SOHC.co.uk Forums > CB350/400

A couple of "tune up" questions...

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Nurse Julie:
It may have been that you had just richened the mixture or it could be worn valve guides.

McCabe-Thiele (Ted):
If it's a relatively small amount of smoke when cold I would not worry about it unless you start using oil in any quantity.

I have found that older engines often give some smoke on start up sometimes due to oil that has found its way past valve guides whilst standing overnight.

Easy to panic and think the worst,

TrickyMicky:

--- Quote from: Mikep328 on September 10, 2023, 01:14:23 PM ---Went out today and checked the tappet clearance - they were .003"+ so I reset them all to .002"  Then I synced the carbs with a gauge set after first calibrating the set so that all gauges read the same when attached to the same cylinder.  I had bench-synced the carbs a few weeks ago and was pleased to see that they all cylinders were within a needle's width of each other.  I tweaked them so they read exactly the same though I doubt that a needle-width difference matters.

I richened the mixture by 1/8 turn on each carb but it was raining so didn't take the bike out to evaluate the change.  In the parking garage it accepted a quick throttle movement from idle with less hesitation than previously.

One thing that I was NOT happy about -  when I started the bike from cold there was visible smoke from the exhaust though it seemed to disappear after it warmed up.  Couldn't really tell the color but it smelled oily to me.  However, the spark plugs all look good color-wise with no sign of oil or fuel fouling.  My wife was behind me on her bike riding to Bristol the other day and she didn't mention seeing any smoke from mine.  I hadn't asked her to look for it but I'm sure she would have said something about it had she seen any smoke.  In any case, further evaluation is in order.

--- End quote ---
You mention that it was raining when you started the bike, meaning the air was quite moist, likely as not, it was steam from the condensation gathering in a cold exhaust. In my nearly days, this cost me a fortune in replacement exhausts (CB750),  as I used it for riding to and from work in all weathers, 5 miles each way so did not get hot.  As much as possible nowadays I do not start the 400 unless I'm going for a long enough ride to get its really hot. I'm still using the genuine Honda system that I fitted in 2006. If you say that someone following you did not see any blue smoke and it's not using oil between changes, I think I would just ride it. Stay safe. Mike.

Mikep328:
Thanks Trick!  I asked my wife specifically today if she had seen any smoke at all from my 400 when she was following me on her motorbike and she replied, "absolutely not!"  So now my plan is to NOT worry about it!

Mikep328:
We went out on a ride of around 30 miles today.  There was a substantial improvement in how the engine would accept opening the throttle so richening the air screws by 1/8 turn definitely helped.  I still have a rough spot - stutter - at around 1800-2000 RPM which continues to make in-town slow speed running at that RPM range a bit annoying.  I'd like to get rid of that...

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