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Discussion Starter · #1 ·
My CB400t from 81 has been acting up lately and seems to be burning oil. i keep needing to put in more oil every 2 weeks, usually 200ml or so. one of the spark plugs is caked with carbon deposits, it isnt wet and oily though, just dry. the other is usually whitish because of an air leak in the carb boot going from the carb to airbox. it runs too lean on that cylinder, i always have to fiddle with the pilot fuel screw. it has 23000 miles, is it possible the rings could be bad? im sure the valves need to be adjusted, but i have no idea how to do that. I have an intermediate level of knowledge with this stuff, is this something I can do myself?
 

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The valve adjustment on the 400 is about as simple as it can get, same setup as was used on cars back to the early 1900s. The procedure is spelled out pretty well in both the Clymer and Honda manuals.

However, I doubt that adjusting the valves will help your oil consumption or plug fouling problems. Do the valve adjustment then do a compression check, then come on back.
 

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Discussion Starter · #3 ·
I checked my PDF of the Clymer manual and it seems easy enough to adjust valves. ive got some time before work tomorrow morning, i try it then. compression seems, fine. i checked both cylinders and they read the same number, however the tool used was made by my grandfather some time in the 60s, so who knows how accurate it is.

the carbon fouling could also be due to the airfilter being old. i had cleaned it recently, but the cells in the 30 year old foam could be bad. plus there is the rip in the carb boot, which could be affecting fuel/air in a number of ways. UNI makes "stock" pod filters, they seem to be more restrictive than standard ones. these may be a better alternative to the airbox without having to rejet. I could put them on a hose to cut down air turbulence going into the carb.

if all it does is burn up a little bit of oil on a regular basis, im not going to tear apart the whole thing or spend money for somebody to take a look at it, as long as it wont do any harm to the engine. I just want to be able to rely on it because i use it to commute. it is 30 years old, i dont expect it to run like new.

i think im just going to tackle the whole thing from the start of the process, starting at the air intake.
 

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Specifically, What were the compression readings? ...ANYTHING below 155/160 PSI indicates the engine is due for re-ringing, and that alone could be your oil-burning problem.....Normal compression pressure for your engine is 185 PSI.......
 

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Discussion Starter · #5 ·
well, the old compression tester read 50 psi, but i have a sneaking suspicion that the 50 year old home made tester has more than a few leaks. i will go to the store and get a real one tomorrow. then we will truly see what is wrong with it.

I honestly dont think it's rings, though. It is very easy to start up and dosent spew out excessive smoke, a little when it's cold or it still has the choke in when it is warming up.
 

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Discussion Starter · #6 ·
ok, i just bought a new compression tester and it read about 155psi in each cylinder. seems to me that if it was rings, they would read different pressures. now im going to adjust the valves and test it again.
 

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Discussion Starter · #8 ·
i adjusted the valves, i got them to the desired gap, the only one that was really off was the exhaust valve on the cylinder with the fouled plug, it was too loose. im going to ride it to work and see how it does, ill top off the oil first. it seems to run ok, no real difference from before.

the compression test now reads 150psi again, no real difference.

i figured if the rings were really bad it would read different pressures in each cylinder because the rings wont wear exactly the same, i think it says something about that in the manual.

but if it does need rings, that's an engine out top, end tear down, right? I dont think im comfortable with doing that myself. what are some other symptoms of worn rings?
 

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If you're talking about a 400T, I believe the top end can be removed without pulling the engine.

The two sides will read different psi as often as they read the same.
Check the compression, then squirt a bit of oil in the spark plug hole.
Check compression again, if it jumps that's a sign of bad rings.
If not, it indicates leaky valves.
 

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Discussion Starter · #10 ·
well, bad news is that it does increase by about 20psi, up to 180psi when i put oil in in through the spark plug hole. at least i know for sure that the rings need to be replaced.

it seems to run better with the adjusted valves, less top end engine noise and smoother running.

unless it seems like a top end job is something I can tackle, i may just deal with it and keep putting oil in it, as long as that wont do it any harm. it still runs/starts well. Ive put rings in another bike before, but that was a 74 suzuki TS100, a tiny little 2 stroker. this is far more complicated.
 
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