Help Needed with Diagnosis, Likely Electrical

Granticus

Granticus

New Member
Joined
Aug 28, 2024
Total Posts
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Location
Hudson, WI
Hello,

I'm a new member, new motorcycle owner, and new rider. So new in every sense. :)

I recently purchased a 1978 Honda CB400A Hawk Hondamatic. It has 11,000 miles on it and overall seems well cared for. It has a Lithium Ion battery in it and a newer LED headlight installed. All the other electrical components seem original.

It ran well for the first few months on short trips around town. When I started going on longer rides I started having some problems.

It would run well on my way to a destination about an hour away. Then, it would be parked for a few hours. Finally, as I was riding it back home it would lose power while riding. I would be going 60mph and suddenly it would slow down and the bike couldn't accelerate over like 30mph for a bit before eventually stalling. I pulled over on the side of the road and tried to start it again without luck. After waiting 5-10 minutes, all the sudden it would start right up and ride like normal. I had this same experience on 3 or 4 different trips.

Then the last time I rode it, after stalling again I let it sit for awhile, but it just wouldn't start again. I eventually drained the battery trying to get it going. I had to get a friend with a trailer to take me home.

A different issue I noticed, that may or may not be related, is my speedometer light or the brights indicator light would not get power. I finally traced it back to the fuse box and saw: top 15 amp fuse looked good, the middle 7 amp fuse was blown, the bottom 7 amp fuse slot had a 10 amp fuse in it. I replaced the blown 7 amp fuse with a 7.5 amp fuse I had on hand. I then was able to get the speedometer light and the brights indicator light to glow but just very faintly like it wasn't getting enough power.

What I'm considering right now:
- Replacing the middle and bottom fuses with the correct 7 amp ones
- Trying to add up the overall wattage use to see if I'm unable to provide adequate power for everything
- Replace the remaining standard halogen light bulb with LED ones (should be less watts and brighter)
- Seeing if I need a new Regulator/Rectifier meant for use with Lithium Ion batteries
- Replacing my ignition coil (possibly with the GM mod mentioned elsewhere
- Replacing my CDI since the symptoms seemed to line up with a failing CDI

I have a multimeter and I'm trying to figure this all out. I would love to know if you think the symptoms I mentioned are pointing to a specific problem? Are the two problems I've mentioned at all related? Do you think any of my next step ideas are worth doing or not necessary?

Thanks in advance! Please let me know if I can provide any additional details.
 
1st off you have more than enough charging power, 170w, to power everything and then some. The ignition system is separate from the electrical system so you can effectively use @130w and have enough left over for keeping the battery charged.
Definitely replace the fuses. Clean the brass clips and pinch together to get good contact.
Here's the diagnosis thread for your ignition system https://www.vintagehondatwins.com/f...cb-cm-400-450-automatic-trans-1978-1983.5757/
Here's an upgrade thread for the electrical system https://www.vintagehondatwins.com/f...ades-to-the-charging-and-ignition-system.164/
Your problem sounds like the gas cap vent may be plugged causing the tank to go into vacuum and stopping fuel flow. Try running with the cap loose or ride until the issue reoccurs and remove the cap.
 
For some reason, I was also going to suggest a fuel starvation issue, such as possibly the tank cap vent not venting...
 

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Ditch the Li-ion battery. even with an upgraded reg/rec, the battery requires a management system to protect the battery from discharge below a certain point (varies for different batteries.) Below that set voltage, the battery will not recover or accept a charge.
 
Here's some pictures of the gas cap. Since I'd taken these pictures, I'd replaced the rubber seal that had cracks in it with a new one. Can I just run a needle or or small wire brush through the hole to clean it out? Or should find a replacement? The bottom of it does look pretty gunked up.

uKWeCJqm.jpg

Iz0aQ21m.jpg


Regarding the battery, I'd looked into it before and it does have a battery management system built into it. Are there other concerns with using Li-ion battery?

697IK0cm.jpg


Fire Power Featherweight 12V / 48 WH
SKU: 490-2524
CCA: 220
MODEL: HJTX14AHQ-FP
- Integrated BMS (Battery Management System) with cell balancing and overcharge protection

Thanks so much for the tips and ideas so far. I'll do some more reading in those threads that were shared.
 
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I have not seen that style battery with the built in BMS. That should solve that issue. However, you need to upgrade the reg/rec and only use a charger designed for Li-ion use.
 
You may have both fuel and electrical issues. I'd soak that cap in a cup of vinegar for a day then rinse with water. Blow 100psi compressed air through the hole to see if the internal spring loaded venting valve is operable. WD40 may work too, instead of vinegar. If it won't pass air then get a new cap. Common problem on these old bikes that have sat a long time. If your tank has the same scale/rust as the cap you need to fill it with a gallon of vinegar and clean it out too, or you will have junk clogging the float bowl valves in the carbs.
 
Your issue sounds like you might have something wacky in your harness. Maybe a loose connection causing the short to blow your fuses and causing issues with power or signal to coils... or a low fuel level issue. Did you try switching to reserve or confirm you have gas?

Id say check your plugs, use LDR link to test your electrical system, use the multimeter to test continuity between components and wiggle the wires as you're checking continuity.

The GM coil mod is awesome and should def be done is still running the stock coil.

Gas cap vent you can test on the road or at home, just run fuel into a container and see if it slows or stops and if removing the cap helps.
 
I confirmed I had gas when it stalled. I'd started the vinegar bath for the gas cap yesterday and all sorts of crud is coming out of it. Looks almost brand new now. Going to let it soak a little longer and then blow it out with the compressor.

I for sure want to do this vinegar bath on the gas tank after seeing the results. I have gas in it right now though, so will need to drain it first. I'll try riding this weekend after it's cleared out.

I've ordered some parts too:
- MOSFET Regulator Rectifier
- Modern fuse box for blade fuses
- LED bulbs for the dash lights
- Dynatek Ignition Coil
- New CDI

One question about the blade fuses, does anyone make these in 7 amp or can you only get 7.5 amp? And is that okay to use 7.5 amp instead?

Thanks everyone for the great advice so far! I'll post again once I learn more.
 
Got the gas tank cleaned out yesterday. Was tougher to drain than I expected. A lot of shaking required to get all the liquid out. I did see some chunks come out of it, and a few pine trees needles that were caught in the petcock filter.

In the process of removing the petcock and then reinstalling, the rubber o-ring inside it must have tore. When I went to put new gas back in it leaked all over the place.

I've ordered a new petcock from Amazon for like $10. Since I've got this holiday weekend to work on the bike and do more testing, I'd love to find a local store that carried that O-ring. No luck with O'Reilly's. Will try Auto Zone next.
 
A NAPA car parts store might be the best possibility for a petcock O-ring. Another possibility is a hardware store. I am fortunate to have a business nearby that specializes in hydraulic and pneumatic repairs and services - Lots of farming equipment gets serviced there. They have a massive amount and types of rubber gaskets and O-rings and hose lines, including viton rubber that is resistant to oils and gas
 
You may have some luck at a Honda Outdoor Equipment store. They had a 10mm circlip I needed for a 61 year old CA95 engine. If Corvette Motherland is Bowling Green, KY., then go to Bowling Green Rubber & Gasket Co. They will copy and make any gasket you need out of a choice of materials and thickness. I lived in BG from late 2007 to late 2010.
 
Granticus said:
In the process of removing the petcock and then reinstalling it, the rubber O-ring inside it must have tore. When I went to put new gas back in it leaked all over the place.

I've ordered a new petcock from Amazon for like $10. But, I'd love to find a local store that carried that O-ring.
Click to expand...
Here's what the genuine Honda petcock O-rings look like. As skinny as they are, yes, DELICATE !
If re-usable, tighten petcock nut only just enough to make the seal.
The O-rings in the cheap Chinese petcocks I bought to use until I rebuild my genuine Honda ones are not only much thicker and fill the whole area between plastic tube and metal nut, but they're also a "square-cut" type compared to the donut or innertube shaped OEM O-rings

 
Keep in mind when replacing O-rings to add some silicon grease or at least Vaseline to matching surfaces. Matching surfaces need to be absolutely smooth as the O-ring needs to be compressed without friction.
 
So an update on the repairs. I was unable to find a replacement o-ring in my area. Tried many store locations as suggested. But, the replacement petcock was only $10 on Amazon so I just did that.

I did another round of cleaning my gas tank and noticed something funny this time. With the petcock removed, at the bottom opening I saw a ton of build up of old gasoline. I had ignored it last time. This time I saw just outside the build up there was an extra layer that wasn't metal or attached to the tank so I start yanking on it with a pliers until I finally got it out in one piece. It was the filter that fit over the old petcock, and it was thoroughly clogged up. The only opening on it was at the top.

In early comments people suggested I ran out of gas. I knew I still had gas in the tank. But this was probably it! The gas level wasn't high enough to get past the clog. So as I ran the bike longer and the gas level went down I would effectively run out of gas because I couldn't access the gas at the bottom of the tank. I'm really close to a test ride to check this theory. Probably tonight.

I also received my new electrical parts and have those installed now, but I'm waiting on the fuses. I opted for Posilock inline fuse holders because the left side panel is jam packed. There's no way I could fit the 4 fuse box I purchased without making some extra room somehow. I'd read somewhere removing the battery box is an option. Anyone have advice or experience with this?
 
Hope this solves the issue,
Removing the battery box? Not really an option unless you went to one of those silly little Li-Ion pieces which are prone to failures like catching fire.
 
LongDistanceRider said:
Hope this solves the issue,
Removing the battery box? Not really an option unless you went to one of those silly little Li-Ion pieces which are prone to failures like catching fire.
Click to expand...
Ok. Thought it was worth asking. The MOSFET Regulator Rectifier is significantly larger than the stock one. So I've had to get creative with fitting everything in.
 
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