1976 XS650 Starving for Fuel?

Wilsonsk

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I've been working through jetting for pods and pipes, and throughout the process, as well as before, it seems if I run at 70 mph for any length of time, it starves for fuel after a couple minutes. If I slow down or stop, it may die, it starts right back up but from then until I park it, it runs rough, like its running out of gas. I have aftermarket petcocks, they had plastic screens on both the ON standpipes and the RESERVE standpipes. The bike sat from 1997 til a couple months ago with fuel in it, the tank was pretty grungy but I've cleaned it a couple times, the second time because I thought newly loosened up crap had plug up my screens. I replaced the plastic screens with stainless screens that were about 7/16 diameter tubes, I pinched one end, flattened them enough to get them in through the hole in the tank, and pinched them onto the stand pipes (I didn't pinch the standpipes). As usual, my first test ride after that seemed like it was ok, but maybe I didn't go 70 for long enough? Seems like if I run 60, I'm ok, 70 it eventually starves for fuel (I think). It still could be more crap coming from places in the tank I can't see with the bore scope and the chain couldn't get to. I've had the carb bowls off several times changing pilot jets, they look nice and clean. Could the petcocks not deliver enough fuel (seems unlikely there are two of them)? Should I put in an inline filter (I don't see crap in the carb bowl, but I'm kind of curious as to what I may catch)?

Thoughts?
 

Jan_P

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If it was dirt it would remain there at next startup.

Please check the tank vent at cap or where it is . So it is open can compensate for the decreasing level
Is there Pri position on these petcocks..?

I would rty these .perhaps run with the gas cap slightly open or someting else in there If it changes anything
 

Wilsonsk

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If it was dirt it would remain there at next startup.

Please check the tank vent at cap or where it is . So it is open can compensate for the decreasing level
Is there Pri position on these petcocks..?

I would rty these .perhaps run with the gas cap slightly open or someting else in there If it changes anything
I'll check that, I put a new seal on the gas from a sloshing standpoint, it doesn't seal well and lets gas leak out when I'm moving the bike around, but I have no idea how well it seals from a vacuum standpoint, these petcocks don't have the PRIME position, no vacuum lines, just basic petcocks, they do seal up nice on the bottom of the tank and they don't leak anywhere other than they don't shut the fuel off completely to the carbs. That sounds like a possible culprit, I could try to get it to starve, then pop the tank lid, see if it fixes it. Something to try!

Another piece of info, choking it helps when it is starving, but only so much, it'll help it recover briefly, but it wont sustain running long with that on either.
 

Jan_P

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Aee ..hh I would check charging voltage across battery
It is simple to do
I would try to measure hot when stumbling also if not a Voltmeter is installed on the Motorcycle
Is this a stock bike stock regulator and stock ignition ??

No difference left and right ?
 

Wilsonsk

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Aee ..hh I would check charging voltage across battery
It is simple to do
I would try to measure hot when stumbling also if not a Voltmeter is installed on the Motorcycle
Is this a stock bike stock regulator and stock ignition ??

No difference left and right ?
I have a handle bar volt meter, 14.2 all of the time, I recently put a VR115 and new 3 phase rectifier. I can’t say that I checked it specifically when it starts starving, but I will. I put the VR115 on last week. The original VR and rectifier were fine but the voltage wasn’t real constant, it did it with that one too. In one of my other threads I went through cleaning the tank the second time because I assumed there were new chunks loose in there plugging up my screens. This vent thing is interesting. I put that new seal on 2 or 3 weeks ago. When I ride that route, It always seems to happen about the same place. Then it’s a battle to keep it running the 2 or 3 miles home, an hour or two later, fire it up and runs great til I run 70 mph for a couple miles.

It just starts to stumble and dies, both cylinders, if I close the throttle quick, it sometimes idle, but usually dies, I pull over, it starts right up but doesn’t want much throttle or it dies.
 
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Wilsonsk

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I dug into my gas cap, the metal center that the seal goes on is two metal discs riveted together with a fiber disc in between. The vent follows the groove to the other end, goes through the hole in the fiber disc, then follows the groove on the other metal disc to the hole on the other end. I couldn’t blow through it, I tried cleaning it with a piece of wire, I got part way through the channel on one side and lost my patience and ground the rivets off to take it apart. Rusted tight, both sides. I scraped it out with the wire but I’ll clean it good and screw it back together, then I need to figure out how to put the latch back together.
IMG_4842.jpeg
IMG_4832.jpeg
 

Wilsonsk

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Got it cleaned up and reassembled. The fiber washer is swollen in the area of the channel, I rotated it and drilled a new hole that lines up with the blind end of the channel. Cleaned everything with a wire wheel and reassembled with screws. I’ll get out and test it, hopefully, this is it, even if it isn’t the root cause, it needed to be cleaned up.
IMG_4848.jpeg
 

Jan_P

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Yes come back if not solved I Would consider disconnecting the charging circuit How many fuses on the Motorcycle
and run on the battery see if any change
Nice job.
 

Wilsonsk

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I rode my loop, it did the same thing, I opened the gas cap and rode that way, still did the same thing. After 2-3 minutes of 70 mph, it starts to hesitate a little, then mostly die, but slowing down will help it recover some, it also runs bad at the bottom end when it does it, although an immediate restart seems to sort of "reset" it?. I have noticed that, after I stop, when it is running rough at the bottom end, choking helps, I tried choking it right when it stumbled out at 70 mph, didn't really have any effect on it. I rode some more and watched the voltage, most casual riding it has always been 14.2 since I put in the VR115 (it did this 70 mph stall/stumble with the old regulator too), but when it was hesitating and wanting to stall, the voltage was moving around between 13.1 and 14.2. When I got back into town and cruising 30 mph in 3rd gear, if I hit the throttle so it lugs a bit, the voltage drops from 14.2 to 13.1-13.2 until I let off the throttle, then right back to 14.2, very repeatable too. It still has the original 1976 points ignition and alternator, new VR115 and new 50A 3 phase rectifier.

Something with the points maybe? I bought aftermarket new points, got frustrated because they didn't mount up well, they are close but not identical, the original points weren't that bad, I cleaned them and that's what I'm running. I bought aftermarket coils from Dennis Kirk, I still have the originals, although the mounting stud broke off of one. My experience with bad coils in old cars with points has been that they shut off, require some time to cool, then work fine. This rarely completely dies, just looses most of its power, pretty proportionally to load on the engine - reduce the load, it recovers some.
 
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Wilsonsk

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....

Something with the points maybe? I bought aftermarket new points, got frustrated because they didn't mount up well, they are close but not identical, the original points weren't that bad, I cleaned them and that's what I'm running. I bought aftermarket coils from Dennis Kirk, I still have the originals, although the mounting stud broke off of one. My experience with bad coils in old cars with points has been that they shut off, require some time to cool, then work fine. This rarely completely dies, just looses most of its power, pretty proportionally to load on the engine - reduce the load, it recovers some.
I have everything I need to convert to TCI, I have a 1981 donor motor and I bought the GN250 module and the Honda coil, I'd really like to identify the problem before changing over, but it would completely replace the ignition system and I can leave the points system installed as well so I could change back relatively easily.
 

Jan_P

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It has signs of being the charging But can be other things as well

A good battery can have the 13 V in a non charging situation

the voltage drops from 14.2 to 13.1-13.2 until I let off the throttle, then right back to 14.2, very repeatable too.

I would look into disconnect the charging circuit. And test ride on the battery only .. Not to far from home .
And service the right hand handlebar inside things

Perhaps that wiring but I cant see the connectors

1692891510827.png
 

Wilsonsk

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the voltage drops from 14.2 to 13.1-13.2 until I let off the throttle, then right back to 14.2, very repeatable too.

I would look into disconnect the charging circuit. And test ride on the battery only .. Not to far from home .
And service the right hand handlebar inside things
I bought a new right hand handlebar switch, not that there couldn't be something wrong there. It does make sense that the 13.1-13.2 I'm seeing could be no charging, the volt meter shows 13-13.1 for a minute or two after I shut it off.

What is the best way to disconnect charging circuit? Disconnect B+ lead coming out of the rectifier? Just unplug the 5 pin rectifier plug? I know some types of alternators don't like running to open.

How long can I run on the battery? I suppose I can get it good and warmed up, confirm the behavior, then disconnect the charging system and try it again? I think the plugs will spark down to 9 or 10 volts?

Whatever it is, it seems to need to run at that ~70 mph/4,000 RPM range for 2-3 minutes to start doing it, it starts by just barely hesitating, so little I'm not sure if it's hesitating or there are little wind gusts, then after another minute or so, then it drops out pretty significantly.
 
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Jan_P

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I have 80 year model and can run ca 30 - 50 km on battery ..But keep it close to home in this case
9 -10 Volts I believe is to little perhaps it runs but wont start On kickstarter even

Please provide the exact machine name / type . So we can find the right wiring. And look if you can spot any connectors Concerning the charging
Others can have a chance to chime in. If there is any risk or clever way to do it.

An inspection of the alternator brushes and measurement slipring resistance is a good thing to do
Might not show cold.

As long as we disconnect in the right way no power is coming in and it cant be hurt

Looking to Disconnect Regulator / Stator / Rotor / Rectifier
Dont do it before we got the right wiring Schematic.
And perhaps pictures.
 

Wilsonsk

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It is a 1976 XS650C, I'll attach the wiring diagram. I checked the brushes, there is plenty left. The voltage regulator and rectifier have been upgraded since this started happening. I used 5twins instructions to convert to VR115. Both the rectifier and voltage regulator plugs have been apart recently and put back together, looked fine. The 5 pin plug to the rectifier would separate all charging from the rest of the bike. Could also unplug the VR115.
d 75_76_XS650 B&C Wiring and Key Updated 3a.jpg
 

Jan_P

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OK here is where I am at
Please wait a while so others can chime In

There are 4 wires going to the alternator rectifier including Neutral switch
There are 3 going to the Regulator.

If disconnecting all these 7 it is electrically speaking not in the circuit. Not Connected as if they were on the workbench
The rotor will spin but not producing anything since there is no power to it.

The ignition should work and bike start on a charged battery.. Making the test possible
1692895662894.png
 

Wilsonsk

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Got out and did some runs, observations:

Rectifier and VR Connected

When cold/warming up, volts stay at 14.2, regardless of load on engine.
Before it started losing power, it started dropping to 13.0-13.1 under acceleration, 14.2 when off throttle
When it started to lose power, I noticed that while it was losing power, the volts were at 14.2, when it would recover the volts would go down to 13.0-13.1 (saw this a couple of times, there’s a lot going on)


Rectifier and VR DISCONNECTED (3 pin VR and 5 pin rectifier plugs unplugged), bike hot, unplugged them on the side of the road
Volts 12.7 before starting engine
Volts 12.7 after starting engine
Volts did not drop with hard acceleration, solid 12.7
Road it 2-3 miles at 70 then it started to lose power
Volts stayed solid at 12.7 regardless of stumbling or making good power
Road about 5 miles total, volts were at 12.5 when I got home.

It seems like there is something wonky with the alternator charging that drops volts when accelerating hard, seems odd that it doesn’t drop at all battery only?

In rereading this, maybe it makes sense that there is no voltage drop on the battery only as whatever happens is happening. It is pulling the charging system down to 13.0 at times, but that is still above the 12.7 volts the battery can maintain on it's own (temporarily). The battery has a lot more current capacity than the charging system so the battery may just be able to deliver the addition current when needed.

Could the voltage drop when it recovers (with the charging system connected) be the extra load (most likely the coil) coming back on line?

Could the drop to 13.0 on acceleration (but not while it is losing power) when hot be a bad coil with high resistance under load? It behaves like an inverse tach sort of, open the throttle, volts go down, close throttle, volts go up?
Does the electrical load increase as RPMs increase? I guess I never really considered that, I always considered coil draw a constant, regardless of load on the engine?

I could swap in one of my old coils, one side at a time, and see if I notice a difference.
 
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