Topic: CB750f 78', newly rebuilt, won't start. Ideas!?!

[Thousandson]

Cb750f 78', rebuilt engine, new o-rings on carbs, new accelerator pump. I have spark on one and four(easy plugs to reach), battery has been filled, charged. I have fuel, I can smell it coming from the open spark plug hole. I know I have air she is sucking, I can hear it. No turn over, not even a sputter.

Give me an idea or two.
Bad connections? I've already swapped point wires no luck.

Oh internet gods, I need to ride.

[nevernoluck]

Absolutely positive you have fuel?

[nevernoluck]

Alright now that its letting me post again, do you have drains in your float bowls? Are they filling up? Sure all cylinders are sparking? Have you tried a very little amount of ether? Or even a little swig if gas in a cylinder?

All it takes is fuel, spark and compression to fire an engine, not saying without the right mix it will stay running but you say no turn over not even a sputter. If I read this correct it does crank over right? I call turning over cranking some others don't, just trying to make sure I'm on the same page

[crazypj]

Cam timing right? Ignition timing right?
re-built motor -  was it running before re-build?

[Thousandson]

Yes I have fuel, I will check all bowls to see if they have fuel.
Motor ran before rebuild. I was very slow on my top end part of my build. Always making sure, #1 piston top dead center, then fire. I will go back through one more time to check again. The top dead center and firing marks, can not be flipped. I tried that and found out its fool proof. I might try re adjusting my point gap, see what happens.

The bike is turning, no kinds of iignition. No bang or any kind of sputter, not even a simple thump.

[Bankerdanny]

Then it sounds like you need to trace your ignition circuit.

Start at the points. Do you have power there? Yes, move to the coils; no, check the stop/run switch.

[Xnavylfr]

You CAN get the can in 180 degrees out from the timing marks.. If you are able to pull the valve cover with the engine in the frame, remove it . Check the right side of the cam (sitting on the bike) The notch should be at 12 O'CLOCK, the 2 dash marks should line up with the line where your cam cap bolts down and the T mark for #1/4 should ALL be in these positions!!! If the notch is at 6 o'clock ,then it is 180 out from the crank shaft.  I'm not positive if the engine would even spin  that far out because of Valve clearance to the pistons!!!

Xnavylfr(CHUCK)

[Thousandson]

I will trace back the ignition circuit, I know that the run/stop switch responds to spark at the piston. I still need to check the 2 and 3 coil. My points is one of my big thoughts, I will check my gap, and i'll tighten it up.

Xnavylfr: I was at a very slow speed, double checking and triple checking cam shaft to points position on install.

After some top dead center, firing position, and more electrical check. I will remove the valve cover off, I may not be able to get it all the way off, but I hope to find a way to check those marks(I'm 99.9% sure it was in the correct position). If the marks are off, the engine will come back out, that will be last ditch effort.

This is my very first engine tear down, this is all part of the process.

Thank you all for advice, questions, and knowledge. If any more ideas pop in your heads I'm all ears. I want my bike.

[crazypj]

You CAN get the can in 180 degrees out from the timing marks.. If you are able to pull the valve cover with the engine in the frame, remove it . Check the right side of the cam (sitting on the bike) The notch should be at 12 O'CLOCK, the 2 dash marks should line up with the line where your cam cap bolts down and the T mark for #1/4 should ALL be in these positions!!! If the notch is at 6 o'clock ,then it is 180 out from the crank shaft.  I'm not positive if the engine would even spin  that far out because of Valve clearance to the pistons!!!

Xnavylfr(CHUCK)

THERE IS NO SUCH THING AS CAM 180 degrees OUT
It turns half crank speed so WILL ALWAYS be 180 'out' when crank is turned a single rotation.
The points cam can be fitted wrong to advance mechanism, that will fire 2/3 when it should fire 1/4, but, there will be a hell of a bang in exhaust if that happens

[Thousandson]

Just came running back in. Its starting to rain and I had a chance to poke at a few things.

Points are looking fine, I adjusted them by eye. Pushed the start button, no luck. After that I got under cover and started replacing my shocks. I took off the muffler, and found out the shocks will need some work. I'll need new rubber eyes, and my clevis needs some shims, and a new tapped hole.

After that failed, I didn't put my muffler back on. I got a little hick up when I tried to start again, she moved and tried to spin. It didn't last more then a 1/4 turn, and no further luck. I returned my points back to stock position, 1 and 4 are still getting spark. My battery was on its way out, and i tired on 2 and 3 but I didn't see a spark.

Since my battery was dieing quickly, I started kick starting. I got two good pops(not in succession, and no real motion.) Looking at my mufler, I think its safe to say, its cloged to all high heaven, time to find a replacement part.

I could feel more suction on 1 and 4 carbs, not much on 2 and 3. Even the exhaust pipe has some suction, I get back pressure, but I'm not getting much pressure build up. Where am I at guys?

[lucky]

Cb750f 78', rebuilt engine, new o-rings on carbs, new accelerator pump. I have spark on one and four(easy plugs to reach), battery has been filled, charged. I have fuel, I can smell it coming from the open spark plug hole. I know I have air she is sucking, I can hear it. No turn over, not even a sputter.

Give me an idea or two.
Bad connections? I've already swapped point wires no luck.

Oh internet gods, I need to ride.

" I've already swapped point wires no luck."  What THE HELL?

[lucky]

Just came running back in. Its starting to rain and I had a chance to poke at a few things.

Points are looking fine, I adjusted them by eye. Pushed the start button, no luck. After that I got under cover and started replacing my shocks. I took off the muffler, and found out the shocks will need some work. I'll need new rubber eyes, and my clevis needs some shims, and a new tapped hole.

After that failed, I didn't put my muffler back on. I got a little hick up when I tried to start again, she moved and tried to spin. It didn't last more then a 1/4 turn, and no further luck. I returned my points back to stock position, 1 and 4 are still getting spark. My battery was on its way out, and i tired on 2 and 3 but I didn't see a spark.

Since my battery was dieing quickly, I started kick starting. I got two good pops(not in succession, and no real motion.) Looking at my mufler, I think its safe to say, its cloged to all high heaven, time to find a replacement part.

I could feel more suction on 1 and 4 carbs, not much on 2 and 3. Even the exhaust pipe has some suction, I get back pressure, but I'm not getting much pressure build up. Where am I at guys?

You cannot adjust points by eye.   cheeeeesssshhh!!!

[lucky]

You need a manual and you need to go step by step and follow instructions.


Just from what you have said I would not trust that the electrical timing is right at all.
It may even need new points and condensers.

[Xnavylfr]

If you have #1 cylinder at TDC and the 1/4 timing marks on the timing index mark and the cam notch on bottom then the CAM is 180 degrees out of time. It's not just having the point wires mixed up or the plug wires on the wrong cylinders.   The Points will be firing when the valves are open instead of closed!! I do believe the cam sprocket has 2X the teeth then the crank sprocket. which means the cam turns 1/2 turn for every full turn of the crank.Cam sprocket has 34t and crank has 17t. Well ,I guess I have to eat crow. With the notch down then it would be timed for 2/3. and fire on #2/3 cylinders.  I bow to you CB GURU.  Live and learn!!!


Xnavylfr(CHUCK)

[Vinhead1957]

Oh no not 180 out?  Like an oil thread! It  will evoke emotion! But I say you are right!

[crazypj]

If you have #1 cylinder at TDC and the 1/4 timing marks on the timing index mark and the cam notch on bottom then the CAM is 180 degrees out of time. It's not just having the point wires mixed up or the plug wires on the wrong cylinders.   The Points will be firing when the valves are open instead of closed!! I do believe the cam sprocket has 2X the teeth then the crank sprocket. which means the cam turns 1/2 turn for every full turn of the crank.Cam sprocket has 34t and crank has 17t. Well ,I guess I have to eat crow. With the notch down then it would be timed for 2/3. and fire on #2/3 cylinders.  I bow to you CB GURU.  Live and learn!!!


Xnavylfr(CHUCK)

Nope, it just means your TDCC on 1 instead of 4, turn crank 360 degrees and your TDCC 4 instead of 1

If you have #1 cylinder at TDC and the 1/4 timing marks on the timing index mark and the cam notch on bottom then the CAM is 180 degrees out of time. It's not just having the point wires mixed up or the plug wires on the wrong cylinders.   The Points will be firing when the valves are open instead of closed!! I do believe the cam sprocket has 2X the teeth then the crank sprocket. which means the cam turns 1/2 turn for every full turn of the crank.Cam sprocket has 34t and crank has 17t. Well ,I guess I have to eat crow. With the notch down then it would be timed for 2/3. and fire on #2/3 cylinders.  I bow to you CB GURU.  Live and learn!!!


Xnavylfr(CHUCK)

 Nope, 180 'out' means TDCC 1 instead of TDCC 4.
Turn crank 360 degrees and the 'other end' of crank would be on compression
You would be 90 degrees out if timed on 2/3 (that would be REALLY BAD and bend all the valves if you turned motor over)
BTW, TDCC
http://acronyms.thefreedictionary.com/TDCC

[Thousandson]

Taking lucky's third piece of advice, I checked spark again(very weak spark, and no spark on 2 and 3), and checked my condensers. Condensers seam to be dead to me, I'll replace my condensers with my points and try and start it again.

On a personal note, Lucky. If your going to criticize and scold me for trying to figure things out by asking. Then I don't need your opinion. If your not going to give me helpful/useful knowledge in these forms. Then please just be quite, I'm looking for help, not your opinions about me doing something wrong.

That being said, and as I've said before, I have a very useful well written shop manual. It doesn't give you all of the helpful information to make my process streamlined. I was trying different things on "My Bike" to see if I could get it to run.