idles then dies with slightest throttle up, valve timing?

...when I turn the throttle a good spray of air/fuel gets blasted the wrong way through the carbs.
It has to be valve timing...

A severely stretched camchain (and worse, jumped a tooth) could delay intake valve closing so much that it would push air/fuel back out before the intake valves close.

If you temporarily adjust intake valve lash to, say, 0.030", and the back-spitting stops, could confirm that...
 
Opened up the intakes to .025" . Seems to improve idle , and throttle no longer kills engine when barley turned ; now it kills it after I open to almost full throttle. Still sounds very bogged down, with a noticeably smaller kickback air/fuel plume through the carbs.
I am thinking ;retard the cam a tooth, since the ignition timing is already fully retarded.
 
There's a small pin in the right end of the camshaft that indexes the backing plate to the advance unit.
If you put the crank at TDC, that pin should point either straight up, or straight down.

The cam runs at half speed of the crank, so 20° at the crank would show 10° at the cam. You'll need sharp eyes, and maybe a protractor to see this. But it is another way to confirm stretched camchain and/or cam mis-timing...
 
Pulled off a.t.u plate. The cam pin seems to be just slightly angled toward the rear of the cycle.
Pulling rocker box to inspect cam location.
 
Cam looks correct.:confused:
If anyone out there has a Haynes Yamaha Owners Manual 70'-83', then maybe you could help me understand page 61 section 36 paragraph 4, which sumed up says;
to set valve timing. L.h cyl. to T.D.C on compression stroke. Alternator timing marks line up. Position in cam with cut out vertical . Which puts both the cam lobes on the left side pointing up.
So the question is ; if the left hand cylinder is on its compression stroke and both valves are there for closed, then that does not make sense because at the same moment my right H. valves are both closed since both those lobes are down.?
 
Last edited:
The pistons move together, so crank TDC for the left occurs at the same time for the right. The cam decides which one is in the compression stroke. Since the crank turns twice for each turn of the cam, you could mount the cam upside-down, so it makes sense to you. But turn the crank another full turn, the cam will turn a half turn, and you'll have that notch pointing up.

The thing to be looking for here is how badly is the camchain stretched. When new, the chain is so tight that it's difficult to attach the master link. When it's badly stretched, the cam can be rotated back/forth up to one tooth's worth of travel. The camchain tensioner needs to be removed to test this 'cam slop'.

For more info, try google search on "cam timing" and "camshaft timing"...
 
Looking at this might help;

http://www.650motorcycles.com/CamTiming.html

Don't confuse cam timing with ignition timing. One effects the other, but they are totally different. Cam timing must be done right first on assembly of the head and the the ignition timing comes after with the installation of points or whatever you are using for this part.

Page 61 is dealing with installing the cam when the pistons are at top dead center. That is all.

Then later you install points and advance unit to look after the timing and it is set to the left cylinder.

This link will show you what it should look like when installed correctly, so can be used to verify your cam timing.

Then you go on to adjust the valves using the standard TDC on the compression stroke of that cylinder.

Then, knowing the cam timing is correct, go on and adjust or check your ignition timing.

The three things are related but all done in a different way, that is perhaps what is confusing you. Think one thing at a time, cam timing, valve adjusting, ignition timing in that order.


Good luck.
 
Last edited:
With cam chain tensioner completely removed there is about a half of a chain link rock in the cam.

This wording is confusing me(quoting the afore mentioned paragraph from Hayens book):
"4. To set the valve timing turn the engine over slowly until the l.h cyl is at TDC on its compression stroke. At this point both valves will be closed and the alternator rotor mark will allign with the t mark on the stator. Position the camshaft so that the cut out in the left-hand sprocket face is vertical. . ."

My confusion stems from the fact that with the cam set in with the l.h cut out vertical, the l.h. cam lobes are up, pushing the valves down. The book seems to say that at this point the l.h cyl is firing (since it's on the compression stroke). Yet at the same moment both valves are garenteed closed on the r.h. cyl. since both those cam lobes are down.

Putting in the cam upside down would make sense, then the cam lobes on the l.h side would be down (valves closed).

Right now the cam is in like the book says ,but both valves are not closed on the l.h. side. I've had the l.h. valves closed with the cutout vertical in the past. But that did not do any good and it sounded terrible with those tappets slapping around in there.
 
Thanks for the link Brian. It seems to answer my questions.
It is weird that the link explains that with the cam cutout vertical the r.h. cyl is on compression stroke not the left-hand cylinder which I interpreted the book as saying.
Un fortunately this is how I have the cam and valves timed now, and when it was idling earlier. Hum...
 
With cam chain tensioner completely removed there is about a half of a chain link rock in the cam...

Well, that's not too bad. Probably about 10° retarded, half life of the chain. Mine's much worse, and it runs fine.

So, maybe should confirm the actual cam timing, or something else, like sticking inlet valves, or something goofy with the ignition...
 
Confirm actual cam timing?
Would disengaging then rotating the cam 180 then reangaging chain put the power stroke on the right side fixing the timing issue? Since the pistons move in unison and I have a dual output coil I would just be moving the power stroke to the other side.
 
Haha, simply rotate the crank 360° and you have the same thing.

Confirming the cam timing involves using a degree wheel and noting the crank angles when specific events occur, like valve opening and closing, and comparing that to the manual specs. A forum google search of "camshaft timing" should bring up some info...
 
So putting the cam in upside down would not change anything?
Timing light is telling me the sparks hitting right on FIRE. Valves seem to be going up and down easily.
 
Last edited:
So putting the cam in upside down would not change anything?

That's right. For every full 360° rotation of the crank, the cam rotates 180°, a half a turn.

Consumer-oriented service manuals generally are not written very well for the newbie mechanic to learn from. As such, the astute members here have acquired all the available manuals to try to cover all the bases when it comes to meticulous procedures. I generally consider these manuals as reference points, reminders if you will, because they're not written very well.

I had one manual that, I swear, was nearly useless. Half of the print was a large cautionary paragraph about 'disconnecting the battery ground', repeatedly inserted before every section, even if you were going to just empty the ashtray. And when it came to doing a complex procedure, it gave no detail, like 'remove engine from car', with no info on how to do that.

Maybe you could use more manuals. Find some here:

http://www.xs650.com/forum/showthread.php?t=41

There's also an online course that will give you a better conceptual understanding of these things:

http://www.xs650.com/forum/showthread.php?t=31638

Back to your problem. One thing you could try, after you get it back together, is to turn off the fuel delivery to the carbs after it starts, so it's running on just the fuel in the bowls. As fuel is used, the fuel level will slowly drop. If the engine runs continuously better, then you may have identified an overfill condition, caused by bad floats, float valves, height setting, crack/leak in carb body, warpage of carb body, ...etc.
 
The issue with the plugs getting wet so quick; the valves are acting correctly, so now it's either fuel delivery or spark malfunction. Right?
 
It's so weird that I have to have my needle e-clip all the way down for it to start and idle , the trade off being that I am getting way too much fuel. I mean it's obviouse that I am getting plenty of fuel to the cylinder, too much fuel really, but when I lean her out it won't start. ????
 
One thing to consider is that you have a bike with a '77 XS650D serial number, but you don't have a 1977 Yamaha XS650. For example, your service manual shows how to service the swingarm, but when you look at your bike, where is it?

Somebody else put this together, right? That means that you'll need to identify exactly what all you have. You're doing carb work on something that resembles carburetors, using specifications that may not apply to these. The tech section has info that can help to positively identify which carbs you have, and their specifications and servicing. If you'd like us to help you identify what you have for carbs, you could post crisp/clear closeup pics of various angles of the carbs, including inside the float bowls and the mating sides of the carburetor bodies...
 
This bike was chopped and falling apart badly before I bought it, but it was running. So I broke it down and built it up to make it more highway reliable. Bought the Hugh's Handbuilt / Pamco Pete combo, sent in my motor to get bored out to a 700, found out it was already a 750, so i know someone was in there prior, anyway, it hasn't run right since I put it back together.
Carbs are the BS38's pictured in the book. They do have 2 different float bowls that look exactly similar (one H and one K).
Thanks for the dans mc link. Looks like I have a lot to read up on.
Sorry no pics right now, for some reason this iPhone won't let me even click on the upload photos botton.
 
Back
Top