Carb problem on yamaha xs750

[matt.k]

I have, what is as far as I can tell, a late 78 xs750. I have recently done some basic restoration work in getting it running and ridable... or so I thought.
I had it out for a maiden voyage a few days ago and it ran great... for exactly 24kms. At which point it sputtered and died while running at about 70km/h and 4500rpm. Seemed almost like it ran out of gas, but the tank was brimmed. Loaded it into a truck and went home.

That afternoon I pulled the carbs, petcocks and filters and all the fuel and vacuum lines. I did the white vinegar method on the inside of the tank ( which had been recently re-painted and cleaned, and looked pretty clean inside still ) and let it sit for two nights. all the jets and carb components, petcocks and filters were nearly spotless.

Today I reassembled everything and went for another test drive. Literally exactly 24kms down the road, same exact thing.

I let it sit for about 25 min on the side of the road, one kick and it fired right up and as far as I could tell it was running fine, perhaps a wee bit boggy in the lower rpms. Pulled up to a red light on the way home and as the rpms dropped to idle it sputtered and died. Let it sit again for a bit however this time it would start but run really rough and would need a fair amount of throttle to even stay around 1500 rpm. Again trucked it home and spent the entire rest of the day looking for answers with nothing remotely definitive.

Could this be an air leak in a carb boot that gets to a certain temp and leaks enough air to lean out the mixture?
Could it be related to old vacuum lines?

All the rubber parts to the carb including boots and everything are stock as far as I know and are worth replacing despite not showing a lot of wear and no obvious cracks. I am worried however the problem is much larger...

Any help or tips on places to start looking would be greatly appreciated.

 

[TheCoffeeGuy]

Sounds like your gas tank cap is sealed. creating a vacuum after a few miles. either make sure the cap is vented or that there is other breathing tube for your tank.

 

[matt.k]

Thanks for the tip, I would not have thought of something like that.

The only "holes" in the tank are for the two petcocks and the filler cap, which I believe is all stock. When I was flushing the tank I do remember that if it was tipped upside down it would leak a bit. Is it possible that it would seem to be vented and then become sealed while driving?

 

[der_nanno]

In short: yes. Try running the bike with the fuel cap open, if the problem persists, it's gotta be something else.

 

[matt.k]

The weather is awful and pouring rain today so I was unable to get out to ride. I did however take the fuel cap off and have a look at it, the rubber seal is very shot so it will need replacing for sure. Am I right in assuming that since this seal was shot it was most likely not sealing anyway? The vent port actually looked alright.

Is it possible that the torn rubber seal is the problem? Its in pretty rough shape and I cant see how it would be sealing air tight in any way against the tank and causing a vacuum in the tank.

Also today in my deductive troubleshooting I tried to start the bike before changing anything from parking it after it died. (thinking that if the problem was temperature or vacuum related, sitting overnight would solve it.) It wouldn't start. At this point I took the fuel cap off and inspected it, put it back on and no change.

I then checked the battery voltage, (cause the starter would only click and not turn over), it was low. So I hooked a charger to it and it read 68%. I did however try to crank it several times both kickstarting and with the starter after it had broke down which I know drains the battery pretty quick. In hind sight I wish I loaded it straight into the truck/ checked the battery voltage right after it died. Its on the charger now and Ill see if it will start once charged, but I am guessing it will fire up no problems.

Could the bike dying after what seems like a specific amount of time be related to the charging system?

Seems strange to me because I was able to get it re-started with one kick, twice, after leaving it sit for a while after it originally died. Electronics are by far my weakness and while I understand the principle of how the charging system works, its intricacies and troubleshooting it remain foggy to me.

 

[matt.k]

UPDATE

First, once the battery was back 100% charged again this afternoon it fired up after only a few kicks. With the key off the battery had 13.2 V and when it was running it was reading 12.6 V. not sure if this is good or bad.

Second, it was running pretty rough, backfiring out of the carbs every 15-20 seconds or so. I don't recall this happening ever before and means its running rich correct? It died when I pulled the choke on fully once it was idling. (with the bike idling the choke always slips to the halfway catch, which it seems to like better anyway)

Due to this I am back to thinking that it is a carb issue with the jets or something, and that the battery was too dead to start it today due to the fact that I tried to revive it several times once it had died on me yesterday. sound logical?

 

[datadavid]

Your bike is not charging and therefore dying when battery gets too low. Leave the carbs alone and aim for 14 volts with bike running.
The fact that you dont know if its good or bad that it shows 12 volts when running is good. Then the good people here know which level to explain on!
I still cant get my 78 to charge right..
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[matt.k]

Fantastic, that makes sense thank you for the reply and giving me a direction to start working in to solve it. Sadly that direction is the one I am least comfortable in...

I will put in some good time online looking into trouble shooting the charging system components and hopefully get somewhere.

Ill keep checking back to this post and updating so if someone has tips on sorting through problems with the generator system that would be greatly appreciated.

 

[matt.k]

UPDATE

I checked the battery voltage again, after watching the full lot of charging system videos on motorcycle MDs youtube channel and knowing a little more about what I was looking for.

Before starting: 12.7V
Idle: 12.3V
2000RPM 12.2-3V
3000RPM 12.2-3V

All this as per my knowledge means a charging system that is not functioning, correct? Now to trouble shoot the REG/REC, stator wiring and the stator itself.

Any where specific I should start or most likely points of failure?

Thank you again for the replys, I feel as though I am pointed in the right direction and the help is much appreciated!

 

[datadavid]

The first check to do is take a size 10 ring spanner, hold it to the generator cover and turn on the ignition. It should magnetize the rotor enough to hold the spanner onto the cover. If not - your regulator is not working. If your bike is completely stock all connectors to the charging components should be behind the left hand sidecover. Usually sprayed with battery acid from the battery vent hose. Inspect and clean those first.

UPDATE

I checked the battery voltage again, after watching the full lot of charging system videos on motorcycle MDs youtube channel and knowing a little more about what I was looking for.

Before starting: 12.7V
Idle: 12.3V
2000RPM 12.2-3V
3000RPM 12.2-3V

All this as per my knowledge means a charging system that is not functioning, correct? Now to trouble shoot the REG/REC, stator wiring and the stator itself.

Any where specific I should start or most likely points of failure?

Thank you again for the replys, I feel as though I am pointed in the right direction and the help is much appreciated!

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[matt.k]

Fantastic I will have a go at that today and post an update. Thanks!

 

[matt.k]

UPDATE

10mm spanner magnetizes to the right side engine cover no problem.

3 white wires from stator harness all have 0.6-0.7 Ohm between them in all combinations. Manual says 0.48 Ohms +/-10% at 20dge if I remember right from what I read yesterday. So it would appear I am a bit on the high side, how bad is that?
As well none of the 3 white wires are grounding.

Everything I found online says that that harness should plug into the REG/REC which I assumed was the one square-ish unit mounted to the frontside of the battery box that has what looks like cooling fins on it. However the plug from the stator with the three white wires (and a yellow wire) plug into a small gold relay looking unit mounted behind the battery box on the left side.

What is this called? I cannot seem to find anything out about it... is plug even plugged into the right spot...?

 

[datadavid]

UPDATE

10mm spanner magnetizes to the right side engine cover no problem.

3 white wires from stator harness all have 0.6-0.7 Ohm between them in all combinations. Manual says 0.48 Ohms +/-10% at 20dge if I remember right from what I read yesterday. So it would appear I am a bit on the high side, how bad is that?
As well none of the 3 white wires are grounding.

Everything I found online says that that harness should plug into the REG/REC which I assumed was the one square-ish unit mounted to the frontside of the battery box that has what looks like cooling fins on it. However the plug from the stator with the three white wires (and a yellow wire) plug into a small gold relay looking unit mounted behind the battery box on the left side.

What is this called? I cannot seem to find anything out about it... is plug even plugged into the right spot...?

Those .2-3 ohms may well be error margin from the multimeter, as long as they are even and not grounding out it should be good. Three white wires can be disconnected and measure output with running engine between wires. Should be 60-70v AC iirc. Gold relay looking box sounds wrong, might be that rectifier part fried and some p.o installed something else, unclear what..
Sounds a lot more like the mechanical regulator part from earlier xs750's..
should have cooling fins if its a rectifier unit. From there it should send positive and negative dc to battery. Does it have two wires coming out of it besides the three white ones?

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[matt.k]

It has three whites and a yellow wire all from the same harness going into that gold relay looking unit, which has a rubber holder and everything and fits into the battery box. Looks stock to me. I’ll get a photo today and post it. No wires come out of this unit and head anywhere else which seems wrong... unless it’s sending back to the right side cover area via the yellow wire? I also have a cooling fin unit on the other side of the battery box that looks exactly like what a reg/rec unit on google. Could something just be plugged in wrong?
I’d have to double check but I believe the black and green wires from the generator head to the rectifier unit and in the wiring diagram from the manual it makes it look like two different units...

It seems to be wired up as per the first diagram but has a unit that looks like that in the second diagram (which is the 2D)


I will take some photos this afternoon and post them

 

[datadavid]

Something sounds very off! The black and green Should go to the regulator component, which looks just like a relay box..

It has three whites and a yellow wire all from the same harness going into that gold relay looking unit, which has a rubber holder and everything and fits into the battery box. Looks stock to me. I’ll get a photo today and post it. No wires come out of this unit and head anywhere else which seems wrong... unless it’s sending back to the right side cover area via the yellow wire? I also have a cooling fin unit on the other side of the battery box that looks exactly like what a reg/rec unit on google. Could something just be plugged in wrong?
I’d have to double check but I believe the black and green wires from the generator head to the rectifier unit and in the wiring diagram from the manual it makes it look like two different units...

It seems to be wired up as per the first diagram but has a unit that looks like that in the second diagram (which is the 2D)


I will take some photos this afternoon and post them

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[matt.k]

Just was doing some more looking online at what that gold unit could be and stumbled across this... which is a blurry photo but I’m 99% sure that is exactly what the plug coming from the stator with the three white wires is plugged into...

 

[matt.k]

UPDATE

alrighty, so it was plugged in wrong... stupid.

The photo below is the gold relay that the stator harness was plugged into. I found the plug with 3 white wires coming off the Reg/rec unit and swapped the plugs around...

Now it seems to be charging great so that’s a plus. On the downside the gold relay has a good rattle to it when I shake it so I’m assuming it’s trash. Also now the headlight doesn’t work or blows the 10A fuse almost instantly when the key is turned on, which is new. I took the gold relay to a shop and asked what’s what and they told me it comes up as the flasher relay.

BUT the signals work just fine. Always have. So I’m guessing that it’s a relay for the headlight and cause it’s broke the fuse is blowing...? Does that make sense?

I attached some photos of the relay and in the background is the holder for it. The second photo is of the plug that was swapped with the stator plug so is now plugging into the relay. I can’t find anything in either wiring diagrams I have that match or look like this plug at all, any ideas?

 

[datadavid]

Maybe someone stuck a flasher relay in place of the headlight relay, seen worse!

UPDATE

alrighty, so it was plugged in wrong... stupid.

The photo below is the gold relay that the stator harness was plugged into. I found the plug with 3 white wires coming off the Reg/rec unit and swapped the plugs around...

Now it seems to be charging great so that’s a plus. On the downside the gold relay has a good rattle to it when I shake it so I’m assuming it’s trash. Also now the headlight doesn’t work or blows the 10A fuse almost instantly when the key is turned on, which is new. I took the gold relay to a shop and asked what’s what and they told me it comes up as the flasher relay.

BUT the signals work just fine. Always have. So I’m guessing that it’s a relay for the headlight and cause it’s broke the fuse is blowing...? Does that make sense?

I attached some photos of the relay and in the background is the holder for it. The second photo is of the plug that was swapped with the stator plug so is now plugging into the relay. I can’t find anything in either wiring diagrams I have that match or look like this plug at all, any ideas?

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[clem]

That is the correct headlight relay. This keeps the headlight off until the bike is running.

Sent from my SAMSUNG-SM-G930A using DO THE TON mobile app

 

[matt.k]

Okay awesome thank you! Would the faulty relay be the reason the headlight fuse blows when the key is turned on? Or do I have some other electrical mystery to solve?

Also I am guessing I will need to replace that relay with a duplicate, not just any relay?