Honda CB550 low to no power Motogadget equipped


New Member
Afternoon everyone

I recently finished a ground up cafe build on a 1975 CB550F. It’s been running like a top and pulls like a small train. I’ve ridden about 150 miles since I finished the build and I’m extremely pleased with the way the bike has been performing, until last night.

Details about the Bike:
1975 CB550 F
Stock Engine internals
Carbs are from a ‘76 550 K (rebuilt and jetted for pods)
Unit foam pod filters
4into1 exhaust w/ high flow muffler
Motogadget M-Unit electronics w/ new wiring
Factory points ignition
Shorai 210cca lithium battery
Regulator-rectifier (eBay special)
Factory style coils (eBay special)

So here’s what happened:

Out for a ride last night and the bike was behaving perfectly. About an hour into the ride I downshift on a quiet bit of road and gave it the beans. The bike pulls normally then suddenly give me a short burst of extra power, like it got a short shot of NOS. I decelerated and everything went back to normal. About 10min later I lost all acceleration, I downshifted and tried to accelerate and the bike was still running but the power delivery was completely flat. I let off the throttle and the engine died.

Pulled over and diagnosed.

Fuel flows from the tank to the carbs (pulled the line off the carbs and observed fuel flow) carbs are filling (checked the drain plug on the #4 carb ((furthest from the fuel inlet to the carbs)). Battery had plenty of power and the starter turned over the engine strongly but wouldn’t start. Pulled the seat and saw the ignition terminal LED is flashing on the Motogadget M-Unit (that indicates, or should indicated a fault in the ignition circuit). Checked all connections on the coils and overall condition and everything seems fine. I pulled the power wire (ignition wire) from the M-Units Ignition Terminal and plugged it into the AUX 1 terminal and the bike fires back up and idles normally. The AUX 1 input supplies the same power as the “ignition terminal” but does not have a “kill switch” feature, so this was only a “get home fix”. I pulled back on the road and acceleration was poor and flat, no power. I got up to about 40mph and when I opened the throttle the bike surged and bucked like the ignition was only sometimes firing. In fear of causing further damage I pulled over and called the wife to scoop me up with the trailer.

I suspect faulty and failing coils and will be starting my full diagnostics with the ignition points and moving through the charging system when I get back home as I’m currently on vacation with only basic tools at hand.

What are your thoughts? More brains are better than one.

Cheers everyone and hopefully everyone had a great 4th of July.
I thought that too, like maybe it threw a spring or the cam lobe wore out on the spark advance. I’ll check and reset that but, it wouldn’t explain the electrical fault I was receiving. After watching a video about the pervious version of the M-Unit I think I may have figured it out.....

One gremlin the bike had (I figured was unrelated) was the horn periodically activating on its own. I figured it was a bad ground or bad connection but it’s actually the M-Unit warning me of something much worse. My regulator/rectifier is faulty and is producing too much voltage to the battery. The M-Unit senses this and activated the horn in an attempt to “drain” the excess voltage. I’m theorizing that the M-Unit went into a full protection mode of sorts and cut the power to the ignition to prevent me from doing any more damage. Without a multimeter on hand I can’t prove much but it’s enough evidence for me to order a new reg/rec from Ricks.

I’ll post up my findings.
Sounds like your on runs well pulls higher revs pops reg and m-unit protects itself....maybe the reg going added voltage to the coils before m-unit kicked in, hotter spark slightly rich mixture should get you a minor boost...
Appreciate that.

Back home from the lake so ill get the multimeter out and see what I can find, the battery is on the charger now. The whole situation of a sudden boost of power reminded me of an old two-stroke engine, runs best right before it explodes. ;D
I think I found the culprit. The white field coil wire coming out of the reg/rec had a small cut that led to a short or a malfunction of some kind, I believe that led to burning out the right coil pack. I found molten plastic oozing out of the coil where the signal wires enter the back of the coil. I installed a Ricks reg/rec specifically designed for lithium batteries (max output is 14.2v) and a new set of coils is on the way.
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