Bridgestone Motorcycle Parts Discussion Board

Bridgestone Tech Talk => 350 Talk => Topic started by: Steve Swan on September 11, 2018, 05:22:22 PM

Title: Re: Slowly yellowing exhaust pipes
Post by: Steve Swan on September 11, 2018, 05:22:22 PM
While the GTR is running great, and almost always starts first kick hot or cold, I would appreciate your thoughts on the discoloration of the exhaust pipes on my GTR.  Although both exhaust pipes are yellowing, the LH exhaust pipe is yellowing more than the RH pipe; the yellowing is light but more with each ride.  I installed new BH8S plugs; they are uniformly the same color, a dark tan with some slight oiling.  I am running Amsoil Interceptor injection oil.  The only things I can think of are the LH mixture screw (I believe is a gas screw) needs to be turned out, the LH ignition timing is advanced more than RH and/or there is some sort of air leak (I see no signs of leakage at cyl/head joint).  The bike came from 500 feet to 5000 feet and was not getting enough air (blubbering and would not accelerate past 4500 rpm), so I dropped the main jet one size, from 130’s that came in with the bike to 120’s (acceleration improved, but still blubbered with 125 mains).  The pipes were not yellow when I took delivery of the bike; the yellowing has occurred since my ownership.  I have not measured each point’s gaps, however I did clean the points and put cam grease on the felt wick.  In all honesty, both gaps looked a bit close so the timing could be a bit retarded….?  Thanks in advance for sharing your expertise!
Title: Re: Slowly yellowing exhaust pipes
Post by: OldSwartout on September 12, 2018, 07:53:03 AM
It sounds as if you have the mixture correct. It's normal to have the head pipes yellow and eventually blue on bikes with single layer pipes, if ridden at normal highway speeds. Both two strokes and four strokes have the problem.  Some brands eventually went to double-walled pipes to avoid the issue. Verify the timing. A sdlight variation probably accounts for the difference between left and right.  Air leaks can also be at crankshaft seals, but probably not enough to affect the mix at road speed.  The mixture screws are air screws and backing them out will lean the mixture, but not significantly at the high RPM and load that yellows/blues the pipes.  The best you can do is occasionally clean the discoloration with Simichrome or other cleaner/chemical made for the job.

If anyone else has more or different experience, chime in.
Title: Re: Slowly yellowing exhaust pipes
Post by: nysz1b on September 12, 2018, 10:36:38 AM
Kawasaki dealt with the yelowing/bluing problem on the 900 Z-1's by using double-walled header pipes.
Title: Re: Slowly yellowing exhaust pipes
Post by: Steve Swan on September 12, 2018, 12:43:14 PM
Karl, thank you for your reply.  My vin is 21F06109, so i believe that makes the GTR a June 1968 unit; in any event the reason i mention the vin is that the timing plug hole in crankcase is 6 mm, not the 8 mm as mentioned in factory literature.  With the 6 mm hole, what do you use for a timing plug?  my idea, i was thinking of using a piece of drill rod that will be a slip fit in the hole...
Title: Re: Slowly yellowing exhaust pipes
Post by: BRT-GTR on September 12, 2018, 05:56:54 PM
            A short piece of 5mm rod about 11/4'' long does the trick. Round off the inner end so it drops into the dimples on the flywheel. Rest a finger on the rod while turning the engine using the rear wheel, geabox in 6th with the plugs out. Points should open when pin snaps into dimple.   Set the points gap to 0.3mm or 12 thou before checking the timing.                                                                                     Brian.
Title: Re: Slowly yellowing exhaust pipes
Post by: Steve Swan on September 12, 2018, 06:05:38 PM
            A short piece of 5mm rod about 11/4'' long does the trick. Round off the inner end so it drops into the dimples on the flywheel. Rest a finger on the rod while turning the engine using the rear wheel, geabox in 6th with the plugs out. Points should open when pin snaps into dimple.   Set the points gap to 0.3mm or 12 thou before checking the timing.                                                                                     Brian.

Brian, thanks for that info; how deep are the dimples?
Title: Re: Slowly yellowing exhaust pipes
Post by: BRT-GTR on September 13, 2018, 06:19:55 AM
           As best I can assess, the dimples are approx 1/16'' deep and were formed by a 5mm drill tip being run into the flywheel.  You will readily feel when the pin locates one and don't worry, there is no chance of the pin dropping into the crankcase. 
        Pin could be slightly longer if you find that easier to handle. Maybe start with a 2'' piece of rod and trim length to suit.
Title: Re: Slowly yellowing exhaust pipes
Post by: Steve Swan on September 13, 2018, 01:15:33 PM
           As best I can assess, the dimples are approx 1/16'' deep and were formed by a 5mm drill tip being run into the flywheel.  You will readily feel when the pin locates one and don't worry, there is no chance of the pin dropping into the crankcase. 
        Pin could be slightly longer if you find that easier to handle. Maybe start with a 2'' piece of rod and trim length to suit.

Thanks Brian, i am glad i asked, i envisioned an actual hole the pin would lock flywheel in place.  i had a piece of 13/64" drill rod that i turned down slightly on a soft abrasive wheel so it would just slip through a 6 mm nut.