Valve adjustment

kickstand

kickstand

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I have a 1971 CB350K that I bought a couple years ago. I haven't put too many miles on it yet. But I recently checked the valves to see if they needed to be adjusted. If I'm not mistaken, the space between the intake valves and the rockers should be 0.002" on this bike. But I can't even get a 0.0015 feeler gauge in between them. I can move the rockers side to side, so they're not touching the valves, but there's very little clearance. I can't move them up and down at all. The thing is, I bought this motorcycle from a guy who had been a motorcycle mechanic for decades. I know he knew this bike inside and out, so I'm wondering if there's any possible reason for the valves to be adjusted this way. The last thing I want to do is adjust the valves to where I think they should be, when in fact he had them adjusted properly all along. For instance, if this engine had a valve job done at some point, would that be a possible explanation for the valves to be adjusted this way? Or should I just go ahead and set them to 0.002"?
Thanks.
 
Just to make sure, you're checking them stone cold, right? (And on the T and LT marks of the compression stroke.) If so, sounds like they are a little tight, but whether that was intentional or not I won't hazard to guess.

Feeling even a 0.002" gap is a subtle thing, but if the 0.002" gauge won't get in there, it won't hurt to adjust them. If the compression is good and it's running well, I could also subscribe to the if it ain't broke don't fix it mantra.

A standard valve job wouldn't require a change to the valve clearances.
 
I am checking them cold. But thanks for mentioning that.

The main reason I am considering adjusting them is that my father told me that leaving them too tight could burn the valve seats over time. My understanding is that the valves help to dissipate heat from the seats while they're closed, so if they're not closed long enough (or, worse, if they don't completely close at all) it could cause damage.

Also, it is a bit hard to start sometimes, and I've heard that can be caused by poor valve adjustment. Once it gets started, though, it runs pretty good.
 
Heat dissipation is a valid concern, although this would be more problematic with a tight exhaust valve since that's where the hot exhaust gasses exit the combustion chamber.

If you set the intake valves to 0.002" there should be no issue, even 0.003". It's always good to recheck after tightening the nut on the tappet to make sure it didn't move while tightening.

Did you check the exhaust valves?
 
Valves - particularly new ones - can “seat” over time. Thus, they require periodic check and adjustment.

Follow the adjustment procedure outlined in the FSM, of which, part is making sure that you’re checking this clearance at the exact point in the stroke that they should be checked.
 
stl360+450 said:
Heat dissipation is a valid concern, although this would be more problematic with a tight exhaust valve since that's where the hot exhaust gasses exit the combustion chamber.

If you set the intake valves to 0.002" there should be no issue, even 0.003". It's always good to recheck after tightening the nut on the tappet to make sure it didn't move while tightening.

Did you check the exhaust valves?
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Thanks, I didn't realize the exhaust side was more likely to have that problem, but it makes sense now. I didn't check them yet, but I'll check them tomorrow.
 
EzPete said:
Valves - particularly new ones - can “seat” over time. Thus, they require periodic check and adjustment.

Follow the adjustment procedure outlined in the FSM, of which, part is making sure that you’re checking this clearance at the exact point in the stroke that they should be checked.
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That makes sense that the clearance could get tighter over time. I hadn't thought about that. It's entirely possible that these are relatively new valves. I know the previous owner had the engine torn apart at some point.
 
Check the correct specs on the clearance per the manufacturer. The .002 is quite tight but maybe correct as I am not a 350 guy. Most Honda settings are a minimum of .003 to . 004 or .005 in my experience on other models. The valves recede into the head over time as a general rule. So such a tight clearance leaves little room for expansion when hot and the receding of the valves over time.
This was already pointed out that you must be on the correct engine cycle and cam lobe placement when checking the valve clearance.
 
kickstand said:
That makes sense that the clearance could get tighter over time. I hadn't thought about that. It's entirely possible that these are relatively new valves. I know the previous owner had the engine torn apart at some point.
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They don't necessarily have to be that new either, they're taking thousands of explosions per minute to the face.

Intake is the .002", exhaust is .004"
 
Another point is they can be slightly more open than spec since the .002 feeler gauge is paper thin. Mike Nixon a mechanic specialist of both Honda and Kawasaki bikes always noted you will get an improvement by opening up the clearance a little bit more from the factory spec. The old Honda Goldwing's always benefitted with better compression readings by using his method. Setting the exhaust at .003 isn't going to cause you any issues and .005 on the intake too. It might be a tad noisier with some more rocker clatter, yet on an old motor there will be other motor noises to mask that in many cases.
 
ancientdad said:
Except it's the other way around, .003" would be fine for the intake and .005" would be fine for exhaust..
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Exhaust was .002 per Pete and intake .004. So my suggestion was increase the exhaust to .003 or .004 and the intake to .005. I would likely leave the intake at .004 the factory spec and just increase the exhaust a tad more to .003 or .004.
 
Flyin900 said:
Exhaust was .002 per Pete and intake .004. So my suggestion was increase the exhaust to .003 or .004 and the intake to .005. I would likely leave the intake at .004 the factory spec and just increase the exhaust a tad more to .003 or .004.
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Pretty sure if he made that mistake, he corrected it based on my screenshot from a minute ago. And it's best to actually check the FSM before repeating something to someone who might not know better. And when has any Honda engine used a larger gap for exhaust than intake intake than for exhaust? The 350 engine has always used .002" for intake and .004" for exhaust.

1780147488319.png
 
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mike in idaho said:
There was a service bulletin from Honda, that first year, regarding the 350s' valve adjustment being a little too tight, causing some idle problems. We were directed to set the intakes at .003" and exhausts at .005".
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I'll have to go through the service bulletins we do have and see if I can find it. By the time I got to Honda shops in 1970 the bike had already been in the US for a couple years, and I never got to see any service bulletins then.
 
Flyin900 said:
Another point is they can be slightly more open than spec since the .002 feeler gauge is paper thin. Mike Nixon a mechanic specialist of both Honda and Kawasaki bikes always noted you will get an improvement by opening up the clearance a little bit more from the factory spec. The old Honda Goldwing's always benefitted with better compression readings by using his method. Setting the exhaust at .003 isn't going to cause you any issues and .005 on the intake too. It might be a tad noisier with some more rocker clatter, yet on an old motor there will be other motor noises to mask that in many cases.
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That jives with something I heard. Someone else said "loose valves make noise, but tight valves break parts." So I don't mind loosening them up. The noise won't bother me as long as it doesn't cause any harm.
 
mike in idaho said:
There was a service bulletin from Honda, that first year, regarding the 350s' valve adjustment being a little too tight, causing some idle problems. We were directed to set the intakes at .003" and exhausts at .005".
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That's great to know. I won't worry about setting them a hair on the loose side, then.
 
kickstand said:
That's great to know. I won't worry about setting them a hair on the loose side, then.
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If accurately done, we're only talking about .001" over spec which won't create very much additional noise. I disagree with the adage "tappy valves are happy valves" because too loose can cause damage long-term, but slightly looser is better after a period of time where a valve had been tight.
 
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I've known people that swear by slightly larger gaps but I have always adjusted to spec on twins and fours and have never had a problem, much less damage done to the engine by adjusting to spec and at the proper intervals. To each his own I guess.
 
kickstand said:
That jives with something I heard. Someone else said "loose valves make noise, but tight valves break parts." So I don't mind loosening them up. The noise won't bother me as long as it doesn't cause any harm.
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It will. Too loose will put excessive stress on the tappet screw and the valve tip. They can’t just be “loosened up” or set at double the spec.
 
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