Charging system mystery!! Professional help needed!! it's....complicated

Hey guys...

Really need some help trying to sort out my '79 XS400. I've had some major issues along the way since day one with this machine and I'm close to calling it quites. If I don't soon my wife is going to force it!!!

Background:
I bought a 1979 Yamaha XS400 with 18000 kms on it. Has never run entirely smooth but it was decent when I got it. In the month before I got it, the owner had a local shop put in a new battery, new starter, carb rebuild, carb sync and a new petcock.

From the first day that I had the bike it was hard to start. Super weak on the electric start, usually had to kick it. And the bike would never start with the choke out. That made it die. Eventually it would start. 3 months in, the bike started running super rough. My mechanic pinpointed the charging system and said that the voltage regulator was shot. We replaced it and a week later the new one was shot!?!?

Before I had bought the bike there had been some sort of meltdown on the bike. There were a lot of melted wiring connectors and even the air filter holders were melted. I figured that the regulator failed, the battery boiled over and the harness got fried! Then the previous owner got out the electrical tape and called it good. It seemed as though there were shorts to ground possible everywhere. So I decided to rewire the bike using a used wiring harness which I refurbished. I checked every connector and every wire and fixed anything that needed it. I also replaced the voltage regulator/rectifier with a unit from Rick's Motorsport Electrics.

I started the bike up and it still ran like crap!??!?! I went through and checked everything again. I got a new battery. Checked the voltage R/R, field coil and stator. Static tests and field tests where possible. R/R checked out good. Field coil magnetized. Stator resistance checks where 1.2 Ohm between each phase consistent and the stator was not grounding out on the bike. But then...I turn on the bike and while it's running at 2000rpm the volts only go up to 12.5V. I take the bike for a ride and the highest volts I can achieve while running is 13.5V. It should be 14.6-14.8V.

I decide that the stator must be faulty. I change the stator and test it. While I'm riding I can get the volts up to 15.0 and it stays there. When I stop, the volts drop down to 12.5V and as I idle down to 12.1V. When the volts are reading 15.0V the throttle is more responsive. When the volts drop, if I crank the throttle...the bike surges and backfires if I crank it too hard. Then as I come to a light it wants to stall out sometimes. The bike is bad in stop and start traffic and takes a bit on a nice straight stretch for the volts to get back up to 15.0V.

Now I'm wondering...is my new battery damaged? Is my stator bad? Or is there something else in this equation that I am not thinking of unrelated to the charging system? The points were set last spring and the compression was good last spring. The bike does not use any oil. Is the surging and backfiring all charging system or something else? Does the coils factor into this? Are the plugs and wires bad? I changed the spark plug caps? Are they a part of this?

Should the volts easily hit 14.8V at 2000rpm or is it normal that the bike needs to drive a bit before hitting that charge? Should I be able to just turn on the bike and once it is warmed up and in neutral, be able to rev it up to 2000rpm and immediately see the volts jump up?

I just don't know what I'm missing in all of this? I don't know enough to figure this out on my own. Any help would be appreciated!! Thx

I know nothing about electrical,...but stalling/backfiring...timing Perhaps?

the rough running doesn't seem like a charging issue to me.

It is normal and expected that these bikes don't produce full charging voltage at 2000rpms. Mine has to be at 4000+ rpms to get above 14.0V and from what I have read that is normal. So 12.5V at 2000rpms is not an issue, however I think 15.0 volts is on the high side, but I don't recall off the top of my head what the manual says. You should check the manual for that info if you haven't already (downloadable in the top stickied thread in this subforum)

As for the rough running I don't have much input other than this: you need to look into carbs first, for cleanliness and also for settings. Does the choke hinder the bike from running when it is cold, or just when it is warm? If it hinders cold running then I would be especially wary of trusting that the shop rebuilt the carbs/set them properly. To be honest, unless you really know and trust the previous owner I would take all the work they claimed has been done to the bike with a grain of salt. Anyway the carbs need to be thoroughly cleaned; every orifice, port etc and then set back to stock (assuming your engine has stock air filters and exhaust, if not then settings may vary.) and also should be synced (I know the PO claimed the shop did this, I don't have much faith in their work based only on reading this thread).

good luck and let us know how it goes!

Yeah, I know the carbs always raise my eyebrows. I know that the shop rebuilt them because the owner had the bike in the shop when I got it. I spoke directly with the shop and since then had another mechanic ultrasonic clean them and sync them. Then after the winter (I had fuel stabilizer in), I ran sea foam through, then ended up taking apart the entire carb and manually cleaned it and checked each jet for factory settings. Then I had my mechanic re-sync it. Is there an unusual gremlin that lives in these carbs that all of us have missed? Maybe a common passage that is easily overlooked? Super perplexed

Sounds like the carbs should be fine. If something isn't running right I generally don't trust other people work unless I have had my eyes on it. But it sounds like you have for the most part. It isn't too hard to make you own manometer and sync it yourself, I would suggest that.

As for passages that are commonly overlooked, I haven't read of any. Hopefully someone who has been feeling with these carbs longer than I can give more details on that.

If the carbs are ruled out and it is still backfiring I would look to spark and timing as suggested above and perform a regular maintenance routine in the points.

Valve clearances can contribute to rough ruining (and really should be checked regardless) as well as poor compression in one or both cylinders. I think it would be good to check compression again do you know for sure what you are working with.

If you set a '79 back to factory carb settings, it has a tendency to run lean. Set the floats at about 26mm and the idles mixture screws to 3 turns out, and see how it runs.

HoughMade said:

If you set a '79 back to factory carb settings, it has a tendency to run lean. Set the floats at about 26mm and the idles mixture screws to 3 turns out, and see how it runs.

Click to expand...

Can I set the float heights without removing the carbs? Cause then i would have to re-sync the carbs! I don't have a sync tool and I just had them done by my mechanic. What is the factory setting for the idle mixture screws? 2.5 right? thanks a lot

Theoretically, the float height can be changed with carbs in situ, but you would have to sync them nonetheless. However, with a "bench sync" (making sure the throttle butterflies are set the same) the bike should run pretty well. Surging and backfiring sound to me like a lean condition which could be settings or dirty carbs, or both. I know the factory setting on a 400F are lean- I have one. My bike has always run best when it was set richer than factory.

The easiest thing is to try turning the idle mix screws out to 3.5 turns (all the what in lightly then out 3.5 FULL (360 degree) turns). Do this before touching the floats and see i it makes a difference. Idle becomes extremely finicky when it's running lean. If is runs well at that setting, call it good.

I notice that your bike has non-stock mufflers. This can cause tuning issues also.