Motorcycle Repair: cb350 engine rebuild, pattern texture, piston ring

Question
QUESTION: Hi Bill,
I have a 1971 cb350 K3 with about 5500 miles on it.  I found that it was shooting fuel out the air intake on the right carb.  A compression test revealed that the left side was at 150 psi and the right at about 95psi.  I tore down the engine and took the cylinder head, cylinders and pistons to an engine shop.  They re-seated the valves (with two NOS intake valves), milled the cylinder head flat, honed the cylinders, cleaned the pistons and put on a new set of rings for me.  
I have it back together now all cleaned up and with new gaskets.  When I first got the engine in the bike, I did a compression test and found it at around 180 for both cylinders.  I have since had idling for about 30 minutes total and have done intermittent compression tests.  The compression has dropped consistently and is now between 150 and 160psi.  Is this normal?  Should I expect fluctuations in the compression for a while until everything is seated or is something wrong?  I also had blow-by coming from the breather tube before the rebuild and still have it.
      Also, When I open up the throttle and let it go the rpms climb and just keep climbing.  I always shut it off before it gets to 4000rpm so as not to damage the rings.  What could cause this runaway rpm after the throttle is released?  
      I love this bike and have done everything by the book and with great care.  I would greatly appreciate any help you could give.  Thank you VERY much.
Pete

ANSWER: Pete... did you do the compression test with carbs mounted or not? Throttle held wide open or not?

Did you clean out the piston ring lands, where the rings go? Varnish/carbon builds up in there and can push the rings out excessively, if not cleaned out thoroughly.

Rings will break in fairly quickly, but it depends on the hone pattern/texture as well as break-in procedure and type of oil. You will have some blow-by during break in, due to the rings still making friends with the cylinder bore surface. Break-in should be about 500-600 miles, with early oil changes. Use 10-30 oil to break it in, if possible. Did you check your end gaps in the cylinders, before installing the piston rings on the pistons? You might want to recheck your head bolt/nut torque, too.

Compression readings through CV carbs tend to run lower than specs due to air flow restrictions of the vacuum piston slides. Even holding the throttle open, just opens the butterflies.

The engine shouldn't run away after a throttle blip. Causes can be air leaks at the manifolds, sticking throttle cable segments, mis-routed throttle cable, insufficient free play at cables or sticking spark advancer. Make sure that the spark advancer is free to advance/retard easily against spring pressure. Intake manifolds have been known to crack inside the spigot mount recesses.

Bill Silver

---------- FOLLOW-UP ----------

QUESTION: Hi Bill,

Thanks so much for you help.  The compression is consistent at 175 now and I have 300 miles on bike since the rebuild.  
I have just a couple problems that I am hoping you can help me figure out.  First, I have oil seeping from the intake rocker arm pin on the left side; the oil collects around the locknut on the rocker arm pin inside the points housing and drips down to the bottom of the points housing.   I pulled off the points housing and the oil seal on the pin seems fine as does the gasket that goes between the points housing and the camcase.  I am considering taking off the locknut on the rocker arm pin, putting gasket cement around the pin where it goes into the housing, and then putting the locknut and washer  back on.  Would that be okay to do??   I have also had a little seepage (much less) from the same place on the right side in the tach housing.  

Secondly, after I ride the bike for about 10 miles, the idle can get perilously low (around 900 rpm)  and occasionally the bike cuts off  when idling and stopped.  It will start right back up and ride fine.  It also does fine if I just give is a little throttle when stopped.  This leeds me to believe that it is getting fuel. I tried raising the idle speed but that makes the idle too high much of the time.  Any ideas??

Thanks again for your help, you got me on the road!!

Pete

Thank you SO much for your help.  All your previous

Answer
Pete....  Did you install NEW o-rings on the rocker arm shafts? Old o-rings can get hardened and loose their ability to seal properly.

http://www.cmsnl.com/honda-cb350-super-sport-350-k2-us_model431/partslist/E++03.  #17.

Check the shafts for wear, if the o-rings are new. Hopefully, you have the right parts installed. Some aftermarket gasket kits have inaccurately made parts.

http://www.cmsnl.com/honda-cb350-super-sport-350-k2-us_model431/partslist/E++02.  #19 is a tach drive seal that may need replacing.

Idle speeds are governed by a number of factors:
Air leaks past the throttle shafts (worn shafts/carburetor body shaft bores.
Ignition timing and spark advancer function (sticking spark advancer or incorrect spark timing)
Air leaks at the intake manifold (cracks)
Gas cap vent plugged up, causing vapor lock -vacuum condition in the fuel tank that lowers float fuel level.
Petcock passageways restricted... remove, clean, overhaul with new parts.

Bill Silver