new guy cb750 build

[crawdadman]

well, this bike might have to sit on the back burner for a while :'( One of the clutch spring bolts sheared off on my dirt bike!! no money, and a big race this weekend... dammit!!

 

[crawdadman]

checked the pulse generator gap, good there. I will need to take the bike to my mechanic to do a compression test, and sync the carbs. Timing is also right on. I thought of something last night though, do I need to replace the Rectifier? I have an 8 volt ballistic battery. I was told that i would not have to replace anything, also that it would turn over the motor alot easier.

 

[coyote13]

Why 8 volt? Aren't all CB750's 12 volt?

 

[Rich Ard]

That's an 8 cell, not 8 volt battery. I would not run it without upgrading to a modern r/r or you will have an expensive doorstop if your charging system is no longer up to snuff.

 

[crawdadman]

That's an 8 cell, not 8 volt battery. I would not run it without upgrading to a modern r/r or you will have an expensive doorstop if your charging system is no longer up to snuff.

i have access to a newer cbr600 regulator. Would something like that work?

 

[crawdadman]

i have access to a newer cbr600 regulator. Would something like that work?

or any other affordable options? couldn't find one under 90 bucks.

 

[crawdadman]

I am baffled! checked both coils, one side wasnt getting spark. bench tested them again and their good. plugged them back up and swapped the blue wire and the yellow wire. Which ever side I have the blue wire on works. So checked the yellow wire.....and it is perfect..spark packs perfect....have no idea what it could be seeing as though i have traced every wire, and found no problems. Any ideas??

 

[notlob]

The black/white wire is pemenantly 12v when the ignition is switched on the blue and yellow are from each ignitor box and control te timing of the spark to match either 1 and 4 or 2 and 3 plugs. You can connect them to either coil and should still get a spark. However they need to be connected to the correct coil for it to run properly. From memory I think blue is 1+4 and yellow 2+3 coils.

When you have a problem like this its a process of elimination. You need to start at the beginning the pulse coils and work your way through. The process is in the manual at chapter 17. Its hit and miss testing random parts of the system.

Once your satisfied the ignition system is functional do the same with the charging system.

 

[crawdadman]

The black/white wire is pemenantly 12v when the ignition is switched on the blue and yellow are from each ignitor box and control te timing of the spark to match either 1 and 4 or 2 and 3 plugs. You can connect them to either coil and should still get a spark. However they need to be connected to the correct coil for it to run properly. From memory I think blue is 1+4 and yellow 2+3 coils.

When you have a problem like this its a process of elimination. You need to start at the beginning the pulse coils and work your way through. The process is in the manual at chapter 17. Its hit and miss testing random parts of the system.

Once your satisfied the ignition system is functional do the same with the charging system.

Thanks man. It probably is something very stupid. though i have gone through the manual.
I am just confused why which ever coil i have the yellow wire on works.
I might just take the whole harness off and attempt to make my own

 

[notlob]

When you say which ever one do you mean swapping between coils or swapping between the connectors on the same coil.

If the former, what happens when you swap the blue do you not a spark?

 

[crawdadman]

When you say which ever one do you mean swapping between coils or swapping between the connectors on the same coil.

If the former, what happens when you swap the blue do you not a spark?

swapping between coils. whichever coil i put the blue wire on works....maybe yellow isn't grounded? my schematic is all blurry and i cant tell what color wire goes were lol

 

[notlob]

putting the blue or yellow wire to either coilshould make no difference to getting a spark all they do is produce the spark at different times to coresspond with which cylinder they are working.
Therefore if the blue produces a spark at the plugs on either coil then that is OK. If the yellow does not produce any sparks on any of the coils that means either the ignitor unit is faulty or the pulse generator coil is faulty.

To test the ignitor just swap the ignitor connectors over. If the yellow starts working and blue doesn't then its a faulty ignitor and you will have to replace it.
If after swapping over the ignitors there is no change still the blue working and the yellow not, then it will be a pulse generator coil faulty.
To check if its a pulse coil, remove the left side cover and put a spark plug in one of the HT leads of each coil and earth them against the engine. then take a large flat blade screwdriver and put it between the metal tab on the coil (the one where you set the coil gap previously) and the cam in the center, earthing each coil in turn should make one of the spark plugs spark or make a clicking noise. If one of the coils does not do that then thats the faulty one. Oh you need the ignition switched on as and all the wiring to the ignitors and coils plugged in in.

 

[crawdadman]

putting the blue or yellow wire to either coilshould make no difference to getting a spark all they do is produce the spark at different times to coresspond with which cylinder they are working.
Therefore if the blue produces a spark at the plugs on either coil then that is OK. If the yellow does not produce any sparks on any of the coils that means either the ignitor unit is faulty or the pulse generator coil is faulty.

To test the ignitor just swap the ignitor connectors over. If the yellow starts working and blue doesn't then its a faulty ignitor and you will have to replace it.
If after swapping over the ignitors there is no change still the blue working and the yellow not, then it will be a pulse generator coil faulty.
To check if its a pulse coil, remove the left side cover and put a spark plug in one of the HT leads of each coil and earth them against the engine. then take a large flat blade screwdriver and put it between the metal tab on the coil (the one where you set the coil gap previously) and the cam in the center, earthing each coil in turn should make one of the spark plugs spark or make a clicking noise. If one of the coils does not do that then thats the faulty one. Oh you need the ignition switched on as and all the wiring to the ignitors and coils plugged in in.

ignitor units are good. must be the pulse generator coil. I tested each coil the way you explained above. trial and error i guess..

 

[Hoosier Daddy]

I must admit, I am having a rough time following along. But it is really early here and I haven't had much sleep.
If it ran before you installed the 600's coils, have you tried putting the old ones back on? Retrace what you did and see if you swapped something or left something off. There are connectors at the small coil wire harness, each spark unit, and at the pulse generator. Check each one for corroded terminals, good connection. I don't think your yellow wire should be ground. (I attached a close up of the circut from the Factory Service Manual wireing Diagram at the bottom of this post... click it to make it full size.)

ignitor units are good. must be the pulse generator coil. I tested each coil the way you explained above. trial and error i guess..

Proper diagnosis is NOT "trial and error"
Notlob is giving good advise to help you out. He is telling you how to determine which part is bad. Don't test just one and assume the other is bad. keep testing until you find the failure, otherwise you may replace a bunch of good parts before stumbling across the final solution if you just S.W.A.G. it.
You said a manual came with the bike, but the schematic is blury... Here are links to the FACTORY service manual chapters I posted here on DTT a while back.
http://www.dotheton.com/forum/index.php?topic=44399.0
Without knowing exactly what year and model of DOHC CB750 you have I can only tell you the wiring schematics are towards the bottom of the list, find the one for you.

Here is a GENERAL color schematic from the Clymers manual for the DOHC 750's that may help but it is NOT exact for every one due to model / year changes. I still recommend the factory ones I referanced above, as well as the trouble shooting sections in Mother Honda's service manual.

 

[crawdadman]

Notlob is giving good advise to help you out. He is telling you how to determine which part is bad. Don't test just one and assume the other is bad. keep testing until you find the failure, otherwise you may replace a bunch of good parts before stumbling across the final solution if you just S.W.A.G. it.
You said a manual came with the bike, but the schematic is blury... Here are links to the FACTORY service manual chapters I posted here on DTT a while back.
http://www.dotheton.com/forum/index.php?topic=44399.0

thanks man! that schematic is alot better than the one that came with my bike, wish i had a printer though.

But i finally found the problem yesterday....one of the wires on the pulse generator coil was mashed. It must have happened when i tore the motor down. Now all i have to do is adjust the valves, adjust the carbs, and keep my fingers crossed." Devil is in the detail" is such an understatement!!

 

[pr design]

just call your local staples or office depot and get their print shop email. Copy the file and send it to them pick it up at 80 cents or there abouts.

 

[crawdadman]

got all the electrical done yesterday...did a compression test today. The most compression i had was 80 psi on number 3 piston. Valves are shot!! :-[ and of course it is cheaper to find a whole head on ebay. Just hope I don't buy someone else's problem.

 

[notlob]

got all the electrical done yesterday...did a compression test today. The most compression i had was 80 psi on number 3 piston. Valves are shot!! :-[ and of course it is cheaper to find a whole head on ebay. Just hope I don't buy someone else's problem.

Did you do the compression test with a warm engine? or just spinning it on the starter?

Could be a false economy to buy another head, it might need the valves doing as well. Would be better to strip your own head down first to see if the valves just need lapping in or if they need recutting. If they just need lapping in then it can be done for the for the cost of a few valve shims, valve stem seals and few hours of tedious valve lapping.

 

[Hoosier Daddy]

Compression test done with the choke off and throttle wide open?

 

[crawdadman]

Did you do the compression test with a warm engine? or just spinning it on the starter?

Could be a false economy to buy another head, it might need the valves doing as well. Would be better to strip your own head down first to see if the valves just need lapping in or if they need recutting. If they just need lapping in then it can be done for the for the cost of a few valve shims, valve stem seals and few hours of tedious valve lapping.

spinning it on the starter, throttle wide open and choke off.... Pulled the head and did a leak test, every valve leaked. I dont have a valve spring compressor, so i took it to a mechanic and was quoted 150 to pull it all apart.