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Author Topic: Carb problem on yamaha xs750  (Read 573 times)

Offline matt.k

  • Posts: 13
Carb problem on yamaha xs750
« on: Sep 01, 2018, 22:58:28 »
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.

Offline TheCoffeeGuy

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Re: Carb problem on yamaha xs750
« Reply #1 on: Sep 02, 2018, 11:06:49 »
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.
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Offline matt.k

  • Posts: 13
Re: Carb problem on yamaha xs750
« Reply #2 on: Sep 02, 2018, 15:03:23 »
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?

Offline der_nanno

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Re: Carb problem on yamaha xs750
« Reply #3 on: Sep 02, 2018, 18:05:13 »
In short: yes. Try running the bike with the fuel cap open, if the problem persists, it's gotta be something else.
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Offline matt.k

  • Posts: 13
Re: Carb problem on yamaha xs750
« Reply #4 on: Sep 02, 2018, 18:37:07 »
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. 



« Last Edit: Sep 02, 2018, 18:42:46 by matt.k »

Offline matt.k

  • Posts: 13
Re: Carb problem on yamaha xs750
« Reply #5 on: Sep 02, 2018, 21:59:09 »
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?

Offline datadavid

  • Posts: 1371
Re: Carb problem on yamaha xs750
« Reply #6 on: Sep 03, 2018, 01:00:19 »
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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« Last Edit: Sep 03, 2018, 01:03:03 by datadavid »

Offline matt.k

  • Posts: 13
Re: Carb problem on yamaha xs750
« Reply #7 on: Sep 03, 2018, 12:25:42 »
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.

Offline matt.k

  • Posts: 13
Re: Carb problem on yamaha xs750
« Reply #8 on: Sep 03, 2018, 19:01:25 »
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!
 

Offline datadavid

  • Posts: 1371
Re: Carb problem on yamaha xs750
« Reply #9 on: Sep 04, 2018, 04:19:48 »
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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