Broken valve

It warmed up enough yesterday. Fitted carbs but still running rich. I have swapped air jets several times, but I'm happy that they are the correct ones.

I tried without air filters. No noticeable difference.

The bike starts with the choke on, but runs without straight away. As the bike warms up, after a few minutes unburned fuel starts coming out of the exhaust. Similar to ticking over with the choke on a vehicle that has warmed up.

I'm wondering if the needle jets are worn. I've fitted new o rings. I think maybe the po had similar problems. I've attached a photo of the piston when I stripped the engine.

I set the timing with a strobe. Full advance was difficult as I had to chase the bike around the garage. It seemed as though full advance was past the mark on the stator. I need to get someone sat on the bike.

I'm thinking maybe condenser or coil. The cost to replace the original ignition components isn't much less than the cost of a Boyer Bransden system.

I think I will be changing to electronic ignition. If that fails I'll rebuild my spare set of 1976 linked carbs, and try them.

Thanks again for all the input, it's really appreciated
 

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I may seem to have given up posting. It's just that I haven't moved forward 😞.

I've ordered a Boyer Bransden ignition. Christmas has delayed delivery.

I'm still working on the carbs. I have a few queries.

The first photo is the 74 carbs with the floats set at 24mm. The needle valve is closed enough to stop the flow of petrol. The floats are actually touching the carb body. The valve isn't closing properly and the carbs seem to be overflowing. The second photo shows 24mm float height on 76 carbs with more clearance.
I've adjusted the float height to around 22mm until the valves stop the petrol flow. Photo three shows the actual petrol level. I think that this will make things leaner rather than rich. Any idea if the carb bodies or floats are incorrect?

Tiggertoo spotted my tickover screw springs almost coilbound as photo three. I think the springs which came with the rebuild kit are formed from thicker wire than standard. It made me think about the butterfly valves. Do they ever close fully? Mine are slightly open at tickover.

I've got a spare pair of 1976 linked carbs. I'm considering cleaning and rebuilding them. I'm not sure if they will fit the 74 rubber inlet manifolds. And more importantly the 74 airbox.

As always sorry for all the questions, and thanks for all the help so far.
 

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I had issues over the last year getting my 77 650D going. I concluded that when genuine Yamaha points and condensers cost about 2/3 the price of a new Boyer Bransden ignition system, then it was sensible to convert to the BB. Given it's a complete new ignition system, I think it's a decent deal. The coil I got with mine is a PVL coil, German good quality.

Having spent far too long and far too much on the 1977 BS38 carburettors, I gave up. I have a set of Kawasaki ER5 carbs that I'm preparing to install. Life has got in the way presently, but in the foreseeable future I'll be fitting them. Good luck sorting your bike.
 
That's just what I've been thinking. I was always thinking about fitting the Boyer Kit. I was just hoping to get it running on the original equipment first. I've fitted Boyer to two bikes previously and was very happy with the results.

I'm getting to the point where I may just cut my losses with the carbs. Life's a bit too short.

Though I don't like to be beaten.

Good luck with your bike and thanks for the encouragement.
 
I've got a spare pair of 1976 linked carbs. I'm considering cleaning and rebuilding them. I'm not sure if they will fit the 74 rubber inlet manifolds. And more importantly the 74 airbox.
I can’t be positive because I haven’t tried it. I expect you’ll find that all that stuff fits. I don’t think there were dimensional changes to the carburetor bodies in that time. The spigots for those years didn’t change. A test fit might be wise, but I’m not sure it’s necessary.

I have a mashup project on hold right now on a 79 Special II. I fitted BS34 carburetors, plastic airbox and battery box all off of an 83. It all fit like they’re made to go together.
 
I may seem to have given up posting. It's just that I haven't moved forward 😞.

I've ordered a Boyer Bransden ignition. Christmas has delayed delivery.

I'm still working on the carbs. I have a few queries.

The first photo is the 74 carbs with the floats set at 24mm. The needle valve is closed enough to stop the flow of petrol. The floats are actually touching the carb body. The valve isn't closing properly and the carbs seem to be overflowing. The second photo shows 24mm float height on 76 carbs with more clearance.
I've adjusted the float height to around 22mm until the valves stop the petrol flow. Photo three shows the actual petrol level. I think that this will make things leaner rather than rich. Any idea if the carb bodies or floats are incorrect?

Tiggertoo spotted my tickover screw springs almost coilbound as photo three. I think the springs which came with the rebuild kit are formed from thicker wire than standard. It made me think about the butterfly valves. Do they ever close fully? Mine are slightly open at tickover.

I've got a spare pair of 1976 linked carbs. I'm considering cleaning and rebuilding them. I'm not sure if they will fit the 74 rubber inlet manifolds. And more importantly the 74 airbox.

As always sorry for all the questions, and thanks for all the help so far.


The float height with the clear piece of tubing attached looks good.
It needs to be below the level of the paper float bowl gasket.
You will know it's too low if it acts like it's running out of gas on a hard pull up a hill on the highway, but it absolutely cannot be above the level of the float bowl gasket.

Aside from Worn or improperly fitting float valves, the only other place fuel will leak from is if there is a crack in the overflow tube. That can happen if water got in there and froze sometimes and the crack can be very very small.

The thing about the idle screw being coil-bound is one reason I don't use those replacement parts out of a cheap kit.
***More likely though: something in your idle circuit is out of adjustment or your timing is off, causing the need to run off the main jet at idle.

I would say install your electronic ignition first and then try and readjust your idle mixture to a place where you can close the main throttle plates more.

I'll bet if this bike does idle when you blip the throttle it hangs at high RPM for a while before settling back down and that's a symptom of a messed up mixture.

The ignition timing does affect the fuel demand for the engine.


Jon
 
Thanks for that Jon. I'm hoping that changing the ignition will help.
And thanks for taking the time to help

You were saying how the advance did not look right with the timing light, the advance mechanism is prone to wear and corrosion as well as the springs becoming weak and I think that the electronic ignition is really the way to go unless you enjoy messing with points.

I used the electronic ignition and TCI box out of an 81 on my 75
 
The advance sorta worked. But I think it advanced too far.
At least I'll know it's working properly with an electronic ignition kit.
I must admit to not enjoying working with points. At fifty years old there seems to be lots to go wrong.
 
I've been away for a while and now I'm going to have to go back and read this whole thread because I really enjoy the forensics of broken valves LOL.

Without reading it I'll bet the person downshifted into too low gear and the 50-year-old valve springs couldn't keep the valves away from the piston.

Most more modern vehicles have a rev limiter (that isn't just the points or the valves floating) so it's not really possible to crash a valve under acceleration but it can be done by accidentally shifting down too many times on the highway.

I've hooked a dirt bike electronic tach to my bike and with the Ninja 500 carbs the power seems to level off around 7,500 RPM. That's as high as I dare rev this design of motor with its terrible vibration and super heavy valves.

Little side note: I tried five different used tachometers, stock tachs, and they were all wildly off! Too high, too low, wouldn't go past a certain RPM... So I confirmed the drop off in power below factory Red line with an electronic tach and now I just run a speedo.


Jon
 
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Hi thought it was about time to post an update. I haven't disappeared.

Over Christmas I was talking to a friend who lives a distance away, so I don't see him very often. He had the same problem with his XS. It turned out to be a dodgy capacitor not working correctly, as it warmed up. Confirming what gggGary suggested way back in post 120.

I've fitted a Boyer Bransden electronic ignition. It took weeks to arrive and then the weather turned very cold. Instead of fitting a relay as suggested by Mailman in his post; https://www.xs650.com/threads/boyer-bransden-ignition-installation-on-my-xs2.60740/.
I've fitted a voltmeter after the kill switch, to monitor the voltage to the ignition and at the same time check the alternator output. I've posted a photo.

I've timed the ignition slightly retarded.
To check the carbs I acquired a Gunsons Colortune. Everything looks fine 😊.

At tickover it's a nice bunson blue flame see photo

Slowly opening throttle nice bunson blue flame

Blipping throttle yellow flame showing rich, just as it should be.

I just need a test run. Unfortunately it's just above freezing here. The roads are wet and covered in salt and grit. Maybe after this weekends storm (you are sending us from the US) the roads will be cleaner.

I'll post after a test run
 

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