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Discussion Starter · #1 ·
Have just finished assembling a sweet 1975 cb360 with parts from two clunkers. My first ever motorcycle. Have referred to this forum repeatedly, love it. Thanks, all, for your wisdom. Have a problem beyond the scope of my amateur mechanicing ability. Can anyone help?

The bike seems to run strong, but acts like the mix is too lean: some backfiring, hesitation and tendency to stall on take-off, some missing while cruising. This last problem happened most noticeably when I had my wife on the back, accelerating up a hill. Sound like lean mix? Clues:
-- One of these bikes had 3300 miles on it, the other about 7000. Used best parts to build with.
-- I cleaned the carbs twice, the last time within an inch of their lives, boiling the gunk out in lemon juice (suggested by someone on line..here?) then soaking with carb dip, followed by compressed air, and careful probing with guitar strings in passageways. Adjusted floats, set both throttle plates exactly the same. Photo below, if I can figure out how to add one.
-- I cleaned out the tank with the nuts and bolts agitation technique, rinsed very clean, installed two in-line fuel filters -- glass cylinder with replaceable screen element, new fuel hoses.
-- Did extremely careful job of timing with the excellent tutorial provided here by tbpmusic.
-- Adjusted valves painstakingly.
-- Adjusted cam chain tensioner exactly as instructed in manual.
-- Installed new after-market mufflers, Dunstall is the name, if I recall it correctly.
--Compression is 155 on the right, 159 on the left, after ball honing and new rings.
-- Pilot screw on carbs, set as in the manual, to maximize rpm at idle, is open on both carbs 2 and 1/4 turns.
-- Strong sparks on both plugs.
-- Here's a baffling one: right cyl. plug is sooty, left one is light tan.
-- Have driven only 200 miles so far. What a thrill.

Anyone have any ideas? One other question: Any way to make this thing run quieter?
A thousand thanks to the moderators and the contributors to this forum. What a delight to read your stuff! Proves two heads are better than one. Onward! :D
 

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I dropped by your other thread.. As for the loudness I'd say at this point you'll have to live with it unless you want to buy different mufflers.. Those Dunstall replicas tend to run a little on the louder side.


GB :mrgreen:
 

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My first questions.....Why do you have such low compression with new rings? (were they COLD readings?)....Warm/hot readings after fresh rings should be 175+, normal running condition for the bike is 170... (150 ish is re-ring time)......Are you sure the rings are in rightside-up?......What was the piston to cylinder clearance?
 

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Discussion Starter · #4 ·
Just posted this exact comment in the "New Members" section. Trolling hard for your opinion!

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Joined: Mon Jan 04, 2010 1:22 pm
Posts: 3
Location: Olympia and Morton, Washington
FIXED!!!
Wow! Thanks for the deluge of advice! Thanks a million. What a treat.
I pulled off the air cleaners, and that fixed everything! WooHoo! It runs beautifully, and the plugs now look identical, a clean brown.
In fastidiously cleaning everything, I had blown the air cleaner elements thoroughly with compressed air, but decided to go one better and I soaked 'em in white gas, scrubbed 'em with a parts brush, and blew 'em dry. An old-timer around here told me it was a mistake to use the white gas, throw 'em away, they're ruined, he said. I was skeptical, but I guess he may have been right. He runs a motorcycle shop, said he had some foam to replace the paper elements in an inexpensive kit. I'm heading to his place tomorrow.
I was skeptical that that could be true, but it was IndieSol's question about the air filters that convinced me to try running it without the filters. Thanks, Man!
Dirtbag said I should have replaced the valve seals....I forgot to mention that I'd lapped the valves. Did not replace the valve seals, but the machinist who helped me with the project said they were ok.

The compression: I'd love your opinion about what's going on there. I re-tested the compression tonight. The machinist who assisted me with this project recommended boring the cylinders and buying new pistons etc., but he knew I didn't have a lot of dough to put into this machine, so he said let's try the ball hone, with new rings. The cylinders had a little rust, all evidence of which was gone after the honing. I don't know the piston-to-cylinder clearance that 665 Sprint asked for. I recall that the machinist saw it as borderline. He had the factory specs. I carefully installed the rings the right side up.
So tonight, elated with how well it was running with the air filters off, I took new readings. Left, 158. Right, 157. Squirted oil in the holes, got 195 left, 178 right. Based on a tiny education in these things, I understand that to prove that the moderately low compression readings are due to rings, not valves. Why so much better on the left? How am I doing?

If you're still with me, one more question: Due to the loudness of this machine, I'm inclined to run it at really low revs in my urban neighborhood, when I'm cruising at 25 to 40 mph, like around 2500 rpm, unless I'm accelerating, I know what lugging is, and this skirts close, but feels ok. If I need power, I downshift.

One more: The bike feels stable at 55, and a little wobbly at 60. I almost never see bikes this size on the freeway anymore, and wonder if I'm unrealistic to consider riding this thing on the freeway. Few people do, so maybe they know something I don't. There must be something that inevitably draws motorcyclists toward bigger bikes.
Thanks, All!
Michael
 
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