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Discussion Starter · #1 ·
Okay, time to tune the bugs out. Here are the problems:

1. Right side is mildly backfiring. I don't believe it is an exhaust leak.
2. The bike starts right up each time, but it won't idle unless I screw down the flippers.
After it's warmed up I back them off.
3. The bike will be idling at 1000 to 1100 rpms and when I come to a stop it suddenly dies out. In fact, just about anytime I stop without giving it gas it dies very abruptly.

I took the carbs apart when I got the bike and just cleaned out the jets and reassembled them. They were pretty clean.

Where do I start?
 

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Sensei
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The advance unit staying advanced (stuck or weak springs) could have the "needs revved, dies at idle" effect...We deliberately welded them at full advance on some racers, but there are almost zero time where the engine is below 2000 RPM during those races......
 

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When you check your timing, observe the advance curve. If the ignition begins to advance before 1800 RPM or if the strobe timing picture dances around at idle speed you have weak springs or worn parts in the advance mechanism. Low speed advance will up the idle RPM and that will result in more advance, raising the RPM even further. If you try to compensate for this by setting the idle lower, the advance decreases and the motor stalls. Your timing light "picture" of the timing marks has to stay constant until you're above idle RPM.
 

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Chris' spark advancer "appeared" to be ok when I inspected it.
Though I did not get to run the engine, and did not disassemble the advancer.

He just had new rings and a professional valve job - I measured 172 psi, immediately after rebuild, cold engine never run, sitting on the bench.
So I doubt if it's compression - he told me himself the bike is WAY faster than it used to be.....

The valves were set at 0.002", so I doubt they're too tight.
That pretty much leaves timing/carbs.........
 

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My CB450 had similar issues and a new advancer made a big difference. My old advancer had weak springs and it caused irregular idle and the bike did stall sometimes. I cut a loop off the sprngs to see what that would do and it helped but I didnt like that as a permanent fix. I bought a new advancer and the bike runs great now.
Don
 

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On your backfiring, Chris - especially if it's on deceleration, it is most likely an exhaust leak.
Check to make sure you didn't distort that copper exhaust washer in the head when you tightened down the wrong pipe before. Recheck the pipe/muffler joint, and check the muffler itself for any leaks.

Check for any intake leaks, too. I recall that your rubber intake manifolds were pretty well fossilized and very hard.
I get the bike running, then point a propane torch around the area of the intakes.
It's not lighted, of course, and not full blast - if there are any intake leaks the gas gets sucked in and the engine instantly runs faster. You can pinpoint it pretty well.
Now, remember - SAFETY !!
I'm sure there's somebody out there capable of blowing up themselves and their bike doing this, so be advised - if you're a geek, don't do it.

Some people spray WD-40 instead, and the engine loses rpm if it gets sucked in.

You never mentioned checking the float level, so I'd do that too.

When the engine begins to act up, see if it helps to nurse the choke a little, that will be valuable info.
Has it been driven enough to get a plug reading yet??
 
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