79 CB750 spark question and engine miss at speeds.

[Hippie459MN]

Well, I got my CB running ok but now I am having another (possibly two) issue. When I am under a mid to hard acceleration the bike runs like a champ but when I am at a constant cruising speed it feel like its missing and only running on 2 cylinders but if I give it gas it will smooth right out. It does have the stock CV carbs on it with pods but all of the jets are factory and I have not changed them. I had to give the carb a really good cleaning this spring and had to readjust the pilot jets to the 1 1/2 turns out that the service manual said they should be at and that seemed to help a lot but now when I am at a constant speed it seems like the bike is missing/not running on all cylinder and when I let off the gas, slowing down, it sounds like I am getting some faint popping out of the exhaust and sometimes at night you can see some flame out of them and its usually the number 1 and number 4 pipes. It does have short straight pipes but I have put in some short (4 inch if I remember right) baffles in each pipe. Thinking a bench sync might be my next try. Going to be ordering a jet kit this week and then I can give that a try too. All of my plugs are kind of a light brown color.

This is the number 1 (right) and number 2 (left) plug. 3 and 4 look pretty much the same.

Now I was checking the spark on each cylinder by pulling all the plugs and turning it over and the no. 1 and no. 4 plug were doing something I think is very odd. Its like there was left over charger after I was done cranking the motor and the plugs would spark a bit a second or two after I was done cranking the motor. I have never seen this and pretty sure that is not supposed to happen. The video is below....

Also, I did ohm out the coils and was getting 2.5 ohms on the one coil (cylinder 1 and 3) and 2.6 (cylinder 2 and 4)on the other but can not find in the manual what it should be and the secondary side I wasnt able to get anything. I did not do the coil test yet like the manual says but everywhere I have kind of been looking it says that my coils should ohm out more in the 3.5-5 ohm range and anything lower would need replacing. I hate to just start throwing parts at this thing and not have it do anything. This thing ran fine all last year (except it idled like crap but was liveable) until the alt rotor went out and then I let it site full of gas for 6 months gumming up the carbs. Stupid me....

Edit: Oh yeah, I had disconnected the plugs for the spark units and they may have gotten switched around but I am not 100% so I am going to maybe switch them around here tonight and see if it runs any different. I am not sure if that makes a difference or not. Maybe someone else knows... Anyone? lol

 

[Hippie459MN]

Any help or ideas would be awesome before I throw this bike away! LoL Its been one nightmare after another this year and I cant even go riding without having issues with it running right. :-(

 

[dan1951]

Plugs look OK, It,s been decades since I left electronics school But from what I can recollect the after spark is normal the high voltage CDI is meant for multiple firings say 1000rpms and on up, By cranking the engine 30 rpms or so Your still charging the high capacitance system to the max as if it were running when You shut it off that excess high voltage discharges even without the hall effect trigger. I have a post on here about the popping out of the 4 into 4 Mine still does it, Off hand I would say carb. problems but seen strange stuff with these bikes. The spark units should be wired right as they are assigned to the specific coil Good luck

 

[Hippie459MN]

That makes complete sense. Also, the steady rpm miss has boiled down to the carbs. Got a few really go replies on another forum and leaning towards the carbs. Ill play around with them this weekend and see what I can come up with.