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

[Cowsertc]

Hey everyone, I'm in desperate need for a little advice with a problem I've encountered with my 75 cb750. I can't get it to start even though I have great spark, fuel, and compression. As the starter is spinning, I can hear pops coming out of the exhaust with every turn over like it's trying to start, but no luck.

A little background:

I did a ring job, and installed dyna coils and ignition on the bike when I first got it, and it ran very well directly after that. I went to adjust the valves A few months ago, and I accidentally dropped a screwdriver bit down the cam chain tunnel from the breather cover. So I drained the oil and hoped to find it on the oil pan, Unfortunately not. So I dropped the motor out of the frame a few weeks later and rocked it back and forth and luckily the bit fell right out of the oil pan. I left the oil out of the motor during this time, but kept the motor sealed up. I took the opportunity to paint the the frame, do an allballz bearing conversion and new progressive forks and shocks while the motor was out.

After I put it all back together and ran a new simplified wiring harness, I've encountered this issue.  I know the wiring is rock solid, soldered and heat shrinked, no crimp-ons. I have excellent, bright white sparks coming out of all 4 brand new NGK D8EA plugs. I have a freshly rebuilt and cleaned carb, with a small float height issue on #4, but that shouldn't stop it from running.

Wondering if anyone else has encountered a similar issue or has any advice on what I should try next?

Thank you for any and all suggestions.

All mods:

Dyna electronic ignition
Dyna 3ohm coils
Mac 4:2 turnout exhaust
Running the stock airbox

[flybox1]

2 things to verify:
~Points gap is as exact as you can get it.
`Clear tube test the bowls....sounds like its starving for fuel

[calj737]

Or perhaps you have connected the coil wires backwards? Now coil A is firing for 1/4 (yellow wire) and coil B is firing for 2/3 (blue wire)? Blue wire controls coil for cylinders 1/4 and so on.

[Cowsertc]

Unfortunately that's not the case. I did swap the bullet connections just for the sake of it and got the same type of pops, but through the carbs rather than the exhaust, which was expected. Thanks for the response though!

[Cowsertc]

I'm going to take the valve cover off this evening just to see if I skipped timing somehow. It was spot on after I did the piston rings, but maybe the jostling of the motor while trying to get the bit out made it skip timing? I don't see how, but I guess nothing is impossible.

[disco]

Starter fluid into the carb throat. If it runs then your problem is fuel related. No run it could be electrical or valve timing.

[seanbarney41]

Give it some throttle while you crank it.  Once its running and off the choke, set the idle speed screw.

[przjohn]

Starter fluid into the carb throat. If it runs then your problem is fuel related. No run it could be electrical or valve timing.

This is what I would try first, you get a quick run out of it then stalls you have a fuel issue.

[Cowsertc]

Starter fluid into the carb throat. If it runs then your problem is fuel related. No run it could be electrical or valve timing.

This is what I would try first, you get a quick run out of it then stalls you have a fuel issue.

I tried the starter fluid and I get some more aggressive popping from the exhaust, but still not running. I have all the air screws out 1 full turn in accordance with my clymers. I know the float height on carb 4 is too high because it leaks out of the overflow, I'll address that once it's running...

I have 12v at both coils, and 12v at each side of the ignition when it should have it. I've got the clutch safety bypassed with the Green/red jumped to the solid green.

One thing I have noticed though is when I test for spark using a screwdriver in the spark plug cap to the motor, when the spark jumps the gap, I am also receiving a mild shock from it even though I'm holding a rubberized handle. Could this be a case of a bad ground? I haven't snugged down the motor mounts yet in case I have to pull the motor again, but I'm unsure if the motor has to be grounded to the frame for it to run?

I really appreciate all the help, this is my first build, so I'm still very new to this. I've tinkered with several bikes before, but nothing from the frame up.

[flybox1]

2 things to verify:
~Points gap is as exact as you can get it.
~Clear tube test the bowls....sounds like its starving for fuel

what were the results of the clear tube test?

[Cowsertc]

2 things to verify:
~Points gap is as exact as you can get it.
~Clear tube test the bowls....sounds like its starving for fuel

what were the results of the clear tube test?

1,2,3 were right below the gasket level and 4 was high.

[flybox1]

Great. 
Bench sync adequately done?

[Cowsertc]

Great. 
Bench sync adequately done?

It should be good. The carb wasn't messed with in anyway once it was removed from the bike when it was running. I will recheck it here in a few minutes just to be sure.

[przjohn]

You can eliminate fuel as the problem.  Your cabs may not be perfect but that is not your problem. You need now to eliminate something else like spark. You need good spark properly timed. You must completely recheck the Dyna timing and make sure you are not off and then check for a good spark on all 4. If spark checks out then the only things left are compression and valve timing.

If I was to place a bet from the symptoms you describe with the popping it would be on a timing issue with either spark or vlaves or maybe both.

[Cowsertc]

You can eliminate fuel as the problem.  Your cabs may not be perfect but that is not your problem. You need now to eliminate something else like spark. You need good spark properly timed. You must completely recheck the Dyna timing and make sure you are not off and then check for a good spark on all 4. If spark checks out then the only things left are compression and valve timing.

If I was to place a bet from the symptoms you describe with the popping it would be on a timing issue with either spark or vlaves or maybe both.

That's where I've come to as well. I'm going to open the valve cover later this afternoon once the heat dies down a bit. I know the spark is good with the ignition, so I may have skipped timing somehow or another on the cam. I'll update once I get that checked.

[calj737]

My vote is you have set the valve clearances on the incorrect stroke. Remove a few tappet covers and position the ignition plate to 1/4 and check to see if you've got the gap you need. If you have fuel, have spark and are getting popping, you don't have compression/combustion.

[Cowsertc]

You can eliminate fuel as the problem.  Your cabs may not be perfect but that is not your problem. You need now to eliminate something else like spark. You need good spark properly timed. You must completely recheck the Dyna timing and make sure you are not off and then check for a good spark on all 4. If spark checks out then the only things left are compression and valve timing.

If I was to place a bet from the symptoms you describe with the popping it would be on a timing issue with either spark or vlaves or maybe both.

Thanks for all the help! This was it... My valve timing was off by roughly 20 degrees or so. Completely unsure how, but hey, it's correct now. Gonna put it all back together tonight and see how it goes. 

[Cowsertc]

Got it all put back together. Still not running, but have significantly more backfire, so now I guess it's a fuel issue? It really sounds like it wants to start, but it won't even with starting fluid. Getting very close though...

[Keith]

Test your ground circuit! Tighten those motor bolts...Can't see how cam timing changed...is chain worn?

[calj737]

My valve timing was off by roughly 20 degrees or so. Completely unsure how, but hey, it's correct now. Gonna put it all back together tonight and see how it goes. 

You can't "adjust" the valve timing without adjusting the valves as the valves can only be correctly adjusted when the ignition timing/cylinder position is correct. Otherwise you will bend a valve or not have compression when the ignition fires (lots of popping and backfires).

[Cowsertc]

My valve timing was off by roughly 20 degrees or so. Completely unsure how, but hey, it's correct now. Gonna put it all back together tonight and see how it goes. 

You can't "adjust" the valve timing without adjusting the valves as the valves can only be correctly adjusted when the ignition timing/cylinder position is correct. Otherwise you will bend a valve or not have compression when the ignition fires (lots of popping and backfires).

Alright, I'm pretty sure I get what you're saying. I corrected the timing marks on the cam with the t mark on the crank, and then I adjusted the clearances to .002mm on the intake and .003 on the exhaust making sure to line up the timing marks again. Is this what you're referring to?

Sorry for any confusion, and again, I really appreciate the help.

[calj737]

It is as long as you reference the timing marks for the correct cylinder pair, and adjust those cylinder's valves per the manual's sequence.

[Cowsertc]

It is as long as you reference the timing marks for the correct cylinder pair, and adjust those cylinder's valves per the manual's sequence.

I did it to spec listed in the clymers.

Still not starting today, I'm moving cross country at the beginning of September and starting to panic a little. 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. They have a proper bench sync, 1/8 in across all four. 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.

[seanbarney41]

left coil wires to cylinders 1 and 4? right coil wires to cylinder 2 and 3?

[madmtnmotors]

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.