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Thread: 1981 Yamaha Seca - Carbon Buildup

  1. #1
    Join Date
    Jul 2015

    Default 1981 Yamaha Seca - Carbon Buildup


    Just a couple months ago I bought a 1981 Yamaha Seca xj750 from a guy in Michigan. I flew from California to Michigan to pick up the bike and ride it 2400 miles back to California. It had a SuperTrapp exhaust with closed end cap and 8 discs. The bike was in good shape, but had sat for almost a year.

    The bike struggled to run faster than 70 MPH and I attributed that to dirty carbs which I planned to clean once I got home. By the time I got to Texas the performance of the bike had dramatically decreased and my top speed was 40 MPH. I got to Tucumcari, New Mexico on Memorial day and the bike refused to go any farther - so I got a uhaul and hauled it home from there.

    Once I got home, I cleaned the carbs, checked the engine and all the electrical, had 2 motorcycle mechanics work on it for a total of 8 hours, and countless suggestions from the guys, but it would not run. Finally I removed the end cap from the SuperTrapp and looked down the exhaust pipe which looked to be clear of obstruction. While I had the cap off, I hit the starter button and the bike started right up and ran well... I examined the exhaust discs and discovered they had carbon buildup that was restricting the exhaust flow - not preventing the flow, but seemingly restricting it. I spent a few hours cleaning the carbon off the 8 discs and now the bike runs great!!!

    So here are my questions: I've been watching the discs closely and after a hundred miles they are getting sooty which means too rich... However my plugs are burning white which means too lean... I'm not sure what to do. The previous owner (who was the original owner) installed slightly larger jets when he added the SuperTrapp (was 120, is now 122) and I believe that was the right thing to do. I'm tempted to put an open ended cap on the bike which - as I understand - would be the equivalent of 10 discs - but that would make it run more lean.

    Any suggestions?

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  2. #2
    Join Date
    Feb 2015
    United States


    Your symptoms indicate to few discs (not enough exhaust flow)...except for the plug burn.
    You could go with an "open" end-cap, which would also provide more sound, or you could pick up a 6 or 12 pack of discs and adding 2-3 at a time until your proper top end and performance are obtained.
    Plug burn (color) can sometimes be affected by modern day fuel additives which vary by brands and location. A good reputable "Tuner" is your best bet for proper air/fuel ratio readings which may make use of a dyno and gas analyzer. In that case you may want an "open" end cap which would allow for the "sniffer".

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