First, I have to preface that I am a complete newcomer to motorcycles. I don't have any people in my circle of friends that ride so I'm just diving in and learning and asking questions on here since I love it so much (so please bear with me!).

I bought a 1972 Honda CB750 and have been doing work on it. I did not receive a manual with it and have found some diagrams online but have some very basic questions.

1) The choke 'on' would be when it's pulled out, correct? I can't seem to find any other chokes online that look like mine but I'm just making the guess by looking at it:

ON? enter image description here

OFF? enter image description here

If this is correct, I left the choke on for like a week since I've had it. I did an oil change and now the engine won't start. It turns but won't fire. My guess is that by keeping the choke on, I screwed up the spark plugs and need to replace those so that's what I'm going to look at first.

I'm just hoping to get confirmation on this (and hoping it will help others with my same type of choke in the future) since I can't seem to find anything definitive online at the moment.


1 Answer 1


Your carburetor looks like a Mikuni VM series, though I could be mistaken. Usually, pulling up the choke engages it, meaning that more gas per amount of air is flowing into the engine (air input is choked). Once the engine is running, you can allow the correct amount of air to flow into the engine (provided your carb is adjusted correctly) by pushing the choke in.

If you ran your engine with too much gas going into the engine for an extended period of time, there are good chances that you will have to change your spark plug as you suggested.

You should be able to roughly diagnose this with a look at the spark plug. If it is covered with black almost fuzzy carbon deposits, it is a sign that frequently indicates that the motor is running too rich. Essentially, the fuel isn't all burning because of an insufficient oxygen supply, and the partially un-burned fuel deposits on the piston head, the cylinder walls, and the spark plug.

A picture from http://www.aa1car.com/library/spark_plug_fouling.htm of a carbon fouled plug:

enter image description here

The carbon will short out your plug instead of bridging the gap correctly, therefore the engine will not start (insufficient spark). The spark plug will need replacing - unless you feel up to attempting a cleaning. (Something I have done numerous times with success, but not something I recommend: spark plugs don't cost that much. :) )

  • Ok great, so it sounds like I was on the right track but I definitely left the choke on and rode it around three or four times with the choke engaged the entire time. The bike turns over but doesn't fire up so I'm guessing I must have done something to the spark plugs so that's the next place I will look. Thanks for the reply! Oct 23, 2016 at 2:36
  • No problem, @MillerMedia! Wish you best success!
    – anonymous2
    Oct 23, 2016 at 2:38
  • I just changed out the four spark plugs and they were all completely fouled (all black and gunked up). Once I changed them out, the bike fired right up. Thanks so much for the help! Oct 23, 2016 at 23:41
  • Was the gunk powdery, or was it kind of, well, gunky? :)
    – anonymous2
    Oct 24, 2016 at 0:57
  • 1
    Okay, black soot sounds fine, @MillerMedia. Just wanted to make sure it wasn't a gooey gunk. Black soot, as I said in the post, would be a sign of running over-rich. It could also mean your engine is running to cold, but in view of the details of the situation, it is reasonable to assume it has to do with the fuel mixture.
    – anonymous2
    Oct 24, 2016 at 11:21

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