Has spark, has compression, but doesn't start...WHY?

There's cam timing and ignition timing and what you did was to adjust the cam chain tension which may have some effect on both of the above, but isn't actually setting or checking either.

What these good people demand to know is how did you set the ignition timing. That would be the exact point at which the points open and cause a spark at the plugs.

They will eventually also want to know how you adjusted the carbs and throttle cables.

Then they will want to know about the air leak in the intakes that's causing that lean mixture and high idle revs. Bikes are not complicated, but they sure can be confusing until you get the hang of it. And get a manual please. It will help enormously.
 
noexit said:
Ok. Before you fuck anything up... Get a workshop manual. Also, learn what timing is.

Timing is important because it sets the point at which the spark plug fires. If this is off, it can cause engine damage.

Your carbs also have to be clean and adjusted right. If you're not going to take to time to learn about how this stuff works, then the only way you're going to get this bike going is to pay someone. If you're in Seattle, you can pay me. I've done it before on these bikes for a friend who didn't know what he was doing. They're a bit finicky, and will be hard to start if everything isn't dialed in right.

It takes three things to make an engine run:
1. Fuel: Carbs have to be clean, gas needs to get to the carbs, carbs have to be adjusted. This is in the manual.
2. Compression: Usually around 135psi or higher, relatively even across all cylinders.
3. Spark: at the right time, anywhere between 5 and 40 degrees before top dead center, depending on the engine and the RPMs.

A manual will explain what these terms mean, what your symptoms like backfiring mean and how to fix them, and how to make sure your carbs are adjusted right, your timing is set right, and you have proper compression. The $35 you spend on it will save you hundreds.
Click to expand...

This ^^^ And sooo true about how finicky these bikes are!
 
Fuel system is clean.

I found a manual for it, and it says to static time it...

10deg BTDC Retarted Ignition
31.5deg BTDC Fully advanced ignition

So.

I think this means I want to rotate the crank shaft CLOCK WISE until I am 10 degrees before TDC, then set the little pointer to that point.

Correct?
What do I do about the fully advanced static?

Also, I read a bit about the valve adjustment. Is it basically using feeler gauges to make sure everything is spaced correctly?

Keep the help coming. Were making progress!

Thanks!

P.S.

Different friend just got a Virago 750 for $100! Bike looks great and wants to go, but the starter, when the button is pushed, dims the lights a bit, but nothing clicks or does anything. So when we tried jumping it, it turns and putters but doesnt start all the way.

However, when we jump it, and its puttering, if you hit the starter button, the starter motor seems to help it along, does this mean the motor is bad? Solenoid? Something else?

ANY and all help is wanted and greatly appreciated.
 
There should be a mark at 10BTDC, so you shouldn't have to measure anything. If you get all three points correct at 10 Degrees and your mechanical advance moves freely, your fully advanced timing should be correct. I wouldn't even worry about it at this point.
 
So, we are more stupid then even we thought.

The points ignition has been replaced with an electric ignition....duhhhhh!

http://s643.photobucket.com/albums/uu154/Sirplus11/Yamaha%20XS/?action=view&current=XS750Dignition.jpg&newest=1

That there is the picture of what is in place of the points.

Now, the manual shouldn't help us as much now that the ignition has been replaced, so how are you supposed to time it with that ignition in it???

Learn somethin new everyday i guess...
 
The manual will help you, do you have the pickup plate aligned with the locating pin in the crankshaft ?
 
OK so it has a late model 750 ignition with mechanical advance. The manual should tell you how to check that for static timing in the retarded (as it stands) position.

IIRC the 850 had electronic timing curve, but don't quote me on that. I just threw one of those ignitions out a couple of weeks ago. It didn't work - probably dry soldered joints.
 
IIRC the 850 had electronic timing curve, but don't quote me on that.

Only on the 81, 78_80 has mechanical.
 
Explain how you got the fuel system clean. In the picture, the carbs are pretty ugly. And (no offense) but you seem like you may not know how to properly clean and tune a carb. Not saying it can't be timing, but you're putting all your eggs in that basket too easily. When you buy a bike that's been sitting, dirty carbs are a more likely culprit than timing. I assume you have good plugs in it? Checking static timing should take five minutes, then on to other problems.
 
We pulled the carbs apart, soaked all the jets in carb cleaner, gently cleaned the inside of the carbs, checked the rubber boot things (they were good) checked the needles (also good). Also used light compressed air to clean out the hoses and blow out any stuff inside the carbs. They were sparkling when we were done.

Tuning them is another story tho...

We just kinda rebuilt em and put them back on...I feel like the issue could very well lie in the tuning aspect of them. All we really need is a carb sync and sync all three carbs to the same pressure correct? I have heard doing it by ear (no pun intended) is the best way to do it. As long as they sound good, the pressure reading is the same in all 3, and the bike is smooth through a full range of travel, they should be all set...correct?

And static timing. No matter how many times I read the manual and look online I am still confused.

Static: NOT Moving.

Can anyone give me info an how to do this? The bike doesnt have points in it anymore considering the new ignition so im not sure how to do it...

Thanks in advance guys
 
noexit said:
Do you have a manual? Their are directions for static timing. Once it's static timed and running, you can time it with a timing light.

For static timing, I just use a 12v light bulb soldered to some alligator clips or a multimeter. There should be 3 marks on the flywheel. Something like F1, F2, F3. Check that the light comes on right as the timing mark lines up for it's set of points, then do the next one. Spin the crank with a wrench on the flywheel.
Click to expand...

in order from left to right on the wheel-thing with markings... (the markings are spaced out about that far from eachother)

|...|...|.............F....T

Are those first 3 lines the F1 F2 F3 you mentioned?

Also, what do we hook the aligator clips to? Sounds like a fairly easy way to do it.
 
That's for static timing when you have points. I don't know if you can static time with an electronic ignition. You'll probably need a timing strobe. You might be able to rent or borrow one from your local auto parts chain.

The T is probably TDC. The F is probably 'Fire', where the sparkplug should fire. The other marks are probably the range for your advance curve. Your manual should show what the marks mean. There should be sets of marks for each cylinder on the flywheel.
 
http://www.dotheton.com/forum/index.php?topic=22783.0

found this.

Should I losen the plate, rotate the crankshaft to the "F" mark, then slowly rotate the timing plate until the spark plug fires?

Its that what that thread is saying?

I have a timing light so the idle timing and advanced timing shouldn't be a problem.

I only have to / can static time the left cylinder, correct?
 
This of any help ? http://www.yamaha-triples.org/forums/thread-view.asp?tid=1363&mid=10245#M10245
 
Tripler said:
This of any help ? http://www.yamaha-triples.org/forums/thread-view.asp?tid=1363&mid=10245#M10245
Click to expand...

That appears to be a Newtronics ignition.
 
http://pinkpossum.com/GT750/carb/BS40carbs.htm that covers the basics of the same carbs fitted to a GT750 Suzuki triple.

To static (bench) synch those carbs, you have to remove the carbs and look at the throttle plates. They should all just be opening the same bypass hole. That way they will all be physically aligned.

Next, gently screw the mixtures screws in until they bottom out and back them out teh specified number of turns. ON teh GT it's 3/4 of a turn. Set all 3 the same. On those carbs, out is more fuel and in is less fuel. Most carbs let in more air, these are different.

Do you know what jets are in those carbs or did someone replace any of them by any chance? Those carbs take a slow (pilot) jet that looks very similar to regular Mikuni pilot jets but they are entirely different in flow pattern. They must be the correct jets. They are shown in that link.
 
Teazer, the point i was trying to make, was that the most people who come across these bikes don't know that when they go to remove the plate, they end up pushing the locating pin into the crankshaft. This pin is shorter than the depth of hole, it is recommended that a 2d nailhead be put in before reinstalling the pin. The out come is that the timing will never be resolved till this is corrected.
 
Tripler said:
This of any help ? http://www.yamaha-triples.org/forums/thread-view.asp?tid=1363&mid=10245#M10245
Click to expand...

So that one there, is basically saying this....(11th page from the top)

http://www.biker.net/750_service/2d.pdf

I need to locate TDC, then adjust the stationary set screw to the "T" on the fly wheel. Correct?

However, whats the best way to REALLY locate TDC? Cam chain adjuster? Or use some sort of feeler gage inside the piston to locate it?

Almost to the bottom of this!!!
 
Tripler said:
Teazer, the point i was trying to make, was that the most people who come across these bikes don't know that when they go to remove the plate, they end up pushing the locating pin into the crankshaft. This pin is shorter than the depth of hole, it is recommended that a 2d nailhead be put in before reinstalling the pin. The out come is that the timing will never be resolved till this is corrected.
Click to expand...

I didn't know that. Glad you raised it again.
 
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