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Motorcycle Repair/CB400F smoke on start


Hi Bill,

We talked before around my exhaust and oil pan. That all went really well, so thanks for all the great advice. I've been riding the bike normally for a couple weeks now.

One thing I've noticed is that the bike feels a little sluggish to me until it warms up. Im not sure if I'm imagining this though as I've also been riding a newer 800cc bike with obviously much more pickup than the 400.

One thing I am noticing though is that on a cold start, the bike tends to have a decent amount of smoke from the tailpipe until it gets warm...a bit on idle, much more as I increase throttle.I'm not sure if this is new, or something I simply didn't notice before. Once I'm going, it seems to go away.
The smoke is white.

I also feel like the bike smells more like gas then it used to while running.
Putting the exhaust back on, I had thought i might be getting some exhaust leaks up front, but I dont think that's the case. I actually dont even know how to check for exhaust leaking around the gaskets.

Thanks again,

Chris, glad that the previous repairs went well and the bike is back in action.

You do have to be accurate about ignition timing and valve adjustments to keep the little engines in top condition. Compression readings should be right around 175 psi, all the way across the cylinder bank.

They are cold-blooded and need some choke to get them going on a cold start. Check the spark timing at idle and at full advance to ensure that you are getting the full 36 or so degrees of total advance. I am finding that increasing the main jet sizes about one step is helpful for dealing with the loss of fuel energy in the alcohol laden gasoline we have now. Carb settings sound be 21mm float level, #75 (or maybe #78 now) main jets, #40 idle jets, clips in the middle of the five notches and the mixture screws turned out just 2 turns.

If the bike has an OEM stock exhaust system (or at least the header pipes), cold start "smoke" is often just condensation being vaporized in the outgoing exhaust heat. If you have the stock muffler, there is a drain hole towards the front that allows the water/moisture to drain out onto the ground.

Exhaust leaks usually show up with some evidence of carbon deposits around the flanges and kind of a spitting sound. Use a long paper tube, like the insides of a paper towel roll, to use to listen around the exhaust ports to hear any leaking sounds.

Bear in mind that the engine is now 38 years old and probably needs a fresh set of valve stem seals and perhaps rings, depending upon the mileage on the engine. Engine oil consumption will normally increase as time goes on and the burnt oil smell in the exhaust will become more evident.

Bill Silver  

Motorcycle Repair

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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.

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