in the stator wiring but those are less common.

I would start with the regulator/rectifier.  I am not a fan of the OEM points type design, but they do work- for a few decades.  I would start by removing the R/R and replace it with a modern solid state unit. 

If throwing parts at a problem is not your style, Start by checking the voltage from the regulator to the rotor when the engine is running with a fully charged battery. 

Then you can test the alternator output which should rise with engine speed starting around 12v or so at idle, rising to 20-30v at 3,000 RPM.

You can also remove the rectifier and test each of the 4 diodes to be sure that it works.

If that's all  a bit much of an ask, start with a new combined regulator rectifier and see if that fixes the problem, but before you do anything, check all the leads and connectors just in case one is broken or corroded.

Also:  https://hvccycle.net/yamaha-rd-charging-problems/

[AAAltered]

I don't know about the 400 but the Yamaha R5 manual has an *18 step* process for troubleshooting the charging system.  Ditch your manual and get a Yamaha manual.

[teazer]

Page 88 of the manual figure 5-1-2 shows the resistance values for the stator and rotor.  It does have the regulator and rectifier mislabeled though.  They got them back to front.

It is pretty comprehensive though.

[1976RD400C]

There should be more than 2 volts at the brushes, especially if the lights are on. You might want to carefully clean the contact points in the regulator. If they are clean there should be no resistance and you can check that with a ohms meter touching on each side of the points (with the plug disconnected from the reg)

[AAAltered]

Here's a link to the 350 manual with the step by step check for the charging system.

https://www.manualslib.com/manual/1091928/Yamaha-1970-Hs1.html?page=30#manual

[JB Weld]

Thanks all for the recommendations! I like the link with diagnosis information.

Also like the recommendation to replace regulator/rectifier with a new/modern option. This stuff is obviously original and I am never against "cheap insurance" by replacing old stuff.

Economy has a replacement I will buy, and will try to source new brushes for the charging system as well. The negative brush is just barely above the minimum 6mm.

If I get some time this weekend I will be doing some of the electrical tears that I haven't performed.

[JB Weld]

Lesson learned! Dig into issues as far as I can!

I was lazy (and busy) about diagnosing my charging issue. Bought a Vape and now I'm pretty sure I know the root cause of the problem. A screw is completely gone from the rotor and another next to it was wedged partially out and scraping on the stator housing.

[m in sc]

ouch! yup.. thats it.

[rodneya]

You wont regret going to the Vape system. Just make sure you take it apart and blue locktite all the bolts including the small pickup ones.

[JB Weld]

Almost done with the install. Been using Chuck's writeup as a guide and definitely adding lock tite. The adapter to base plate bolts actually had some type of lock tite already, but the three bolts holding the coil were not real tight. Glad this forum still exists.