Topic: CB350K4 low compression

[kuyarico]

I've been working on my CB350 for about the last couple of months. I had blown a cylinder about a month ago and replaced the engine with a Craigslist find. The new motor was very clean. It came with carbs, coils and wiring. I installed the motor and did a full tuneup on it. Cam chain adjustment, valve clearances, ignition points/timing, cleaned the advancer, carbs cleaned and floats adjusted. I could never get the bike to idle properly. I eventually identified an air leak at the left carb boot. I was certain that it would solve my high and erratic idle, and it did to a degree. But the bike still wouldn't idle very smoothly. The idles would rise dramatically 4 - 5K rpm as the engine warmed up.

Anyways, after rechecking my valve clearances and timing(which was a PITA with the idle being all over the place), I decided to buy a compression gauge and check the compression. I found that the left cylinder has about 128 psi and the right was about at spec at 170.  I put a few drops of oil in the left cyl and checked it again. It came up to about 167. I'm going to start pulling the motor this week. Is there anything (besides the piston/rings) that I should look for? I've worked on my own bikes for the past 5 years, but have never cracked open a motor. I would welcome any expert advice from this forum.

I'm seriously bummed about this mostly because I had planned on riding the bike down to the AMA Vintage Days this coming weekend. But instead, I'll have to ride my 2007 BMW (how embarassing). In all honesty, the enjoyment that I get from riding my vintage scoots goes way beyond any modern bike that I've ridden.

Here's a pic of my ride:

[cycleman]

Before I tore anything apart I would drive it for a few hundred mile or so.

Your post doesn't indicate how long the engine you put in has sat, without being run.  Any engine that has sat for awhile, especially if its sat for several years will show low readings.  Also vac gauges designed for the auto world are not going to be that accurate. Engines that have sat, have dry cylinder walls and the rings are not working properly, thats why you get low readings. 

The higher idle as bike warms up indicates that the bike is running lean or the idle screw is not set properly and has been adjusted to compensate for the rough idle.  I supect that the idle circuits. pilot jets are not really clean. If your gas tank is clean, I would put some Sea Foam in the tank and run it.

Your best bet is to put fresh oil in the bike, some Sea Foam in the tank and take the bike for a ride before you do any tearing down.  If it is just minor the rings willl free up & the Sea Foam will help to clean the carb. I suspect the poor driveability is carb related not engine related.  ( Providing points/timing are OK ). As far as adjustments go its always valves, ignition and then carbs with motorcyle engines.

[kuyarico]

I cleaned out the the carbs and verified that the idle circuits and pilot gets were clean. The lean condition concerns me and since I blew a motor with a similar condition a few weeks back, I'm a bit reluctant to take it for an extended shake down ride. That being said, It seems like the compression reading could be worse. I'll try some fresh oil and seafoam like you said. In the meantime, I'm in the process of tearing down the old motor since it's already out to find out exactly that motor blew. Thanks for the advice.

[kuyarico]

Well, I found out what happened to the left cylinder on the old motor.

[bryanj]

Typical points slipped hole that, polish the ally out of the bore and fit a new piston.

NOTE:-

These motors DO NOT like to be thrashed from cold as the rings tighten up and shatter---even after 30,000 miles. Used to be a common fault on the UK 250cc version

[kuyarico]

I opted for the oil change/seafoam suggestion. I also rechecked the points and was able to get it running smoothly. It's running pretty well at the moment. I just took it for a 40 + ride. I'm going to recheck the compression after I've put a good amount of miles on it.

As for the motor that I took apart, I'm going to rebuild it to have a spare. Does anyone have any opinions on http://bore-tech.com/CB.html