Misfire under load

[this old xs]

Might have to look I to it, thanks gentlemen, I think it would have saved me a bit of time and stress. I thought I could borrow the tap and die from work, but sadly not. They did have thread files I could borrow but not metric.

 

[this old xs]

Engine in, oil in. Kicked about 20 times but no oil to the top end (right hand exhaust valve cover off). Put the battery in and cranked it, oil aplenty! That's enough for tonight.

 

[Its been a long time]

Engine in, oil in. Kicked about 20 times but no oil to the top end (right hand exhaust valve cover off). Put the battery in and cranked it, oil aplenty! That's enough for tonight.

Good luck with the start up!

 

[this old xs]

Thanks. I'm going to be nervous!

 

[this old xs]

Ok, finished putting it back together. Timed right hand cylinder. Left hand points gap was barely there and correcting it put the static timing about where I needed it.

Fired up easy and runs smooth. No bad noises which is a huuuuge relief. Like, HUGE. Checked timing with my mates timing light and the left cylinder is a degree or two advanced from the right but the adjustment screw chewed out, so although the difference annoys me I'm calling it good for now.

Also the right hand points are arcing pretty good even though I replaced the condensors when I bought it. I fixed that for now by putting the points cover on

I've had a bit of a fuel leak on and off from the hose between the carbs for a while, but I noticed it was pretty bad. Off with the carbs a few times until I finally figured out the brass press in fittings on the carbs were loose. No wonder I couldn't stop the leak with hose clamps.

Charging system seems good, 13.3V at idle. There may have been an accidental incident between a hammer and the face of the rotor so it was a relief to see it appears to have survived.

Ready for a test ride. Just catching my breath!

 

[this old xs]

Test ride done. Runs good, shifts perfect. Heading up the road for a cruise now.

 

[Its been a long time]

Test ride done. Runs good, shifts perfect. Heading up the road for a cruise now.

Well done, huge relief

 

[this old xs]

Thanks!

Cruise was so great. Did 80km/50m or so. Too scared to look in the oil.

While I had the cases split again I had a good look at the shifting. I couldn't see any reason for it to not work, other than 2 big dogs sometimes butt together and have to slide past each other for the gear to engage.

Maybe all the debris in the oil was stopping them. Speaking of, shout out to Jan P for being polite enough to refrain from "I told you so!" Hopefully I got it this time.

To the subject of this thread - the misfire is still there, but, I'm optimistic I can finally fix it. Maybe. What 5twins said (and I did drop the needles one groove) got me thinking. I've been chasing what I thought was a lean misfire. I bought a couple of second hand Chinese pit bikes off marketplace a while back and they needed a lot of carby work. While tuning them they misfired terribly up top, but it was too much fuel. Leaned them out and they run great.

I'm going to go back to the 135 mainjets and leave the needles where they are to see how that goes.

 

[5twins]

Something to consider is that the needle jets do wear on these carbs over time, getting gradually richer. It's hard to tell because the hole through them is so big, but there is a slight "lip" around the outside of it. Make sure it's there, all the way around. They tend to wear oval over time.

 

[this old xs]

Hmm. Interesting, thanks 5twins. I don't know how many kms are on this bike. The speedo was showing around 34,000km, but that doesn't seem right for first oversize. Compound that with the fact that I bought a replacement speedo from the USA and its now showing 22,000 miles!

Long way of saying it's definitely worth me checking.

 

[this old xs]

Well Mr 5twins, it seems I owe you a beer if you're ever down this way. Checked the needle jets and they both have visible wear, though not a huge amount. One seems slightly worse than the other, and to be honest I often thought maybe it's only missing on one cylinder.

I put 135's back in but left the needles one notch lean. Misfire barely there. I think replacing the needle jets should do it. Should I replace the needles too? They look OK. Anyone got any part numbers? I had a quick look on mikuinioz but no luck.

While I was tidying up the bench to work on the carbies I found this:



I had a sinking feeling I knew where it belonged. Let's check:



Yep.

Well I guess I can inspect the oil after all. I'd say less than 100ml came out due to putting wood under the wheels so it leans way over on the sidestand.

The good news is there were no big gold flakes like before. There definitely is something in there, some super fine particulate is suspended. It's much too fine for either strainer to catch, and you just know I'm running a paper filter sleeve. At this stage I've done all I can do so I'm going to call it good unless it breaks.

 

[5twins]

Unfortunately, these Z-2 needle jets aren't available anymore. I think repops are available though. Or you can watch eBay for some N.O.S. originals. I've managed to score several over the years.

The '78 carbs saw big changes compared to the previous '76-'77 set. In particular, the main jet size was increased substantially, from a 122.5 to a 135. That's 5 sizes and that's a lot, lol. In order to do that, the midrange had to be leaned out, and that was accomplished by changing the needle jet from a Z-8 to the Z-2. But even doing this, the upper midrange on the '78 carbs is still right on the verge of being too rich. This is why any main jet size increase at all, even just one size, will push it over the edge and cause stumbling unless the needle is leaned a step. In fact, you may even be able to coax some stumbles out of it under heavy or full throttle applications even with the stock 135 mains. So, once the needle jets begin to wear and get richer, you'll definitely get some stumbles.

 

[this old xs]

Turns out I can get new needle jets from Geoff (substitute Mikes XS or XS650direct depending on where you live). $20 a pop. He also has some Keyster tuning kits but they're spendy and I don't want to fiddle, near enough is fine as long as that misfire is gone.

https://www.geoffsxs.com.au/en-gb/product/48-0658?search=needle&description=1&page=2

 

[this old xs]

Tough day in the shed yesterday. This time I didn't take care to keep the clutch plates in the same orientation. I was more concerned about cleaning them, so they got moved around a notch or two. They've worn in so I had to adjust the clutch. Trying to undo the locknut and I pulled the mechanism off the cable. No harm done fortunately, but had to take the cover off to fix it.

Next the needle jets. I don't have the right orings but I've bodged it before. Not this time. Mucked around for ages wrecking orings. Ended up shaving them down with a knife. Will get the right ones later.

That stupid fitting was still weeping fuel. Opened it up for a tight fit. Still weeping. Shoved an oring over it which is pressed on by the fuel line. If it's stupid and it works, it ain't stupid.

Fired it up and it was running terrible, missing badly. Then I noticed a puddle forming under the bike! Fuel was gushing out the left hand airbox. Shut it down and tapped the bowl, same thing. Carbs off again and I couldn't see any reason for the flooding. Blew it out with compressed air anyway, been behaving since.

Then I spent a really, really long time gapping the points and setting the timing. Took it for a spin today and the misfire is just a flutter from 4.5krpm to 5 or 5.5krpm and only absolutely wide open. Back off a whisker and it stops. Pulls like a train.

Someone carved air filters out of big blocks of foam. I'm thinking if I make some new ones out of the air filter foam I bought ages ago, that might do it.

 

[this old xs]

Forgot the most important part! After stopping the fuel gusher, it was idling away smoothly when the left cylinder, slowly at first, starts blowing smoke. White oil smoke! You could smell it, and it built up so it was really fogging.

My heart was in my boots let me tell you. After a minute or two it cleared up and stopped but my confidence is shaken. I can't understand how even with bad cylinder washing it would cause oil to get into the combustion chamber.

Hopefully it was just a weird anomaly that never happens again. Ever.

 

[Its been a long time]

Forgot the most important part! After stopping the fuel gusher, it was idling away smoothly when the left cylinder, slowly at first, starts blowing smoke. White oil smoke! You could smell it, and it built up so it was really fogging.

My heart was in my boots let me tell you. After a minute or two it cleared up and stopped but my confidence is shaken. I can't understand how even with bad cylinder washing it would cause oil to get into the combustion chamber.

Hopefully it was just a weird anomaly that never happens again. Ever.

In my experience white smoke isn't oil, it's excessive fuel? Good luck sorting it out, it's been an adventure!

 

[this old xs]

Smelled and looked like a clapped out old Briggs and Stratton. I've seen white fuel smoke in diesels but normally black fuel smoke for gasoline? I dunno, as long as it doesn't do it again!

 

[this old xs]

I think I solved the mystery of the great smoking cylinder panic of 2 weeks ago. Haven't been able to hardly ride because of work etc, but the other day after some ahem, spirited riding, as I was parking I caught a whiff of burning oil. Worse, oil had dripped from the join between the left hand carb and air box.

So the penny finally dropped on two things. Oil is being blown/sucked into the carb by the crankcase ventilation. Also, when the carb was flooding the other day the fuel mixed with the oil in the air box to cause billowing clouds of smoke.

I did my research here on the crankcase ventilation. I have a spare set of reeds from those 50cc Chinese pocket bikes and I was thinking about putting it in where the baffles are, except when I took the baffles out and had a good look in there, I found something I thought my bike didn't have:

If you look closely you can see the little rubber dealy. I pushed it down where it should be, but the left cylinder had none. A piece of low pressure fuel/oil line jammed in there pretty good, even if the hole is a tad big.

I haven't checked the air boxes for oil yet, but hopefully it's sorted or improved. I also ordered some inline check valves so I'll see about maybe sliding those in.

 

[this old xs]

For anyone wondering if paper filter sleeves work, yes they do. First one I've had block, and the resulting pressure partially collapsed the strainer.

It broke the adhesive and bent the perforated metal backing inside so I reckon my oil pump is probably good. Was it bypassing oil? Probably. Am I worried about that? Not really.

The paper (not shown) was black. I assume from the rings bedding in after honing? All magnets (both drain plugs and the two on the pick-up strainer) had a small amount of black sludge. A bit like molybdenum grease, or maybe if you made a slurry out of graphite powder. I'm guessing (hoping) this is from the liners and should be finished now.

I still had the OEM strainer but it had blobs of what I assume is yamabond from a previous assembly in it. Managed to clean it up a bit and put it in. I debated about a paper filter sleeve but ended up fitting one. Also noticed the OEM unit is reinforced and flat, no pleats, where the aftermarket one got squished. It's directly opposite where the oil feeds in.

I've had about 4 paper filter sleeves and this is the only one to clog, so I'm calling it an anomaly.

 

[this old xs]

Now I realise this is 100% on me, for looking at these air filters and going, meh, they'll be fine:



Because a you can see, I finally made new ones and I'm calling it misfire fixed! You can go full throttle and it just goes. Great success!

So now I've got a new problem (of course). I think it's fuel starvation. Prolonged full throttle runs, 80+mph, really giving her all the way, it just dies after a short while. It just loses power, which I can tell you, is quite unsettling when thinking about all the money, time and effort that could be self destructing. It regains power after backing off and rides normally.

This all screams fuel starvation to me, but weirdly it never did this before. I haven't checked the fuel tap strainer yet, but I will. The question is, does the XS650 need two fuel taps? Mine had two, but after multiple rebuilds, cleaning and polishing (they're the dreaded vacuum operated ones) I could only get one to work reliably. I blanked off the tank where one was, and I teed off the delivery so it feeds both carbies, and the balance pipe between the carbies is still there too.

I would be surprised if one tap couldn't deliver enough fuel, but it came with two and I suppose Yamaha didn't do that just for fun? If two are needed, I'll ditch the vacuum ones and fit normal types.