Hello everyone, I'm Lennart and I'm from Germany. I have a 1975 250B imported from the USA for a few days now and I'm a total Yamaha newbie. I have the problem that despite a new battery and an intact fuse, some parts of the electrical system do not work. The bike can be started and the horn works. However, the brake light, indicators, front light and speedometer light do not work. The clutch has also stopped working since the bike was transported. The lever on the engine is pulled by operating the hand lever, so the Bowden cable is intact. On the clutch side, you can also hear a clicking noise, but the clutch does not disengage.Fortunately, I can download some useful diagrams from the great database here, as I couldn't find them anywhere else. I will remove the side cover to have a look at the clutch. It can only be something small, I did a test drive just before and it worked. But not since the transport.  Because of the electrics, I thought I might have a look at the ignition switch. Or do you have any tips? :-) 

Lennart

 from near Salisbury, UK.

First thing I would do is to check the bulbs in each of the items that you mentioned. In fact, check that all of those bulbs are present!

1975 bikes should, I think, have the voltage regulator fitted as standard, but worth checking that your bike has one.

How comfortable are you, in general, with testing electrics?

If you want to post photos, then you need to use a third party host such as imgur.

I'm pretty sure 1976 was the first model year that a magneto enduro came with a voltage regulator in the US. 

The brown wire powers the horn and other battery-powered accessories so checking the bulbs as recommended is great advice. 

The chances are great that the tail/brake and headlight/gauge illumination bulbs are blown if the battery was bad or disconnected and the engine revved....  (signals might be a different issue)

Note that headlight and gauge illumination lights only work with engine running.

Thanks for confirming that, Mark.

Could be that a new headlight might be needed.

The online combined "digital" 75-76 250-400 b-c shared parts lists could be a source of the confusion of when the voltage regulator became standard?

Anecdotal evidence that the 1976 model year was first at having a regulator can be found:
*In the wiring diagrams, 1975 models have no regulator shown
*Yamaha Service Bulletin 05-84 "All Pre-1976 A.C.Magneto Models Voltage Regulator" (instructions for adding a regulator)
*Yamaha (paper) Parts Book for the 250/400 B-C shows the regulator and associated mounting hardware in the "C" model columns only.

It is possible that some very late-production 1975 models out there came with a factory regulator... (not terribly likely as usually Yamaha would have a bulletin with a starting VIN published for a fairly major model change like that.)

Hi! You were right, the bulbs in the front and rear lights were broken. The flasher relay was also broken, at least the left side flashed with a different one. The right side still doesn't work, I'll check for a broken cable or defective switch.
Regarding your question Gr8uncleal, I should have figured out the faulty bulbs myself I have a very basic understanding. But there are better topics for me, its rather annoying.
Next time I'll check the clutch. In the service manual draining the oil before dismantle the cover plate is missing, but the oil filler opening tells me that the clutch has lubrication.  At best, the disks are gummed up and I just have to clean them. I will see if the push rod separates the disks.
I will also measure the disks and springs.
According to the repair instructions for the gearbox, SAE 10W30 oil is suitable for refilling. The same is also used as fork oil?

Another question :-) The rubber flange between the carburetor and the diaphragm is very porous. I can hardly find any offers for the 250B, and when I do, these parts are extremely expensive (approx. 200 euros)
Is there an alternative or are these parts available outside Europe at a reasonable price or will another flange fit?

Hi Lennart,
welcome to the forum from Germany (Schwerte)
The rubber intake manifold, you can purchase as a reproduction here www.claussstudios.com/store/c49/Yamaha.html
Including customs and shipping, you should end up at around 120€.
Regards, Guido

Hey Wessi, When I read your name, I probably should have called myself Ossi (Dresden)  Thanks for the tip and viele Grüße nach Schwerte!  
So I dismantled the clutch and cleaned the plates. The plates are 3.00 mm thick, so they are as good as new. The springs are 35.5-35.7 mm thick, which is borderline. But I think they would lead to the opposite problem, the clutch would slip.
I have fitted everything and topped up the oil. Unfortunately, the clutch is still not disengaging properly. When you pull the clutch lever it doesn't disengage, only when you push the bike backwards and forwards with the gear engaged does it start to disengage. 

So the next thing I tried was to adjust the clutch. I think this could be the problem: The lever on the underside of the engine is not in the right position. It is in front of and not behind the main bolt. I loosened the locking screw and used the locking screws on the Bowden cable to adjust the lever so that it is slightly behind.
The instructions then describe how to unscrew the eccentric bolt until the clutch push rod makes contact. But don't you have to screw the eccentric screw in? That doesn't make sense to me. So I first unscrewed it, then screwed it in again until I felt resistance. I followed the video below.


However, I now have a lot of play in the clutch cable. When I remove it using the locking screws, the lever on the underside of the engine sits in front of the main bolt again. 
I made a photo of the situation:
ibb.co/GQWkKV0
So maybe i didnt understand the manual correctly?

That video has nothing to do with your model.  Completely different system.

The eccentric screw raises and lowers the entire shaft and lever on your model.  The complete range of adjustment is achieved with only 1/2 turn of the screw!

First you must make sure the pin on the screw is engaged in the slot in the shaft.  Moving the shaft up and down slightly as you turn the screw is sometimes needed.  Once the pin engages, then turning the screw will raise and lower the shaft and arm. 

Maximum freeplay is when the shaft and lever is up inside the case as far as it will go...  turning the eccentric will drop the lever down out of the case and reduce freeplay.  After 1/2 turn on the adjustment screw from maximum freeplay you will be at minimum freeplay.  Turning the screw more will start to increase freeplay again. 
 

Whatever model were talking, the hub must be able to turn freely without the plates in because on some models it's easy to accidentally trap things behind the spacer through the middle & lock it up from free wheeling when hub nut is tightened.