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

[PancakeShake]

Alright,

Me and my buddy just picked up an XS 750 for a project bike.

Owner said:
- Dont "really" know whats wrong
- Might be the points
- Has new stator
- Has new plugs
- Has great compression (said his mechanic)
- Runs but not great (when running)

What we found out after looking / cleaning / trying to start it:
- HAS Great compression
- HAS spark in all 3 heads
- Carb's seem pretty clean (enough to at least start the bike)
- Has pods, they are junk, but the carbs DID have 140 Jets so they were re-jetted properly
- Turns over and over, but doesnt wanna fire up
- Backfired once (good, bad, dont matter?)

Our assumptions:
1) Cant be the points, cause we get plenty of spark. TRUE or FALSE?
2) Shouldn't be the carbs because they seem clean enough to at least start (cleaning them anyway) TRUE or FASLE?
3) Compression seems pretty damn good.
4) Flooding? TRUE or FALSE

Our Questions:
5) Could it be something inside the cylinders?
6) Could it have been bad gas?
7) ANY other ideas?

Were pretty new, but loving every minute of the struggle so let us know if you have any ideas! Thanks!

Middle carb has some gunk on the floats, but still not too much, is that enough "stuff" to prevent it from starting/running?

Here she is!

 

[wcz324]

Junky pods may mean it's not getting enough air --> Flooding, like you said.
I would think that good spark means the points are fine, BUT make sure the timing is right, that could be causing you trouble.
I'd say if you're cleaning the carbs anyway, just clean them and then make sure fuel is flowing through them properly and you're probably good to go on those.

 

[rexsk8er]

sounds to me like the carbs are gummed up. just because they look clean inside doesn't mean the passages flow freely.


here's a test. get some either or starter fluid of some sort any kind of choke carb break cleaner will work. spray it into the open throats of the carbs under the slide with the throttle wide open. roll it over and if it fires and stays running for a couple seconds its the carbs.

if the carbs are fully cleaned with all passages flowing plenty of cleaning solution use an alternate fuel source to the carbs, could be a clogged filter petcock or pump, not sure tho if those have a pump. doubtful. anyway some of those models do have a vacuum operated fuel valve. basically it opens once the engine starts to prevent flooding it clogs easily.


if it doesnt do jack, it could be weak spark. points tend to get a tit on them and they have to be periodically filed down otherwise the fire will be weak.

 

[doubl3clutch]

X2 for timing. I would get the timing and points adjusted perfect. That would be my first guess.

Also check your fuel delivery, make sure you carbs are getting enough gas.

Tank > petcock > lines > carbs

 

[PancakeShake]

Any good way of adjusting the timing? Everywhere I read basically requires a running bike to check it.

And we drained the tank after half filing it (idiots) so the petcocks definitely work fine.

 

[doubl3clutch]

http://www.youtube.com/watch?v=HQUBA3wZwCQ -

This vid shows you how with a simple test light. This vid is a basic walkthrough on the cb350. I'ts dark, but its simple.

I'd buy a manaul or try to find one online. I don't know anything about xs750's, but timing should be similar to the cb350 process with a points ignition.

 

[noexit]

These bikes have a vacuum operated petcock. Make sure that it's either working, or your have your petcocks turned to "prime". This site has a tutorial on how to convert it from vacuum operation, or you can replace them with normal on/off/reserve petcocks if they're giving you problems.

 

[MotorbikeBruno]

I'd do what rex said as well. And timing on these bikes really needs to be ON for it to run well. Get to it!

 

[PancakeShake]

I'd do what rex said as well. And timing on these bikes really needs to be ON for it to run well. Get to it!

I think we have timing light ready to go, but the bike doesnt run, so how should we time it?

I know the idle has to be the "|" and the "F" for timing, but how do you line it up when the bike doesnt run?

Can we setup the light and engage the starter and turn it over for a few rotations and do it / check it that way?

 

[MotorbikeBruno]

You get a wrench on the nut that is in the center of the points plate. Here's a little how-to for the cam chain, but you can see the points plate in the first picture.

http://xs850.minek.com/pdf/manuals/xs750_2d_user_manual/2d_owners_period_maint4.pdf

And join http://www.yamaha-triples.org/ There is an insane amount of info there, if I remember there is a few step-by step instructions there.

Have fun! Get her running man!

 

[noexit]

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.

 

[noexit]

Don't try cranking it over with the starter. You'll burn it out.

 

[MotorbikeBruno]

Yeah, definitely don't use the starter....just do it by hand with a wrench, I want to say that was 17mm but I may be mistaken, as I sold my Triple a LONG time ago.

 

[Tim]

Definitely do the static timing, and also adjust the valves. Always do those things before bothering to try and start a new-to-you bike.

You mention spark and compression, but have you confirmed there is fuel in the carb bowls and have you cleaned the jets out?

Also get brand-spanking new spark plugs and gap them to spec.

 

[rexsk8er]

Kind of a random detour here,

I worked on a 1983 Triumph Bonneville 750 T140, and it lost fire. Well I installed a new ignition system and it fired but it was hell getting the static timing right

The damn thing had no other mark than TDC. Older model bikes had a plug in a hole that you would install a stopper to set your static timing. Simply put the motor at TDC install the tool (just a threaded stud to an according length) and roll the motor over gently backward using the wheel or a socket on the motor until it just stops on the tool. it actually stops the crank from rotating at something like 24degrees BTDC. Than you just set the pickup or points gap, or whatever.

The other method if there are No timing marks is to use a stick. You have to do a bit of math to calculate how many degrees of rotation of the crankshaft per inch of piston travel. I wont go into all the formulas but you can look up calculators on the net. Than you put the stick in the piston hole on TDC compression, make a mark at a set point on the head (spark plug hole is good) than measure from that mark up the stick whatever the measurement per degree x degrees BTDC for specified engine. when you re install the stick you just roll the engine backward untill the 2nd top most mark you just made is at the exact position you put the first mark.

Sounds like a PITA and it can be, but take your time, do some research and stick to the suggestions here they all seem spot on

 

[PancakeShake]

Success!

We got it to run!

However, it usually needs some starting fluid to go, and when it does it backfires and pops like a mother, until finally is chugs, chugs, and goes.

Once it starts going (on full choke) it starts to rev up pretty high. We dont have a tach but I would say around 5 or 6k.

Then as we take the choke down to zero, the bike drops RPM, but only a little bit. Maybe .5K or so.

SO.
-Runs
-Backfires when starting (and when choke is on full)
-RPMs really high

Any thoughts on the backfiring?
Any thoughts about the RPM's?

I have some videos I will post up later and you guys can see what im saying.

 

[noexit]

Can you loosen the idle adjuster?

It still sounds like timing as far as backfiring is concerned.

 

[Tim]

Can you reconfirm what you've done for timing and valve adjustment?

 

[PancakeShake]

Not sure what you mean by valve adjustment...new to this haha.

And for timing, I found a place online that said to loosen the came chain tensioner, rotate the crank till the "T" lines up with the timing notch, then re-tighten the cam chain tensioner. I did that, along with giving the points a good cleaning.

So I havent really done to much timing wise. Just kinda re-located TDC (I think?)

Sometimes it gives a SOLID clunk in the first couple seconds of engaging the starter. Then it goes away, and fires up.

The videos should help (when ever my buddy emails them to me)

Thanks for the help guys!

 

[noexit]

Not sure what you mean by valve adjustment...new to this haha.

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.