Topic: CB400f won't start after stopping when HOT!

[redlabel]

Hi!
I have a 1975 CB400f in good condition. Clean tank and carbs. Have been using the bike around town without any problems.
Yesterday I took her out on the road and drove about 40KM with revs steady around 4000rpms. I stopped for a chat with a friend and when I tried to start it she didn't fire at all! Cranked over, but no ignition.
I let her cool down an hour and tried again and she fired right up and I rode all the way home for another 40KM without a hick up.

I run straight pipe and open filters with #38 Slow jet and #85 main jet.
Anyone with any insight to my issues?

[HondaMan]

Hi!
I have a 1975 CB400f in good condition. Clean tank and carbs. Have been using the bike around town without any problems.
Yesterday I took her out on the road and drove about 40KM with revs steady around 4000rpms. I stopped for a chat with a friend and when I tried to start it she didn't fire at all! Cranked over, but no ignition.
I let her cool down an hour and tried again and she fired right up and I rode all the way home for another 40KM without a hick up.

I run straight pipe and open filters with #38 Slow jet and #85 main jet.
Anyone with any insight to my issues?

The first things that come to mind are:
1. Old condensors?
2. Bad coil. When these get old and the insulation breaks down in them (on the HV side), they lose voltage when hot. Typically it drops to about 3000 volts (less than 50% of normal). Sometimes it also starts breaking up or gets intermittent. When cooled off, it runs again, or even if continuously run, it will keep going. But when stopped, the heat builds up more under the tank and they show their "true colors" in secret!

[redlabel]

Thank you Hondaman!
I have Dyna S ignition so the Condensors is not the problem, but I do run the stock coils.....
Only way to test  is to run it as hot as last time and check for spark?
Should I get the Dyna 5ohm coils? I also run the stock spark plugs, NGK DR8ESL. I shouldn't change these for another heatrange when running straight pipes and open filters?.....I just have to ask. 

[HondaMan]

Thank you Hondaman!
I have Dyna S ignition so the Condensors is not the problem, but I do run the stock coils.....
Only way to test  is to run it as hot as last time and check for spark?
Should I get the Dyna 5ohm coils? I also run the stock spark plugs, NGK DR8ESL. I shouldn't change these for another heatrange when running straight pipes and open filters?.....I just have to ask. 

Have you checked the resistance of the sparkplug caps? They should be (if OEM) between 7500 and 8500 ohms, and on any one coil they must be within 800 ohms of each other on the stock coils (if stock plug caps). If newer NGK caps, they should be 5000-5800 ohms, and not more than 500 ohms different from each other.

The Dyna S also has a long history of "dying" when hot. What actually happens is: the transistor switches (actually they are Hall Effect magnetic devices) leak electrically when hot. This causes them to not turn OFF all the way when the magnet swings by, so the spark output is weak, and very short. After it cools back off, they work again.

Historically speaking, each time this happened to my own Dyna (twice), they died soon afterward. That was what finally drove me, in 2006, to bring my old Transistor Ignition system "back to life", so to speak.

[redlabel]

Have you checked the resistance of the sparkplug caps? They should be (if OEM) between 7500 and 8500 ohms, and on any one coil they must be within 800 ohms of each other on the stock coils (if stock plug caps). If newer NGK caps, they should be 5000-5800 ohms, and not more than 500 ohms different from each other.

The Dyna S also has a long history of "dying" when hot. What actually happens is: the transistor switches (actually they are Hall Effect magnetic devices) leak electrically when hot. This causes them to not turn OFF all the way when the magnet swings by, so the spark output is weak, and very short. After it cools back off, they work again.

Historically speaking, each time this happened to my own Dyna (twice), they died soon afterward. That was what finally drove me, in 2006, to bring my old Transistor Ignition system "back to life", so to speak.

The caps are new NGKs VD05F so the 5K Ohm should be correct. I will check. The plugs are standard NGK DR8ESL. I really hope it's not the Dyna S! I thought that the Dyna was the way to go to have a safe ride
It happened again today, but fired right up again after 10min cooldown.