Ignition problems, so confused, please help.

[The Fabricator]

This is my first post but not my first forum so I'm hoping you guys can help me, I'm completely confused . I have a 71 CB750 that I bought last fall and the guy who had it before me had it for 4 years but he never tried to start it. I'm trying to get it started so I can work on it through winter so its ready for next summer. I'll tell you everything I've done:

- new batter fully charged
-Gapped the spark plugs to .026"
-gapped the intake valves to .002" and the exhaust to .003"
-gapped the points to .012"
-tune up kit from Charlie's Place that had new points and condensers
-rebuilt the carburetors, set the floats to 26mm and turned the air adjust screw out one turn from bottoming out
-new spark plug caps and the correctly oriented to the right cylinders with clean spark plugs
-I didn't check the compression but the bike has 26,xxx miles on it, is very clean and when I kick it over it feels like it has good compression
-The yellow wire is going to the coil for cylinders 2-3 and the points, the blue wire is going to the 1-4 coil and points

I have good spark, I have fuel and air so the recipe for it to run is there but its spitting out the carbs. I did a static timing adjustment using a test light and I have it so the the points open on the "F" mark for both 1-4 and 2-3 so the time should be good enough to get it running so I can fine tune to full advance with a timing light. A local dealer said maybe I switched the cam around on the spark advance when I took it apart to cleaned it. I rotated the the cam 180 degrees but then the points for 1-4 were closing on their respective "F" mark and 2-3 points were closing on their respective "F" mark. That's not right so I turned the cam back around and was able to time the points correctly to how the four manuals I have say they should be. When I go to start it its spitting out the carbs and I might get a pop out of one of the exhausts. I've been banging my head against the wall with this and I have no idea where to go from here. As far as I know everything has been done correctly but something seems like its 180 degrees off from where it should be? Did someone put the cam in backwards?!? That's a joke since I don't believe its possible and I see no signs that the engine has ever been taken apart in anyway. Any help would be greatly appreciated and I'm completely open minded to ideas; thanks

 

[dirtdigger]

I would verify that cylinder 1-4 points are opening on cylinder 1 compression stroke. Put cylinder to tdc 1 and check that both intake and exhaust valves are closed on cylinder 1. Then verify that the 1-4 points are indeed opening on compression stroke of cylinder 1. Then verify that the wiring for 1-4 points is going to the coil for cylinder 1-4 and that it hasn't been swapped for coil 2-3. It sounds like you have a decent handle on the mechanical aspect of things so if it sounds like its 180 something with the points or wiring swapped. If by some chance the cam was installed 180 off of correct in theory all you would need to do is swap the wire from the point to the coil because that will in effect put the cam in time with the points once again because they both can either be 180 off or they are in time.

 

[The Fabricator]

Thanks for the response. I don't think the came has ever been removed but I was at my wits end and was brainstorming. I checked and the wires from the coils to the points are correct. When sitting on the bike the left coil is for cylinders 1 and 4 which contains the blue wire that goes to the points for cylinders 1-4. The coil on the right has the yellow wire that goes the 2-3 points. If by some chance the cam was wrong I see what you're saying about just switching the wires to put the cam and ignition in sync. None of the manuals said to check that the pistons were at TDC while checking if the points were opening at the same time. I might try flipping the cam on for the time advancement just to see what happens. If it doesn't work its one less thing to consider.

 

[The Fabricator]

Well I got it running! Instead of turning the cam around I took the blue and yellow wires that come out of the harness by the coils and switched them and it started right up without hesitation. I looked to see if maybe the wires got switched around where they come out by the rear brake switch and they were correct so I have no idea where the wiring was mixed up. I could put the switch the wires back and then switch the advance cam but then I would have to do the timing opposite so I'll just leave the wires switched. I'm just glad its running.

 

[K2750tracy]

on my 72 the blue wire from the points conects to the yellow on the coil, same with the yellow form points to the blue on coil. thru me for a loop too. i had same problem when i did my wiring. took the bike to a 750 guru he laughed , switched the wires and started it right up. just me but i thought yellow to yellow

 

[The Fabricator]

That's really weird that they would not match the colors. Its not just you and I'm sure there others because to me, common sense would have yellow to yellow and blue to blue. It caused me an awful lot of grief trying to figure it out what was wrong when everything else was as it should be. My neighbors probably heard all my guttural sounds, yelling, and swearing as I was pulling out my hair haha.