yamaha xs750 runs great for 24 km then dies

  • 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…

    Today I went to start the bike after changing nothing and it it wouldn’t start. I figured if it was a temperature related issue with an old rubber component or a vacuum issue it would have sorted itself overnight. I checked the battery voltage on a charger and it was at 68%. I did try to start it both with the electric starter and kickstarter a few times after it died the second time yesterday, which I know drains the battery pretty quick.

    Could this problem be related to the charging system?

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


    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?

  • administrator

    hmm, this is a tricky one, I would do a few things

    1. check to make sure your floats aren’t sticking, this could cause a “too much fuel” problem.
    2. I would also do a carb sync to make sure they are all running consistently.
    3. check the plugs to see how they look, are they gapped properly?
    4. are all 3 cylinders firing? check to see if all three header tubes are getting warm.
    5. are your vacuum caps in good shape? plugged?

    if you get it running good again and can take it on another ride, try riding with the gas cap popped (make sure the tank isn’t super full so you don’t get spillage out the top) and see if it dies again at that 24km mark.

    I experienced something similar to the dying while riding problem, the gas cap on my bike wasn’t venting properly/quick enough for whatever reason and it would actually prevent the gas from flowing down into the carbs and would cause it to starve for fuel, it would then eventually equalize pressure and flow down and was able to start again.


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