CDI?

[mr_josh]

Got a buddy with a '79 CB750 that has an '80 motor.  Bought it a couple of weeks ago, immediately started losing power at about 4500 RPM.  Sometimes you can pull through 4500, but most of the time it'll hit that point, start bogging, spitting, popping out of the pipes, etc.

Today he took it for a drive and it quit on him.  Parked it for 20 minutes, it fired back up, rode it to his destination, never made it back home.  It spit and coughed and eventually died.  I went out and took a look at it, pulled a plug and grounded it (far-right side cylinder), noticed a really weak, intermittent spark.

I really want to say this is a CDI unit, but just can't say.

I guess what I'm asking is, where do I start looking?  Any resistance values for the spark trigger(s) on the crank?  Any way to test the CDI units?  Any tests for the coils?  Have multimeter, want to fix.

TIA, folks.

Josh

[TuNguyen-2]

It could be the problem with CDI or coil also with the carb too dirty. So I suggest to open carb and clean it up first. One more thing is have you check the fuel line ? if the tank is stuck fuel, the bike will turn off dueto it is not have gas enough.

[blackhawkrider]

Start with the easy cheap things like spark plugs. I have found several times that replacing spark plugs will help with weak spark. If they are new plugs than go to checking your wires and coils. If new plugs fix it though and it starts doing it again your carbs are suspect. Hope it helps.