You are here:

Motorcycle Repair/1980 honda cb900


The atu unit is the automatic timing unit on left side or pulse generator. No i never had the engine apart itself i had remove the pulse cover and the ATU in doing so i barely turned crank shaft tryn to loosin bolt its bein a pain in my rear to try to realign. I removed valve cover so see where two pin points were on timong sprocket when the T is alingned on pulse cover and all seems to line up. But it went from runnin like crap to when i removed that and reinstalled now it doesnt want to turn over at first was backfiring until i did what i did and aligned all the points the book said to. Whats so irritating is a can fix vehicle engines like there nothin but this bike is seeming impossible lol. I can provide some pics if that would help you determine the timing issue. I do appreciate all the insight u have provided.

The spark advancer is pinned to the end of the crankshaft, so you can't go wrong there. It may be difficult to turn the engine backwards due to the starter clutch drive being engaged, so just turn the engine forward until the F mark aligns with the mark on the case, then move the pulse generator plate until the magnets line up squarely. This assumes that the spark advancer is functioning normally in advance/retard function and is indexed back on the end of the crankshaft with the locating pin.

If you have backfiring all of a sudden either spark advancer trigger is on backwards or the coil wiring is reversed from 1-4 to 2-3 coil sets.

Fully charge the battery and load test it before going further. The transistor ignition needs a minimum of about 11.5 volts to be happy. If you are cranking the engine with the H4 headlight glowing and running the battery down while you check for spark or other tests, the battery will drop below the cutoff point for the electronics.

Again, spark units can fail, battery voltage needs to be 12v+, spark timing set correctly and spark plug caps checked for 5k ohm resistance...

We are assuming that the engine has good compression in all cylinders....

Bill Silver  

Motorcycle Repair

All Answers

Answers by Expert:

Ask Experts


Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

©2016 All rights reserved.