Air Box Oil Leak and Tightening the Oil Filter


I’ve been having oil leak from my PC for several months now.  Last year, I ended up having to replace the seal around the shift rod.  This time it doesn’t look like it’s coming from there.  Instead, I found two separate places that oil was leaking from.  The oil filter had become loose and appears to be the main source of oil.  I tightened it nearly a full turn.  Also the air box has developed a bit of a leak.



The telltale signs of an oil leak.




Oil was leaking from where I’m pointing my finger at.  The carb boots in the air box are not an integral part of the air box.  They are molded separately and then inserted after the fact.  Some sort of glue is used to seal them.  The glue on mine has gone bad.  I cleaned it up as best I could and re-installed it for now.  In a few weeks when I replace the air box with something different, I will make sure the new air box does not have a leak.  Incidentally, the reason that oil is coming out of the air box is because of the crank case breather tube.





12 Replies to “Air Box Oil Leak and Tightening the Oil Filter”

  1. Douglas,
    Your site and detailed photos has inspired me to fix my ’94 PC’s (136K mi.) cam plugs. My bike also has the broken airbox seal, and I intend to do this fix… but the design of this airbox puzzles me. I can’t understand why the one small pipe would seemingly redirect oil directly into the carb. Can you comment? Also, how much oil should that breather tube be passing? There was a great deal of oil staining on the rear cyl head cover, and the hose was rotted and will be replaced. Thanks for your insightful site.

    1. Hi Karl,

      The oil that gets redirected into the carb is burned by the engine. It”s the same way on all of the motorcycles I’ve worked on from the 80s. Well, okay… all of my bikes are from (or were designed in) the 80s or late 70s… The crank case breather tube just sucks any fumes or vapor off from the crank case and lets the engine ingest and burn it rather than have the oil fumes go to atmosphere directly. I was surprised with how much oil was in my airbox but the oil level on my bike has stayed rock solid except in the most extreme heat situations (100+ F and very hard, fast riding). It seems that the hotter the engine is, the more oil gets pushed up and into the airbox. It’s buy design though. I always carry an extra quart of oil with me just in case I run low.

      Good luck!


      1. Thank you! That gibes with the conditions in which I was losing oil (100 degree MD summers).
        I didn’t want to pull the engine out to check the condition of the head gaskets, and the pattern of oil staining and your explanation about the airbox/carb leak give me hope that I may not have to.

        1. Unless you lost coolant, the fan stopped functioning, or the thermostat broke, it would be exceedingly unlikely that the head gasket was toast. It’s probably the air box and/or cam seals that have cause your staining. They’re both pretty easy fixes.

          If you happen to have the front cowling off the bike at some point, you might go ahead and test the fan and the thermostat as per the manual’s instructions. I need to do that the next time I have my bike apart. After all these years, those components might start to go bad soon. Although your bike will be several years behind mine with things starting to fail.



          1. Well, after several months of on-and-off work, I have completed my engine repairs- cylinder head covers off for a reseal and cam plug replacement, 3 radiator hoses and breather hose replaced, and carbs sea-foamed (after all that work, I didn’t want to get into a carb rebuild.) She restarted, and nothing went BANG, so I guess I did it right! Going to leave the fairing off for a couple days’ drip-test. Based on your experience, is there anywhere else I should go prospecting for leaks? Thank you again for your expertise- I am amazed at your fuel injected PC project, by the way.

            1. Hi Karl,

              I’m glad to hear that you fixed all your leaks! Last night I pulled the plastics of my bike to install LED bulbs (more on that in a future post 😀 ) and found a significant oil leak. I am hoping that the leak is originating in the air box. Tonight I am going to pull the air box and use some RTV on the broken seals. There sure is an awful lot of oil swirling all around inside the plastics just to be coming from the crank case breather tube and air box.



              1. Yeah, I also resealed the airbox while I had it off. The boots were a giant pain to take off and clean up. I am perplexed by the volume of oil that makes its way into the airbox. Having seen what’s on the other end, I don’t see how that much oil should be burping through.

                1. It makes me wonder why I don’t have to refill the oil on my bike more often. In fact, I hardly ever do unless I’ve been running for a long time at high speeds in hot weather over hilly terrain. The last time I can remember topping off my bike was last summer after I had done about six hours at freeway speeds through the mountains of Southern Oregon in 95 degree weather. My suspicion is that it is a heat thing and our bikes are much more prone than most to heat-related phenomena. There is a thread on the IPCRC at the moment talking about rear wheel temperature and the “chunking” issues that Metzeler tires have, for instance.

                  I wish I had the ability to put a small flow meter in the oil breather tube to record just how much fluid or gas is moving up through that tube during normal operations. Also I wonder if there is any correlation between the air filter state (how dirty it is, etc) and oil consumption…

                  1. Douglas, i bought a “well used” 89 pc and because of starting problems have gotten into the carbs. First thing i noticed that really surprised me was the amount of oil in the air filter. Apparently this is not unusual with pc’s. Being a participant on the Volvo “turbobricks” forum we have been experiencing a very similar problem with older volvos. A response to this problem that a lot of volvo owners have applied is the installation of a “catch can”. Have you heard of this being tried on a pc? When i put mine back together i am hoping to eliminate the charcoal canister and as many associated vacuum hoses as possible, Calif version. That space could be utilized by a catch can. What are your thoughts on this? Thanks, jim

                    1. Hi Jim,

                      I’ve never heard off having an oil catch can but I suppose it would work. The way the air box is setup, the oil ends up getting burned in the engine. After I resealed my airbox, I haven’t had much problem with oil leaks from the airbox.

                      Regarding the California emissions equipment, aside from simplifying the design of the bike, removing it won’t get you a noticeable bump in power. If you want to remove it, check out and the subsequent pages. Other people on the IPCRC ( have removed the emission system without too much fuss. One of my two PCs has an emission system on it. I haven’t bothered doing anything to it.



                    2. Thanks for the speedy response, Douglas. My intentions are to eliminate as much of the mess as possible. I’m a bit of a perfectionist when it comes to those things. This is not an uncommon occurrence with older cars. Generally it makes a mess in the intake manifold and throttle body. These catch cans are for sale all over e-bay but i made my own from a screw top plastic bottle, 2 plastic fittings ans some reinforced plastic tubing. Have experimented with different types of disposable media as the inside baffle system didn’t seem to be very effective. If it doesn’t cause gumming up of the carbs i may be just as well off following your method. Not looking for bump in power as much as just simplify things by getting rid of some of the “clutter”. Thanks much for the tips I will check them right now. BTW while i have the carbs apart are there any mods or jetting changes you would recommend? I live at sea level, SoCal. Cheers to you, Jim

                    3. Hi Jim,

                      I would recommend against anything plastic that isn’t rated for high temperatures in the engine compartment on a PC800. It gets very hot in there due to the completely enclosed engine design. A metal can with automotive hose (maybe fuel hose?) going to it would probably survive. Having had things melt in that general area before, I can say that it’s good to plan for high heat.

                      The only thing I would do while servicing the carbs is to verify there are no tears or holes in the diaphragms. The jetting should remain the same. I am not 100% sure if the 49 state vs CA model PC800s had different jetting or not. You might check it out in the service manual. Did you buy a carb rebuild kit? It’s not a bad idea to replace the jets if you’re already in there but it isn’t necessary. My bike has somewhere between 50 and 60k on it and has never had jets replaced.



Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.