Wednesday, February 15, 2006

Modified a Mac to fit the 650

The bike is approaching finished now. The biggest mod this time is that I got rid of the 2-into-2 exhaust that I built previously. It was too loud, and since I decided to get rid of the sissy bar there was no good place to attach a bracket to keep it from moving around everywhere. So I shelved that exhaust, and decided to use the MAC exhaust that I had from a parts bike I acquired some time back.

First off, the Mac exhaust for a CX500 will NOT fit on a CX650 without major modifications. The heads are about 1/2" taller, AND 1/2" wider. Add to that an extra 2" of oil pan sticking out the bottom of the engine, and there is no way the exhaust will fit. Fortunately, I have a wire-feed welder.

First thing I did was cut the muffler off of the MAC. The MAC exhaust system comes with the muffler welded in place, and it's a big megaphone that just screams "1970's Honda". Out comes the angle grinder with the cutoff wheel, and the muffler goes in the trash can. Next step was extending the exhaust headers. I cut the headers off about halfway down their length. I bought two exhaust pipe couplers from Advance Auto that fit over the cut-off Mac pipes snugly. Then I took my spare CX650 engine and flipped it upside down on the garage floor. (Note: Make SURE all the oil is drained before trying this!!!) I mounted the exhaust system with the couplers and once it was all in place and looked good, I tack welded the pipes in place. Then I took the headers off the spare engine, put it back upright, and got a bunch of paper towels to clean all the oil off my garage floor.

After I cleaned the oil, I went back and welded the pipes together properly. Then I test-fitted them to the chopper. Well, the bottom bracket didn't line up correctly, so I cut it off and welded it back on in a new location, so it now bolts to the bottom of the engine. Then I tried test-fitting the JC Whitney muffler I was planning on using. It fit perfectly, without any spacers or anything.... but the angle of the Mac pipe was such that the muffler hit the axle and swingarm. So I took the mac off again, and cut through half of the big 2" diameter pipe, right under the engine. I cut a slot about 1/8 to 1/4" wide through half the pipe. Then I bent the pipe to the right, which closed that newly cut slot. Then I welded it back together.

After I was done I wrapped the headers in header tape from JC Whitney, and attached it with stainless steel hose clamps. Then I painted the wrap with high-temp exhaust paint. Then I finally mounted it correctly to the bike, with the crush rings and everything. The muffler bolted right onto the pipe, but there was no support for it bouncing up and down. I welded a stud to the back of the pipe, and used a simple chrome bracket to hold the muffler in place. It looks great.

Now that the new exhaust was on, it was time to correct the jetting. The bike had always run a little lean, so when I installed the 650 engine I also installed #130 main jets. That really helped wide-open power, but it was still hesitant and didn't run well between idle and 4000rpm or so. I rode the bike with my new Mac exhaust like this for a while. I experimented with .020" shims under the needles, and that helped move the hesitation lower in the rpm range. Screwing the pilot jets out as far as they'd go also helped matters. This told me right away that I was running too lean just off-idle. So I repalced the stock #78 slow jet with a #85 slow jet. Instantly the bike ran PERFECT. It's been running that way ever since, though it may be a little rich in the midrange. Going to remove the shims and see if that makes a difference.

Parts Breakdown:
•  MAC Exhaust $Free (got it off a parts bike, been saving it for a while)
•  JCW Muffler $35
•  Misc jets $15
•  Clamps $10
•  Crush rings $20
•  Exhaust couplers  $8
Total $113

Running total for Chopper: $1971

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