Topic: CB400F Not Firing On All Cylinders - Weak Spark?

[RickB]

Hi guys,

I hope this will be my last plea for help. Two years into rebuilding my first bike, I'm due to move to the USA for good at the end of the week. I've yet to ride it. This is my last problem, I hope.

I have been scouring the forums for solutions to this and have read a few helpful things, but I need some hand holding.

I have a 1975 CB400F. Rebuilt motor, new pistons, rings, camshaft, rebuilt carbs, new needles, jets etc, stock airbox, new fully charged battery, Dyna S ignition with timing set, new Dyna 5 ohm coils tested and passed for primary and secondary resistance, 7mm wires (black suppressor core DW-600), new DES8L plugs, stock wiring harness with relays for push button starter and to take power from the stop/run/stop switch.

The issue. The bike will start easily with full choke, which I back off promptly. It will idle for short time, then it will pop and fart out the airbox, then bog down and die. The 1 and 4 headers will stay cold. Or occasionally just the number 1.

I have pulled the plugs, laid then on the engine casings and watched the spark. It is a faint purple spark. I have 9 new spark plugs. Sometimes one won't fire. I'll swap it for another one and it will. It appears to be an intermittent problem. I've inspected, disconnected and reconnected the coils and leads. All appears good.

It has idled and run well in the past, but only when I've had the battery directly wired to the ignition. If not, after a couple minutes it will foul the plugs and stop starting all together.

I've probably pulled the plugs 20 times trying to read the tea leaves. When it pops out the air filter the clip on cover almost pops off and the engine bogs down.

I know these plugs look a bit sooty but having seen similar photos from other 350Fs and 400Fs I think this is acceptable. Except for that real clean one!



#1 doesn't even appear to be firing.



#1



#2



#3



#4

Everything is stock besides the muffler. It doesn't have any direct baffling (ie you can see straight through it).

The Dyna S ignition is firing the plug, I've tested it with a plug in it grounded against the engine and shocked myself more than once. I've tried a total of 9 different plugs in it too. However as I said, it appears a faint purple, not a thick white spark.

I suspect the coils aren't getting enough voltage. There is a voltage test procedure given in the Dyna S instructions, but as I didn't install the Dyna S system, it's a bit foreign to me. I will read through it again tomorrow and attempt to test it, but has anyone hear got any suggestions for what could be causing a weak spark, and hence misfiring, on a CB400F with Dyna S ignition and 5 ohm coils? Bad/corroded terminals? Incorrect adjustment of the ignition? Faulty plug leads?

Once again, sorry for the novel of a question, just wanted to get all the information out there in case someone has had a similar problem and could walk be through the trouble shooting procedure.

Thanks in advance,
Rick.

[DarcyCB400F]

Rick,
You have covered a lot of points except the carb balance? Have you performed a carb balance?
FWIW unbalanced carbs can create a lot of havoc!

[RickB]

Darcy,

The carbs were balanced by my mechanic yes. He is fantastic with the mechanical stuff, set all my floats, idle screws, mixtures etc when he had it running. He's just not great with electrics. Hence why Chickenman_26 and TwoTired were able to get my battery charging!

The mechanic reckons the coils are under powered, but Dyna says the 5 ohm coils are recommended for 'street bikes'. I've had issues with the electrics, that's why I suspect it's something relatively simple causing a weak spark. Of course, I could be, and often am, wrong.

Rick.

[HondanutRider]

Rick,

You've probably covered this already, but with the popping and dying, have you checked your valves and verified that the tappet clearances are good?  I have a 350 that was rough at first and although I was certain my tappets had been done, when I redid them it solved the problem.

[Doctorlumen]

If 1 and 4 are the issue (but it sounds like more 1 than 4?) my first instinct is the coil that feeds them. But, you tested them. And they are new. So, what about the lines from the harness to those coils?
Is the #1 plug wet? Does it smell of gas?
Could a main jet or something be clogged in one of those two carbs? Not to short your mechanic, but as Darcy mentioned, these carbs create a LOT of woes seemingly out of nowhere. You can check the jets on 1 and 4 fairly easily with the carbs on the bike. Maybe give them a squirt of carb cleaner? Or OR....I like this idea...when you get her to idle, tap the 1 and 4 carb floats with the end of a ratchet and see if the problem occurs, or more importantly if when it occurs, the problem works itself out. Sounds stupid, but if there is a little wad of crap floating around in there somewhere, this might be your chance to help it along into oblivion.

I also agree with the valve adjust. Check them just to be sure.
There is one other thing...this one might be far fetched. You may have a bent valve. I've worked on bikes that were built fresh, timed and tuned to spec, but seemed to just run lean...like the lean you are describing. Turned out, one of the valves was bent just enough to stick when it warmed up. But those plugs were all dark to a point, not perfectly clean.

Please tell me you don't have to leave the bike behind when you move. 

[Mars]

Are you positive that you hooked up the spark plug wires correctly? My CB400f was said to "bog out in the top end" by the PO. Things got a lot better when I attached the spark plug wires correctly..

[RickB]

Thanks guys,

Yep, the tappets are set, I watched the mechanic do them. Then we rechecked them again, just to be sure.

When I pull the plug from the offending cylinder, it's always wet with fuel. I agree doctorlumen, I suspect it is the 40 years old wiring to the coils and/or ignition rather than the brand new components. At least that's what seems more plausible.

So I'd like to go through the wiring harness and clean/check all the terminals that supply power to the coils. Trouble is, I don't really know what those terminals are. I learn't a lot about the charging system when I was hunting down my bad rectifier, but the ignition is foreign to me. So if anyone has a list of the terminals/coupling I should be checking on the way to the coils, that would be most helpful.

I'd also like to test my spark plug leads to verify they are functioning correctly. What resistance should they have?

Rick.

[RickB]

So today was an interesting day.

So I did the voltage tests Dyna give in their support documentation that come with the ignition, everything checked out. I polished/cleaned the couplers at the solenoid, the solenoid/harness and key switch. I then took the leads off and measured them for resistance...

Spark plug lead resistance with tester on 20k ohm setting:

#1: 6.95
#2: 5.89
#3: 5.62
#4: 6.79

I immediately thought this was odd, given that I'm having trouble with the 1 and 4 cylinders. I then checked the length. Yep, 1 and 4 leads were a good 3 inches longer than 2 and 3. So I cut them all down to the same length (about 25cm/11inches). This was the shortest I could safely go with number 4, so I decided to make them all the same length.

I then checked for resistance again. They were all around 5.30 ohm +/- .20 or so.

I reinstalled them, started the bike and wouldn't ya' know it, it ran like a dream. Thick spark and firing on all cylinders!

I would never have thought cutting my leads down by 4 inches would make such a difference. Anyway, it is chalk and cheese now. I even went for my first ride!

I made it half way home and then a fuse blew. I had to push the bike 2km home in the dark. The lower 15A fuse now blows as soon as I turn the key. No power, no lights, nothing. No idea what has changed, I was just riding along then it cut out. Maybe something shook loose and shorted?

Anyone know what circuit that lower 15A fuse is on? That should help me troubleshoot. Could it be the battery is overcharging from incorrect adjustment of the voltage regulator?

So I'm on the hunt for more electrical gremlins but feeling very positive.

Thanks again for all the help guys,
Rick

[MoMo]

Rick,  does it blow as soon as the key is turned? Normal culprits can be the fuse box, ignition switch and a pinched wire anywhere.. Glad you finally got to ride...Larry

[DarcyCB400F]

I have a very gooood wiring diagraam. If you PM your email address, I will send it.
Glad to see you figured out your spark issue!!

[RickB]

Hi guys,

I fixed the fuse blowing issue. It was an unshielded section of wire from the neutral light switch down near the starter motor. I didn't find it until I'd checked the ignition and both switches, but once I'd covered the exposed wiring with some heat shrink, I was good to go.

Thanks again for all the great help,
Rick.

[DarcyCB400F]

Great news Rick!
Those electrical gremlins can be a pain for sure!