CB750 Rebuild won't start

Buff CB

Buff CB

New Member
After doing a full rebuild on my 78 CB750F my worst fears are becoming reality. I can't get her to start for anything. Here are the bullet points to help describe what I've done so far.


-Full engine tear down. I've taken everything apart down to the crankcase separated.


-Rebuilt engine using pictures taken during tear down and a Clymer manual

-Took great care to position camshaft in correct timing with points and pistons.


-Carbs were taken off and cleaned by my father who only cleaned two. Those two were in great condition so he didn't touch the other two.


-Stock headers replaced but installed baffle for a shorter exhaust.




That's the build part here are the symptoms


-No burp or cough of any kind just constant cranking using the starter


-Backfires loudly when timing is retarded but nothing when "right on" or advanced.


-There are no pods or filters over the carbs right now but always starting with full choke.


-Double and triple checked that I'm getting spark at TDC when on the compression stoke on piston #1.


-All four plugs spark strongly.


-Have used starting fluid only to get more backfiring


-The engine does get flooded if I go at it too long so I know it's getting gas. It's new gas too.


-I've checked the timing with timing light while starting and it's bang on.


-I've checked the valve clearances all over and they are up to spec.


-I've checked the points clearance and they are also up to spec.


My dad is a hobby mechanic and he and I are both stumped profusely. The one thing I haven't checked yet is the compression due to me not having the right size plug to test it. I will have one next week and will test that as well.


Thanks for any info or insight anyone might have. I've watched this entire riding season go by and it's killing me.
 
Are the coils attached to the correct points or triggers? Sounds like it's trying to fire the wrong holes.
 
I've mapped the coils to the correct cylinders. There happened to be tags on each spark plug wire indicating 1,2,3,4.


There is only one way for the rotor to go in because of a set pin I do believe.


I also don't have the kill switch on accidentally.
 
I just finished my first 750 rebuild last week same thing was happening. I found out there was two "T" and
"F" timing marks set timing to other one which was 180 degrees difference and it started right up. Mine had intake opening instead of exhaust and exhaust instead of intake. Maybe its your prob maybe worth a shot. What gave mine away is I put my hand over exhaust and felt it sucking. Hope it helps.
 
To make a motor run, go back to the basics

Timing (Spark)
Gas
Compression

If you have these 3 things - motor will start, could always be other problems, but always start here. Learned this lesson the hard way TOO many times. Stick your finger over the spark plug hole, hit the starter button, if the compression blows your finger off the hole, you have enough compression to run. Try dropping a few drops of oil in each cylinder to build up compression - as this is the first time the engine will have run after rebuild. There are always tips you learn with each rebuild.

Good luck and keep us posted.
 
Try swapping the coil to trigger/points leads from one side to the other just to see.
 
Just checked compression and I am getting 60-80 psi on each cylinder dry and 130 wet. Since It's not a warm engine I believe that's pretty good.

Also checked the camshaft and found it was set just as the manual stated. Pistons 1 and 4 at TDC, notch on camshaft up and the timing mark on T for cylinders 1 and 4. There is nothing else I can think of that would indicate that the valve timing is 180 off.

I will try flipping the ignition wires again but now I am truly stumped since I had doubts about the camshaft.
 
With many hours of head scratching I finally got it running. The valve clearences were a bit off which didn't help but it turned out to be the wires from the harness going into the coils. I had yellow to yellow and blue to blue but after swapping them she started roughly but then got a lot better after warming up.

I don't understand why it works but it does, so I'm happy with that.

Thanks for the ideas and support guys!
 
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