Topic: Can't get my 75 cb750 started, need help!

[Cowsertc]

I'm starting to wonder if the carb is flooding the spark because after a few turn overs, the backfire and popping goes away. I'm getting some fuel blowback from the intake of the carb and some flames coming out of the exhaust without backfire. I know I shouldn't have fuel blowing out of the carbs. So maybe the float level is too high? I did the clear tube test, but I may have mis-judged it? I'll be doing it again this evening to be sure.

Forget about fuel. Sounds like one or more of the following (in no particular order):

>Cam not properly timed
>Ignition not properly timed
>Ignition not properly wired
>Ignition wire(s) shorted or broken
>Valves lash not set properly
>Mechanical damage or failure of one or more valve train and or ignition components

I would focus on those items you can get to with the engine still in the frame. Check, double check, triple check, and quadruple check all of your ignition wiring and components first.

Thanks for the input, I really appreciate it.

-Cam is timed properly
-Ignition is timed properly
-igniton wiring is correct to the diagrams sent from dyna

Unfortunately I am unsure as to what the valve lashing is, I will do my research and diagnose that in the morning

-there is no damage to any of the valves or ignition components.

This is even starting to stump some of my much more mechanically inclined friends and family.

Sorry if it seems like I negated most of what you said, but this is week 3 of this persistent problem and I've gone over the list several dozen times now.

I'll check into the valve lashing soon.

[madmtnmotors]

Valve lash is the gap between the rocker arm and the valve.

Backfiring is most commonly a result of faulty ignition wiring or improperly timed ignition. Improper cam timing can have a similar effect, but my money is on ignition wiring, especially based on this statement in your original post:

"After I put it all back together and ran a new simplified wiring harness, I've encountered this issue."

I still have my first CB750 that I bought back in 1986, still in good working order, and yet I can still manage to cross up the ignition wiring once in a while. 

[przjohn]

I think your problem may be timing related and I don't believe you understand the difference between cam timing, valve adjustment, and spark timing. Stop chasing fuel system Butterflies and troubleshoot the problem. Start with the valve timing and describe exactly how you are timing it by referencing what marks line up where to the folks here so that they know you actually did it right. Then go from there and explain how you did it.

[Cowsertc]

Valve lash is the gap between the rocker arm and the valve.

Backfiring is most commonly a result of faulty ignition wiring or improperly timed ignition. Improper cam timing can have a similar effect, but my money is on ignition wiring, especially based on this statement in your original post:

"After I put it all back together and ran a new simplified wiring harness, I've encountered this issue."

I still have my first CB750 that I bought back in 1986, still in good working order, and yet I can still manage to cross up the ignition wiring once in a while. 

Thanks for the info, the valve lashing is set properly. .002mm at the intake and.003 on the exhaust when each cylinder was at TDC.

I was running the simplified wiring before all of this, but that was with butt splices and electrical tape, when I put it all back together I soldered and heatshrinked. I may take it all off and rewire from the ground up again. In case it may eliminate and issue that may be present.

I've also been questioning the idea if the freshly painted frame may have cause a grounding issue somewhere as well. I've removed the paint from where the coils mount and from the frame ground to the battery.

I'll probably tackle that tonight since I know the mechanical side of things is up to snuff.

I really do appreciate the help, I'll post updates as soon as I can.

[Cowsertc]

I think your problem may be timing related and I don't believe you understand the difference between cam timing, valve adjustment, and spark timing. Stop chasing fuel system Butterflies and troubleshoot the problem. Start with the valve timing and describe exactly how you are timing it by referencing what marks line up where to the folks here so that they know you actually did it right. Then go from there and explain how you did it.

Sorry if I'm not being clear, it's the frustration setting in.

I set the crank to the 1-4 "T" mark and then alligned the horizontal line on the cam with the cam tower? With the notch at 12 o'clock.

Once that was assembled, 1 adjusted the valve lashing across all 4 cylinders in accordance with the clymers manual.  .002mm at the intake, .003mm at the exhaust.

After that I set the crank to the 1-4 "F" mark and set my dyna igniton until it read 12v going to the 1-4 coil, then repeated the process for the 2-3 side.

Again, I apologize for any confusion I'm causing. I'm not the best at keeping several conversations going at one time.

[Cowsertc]

I think your problem may be timing related and I don't believe you understand the difference between cam timing, valve adjustment, and spark timing. Stop chasing fuel system Butterflies and troubleshoot the problem. Start with the valve timing and describe exactly how you are timing it by referencing what marks line up where to the folks here so that they know you actually did it right. Then go from there and explain how you did it.

Sorry if I'm not being clear, it's the frustration setting in.

I set the crank to the 1-4 "T" mark and then alligned the horizontal line on the cam with the cam tower? With the notch at 12 o'clock.

Once that was assembled, 1 adjusted the valve lashing across all 4 cylinders in accordance with the clymers manual.  .002 in at the intake, .003in at the exhaust.

After that I set the crank to the 1-4 "F" mark and set my dyna igniton until it read 12v going to the 1-4 coil, then repeated the process for the 2-3 side.

Again, I apologize for any confusion I'm causing. I'm not the best at keeping several conversations going at one time.

To be little more specific for the cam timing, I span the crank until the index mark on the cam lined up with how it should have been, then I unbolted the cam chain from the cam and span the crank until it was at the 1-4 t mark, then bolted the sprocket back onto the cam.

[madmtnmotors]

I would recommend troubleshooting the wiring that you have in place. Electrical troubleshooting often involves figuring out what it's not. Break out the multimeter and start tracing out your ignition connections to see if you crossed wires between the ignition and the coils. Earlier this year, at the Manufacturer's Cup event in Valdosta, I helped a fellow member here trace out this very issue. He is a die hard Honda wrencher and spent a good part of his career as a service manger for a Honda motorcycle dealership. He added a short length of yellow and blue wire to the ends of the wires from the electronic triggers at the ignition plate, but in what I can only guess was a moment of dyslexia, he added the yellow extension to the 1/4 trigger and the blue wire to the 2/3 trigger. When hooking everything up this led him to wire the ignition backwards which I discovered quickly when checking the static timing with a multimeter. When properly wired it fired on the first kick. My static timing was also proven to be within a few degrees of spot on when checked with a timing light once it was up and running.

If you can find out what's wrong with what you have you will have learned a valuable lesson in what not to do. If you replace all of the wiring and everything ends up working like it should then you won't know what you did wrong... 


Regarding your method for timing the cam, how far did you turn the crank while disconnected from the cam? Was it more than 180 degrees? If so, were the rocker arms in place? If so, you could very well bend valves doing this. The crank and cam should never be turned independently of each other when any of the valves are open. If the rocker arms were removed then this is not an issue since all of the valves would be closed.

Again, never turn the crank independently of the cam when there are valves being held open by the rocker arms. Remove the rocker arms before spinning the crank independently of the cam. Get your cam timing set properly before installing any of the rocker arms. 

[Cowsertc]

.

Regarding your method for timing the cam, how far did you turn the crank while disconnected from the cam? Was it more than 180 degrees? If so, were the rocker arms in place? If so, you could very well bend valves doing this. The crank and cam should never be turned independently of each other when any of the valves are open. If the rocker arms were removed then this is not an issue since all of the valves would be closed.

Again, never turn the crank independently of the cam when there are valves being held open by the rocker arms. Remove the rocker arms before spinning the crank independently of the cam. Get your cam timing set properly before installing any of the rocker arms. 

It was less than an 1/8 of a turn counter-clockwise, very, very slowly. There was no resistance at all when I did it, but I definitely won't be doing that again. I see now how that could've cause a major issue.

[PeWe]

Std CB750 valve lash: IN: 0.05mm EX:0.08mm  With many hotter cams 0.10-0.15mm. My DP315 0.3mm.
Maybe the blue, yellow wires are mixed from ignition to coils? Swap them and see if it works better. (same as if the ign rotor is set 180 degrees wrong.)
One thing is to check ignition on all 4. Use an additional plug and connect it to all caps, one by one and see if sparks will happen when running on the starter.
If OK, use starter spray in all carbs. Start and keep it running by spraying...

[madmtnmotors]

It was less than an 1/8 of a turn counter-clockwise, very, very slowly. There was no resistance at all when I did it, but I definitely won't be doing that again. I see now how that could've cause a major issue.

You should be fine then. The same rule applies to all four stroke overhead valve engines. 

[Cowsertc]

Std CB750 valve lash: IN: 0.05mm EX:0.08mm  With many hotter cams 0.10-0.15mm. My DP315 0.3mm.
Maybe the blue, yellow wires are mixed from ignition to coils? Swap them and see if it works better. (same as if the ign rotor is set 180 degrees wrong.)
One thing is to check ignition on all 4. Use an additional plug and connect it to all caps, one by one and see if sparks will happen when running on the starter.
If OK, use starter spray in all carbs. Start and keep it running by spraying...

Thanks for the reply,

When I swapped the blue and yellow wires, I got backfire from the carbs and even a little bit of flame.

I have a bright white spark across all 4 cylinders with new plugs and a bright yellow spark when I use the old ones that came with the bike when I purchased it.

I did notice last night that my new dyna coils get pretty hot with just the key in the ignition space. I'm not sure if this is normal or not. Also, I know it sounds weird, but they were also magnetized while they were hot, or at least the mounting bracket was.

Today I will be getting rid of all the plastic connections that dyna sent with the coils and ignition in order to make sure there isn't a loose connection anywhere.

[calj737]

If you have bright spark on all 4, and you've swapped the wires with worse results, then I submit you either have no fuel or no combustion. Use a piston stop to measure TDC thru the plug hole and COMFIRM your ignition timing is 100% correct and then your valve lash.

[madmtnmotors]

Post some close up pictures of your coils and spark plug wires.

[Cowsertc]

Post some close up pictures of your coils and spark plug wires.

I will here in a little while. I may have found another piece of the puzzle. I also have an 85 vt500 bobber build I'm taking on at the same time and it appears that I may have mixed up the spark plug caps. Two of the caps I had installed on the cb750 did were not resistors and belonged on the other bike. So I fixed that situation and after a quick brake job on my truck, I'll be able to see if that could have fixed anything. Pictures will come shortly.

[przjohn]

Post some close up pictures of your coils and spark plug wires.

I will here in a little while. I may have found another piece of the puzzle. I

also

have an 85 vt500 bobber build I'm taking on at the same time and it appears that I may have mixed up the spark plug caps. Two of the caps I had installed on the cb750 did were not resistors and belonged on the other bike. So I fixed that situation and after a quick brake job on my truck, I'll be able to see if that could have fixed anything. Pictures will come shortly.

Sounds like you have alot of balls in the air right now.

[my name is nobody]

Late to the party, but plug wires are going to the proper cyls, yes? assuming you
have new coils installed with new wires..

[Cowsertc]

Late to the party, but plug wires are going to the proper cyls, yes? assuming you
have new coils installed with new wires..

Yup, 1-4 and 2-3. Triple checked my wiring today with multimeter and everything is spot on. No crossed wires, no grounds left out, everything that should have power does.

[Cowsertc]

Alright, here's a list of updates:

All of my wiring checks out perfectly, no crossed wires, no loose connections, everything is routed exactly where it should be.

Both valve and ignition timing are set as close as I could possibly get it, near perfect for static timing.

All cylinders are at TDC when they're supposed to be.

If I take the carbs off and spray starting fluid directly into the intake boots, the engine speeds up and pops out of the exhaust with little puffs of smoke. Also some smoke puffs out of the intake boots. But still not running.

When the carbs are reinstalled, it doesn't want to do anything, even with starting fluid. Which kind of sounds like an air problem to me?

Just for good faith, I swapped the blue and yellow connections and got flames out of the carbs, so I know it's not that being swapped.

The coils are going to the correct cylinders. 1-4 and 2-3.

Sorry I haven't posted any photos, I'm not sure how to do it. Very unfamiliar with photo bucket and I can only work off mobile right now.

[seanbarney41]

Did you have the ignition advancer apart?

[Cowsertc]

Did you have the ignition advancer apart?

It was never removed from the bike, I only had to mess with the ignition cam to put the dyna igniton spindle? The part with the magnet that replaces the cam lobes for the points. The actual plate was never tampered with, and I've had the bike running after the ignition installed.

[Keith]

Assuming your cam timing is correct, first thing I would check is valve adjustment. Too tight, no compression. Do you have a compression gauge? If not, pull plugs, cover one plug hole with a finger tip, crank engine over. You should feel each compression stroke.

[my name is nobody]

Another thought...On the 750. when the cam notch is exactly at 12 o'clock, horizontal line dead nuts flat, 1-4t aligned, the engine should have correct cam timing.
HOWEVER, At this position, the #4 cylinder will be at tdc of compression,
NOT #1. (I don';t know why it's like this, but it is) You have to turn the crank another 360 degrees and #1 will be
at tdc compression. The cam notch will be at 6'c;ock. Are you going straight to #1 and setting your valves
after aligning your marks? If so, a valve for #1 will be open, and your adjustment
is dead wrong...check it incase you haven't and it'll make sense..

[Cowsertc]

Another thought...On the 750. when the cam notch is exactly at 12 o'clock, horizontal line dead nuts flat, 1-4t aligned, the engine should have correct cam timing.
HOWEVER, At this position, the #4 cylinder will be at tdc of compression,
NOT #1. (I don';t know why it's like this, but it is) You have to turn the crank another 360 degrees and #1 will be
at tdc compression. The cam notch will be at 6'c;ock. Are you going straight to #1 and setting your valves
after aligning your marks? If so, a valve for #1 will be open, and your adjustment
is dead wrong...check it incase you haven't and it'll make sense..

That makes sense to me. I'll go out and check it in a few moments.

Sorry about the sudden absence everyone. The wife wanted to go see the eclipse, so I lost two days due to travel. Back now and getting ready to start tackling it again.

Thanks again for all the support.

[madmtnmotors]

Rather than taking a haphazard approach, which is easy to do when advice is running the gamut of all the potential problems, the most productive (and logical IMHO) approach is to address the three main components required for an ICE to function properly:

1. Compression. Do a compression check first and rule out bent valves, broken piston rings, improperly timed cam, etc. Without compression it will never run. Be sure to have the carbs wide open (full throttle), or remove the carbs completely for the most accurate results.
2. Properly timed spark. You seem to have done due diligence here. What method are you using for static timing? Close ups of all your ignition components from the points/trigger plate all the way up to the spark plugs has proven helpful in the past.
3. Properly metered fuel. Do the carbs last. Unless the bike has been sitting for years, if the bike ran before the engine work was done then the carbs should still provide enough fuel to run the engine now. The fact that it will not run even with starting fluid leads me to believe that something else (not carbs) is amiss.

Post as many pictures as you can. 

[Cowsertc]

Another thought...On the 750. when the cam notch is exactly at 12 o'clock, horizontal line dead nuts flat, 1-4t aligned, the engine should have correct cam timing.
HOWEVER, At this position, the #4 cylinder will be at tdc of compression,
NOT #1. (I don';t know why it's like this, but it is) You have to turn the crank another 360 degrees and #1 will be
at tdc compression. The cam notch will be at 6'c;ock. Are you going straight to #1 and setting your valves
after aligning your marks? If so, a valve for #1 will be open, and your adjustment
is dead wrong...check it incase you haven't and it'll make sense..

Everything checks out, the marks are where they're supposed to be at all locations. Correct cylinders on compression when they're supposed to be.