1983 CB450SC oil leak, gas smell in oil, and backfiring
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  1. #1
    Member srahome's Avatar
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    1983 CB450SC oil leak, gas smell in oil, and backfiring

    Hi all,

    I started this discussion on the end of another thread before I realized I where I should be posting (newbie!). Anyway, I picked up a 1983 CB450SC Nighthawk a week ago and have experienced some things that concern me. I'm a complete engine newbie so I'm not sure how serious any of this could be.

    So here goes: The bike has about 38,000 km on the odometer and seems to run fairly well, if a little balky at a steady throttle. I took it for a slow spin around the downtown core on the weekend and noticed some smoke coming from around the engine block. Seems there was some oil seeping from either the bottom gasket or the middle gasket. I cleaned it up when I got home with some water and diluted dish soap to cut the grease and then took it for another run.

    I ran it up under load to about 5000 rpm. When I got to the next light, I noticed a fair amount of oil had splashed onto the engine block and was smoking. I pulled over and turned off the engine to check the oil to see if it was low and I couldn't get the dipstick cap off. Anyway, I rode it slowly home and checked the oil again - a little high (about 5mm over the high line but unchanged from before the ride). Could too much oil cause the leak? With the oil spray getting on a few other parts of the engine block, I can't exactly tell where the oil is coming from. The fins above the exhaust pipe outlet are totally dry so it doesn't seem to be the top gasket. There seems to be oil originating from between the cylinders too, from the bottom gasket, and possibly from the oil pipe from the base of the engine to the cam cover. Sorry if I'm not great with the technical terms - my first bike and I'm completely new to tinkering on engines.

    I checked the breather hose and it's clear. I checked the air filter and it's clear. Is it possible that the oil line from the base of the engine to the cam cover is blocked? Could that cause leakage?

    The latest development is that I rode it about 30km (20 miles) today and noticed when I got home that the oil reservoir had a slight gas smell. I haven't changed the oil since I bought the bike since it looked clean. I also make sure to turn off the petcock every time the bike is parked. Today, it ran a little rougher than the first outing and started backfiring out of the right exhaust pipe during deceleration (downshifting).

    I love the bike - the look, the stance, the riding position, the sound, the size, all of it - but I don't really want to sink a pile of dough into a lost cause. I'm trying not to feel like I was taken advantage of by the seller. He claimed it was running perfectly and was used regularly until the end of the last season (said every previous owner, I know!). Anyway, tips are appreciated. I've attached a few pics - sorry but I can't figure out how to get the rotation right.
    1983 CB450SC oil leak, gas smell in oil, and backfiring-cb450sc-1.jpg1983 CB450SC oil leak, gas smell in oil, and backfiring-cb450sc-2.jpg1983 CB450SC oil leak, gas smell in oil, and backfiring-cb450sc-3.jpg

  2. #2
    Super Moderator longdistancerider's Avatar
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    Gas in the oil is a really bad thing, dilutes the oil and causes bearing failure. So get that changed with a new filter. Use 10-40W JASO MA rated oil, automobile oil will not work with the clutch.
    Once that's done clean the engine with a degreaser found in auto parts stores, follow the directions. Once the engine is dry throw some baby powder on the suspect areas for leaking and run the engine for a few minutes. The powder will turn into a gooey mess where the leak is.
    Do you have the FSM, factory service manual?
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration


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  3. #3
    Member srahome's Avatar
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    Thanks, Jim. I bought some LucasOil 20w50 for the oil change since I heard it may reduce leaks, lower oil temps, and make the engine run quieter. Is that a good move? It looks like the HiFlo HF-111 and K&N-111 oil filters will work but I'm not 100% sure - do you know how I could find out? I have the Clymer Manual and just downloaded the shop manual posted by another user (thanks for that!). I will change the oil tomorrow and go from there. I assume I should probably wait until after the oil change to ride it again - esp if the gas / oil thing has thinned the oil so significantly that it will cause damage. I guess it's too early to think about potential causes for the gas in oil, oil leak, and backfiring?
    Cheers,
    Scott

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  5. #4
    Banned Mayhem's Avatar
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    It is a safe bet all things considered, If you have Gas in your oil, then Your rings are likely cooked or weak.
    The cylinder bore in the only place on the entire engine that certain conditions would allow fuel into the oil, much the same it would let oil in to be burnt.

  6. #5
    Super Moderator longdistancerider's Avatar
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    Either of those filters will be fine since they are the same ones in different boxes but you do have to re-use the O-ring since they don't include that.
    20-50W is specified for riding in 85 degree plus temps, 10-40W otherwise but you'll be fine using it IF the oil states JASO MA on it for rating. Oil viscosity will not solve any oil leak, might slow down seepage but you have a leak not seepage.
    Clymer is fine for locating where something is but it's got so many errors it's a good door stop or fire starter in most case. Rely on the FSM.
    Yes, it is a possibility as mentioned there might be ring damage. You won't know until you've got it running properly and do a hot compression test.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration


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  7. #6
    Junior Member srhaome's Avatar
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    Thanks again - I have a friend coming tomorrow with a compression hose / gauge to do the compression check. We'll change the oil first but it sounds best to stick with 10W40 for now. Didn't know Clymer was so bad so now I'm really glad for the FSM posted on this site. Forgot to ask - do I need a new crush washer for the plug when I change the oil?

  8. #7
    Super Moderator longdistancerider's Avatar
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    It's a good idea to replace it but I have re-used mine several times, especially while travelling.
    You did get the 450 FSM correct, different than the 400 in some areas
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration


    https://www.hondatwins.net/forums/22...d-attempt.html and https://www.hondatwins.net/forums/1-...r-attempt.html
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  9. #8
    Member srahome's Avatar
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    I got the manual from the post by SeEyEeS in the SOHC group. It's labelled as CB/CM450 shop manual and lists an addendum for the '82 and '85 CB450SC so I assume it's the right one. Some of the instructions are preceded by a combo of CB450T, CM450C/E while some say only CM450. I assume i should follow instructions for the CB450T. None (other than the short SC-specific addendum) refer to the CB450SC specifically.


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    -- srahome --
    1990s Honda CD100
    1983 Honda CB450SC Nighthawk

  10. #9
    Super Moderator longdistancerider's Avatar
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    Yes, use the basic manual sections after checking the addendum section first to see if there is anything specific listed there. Addendum's are specific year/model changes from the basics listed
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration


    https://www.hondatwins.net/forums/22...d-attempt.html and https://www.hondatwins.net/forums/1-...r-attempt.html
    Road Trip https://www.hondatwins.net/forums/11...here-i-go.html or "where's Jim now?"
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