Honda CB750F 1981 does not start (plugs, wires, spark, coils all fine)

[RobertV]

Hi everyone,

I have a problem with starting the bike. Suddenly it does not want to start any more, before summer all worked fine. I have checked compression (110-120 PSI /wet), spark plugs are new, caps and wires from coils are new. There is strong visible spark on all 4, coils voltage is in spec also sparks unit are fine also in spec. I tried to spray start fluid into carbs and only fires for 1 sec, then nothing. I completely emptied the start spray fluid and nothing happens. Start/kill switch is working fine. I am out of ideas and I would appreciate any good information.
I have checked the forum thread I found a lot info and checked everything what I could read and think of. Help please.

Regards,
Robert

 

[Bo82CB]

Check did you connect plug wires to correct coils!

 

[RobertV]

Check did you connect plug wires to correct coils!

This is how it looks on the coils, it should be ok but unfortunately is not working, but as I said sparks are working fine.

 

[RobertV]

View attachment 13843
This is how it looks on the coils, it should be ok but unfortunately is not working, but as I said sparks are working fine.

Left: cylinder 1 and 4
Right: cylinder 2 and 3

 

[fallawc]

Random question, how do you check voltage on the coils? I want to check this on mine..

 

[RobertV]

Random question, how do you check voltage on the coils? I want to check this on mine..

Hi,
i took it to local car electrical service where they have checked it. You can also check on youtube "mechanic mike" where he shows you the procedure with multimeter ofcourse, its simple and I did it as well. For double check I took it to local car electrical service just to be sure.

 

[SenseAmidMadness]

You've already eliminated compression and spark as possible causes. The only remaining possibles are fuel, air, and timing.

If the bike sat for a while without running it's possible that the carburetors are clogged up with bad fuel and will need cleaning. The fact that the bike fires on starting fluid tells me that it's probably timed right.

 

[Biebs]

Classic air filter clogged check for mice / rats.

 

[brettp]

Check the fuel delivery diaphragm on top of the carb. I removed mine and she fired right up. If the hoses are connected to that diaphragm have leaks they won't produce the vacuum necessary to deliver fuel.

 

[mrtwowheel]

Petcocks can clog. Unhook the petcock hose see if it flows on ON and RES. Don't start a fire or explosion. Next, fuel filter, does it flow? How big is your fuel fillter? The big diameter filters cause air locks. If you suspect an airlock open the gas cap and blow into it just enough to cause a little pressure in the tank. DON'T INHALE GAS FUMES, that will make you stupid.

Scott

 

[RobertV]

Check the fuel delivery diaphragm on top of the carb. I removed mine and she fired right up. If the hoses are connected to that diaphragm have leaks they won't produce the vacuum necessary to deliver fuel.

How do you mean remove it, I don't understand how should this work. Do you have any picture maybe, it would help me a lot. Thank you.

 

[RobertV]

You've already eliminated compression and spark as possible causes. The only remaining possibles are fuel, air, and timing.

If the bike sat for a while without running it's possible that the carburetors are clogged up with bad fuel and will need cleaning. The fact that the bike fires on starting fluid tells me that it's probably timed right.

Thank you for your answer, just to be sure could you advise me how to check if the timing is ok. I tried again with starting fluid but now nothing happens.

 

[brettp]

1. Follow the fuel line from the petcock to the fuel diaphragm on the top of the carb.
2. Find the fuel line that goes into the carb from the diaphragm.
3. Now make the fuel line from the petcock go straight to the carb.