First motorcycle and a cylinder not firing.

Thursty

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Recently I acquired my first motorcycle - a 1976 xs650c. As a bricklayer by trade, I am completely new to 'mechanics', and have decided that I want to figure out the bike for myself, and it be 'built' not 'bought'. I didn't buy the bike so that someone else can get to know it and fix it for me. I bought it to learn about for myself.

I originally had hoped that my first post on this forum would be the backstory of the bike, as it's quite fascinating (I bought it off of the original owner, and it hadn't ran since 1982), but my desire to get it running at this moment outweighs story time. However, I feel it necessary to at least bring you all up to speed in order to rectify the current state of the bike:

First thing I did when I got the bike was rebuild both carbs (bs38). I then rebuilt the petcocks, and cleaned the gas tank. Before trying to start the bike, I made sure to set cam tension, valves, timing, and bench sync the carbs, all in that order. I also installed new spark plugs, and pod filters, as the 40+ year old filters that were on the bike turned to dust on contact. Through just a small amount of research, I assumed that the bike was going to be running too lean after installing the pod filters, and would need to rejet the carbs. Oh yeah - and the bike has spark. After a bit of kicking, the bike started up for the first time in 40 years. The exhaust was quiet, but seemed to backfire quite a bit. I wasn't too worried about it, as i figured i would just need to do some more fine tuning, and so moved onto installing a new (to me) chain. In the process of aligning my rear wheel, I found out that my bike has an aftermarket rear shock mount on the swingarm, which obscured the alignment markings above the axle that are needed for aligning my rear wheel. In order to remove the shock mounts, I also had to remove the mufflers. Here's why I'm mentioning this - the mufflers had some bad rust holes, and removing this solved 'some' of the backfiring. However, not all. Upon starting the bike again, I noticed that the right carb seemed to 'pop' a lot. Upon further investigation, I noticed that the right exhaust pipe was not near as warm as the left, and when I placed the backside of my hand at the end of the left exhaust pipe, it didn't feel as though it was firing consistently. At this point I also thought I could hear some sort of 'clanging' sound coming from the right side of the engine. I shut the bike off as quickly as possible.

A couple of days passed, as I tinkered with other little stuff on the bike - such as throttle cables, clutch cables, shock mounts, etc. Last night I picked up a compression tester, as I figured this would be a logical next step. 145+ on both cylinders. That's pretty good isn't it? So I started the bike up again, tried to listen more intently, and took advantage of the low light (it was getting dark out) to see if I could observe anything such as sparks or flames. It seems to be backfiring out of the right exhaust pipe, and right carb, and the consistent clanging sound seems to be (as far as I can tell) coming from the upper, right side of the engine. I've never disassembled an engine before, and don't really know what to do next. I've gotten this far by 80% service manuals, 10% youtube videos, and 10% on these forums. I figured that this would be a good time to get involved with the forum and ask for help. The only thing I can think of that I haven't tried yet is switching the spark plug wires to the opposite cylinders to see if its a wiring issue associated with the coil/spark plug wires.
 

Jim

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First, welcome to the forum. Golden rule here... lots of pics.
Not just to oogle, but sometimes there's clues in 'em.
Any chance you can give us a video of it running?
Your description is good... hearing it would be better.
 

Thursty

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20220917_170817.jpg
 

Thursty

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Ill post the video tonight after work. OH, and one more thing! The last time i started the bike, after it had been running for about 2-3 minutes, i touched the throttle a bit, and when i let off, the rpms didnt drop but rather climbed. I didnt like it, so again, i killed it.

Anyways, ill get back to you with a video tonight. Thanks.
 

jpdevol

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nd the consistent clanging sound seems to be (as far as I can tell) coming from the upper, right side of the engine. I've never disassembled an engine before, and don't really know what to do next.
First, to address the clanging, when the engine is cold before starting, perhaps adjust cam chain and valve tappet clearance per book. Tools needed: 22mm wrench (or adjustable) to remove cam chain tensioner cap, rotate engine at rotor nut with 17mm wrench and adjust plunger with 10mm wrench. Then valves per book, needed: feeler gauge set, 12mm wrench and pliers for square drive tappet adjuster. Clearance cold intake .003", exhaust .006".

After running and video sounds like, prepare to set points gap (feeler gauges) and timing (need timing light). Likely need to synchronize carbs. (need vacuum gauges).

All that is just an overview of how I'd proceed - folks here can walk through and answer specific questions.
 

Thursty

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I took a new video tonight, while trying my hardest to capture the 'clanging' sound as good as possible. I will also add (if i didn't already mention it), that I also installed new points and set them 'per book', along with valve clearance, cam tension, timing, etc, etc. (not in that order, but rather in the proper order: cam tension, valves, points/timing, then carbs (or at least i think thats the proper order?). And I set everything by the book (i actually have 2 books to compare and confirm).

Before this video was taken, I tried starting the bike with the spark plug wires swapped to opposite cylinders. The bike wouldnt fire... Maybe because the coils aren't ambidextrous haha? Switched them back and bike fired up after a couple kicks. Compression is at 150 on both cylinders. An observation that I also made, was that the right cylinder, which wasn't firing very good/consistently, also wasn't burning fuel as fast as the left cylinder. I made this observation when shutting off the fuel to both carburetors while the bike was running, and keeping an eye on the fuel levels in the fuel lines. The right fuel line hardly dropped at all by the time the left fuel line was empty. Not sure if thats any help or not? Before i even started the bike for the first time, I knew that the bike had a 95%+ chance that it would be running lean. Thanks for the help.

Note: remember when watching the video, that I am a bricklayer, not a mechanic. This is all new to me, and my problem solving may be completely off track and irrelevant to the situation at hand.
 

Jim

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So first off the clanging sound. Sounds to me like the cam chain. Either you didn't adjust the chain correctly, or the rubber on the front guide has let go. Both are common problems unfortunately.
I made this observation when shutting off the fuel to both carburetors while the bike was running, and keeping an eye on the fuel levels in the fuel lines. The right fuel line hardly dropped at all by the time the left fuel line was empty. Not sure if thats any help or not?
Yes. You may just be a bricklayer.... but there's a potential in you young Skywalker... ;)

I'd guess your pilot circuit is plugged in the right carb. That or the float valve is clogged. And once again, that's not unusual. It's also not unusual for it to take two or three tries before you finally get 'em clean enough to work. I'd take em back off and give 'em another scrubbing. In the Tech section you'll find the carb guide. It's the bible for those carbs. Read through it several times.

Last... with the points ignition, the coils fire 360° apart. You can't just swap the HT (plug) leads, you'll also have to swap the wires to the points to get 'em to swap sides. I'd go through the carbs again first though.
 
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Jim

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Also should have mentioned...
These engines are a 50+ yr old design of an air cooled engine.... they're noisy by nature. What you're hearing isn't terrible.
 

Thursty

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Ill go through the carbs again tomorrow forsure. Speaking of the carbs, the rebuilt kit I got came with a #27.5 pilot, and #135 main jet. One of my manuals says that my 1976 XS650C, with BS38 carbs, runs a 122.5 main, and a 25 pilot. HOWEVER, my other book states that 650s come standard with a no. 130 jet, and says nothing about the pilot (from what I've read so far). Which is correct? and where do i go from here with the current jets I have in the carb? Just run them for now until i rectify the other problems?

And as for the cam chain, how do I go about the rubber on the front guide? I'm pretty confident i used the proper procedure for setting it, as I did it twice, and both times very carefully. I turned the engine over counterclockwise until the pushrod was at its furthest point. I then adjusted the pushrod untill it was flush with the end of the adjuster. I will note that the second time I set it (it wasn't in need of much adjusting, but did it anyways to make sure), upon loosening the adjuster, the pushrod was loosening WITH the adjuster, and maintaining its same status (if that makes sense?). I then tightened the adjuster more than it was before, and then backed it off again, finally getting the pushrod to sit flush again. I know thats probably confusing, but just trying my best to illustrate my findings. Thanks again.
 

teamWicked

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Ill go through the carbs again tomorrow forsure. Speaking of the carbs, the rebuilt kit I got came with a #27.5 pilot, and #135 main jet. One of my manuals says that my 1976 XS650C, with BS38 carbs, runs a 122.5 main, and a 25 pilot. HOWEVER, my other book states that 650s come standard with a no. 130 jet, and says nothing about the pilot (from what I've read so far). Which is correct? and where do i go from here with the current jets I have in the carb? Just run them for now until i rectify the other problems?

And as for the cam chain, how do I go about the rubber on the front guide? I'm pretty confident i used the proper procedure for setting it, as I did it twice, and both times very carefully. I turned the engine over counterclockwise until the pushrod was at its furthest point. I then adjusted the pushrod untill it was flush with the end of the adjuster. I will note that the second time I set it (it wasn't in need of much adjusting, but did it anyways to make sure), upon loosening the adjuster, the pushrod was loosening WITH the adjuster, and maintaining its same status (if that makes sense?). I then tightened the adjuster more than it was before, and then backed it off again, finally getting the pushrod to sit flush again. I know thats probably confusing, but just trying my best to illustrate my findings. Thanks again.
There is a ton of knowledge and documentation on carbs here https://www.xs650.com/threads/carbs-carburetors.43/
Each carb set is a little different but trying to mix and match is maybe a bad idea. If you have 76 (76-77) the standard main is 122.5. Mym77 BBK 750 pods and open exhaust likes 130 on the mains. I would think you need a radical cam and porting to need more.
 

Jim

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The book's not real clear on adjusting the cam chain and there's various ways to do it.... the book way being the worst way. Here's how I do it:

Remove both plugs and pull the alternator cover. Put a wrench (17mm) on the rotor nut and rotate ccw until you reach TDC (top dead center). As soon as you reach TDC, the crank will want to go past that slightly. Let it. The cam is now on valve overlap where both intake and exhaust valves are open (slightly). Trying to rotate the crank either way now becomes harder because no matter which direction you go, you're fighting against the valve springs.

Now you can use your wrench and rock the crank back and forth slightly. Watch the cam chain adjusting pin as you do this. You want to see it move out to about flush and back in 1-2mm as you rock back and forth. If it stays flush, the chain is too tight.
Do that and see where we're at as far as the noise goes.
 

Thursty

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One last thing before I work on the bike tomorrow, I have 2 sets of uni pods to choose from for the bike. I will add the links to them in this message. If I have 135 jets, but standard is 122.5, does that mean i may need to back off a couple of sizes?

https://fortnine.ca/en/uni-filter-universal-2-stage-clamp-on-pod-air-filter
https://fortnine.ca/en/uni-filter-universal-hi-flow-clamp-on-pod-filter-kit

my uncle runs the hi-flow filters on his 81 xs650 (BS34 carbs) and it runs good. I'll hopefully give yous an update tomorrow evening.
 

5twins

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Oh boy, where do I start, lol. Don't get the "high flow" UNIs like your uncle has. They don't really flow any more than the stock airbox because they're so short and have solid end caps. For your BS38s, you want the UNI UP4200 or UP4200ST (dual layer, shown in your 1st link). You don't really need the dual layer version, it costs more. Here's the more basic single layer UP4200 on my '78, it's all you need .....

Uni Pod.jpg


Rebuild kits aren't recommended because, as you've discovered, many come with the wrong size jets. They've been made and sold this way for many years and none of the sellers seem to care, lol. Over the years, the 650 came with no fewer than 5 different carb sets. There were 4 different sets of 38s and the BS34s that came on the '80 and later models. Jetting is different on all sets. The '76-'77 kits you got come with jetting for the '78-'79 carb set.

Carb Specs Reduced Size.jpg


Since you're modding the intake and exhaust, the 27.5 pilots are probably good, but those 135 mains are going to be too big. The most I've been able to use in a carb set like yours is a 132.5. With the larger mains, you will probably need to lean your needle setting a step as well.

You said you installed new points. If you used a feeler gauge to gap them and didn't clean the oil off the blade before using it, you've probably contaminated the points faces. This will caused them to misfire or not fire at all. That could explain the running on one cylinder.
 
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Thursty

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I'm fairly certain that the feeler gauges were clean before setting the points. Ill inspect them again. I didn't end up having time to work on the bike today, however, I did do some 'thinking' throughout the day, and am hoping to investigate a few things that i thought about. Teamwicked provided me with that link to some super good carb info, that I believe may be of good use to me. The XS650 Garage Carburetor Guide, prepared by Grizld1 and 5twins, pointed out on page 6, paragraph 3, that there are little holes on the sides of the slides. Holes that I didn't know existed. Something else I've considered, was Jim's advice to clean the carbs again. These observations led to my next thought, which is a memory of when i first got the bike a month ago and first stripped the carbs. The slide in the right hand carb was seized. In the process of disassembling the carb, I actually had to remove the needle, slide and diaphragm, by first accessing the float bowl and cleaning things from that end. At this point I had concluded that the cause of the 'seized' slide, was varnish caused from improper storage. I bet cleaning the carb a couple more times would really help. This would make sense as the left hand carb was stored fairly good, and it seems to run good. ALSO, remember how i pointed out that when i shut off fuel to both petcocks while the bike was running, the left carb seemed to burn off a lot more fuel than the right one, which would hardly use any at all?

I bet my right carb is one of the major culprits. I'm going to completely go through everything again as far as cam tension, valves, points, timing, and then carb. I'll hopefully spend most of the time on the right carb.

This still doesn't rectify the clanging sound. Do you think this clanging could cause further damage and that I should refrain from running the bike until solved? Again, compression in both cylinders is 150.

Also, am I good to continue troubleshooting the bike for now while running the 135 main? Or again, should I refrain from running the bike until rectified?

Thanks again.
 

Thursty

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Also is the UP4200ST literally the exact same filter as the UP4200, only with an additional sleeve over the main part of the filter? Therefore could be run without the sleeve if so desired? The reason I ask, is that I have 2 UP4200ST filters, and was wondering if by removing the outer sleeve, it would run leaner to help (even just a little) compensate 'temporarily' (just for tomorrow), the 135 main jet?
 
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