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Discussion Starter · #1 ·
Fall is in full swing here, so it's time for me to start considering winter projects. My '72 K5 has been great all summer, but has developed few little niggling problems.
Last weekend I tuned her up. Set the valve clearances (what a difference that made!), timing and points, and adjusted the cam chain tension and carbs. Now she's running much more smoothly, but when warmed-up the engine cuts out after 15 -30 seconds at idle, like at a stoplight. She also takes a bit longer than I'd like to return to idle before cutting out.
The right cylinder hs been running rich. The plug was fouled with carbon buildup, and the exhaust smokes a bit on that side. I tried using the pilot screw to adjust the mixture, as per the service manual, but leaning out the mix drops the rpm.
How she starts has also changed slightly. Before being tuned, she needed full choke and would turn over several times before firing, usually on the first try. Now, she doesn't want to start right away, requiring several tries with the starter button with no sign of firing. Then she fires as soon as you thumb the button and runs quite smoothly but with the occasional cough.

Could these things be related? Could they be related to low compression? What steps might I take to diagnose the problem(s)? Any thoughts you might have will be greatly appreciated.

Since posting earlier this morning I went out, cleaned up the right plug and fired her up. Started the second try (sometimes my kill-switch is hinky), with no smoke that I could detect. Right cylinder is still not running as smoothly as the left, (I can feel "popping" when I put my thumb over the exhaust)but she didn't cut out even after 5 minutes or so at idle. Played with the pilot screw but it didn't seem to make much difference until over 2 turns out - at which point the engine cut out. I left it at 1 turn out. I'd take her out for a spin, but it's raining.
 

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You should gather some basic diagnostic info first.
Check the compression - warm engine, throttle wide open, NO CHOKE.
If you don't see a minimum of 160 psi, you need top end work, that simple. With poor compression, you'll just spin your wheels, wasting time and money.
Or if you can run a leakdown test, that's even better.

And, as always on a 450, CHECK THE BATTERY - a 450 will not start or run well with a compromised battery. Running these batteries all the way down just a few times will hurt them bad. Just because it's new doesn't mean it's good. They can be bad right out of the box.
Get yourself a Battery Tender Jr., and use it religiously. They can be had for less than $20 here -
http://www.derbycycles.com/cgi-bin/eShop/index.cgi?pid=2601
Hardwire the connector to your battery and plug it in when you park the bike.

Make sure you've done the timing properly - there's a specific, non-intuitive procedure.
http://www.hondatwins.net/forum/viewtopic.php?f=18&t=883

If all those things check out, that's the time to move on to the carbs.
http://www.hondatwins.net/forum/viewtopic.php?f=29&t=537
With all other systems checking out ok, reluctance to idle down properly generally indicates the carbs are "out of synch".
Failure of a mixture screw to affect the rpm at idle generally indicates a dirty carb. The exact turns out of the mixture screw is fairly irrelevant within extremes.

And 450's are extremely cold-blooded, almost always require full choke to start if the engine isn't warm.
And they get kind of bitchy if they're ignored and not run regularly. I have to go out to my unheated garage all winter and start and run mine at least once a week, or it gets pretty snotty about things.

Let us know what you find with the diagnostics.
 

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Discussion Starter · #3 ·
Bill,

Thanks for the reply. Good to hear from you again. I haven't hung out here for most of the riding season - too busy riding! I know my compression is low. What will a top end rebuild (if that's what I need) cost? The local bike shops won't touch a bike this old. There's a local shade tree mechanic, but I'm leery of him.

While I'm in there, what else should I look at to improve reliability and performance?

Stafford
 

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Shaky Bones said:
Bill,

Thanks for the reply. Good to hear from you again. I haven't hung out here for most of the riding season - too busy riding! I know my compression is low. What will a top end rebuild (if that's what I need) cost? The local bike shops won't touch a bike this old. There's a local shade tree mechanic, but I'm leery of him.

While I'm in there, what else should I look at to improve reliability and performance?

Stafford
Do check the compression anyway - then pour a little bit of oil in the cylinder, check the reading again. If it jumps up dramatically, it's a sure sign of bad rings. Otherwise, the valves are the cause of your bad compression.

These are prices from the local machinist I use.........
Bore jobs are typically $40-$50 per hole.
Valve work runs me about $35-$50 per valve, to cut new seats and lap them in, then vacuum test.
To just lap them in he charges $15 per valve.

Once you have it torn down, turn the head on its "side" and pour acetone into the intake/exhaust ports - if you see any leakage/seepage at all around the valves (in the combustion dome area), you need a valve job. If not, don't touch them.
Have someone "mike" your cylinders and compare to the specs in the manual. Pay special attention to the cylinder "taper" - it's an extremely small spec on the 450. Check the cylinders for any scoring. If anything is out of spec, you need to go oversize.
If you're able to get away with just new rings, you must hone. Do not hone if you don't replace the rings.
Do not put any oil on the pistons or rings or cylinders when you reassemble - leave them dry.

Put in a new cam chain and check all the rollers and tensioner. Check the tappets, cams, and cam bearings for wear. Replace anything that shows undue wear. Do not get the torsion bars or valves mixed up - put them in baggies and label them "left intake", etc.

There are posts here in the 450 section to guide you through it all.
Or get the engine to me and I'll do it for you........or to Steve, he's closer to you.
 
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