Topic: Need ignition help for 400F

[Joel]

I've been having trouble getting my 400F to idle and run smoothly.  Lately, it hesitates under load when throttled up.  I pulled the plug boots one by one and the engine would die almost immediately when 1 or 4 were pulled.  It would keep running, sometimes with no change in RPMs, when 2 or 3 were pulled.  I unhooked the coils and removed the plug caps from the wires to test the coils.  The 1/4 coil was about 4.8 ohms on the primary and about 15k ohms on the secondary.  The 2/3 coil was just over 5 ohms on the primary and just under 21k ohms on the secondary.  I'd like to know if the high secondary resistance on the 2/3 coil is a sign it will need replaced soon.  I tested each of the plug caps and they were about 10k ohms each.

[HondaMan]

Sounds like you need a new 2/3 coil...

[Joel]

I was afraid of that.  Are there any options other than used OEM or 5 ohm Dynas?

Also, are the plug cap resistances correct?  I don't see a specification in the shop manual.

[BVCB650]

Bikebandit has the right side one for $95.

[HondaMan]

I was afraid of that.  Are there any options other than used OEM or 5 ohm Dynas?

Also, are the plug cap resistances correct?  I don't see a specification in the shop manual.

Any of the used (or new) SOHC4 coils will work fine. Their primaries are all within 0.4 ohms of each other, and the 750 has the weakest voltage, at 7500 volts (but longest duration at 1.5mS discharge). The 500/550/400F are all higher voltage, about 8000 volts in actual tests. Their duration is shorter, at about 1.2mS discharge time.

The upper limit for the 750/500 cap resistance is 9500 ohms, the 550 a little higher (TT says 10k is new on those). I don't have a reference for the 400F, but imagine it to be almost identical to the 550.

[Joel]

I bought a used pair off eBay and used the left-hand coil to replace the installed one.  I'm still having the same problem.  The symptom only seems to appear after a few miles of highway riding and a stop.  The first time, I stopped at a red light after about 3 miles of riding.  When the light turned green, I took off and the bike had uneven and slow acceleration.  The second time, I stopped at a red light to turn left and the same symptoms appeared when the light turned green and I accelerated away.

I have the right-hand coil from the eBay pair I bought and once I can break loose the frozen mounting bolts I'll swap that one into the left-hand position and try it again.  Are the factory coils on these bikes prone to problems when warm?

I tested the used left-hand coil before I installed it.  The secondary was good at around 15k ohms but the primary fluctuated rather significantly.  The reading varied when the wires were moved around.  The primary wires look to be soldered onto the coil.  I thought I remembered seeing a thread where someone had cut the plug wires off the secondary on an OEM coil, drilled and tapped holes and put replaceable wires on them.  I'd try that if it might help.

[HondaMan]

If you are running points, try swapping the condensors to see if the problem follows a possibly bad one.

If you are running Dyna S, make sure the gaps between the pickups and the trigger magnet are both equal and within Dyna's distance spec.

Also, check the connectors at the right rear top of the engine: these can get loose or dirty and cause issues (sorry, I thought you had checked these things before the first post...  ).

[Joel]

I still have points.  I replaced them and the condensers last summer with new items.  I'll try swapping and see what happens.  I'll also check that connector.  I didn't know there was one back there. 

[TimV840]

So, are you not getting any spark from these 2 cylinders? I didn't read anywhere that you actually checked that. Check spark right out of the plug wire/boot first before anything else....

Tim

[Joel]

Ok. Finally had some time to tinker. I swapped the condensors and remounted all the plug boots after cutting about a quarter inch off each wire. It still idles about the same, which is rather lumpy. It will now sputter and die no matter which plug boot is pulled.

Is heat a bad thing for the plug wires and boots? The boots were all pretty warm, especially on the plug end which if expect. The wire for cylinder three runs almost against the head and seems to be quite a bit warmer than the others.

I may get a new set of boots and see if that helps or clears it up. I expect the set on it are probably 1975 vintage.

[HondaMan]

Ok. Finally had some time to tinker. I swapped the condensors and remounted all the plug boots after cutting about a quarter inch off each wire. It still idles about the same, which is rather lumpy. It will now sputter and die no matter which plug boot is pulled.

Is heat a bad thing for the plug wires and boots? The boots were all pretty warm, especially on the plug end which if expect. The wire for cylinder three runs almost against the head and seems to be quite a bit warmer than the others.

I may get a new set of boots and see if that helps or clears it up. I expect the set on it are probably 1975 vintage.

If you take that ohmmeter to the plug caps, you can check them: test from the little screw on the wire's side to the brass contact on the plug side. It should read between 7500 and 10,000 ohms, no more. If you get new ones, get the "generic" NGK caps (in the right shape) in 10,000 ohm resistance for the 400F.

Oddly enough, I have replaced far more left-side coils than right-side, over the years. No clue as to 'why', though. 

[Joel]

I tested all the caps. They were 11.08k, 9.98k, 9.8k and 9.15k.

I haven't had a chance to test spark at the plugs yet. Does it matter if I test spark from the wire or should I pull each plug and ground it against the engine?

I looked for the connector at the back of the engine. The only one I saw was an 8 conductor (4x2) white plug on the rider's left side at the back of the engine case. It looked clean and firmly connected.

With a fully charged battery, I'm getting 12.51V across the battery, 11.45V from battery source to engine ground and 9.98V from battery ground to coil source. Both coil primaries are 5 ohms or less and both coil secondaries are 14.5k ohms or less.

And thanks for the help so far fellas. I'd be pulling hair out without this place.

[Spanner 1]

I think your 2+3 slides in your carbs are bottomed-out at idle...same symptoms as no spark, slides sitting on the bottom = no run, not ignition at all.
Done a carb sync....?

[HondaMan]

I tested all the caps. They were 11.08k, 9.98k, 9.8k and 9.15k.

I haven't had a chance to test spark at the plugs yet. Does it matter if I test spark from the wire or should I pull each plug and ground it against the engine?

I looked for the connector at the back of the engine. The only one I saw was an 8 conductor (4x2) white plug on the rider's left side at the back of the engine case. It looked clean and firmly connected.

With a fully charged battery, I'm getting 12.51V across the battery, 11.45V from battery source to engine ground and 9.98V from battery ground to coil source. Both coil primaries are 5 ohms or less and both coil secondaries are 14.5k ohms or less.

And thanks for the help so far fellas. I'd be pulling hair out without this place.

Based on these readings, here's what you can expect to find/get:

Plug caps: I'm assuming (uh, oh...) that you have them listed as 1-2-3-4 or 4-3-2-1 in order, there: the #1-#4 pair will have weak spark on the side where the 11K cap is. Replace at least that PAIR of caps, together. NGK makes some nice-looking 10k caps for about $5 each.

The coil voltage is a bit lower than it should be: this circuit goes from the BLACK wire in the headlight to the KILL switch, then back to the headlight, then to the coils. Follow those connectors and clean each one. The KILL switch itself can have some corrosion on it, but this will actually wipe off is you operate the switch back & forth 20 or 30 times, which I do to every bike that comes by here. It's interesting how many of them end up with better RPM afterward. The voltage at the coils should be no less than 1.0 volts below battery voltage for best results (10.45 volts minimum in this case). The coils will still run down to 7.0 volts, but the engine won't rev past about 4000 RPM at this voltage.


Also, try to find the GREEN ground ring to the frame: it's either near the coils or under the seat somewhere (I forget where the 400 has it). Clean this well: it serves the whole wiring/charging system, and is equally important to the engine ground cable (did you pull that one off and scrub it clean, too? It should only drop about 0.1 volt).

[Joel]

My plan was to pull the wiring and rebuild a harness using new wire over my Christmas break.  I probably won't do much riding on it between now and then since the 40 degree weather seems to be here to stay.  The carbs need gone through too and I'd like to get them rebuilt over the break as well.

As for the resistor caps, I checked Z1 and they said theirs are 5Ω.  Perhaps he meant 5kΩ.  I e-mailed him for clarification.  What would the effect of using lower resistance caps be?

NGKs site wasn't much help in locating the proper resistance caps.  Any good source?  Any opinion on Dyna replacement coils as opposed to new stock replacements?  I didn't see much else in the way of universal motorcycle coils with the proper specs in an internet search.

[HondaMan]

My plan was to pull the wiring and rebuild a harness using new wire over my Christmas break.  I probably won't do much riding on it between now and then since the 40 degree weather seems to be here to stay.  The carbs need gone through too and I'd like to get them rebuilt over the break as well.

As for the resistor caps, I checked Z1 and they said theirs are 5Ω.  Perhaps he meant 5kΩ.  I e-mailed him for clarification.  What would the effect of using lower resistance caps be?

NGKs site wasn't much help in locating the proper resistance caps.  Any good source?  Any opinion on Dyna replacement coils as opposed to new stock replacements?  I didn't see much else in the way of universal motorcycle coils with the proper specs in an internet search.

If you go to 5k ohm, use that same value on both wires on each coil to keep it balanced. The earlier bikes were all 5K-7.5K new, but this went to 10K when the FCC started investigating complaints across the country that motorcycles interfered with AM radio reception on lonely highways (remember AM radio?) during the 1970s. The Fours were part of that "menace". You also could not run an AM radio in a Vetter Fairing Radio Pack on most Fours, and the 10k caps helped quite a bit. Those NGK caps come in 0k, 5k and 10k resistances: i've had all three types here for different purposes. Locally, RPM Motorsports Honda and G-Force Kawi usually stock them.

If you go Dyna on a 400F, make it the 5-ohm primary coils or you will run out of electrical power. Those coils have replaceable wires, not a bad thing...they also come in either copper core or suppression core (which is resistance wire, like in cars). The latter is about 2k ohms in the bike-size lengths  we use, so that plus a 5k cap would put you spot-on for a brand-new spark setup.

If you have to end up with zero ohm caps for any reason, use the DR8ES-L or XR24ES-U plugs, which have about 6k ohms inside. Just don't run without resistance, as it yields less performance (long story...).