Engine cutting out / dies on throttle

Fingerscrossed

XS650 Enthusiast
Messages
48
Reaction score
30
Points
18
Location
Sydney, Australia
Hi, I've posted a few times recently, so to go through what I've done:
Points, valves, timing, cleaned out tank, new coils and condenser.
I cleaned the '78 bs38's and put in new butterfly seals. I'm running one petcock into right side carb and using the fuel crossover tube.

I took it out this morning and it ran great for the first hour and a half, started first kick. Only thing was some popping through the left pipe, and it didn't sound as strong as the right. Anyway, about 1hr 45mins in the engine started cutting out at consistent throttle. I picked up some new spark plugs and put them in as I was going past the store. It seemed to work for a bit then came back. After that it died on idle, going from sounding really strong to nothing, and it only got worse from there. More cutting out and then in the last 10 mins from home it would die on throttle and then start right up, go for 20 seconds and die again, even just die when rolling and no throttle. Luckily I got back just as it got really bad but I'm not really sure where to start. I tried having the fuel cap open but that didn't do anything (I did take it apart and put that back together recently). I tested the battery when it started happening (before putting in new plugs) and it was 13.5V. On the plugs, the right side was beautifully tan while the left side (popping one) was a bit black.

I'm thinking it could be something weird with the oil: unfortunately, the bike was backed into by a car a little while ago and was left on its side for about 24hrs. Could it have been something has leaked where it isn't supposed to? and it only appeared when heated up?

Any suggestions would be appreciated!!
 

Attachments

  • 47C033CB-7EC8-4096-B64B-4CCAEEF87114.JPG
    47C033CB-7EC8-4096-B64B-4CCAEEF87114.JPG
    526.2 KB · Views: 52
Just to be on the safe side 13.5 can a good battery have without the machine is charging
Please measure charging voltage across battery at different rpm
And when hot and the problems is there

And then I would still focus on the left side having problems .
 
Ok, just checked the battery. 13.5V off and then 12.5V when ignition turned on. It started fine again, but while revving (don't have a tacho so not specific) the reading didn't change. I didn't hold for too long, but it just reading 13.49-13.5V accross all rpm.

I charged the battery before taking it out today as it's been about 6 weeks since it has been running. This same thing happened the last time it ran, where it started cutting out (I thought it was from rust blocking jets), but I changed spark plugs and it was all good.
 
Last edited:
Check and see what you have for fuel flow to the carbs. The filter in the tank could be plugged or if you have a inline filter it could be plugged.
 
Ok, just checked the battery. 13.5V off and then 12.5V when ignition turned on. It started fine again, but while revving (don't have a tacho so not specific) the reading didn't change. I didn't hold for too long, but it just reading 13.49-13.5V accross all rpm.

I charged the battery before taking it out today as it's been about 6 weeks since it has been running. This same thing happened the last time it ran, where it started cutting out (I thought it was from rust blocking jets), but I changed spark plugs and it was all good.

In my Book that is to Low It might not charge sufficiently or at all .. A voltage drop downstream from the 13.5 and it can be low at coils and so others please Chime In I would like to se 14 .2 .. ish
 
Ok good call, I'll go through the fuel line and also check if it runs any different when cold.

There's a battery testing store down the road so I'll bring it there to check, thanks!
 
Tested the battery and that’s healthy, I’ll have another look at the fuel line and change the oil just to rule it out (won’t be able to for a few days).

I don’t think it’s fuel related because it happened before I changed (and cleaned) carbs and replaced the fuel line. Also before I cleaned out the tank. Although when it happened 6 weeks ago, I just put in new spark plugs and it ran fine for the 3 hour ride. Hmm not sure.
 
Last edited:
Another update... I went through and trimmed the ends of spark plug cables, cleaned the connections to caps/coils, and sprayed some other connections with cleaner. It started strong on the first kick this morning but then died after two seconds. I started it again and it was idling fine with some throttle blips, but still seemed to be cutting out a little under consistent throttle. I'll try and take it around the block to make sure. The headlight was also getting brighter under throttle + the battery is healthy, so I think that means it's either ignition or fuel-related.

From this morning, it means it's happening when cold too. I can see fuel going past the inline filter, maybe I should try a T connection instead of just using the passage between the carbs? It was happening when I was using a different set of carbs and 2 filters though too, although maybe it was different issues with similar symptoms.

It's weird to me that putting in new spark plugs seemingly fixed it for a while before, but maybe it was a mix of things, I have no idea.
 
Not Yet seen any measurement of Charging Voltage measured across the battery at different rpm or have I missed it
Simple to do a couple of minutes

If weak spark the plugs can soot up replacing get fresh clean ones that soots up again
 
Ok! Some more info...
Not Yet seen any measurement of Charging Voltage measured across the battery at different rpm or have I missed it
Simple to do a couple of minutes

If weak spark the plugs can soot up replacing get fresh clean ones that soots up again
It's reading around 13.2V at idle and going up to 13.8 when revving (not sure on rpm, don't have tacho with me).

I pulled the new plugs and as seen in the photo, they are bad. They look a little wet? I think it must be something fouling the plugs, which is probably not carb related as I've changed those (although I have realised the '78s I got don't have O rings on the mix screw so I'll fix that). I've changed the coils, condensers, and points. It's getting spark, it shocked me when I was doing the test against the engine.

I took it around the block: a lot of popping from the left exhaust, and it started cutting out straight away. If I hit the starter, nothing would happen but if I waited 5 seconds and hit it, it would start right up again.

Thanks for all the help so far!
 

Attachments

  • IMG_7721.jpg
    IMG_7721.jpg
    535.3 KB · Views: 47
and for reference, here are the plugs I took out when it started cutting out yesterday. I haven't touched the mix screw on the right side since but was playing with the left due to the popping. Any idea why it would go from tan to black without adjustment?
 

Attachments

  • IMG_7723.png
    IMG_7723.png
    2.1 MB · Views: 23
  • IMG_7722.png
    IMG_7722.png
    2.3 MB · Views: 41
Ok, so I did some measuring but didn't get very far as I suspect something is wrong with the charging system.

I have one cap rated at 9k and one at 5k.

With the ignition on, I measured 12.55 at the battery and 10.4 at the brushes. 'The Yamaha XS650 Engine' book says anything under a .5v drop is bad, so I assume something there is bad... Would this cause spark plugs to foul? And the battery is charging, it just doesn't seem to be getting a hot enough spark to the plugs (they do seem a tiny bit wet, but they're a lot more sooty).

I was thinking about going with a PMA anyway, so I could keep going and measure the rotor, stator and rectifier but I think it would be better and cheaper in the long run to replace it all at once. What do you guys think? Or should I keep testing and not assume that buying the PMA will fix all my problems...
 
Ok, so I did some measuring but didn't get very far as I suspect something is wrong with the charging system.

I have one cap rated at 9k and one at 5k.

With the ignition on, I measured 12.55 at the battery and 10.4 at the brushes. 'The Yamaha XS650 Engine' book says anything under a .5v drop is bad, so I assume something there is bad... Would this cause spark plugs to foul? And the battery is charging, it just doesn't seem to be getting a hot enough spark to the plugs (they do seem a tiny bit wet, but they're a lot more sooty).

I was thinking about going with a PMA anyway, so I could keep going and measure the rotor, stator and rectifier but I think it would be better and cheaper in the long run to replace it all at once. What do you guys think? Or should I keep testing and not assume that buying the PMA will fix all my problems...
Voltage drop across the ignition switch? 10.4v isn't good.

Are the brushes worn? Before spending money on a PMA, I'd pop the stator off and clean the copper tracks on the rotor. Measure the resistance across the rotor to make sure it's in spec. Then refit the stator with new brushes. That might give better charge voltage and is only a few $.
 
Voltage drop across the ignition switch? 10.4v isn't good.

Are the brushes worn? Before spending money on a PMA, I'd pop the stator off and clean the copper tracks on the rotor. Measure the resistance across the rotor to make sure it's in spec. Then refit the stator with new brushes. That might give better charge voltage and is only a few $.

Since the charging voltage is low with the bike running it is something to look into Can be OK but still looks low
It's reading around 13.2V at idle and going up to 13.8 when revving (not sure on rpm, don't have tacho with me).

That also looks wrong should be the same perhaps the 9 k is bad
I have one cap rated at 9k and one at 5k.

PMA does not have a very good reputation the stock is better
 
Since the charging voltage is low with the bike running it is something to look into Can be OK but still looks low
It's reading around 13.2V at idle and going up to 13.8 when revving (not sure on rpm, don't have tacho with me).

That also looks wrong should be the same perhaps the 9 k is bad
I have one cap rated at 9k and one at 5k.

PMA does not have a very good reputation the stock is better
I agree with you. I'm bringing a 77 XS650D back from the dead and I am doing what I can to make the charging system as good as it can be. Likewise the points ignition system too. It's all very simple and should still be pretty robust if it's looked after properly.
 
Back
Top