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Exhaust Differential

Discussion in 'The Garage' started by arfingdog, Jun 18, 2020.

  1. arfingdog

    arfingdog XS400 Enthusiast

    That is, I have stronger exhaust coming out of the right side than the left. Is this indicative of a problem in cylinder 1?
    The bike, a 79 XS400F, starts easily and idles nicely. But even at idle I can feel a marked difference between the two sides. I can't answer much about how it is on the road, as it isn't registered yet (next week!) because our DMV has been closed. But I have done a few miles and it isn't particularly rough running.
    Background: The exhausts are not stock, but are about stock length. The baffle system is simply a single disconnect/overlap the makes the exhaust fill a 10" area before moving on. New coils, connectors, plugs, and points. Valve clearances good. Static timing good. Carbs recently cleaned by me with new jets. I balanced the carbs today. The air filters are new and the same on both sides.
    Thanks for your thoughts on this!

    Tom
     
  2. xschris

    xschris a lifestyle not a trend Top Contributor

    Synced with a manometer? Cold compression test on both sides? Those are the things I would do. Nothing aftermarket will act or work like stock exhaust. I have found lots of them don't seal well at both ends either. Post some pics of the bike.
     
  3. arfingdog

    arfingdog XS400 Enthusiast

    Synced with a manometer, yes. Have done no compression tests yet. I hadn't thought of exhaust leaks as a possible cause. Given that it was sitting in a barn for 17 years, maybe I should make sure there isn't some mouse nesting material up in the header. :)
     
  4. arfingdog

    arfingdog XS400 Enthusiast

    I'll be doing a compression test this weekend. I didn't want to suspect that the cause could be mechanical underperformance in cyl 1--was hoping for something like "Oh, they all do that!" ;-) IMG_20200613_080218 sm.jpg
     
  5. Graham04

    Graham04 XS400 Member

    It's not so bad, at least you can take the head and barrels off without removing the engine. Try letting something like redex soak into the piston rings, just in case they have gummed up over the years. A bit of running might free them up.
     
    arfingdog likes this.
  6. arfingdog

    arfingdog XS400 Enthusiast

    OK, prepare for some dumb questions, because I haven't done a compression test before. Plugs are out, carbs have been drained, Ignition fuse removed. But I can't use start button without the ignition fuse...will kick starting provide enough force? If not, I don't want to hurt the ignition system: Will it be OK if I connect the spark plugs and ground them to the engine?
     
  7. xschris

    xschris a lifestyle not a trend Top Contributor

    I would jump the starter directly. Do the compression test with your throttle at full open. Leave the other plug out also as this will put less stress on the battery your jumping it with. I use a jump pack for this. You can just kick it but it's just more work.
     
  8. arfingdog

    arfingdog XS400 Enthusiast

    Thanks Chris. Before I saw that I went out and did it the more work way. So, what is the compression supposed to be? Doing it cold, wide open throttle, both plugs out, I got 145 psi on the left and 160 psi on the right.
     
  9. xschris

    xschris a lifestyle not a trend Top Contributor

    125-155 is the low any high for these. They should be only a few lbs from each other. I would do a wet test on the 145 one. If it goes way up your rings could be a bit worn or sticky. If it stays the same I would check your valve adjustment. It might be just a matter of driving the bike at highway speeds many times and miles to clean the motor out. I had my 80g go from 125/127 to 147/145 after a few thousand miles and a few oil changes.
     
  10. arfingdog

    arfingdog XS400 Enthusiast

    Thanks as always, xschris. I feel good about the valve clearances. I had done them a month or two ago when I got the bike, but I rechecked #1 a few days ago to make sure. I think I'll first do a new cold dry test using the starter (alas, I have a jump trigger but not here). I want to see how the pressure climbs...when using the kick it seemed like #1 climbed in baby steps more than #2. And then I'll do the wet test. If this is how it stands, I believe I'll go with the run it for a while method, after, as @Graham04 suggested (thanks!), adding a little something (Seafoam) into the pistons and letting it sit. The engine did sit for 17 years, after all, and has only done 6 or 7 neighborhood miles since, so not surprising that it might need some loosening up.
     
  11. arfingdog

    arfingdog XS400 Enthusiast

    Reporting back, and with some puzzling results. I tested both cylinders dry again, WOT, but using the starter instead of kick: #1 = 170 psi, #2 = 180 psi! Of course, I suspected the accuracy of my Mityvac gauge, so I attached it to my compressor and the Mityvac reading matches the output readings on my compressor exactly. So, what do I make of such high compression readings?
     
  12. xschris

    xschris a lifestyle not a trend Top Contributor

    Could be full of carbon and oil. With the 6 motors I have and one that had 9k miles on it I never got more than 155 lbs cold and dry.
     
  13. arfingdog

    arfingdog XS400 Enthusiast

    Ok, well, I've squirted some Seafoam in each cylinder and cranked it over, and for fun I might take a look inside with my cheap borescope. After tomorrow, I'll try to put some runtime on the engine and see how it fares.
     
  14. xschris

    xschris a lifestyle not a trend Top Contributor

    Just drive it for a while. Put long rides on it and avoid short runs or idle time as those are the worst thing for these old motors. I don't even start the bikes unless I can put at least 10 miles of 55+ mph runs on them.
     
  15. arfingdog

    arfingdog XS400 Enthusiast

    Sounds like a plan, Chris. Borescope showed plenty of carbon in there!
     
    xschris likes this.
  16. arfingdog

    arfingdog XS400 Enthusiast

    Just to close out my end of this thread by getting back to the exhaust.. just in case this hasn't been said before, @xschris knows all! I checked around the header to exhaust bell joint and there's a pretty good leak happening on the left side, as you suggested. Mostly answers that mystery.
    Now I run it for a while.
     
    Gra900 likes this.

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