cb350 problems with left cylinder

S

space pup

Active Member
so i finally got my 71 cb350 up and running, safetied and on the road. i did about 100 miles over several trips, when my father noticed while riding behind me that i had smoke coming from the left cylinder.

so after i got home, i did a compression test to see if i had a sticking valve or maybe a worn out ring. i have been getting pressure reading all over the place from 0psi to about 75psi

when i put the plug back in, it doesn't seem to be firing at idle, ut it is burning oil at revs.

can anyone make some suggestions on how to go about trouble shooting this. i has hoping to not have to pull the engine out of the frame (as i am a beginner and kind of daunted by idea).

what are possible causes of the issue i seem to be having?

any help would be appreciated.

thanks
 
Did you have the throttle and choke open when checking compression? Just have to ask. And how did you turn the motor over - kick or electric?

Might be the valves needing adjustment or bad piston rings. Double check the compression til you get a solid reading.
 
i got it running on the right cylinder and checked the compression with the choke open and both reving the engine as well as just idling.

my gut is telling me its piston rings... :( prolly have to break the engine down... sigh and just after i got it running
 
Um - that's not good. You're supposed to get the engine warmed up, shut her down, then check for compression. With both plugs pulled (electric start) or with the ignition "off" (kickstart).

Make sure the choke + throttle are wide open and try again.
 
just out of curiosity, what difference does it make if both plugs are pulled? shouldn't one cylinder's compression be independent of the other?

why can't i check the compression with the engine running on one cylinder? my gauge marks maximum compression, so shouldn't it be the same level of compression when its running or just kicked over.

i try just kicking it over with both plugs out, afterward too and was getting little to no compression registering.
 
Plugs, plug boots, coil wires, doesn't make a difference. The point is, the motor should not be running (or able to catch fire) to do the compression test.

You'll reach max compression within 7-10 kicks or a couple seconds with an electric starter. Feel free to google "how to compression test".
 
ok, i guess ill check the compression again tomorrow and see where that gets me. my gut says its gonna be piston rings tho.

i guess if i have to open the top end there is a whole slew of stuff i will need to get and check/ might as well change

pointers please
 
With the engine running the throttle valve restricts airflow so it wont be accurate. It should still go way over 75psi though.
Do it properly, both plugs out, kick 5 or 6 times quickly with throttle wide open.
If you want to cut corners and do it your way, fine, do it your way and don't bother to ask for any help because you already know whats best

PJ
 
Im super curious how you got the engine to run on one cylinder and do a compression test at the same time? On the parallel twins the "live" cylinder needs to be ticking over pretty quick.

Heres what you do: Run the bike until its warm. Turn off engine. Pull plugs. screw compression tester into plug holes. pull throttle wide open. Kick 5 or six times, until the needle on the gauge gets to its highest point. Unscrew compression tester. Do the same on the opposite cylinder. If you are getting 0 psi as your reading, its definately not the rings or your gauge isnt sealing properly.

If you are really sure its the rings, and you get a low compression reading, dump a tablespoon of oil down that cylinder and check compression again. If its the rings, your compression will now be in spec, or fairly close to it. If it doesnt change, its not the rings.

To diagnose your problem, heres what id do:

1) Adjust valves
2) Check points/timing
3) Take carbs off, clean thoroughly, reset pilot and throttle stop to factory settings
4) Tune carbs
5) If its still not fixed, then start suspecting the engine

Trying to get an engine running properly in any other order is just basically wasting time (unless you can absolutely pinpoint your problem without a shadow of a doubt). You want to rule out the easy things first. Doing steps 1-4 should take you between 1 hour to 1 day at the most. Tearing down the engine will take you from 1 day to months at the most. And you will be really really disappointed if you tear down the engine and it turns out that it was just the valve clearances after all.
 
so i redid the compression test as instructed. seemed to be getting consistent 25 psi reading on the LH cylinder (with occasional spikes up to 75 psi, weird but i have a theory) the RH is reading consistent 125 psi.

poured some oil into the cylinder and the compression read 125 psi on the left. so its rings?! my theory as to why the reading is jumping up sometimes is that the rings have broken into a few parts which are a bit loose and can occasionally carry up enough oil with them to make enough of a seal to get some compression. does that make sense?

anyway... i have a spear engine on the side hereoff a 73 cb350 (picked it up last year for FREE) the odo reads only 3000 miles or so. the outside is a bit dirty, etc but i took the head cover off and everything looks exceptionally clean.

my thought is to check it out as best as i can and if it looks good, to swap it into the frame of my 71. i mean if im gonna take the engine out of the 71 to work on it and change the rings, i might as well try and drop the other one in, no?
 
today, i pulled the engine out of the frame and swapped in the spare i have. got it to start up and wouldn't you know it im having trouble with the firing on the left cylinder!

this time i seem to be having a bit of an electrical issue that i will track down. popped my inductive timing light on the spark cable and seem to be getting only intermittent current flow. ill have to check the points first and work from there, but im out of time to work on that today.

this engine sounds way quieter than the other. i guess that what having 1/3 the millage will do!
 
ok, so i tore down the engine today. took a look at everything. was surprised that the rings on both cylinders were intact. there was a lot of carbon on the left piston and in the combustion chamber and the valves.

i tried to check so see if the rings were within wear spec like my manual tells me to, but like an idiot i broke one of the oil rings... so i guess i am going to need to get a new set after all.

the rings i did manage to check were still within the wear limits. the cylinders are also within spec and have not been bored out.

my question is now, when i search for piston rings for cb350s they always seem to come in overbore sizes (+0.25mm, +0.5mm, etc.)

should i not be getting +0.0mm since the cylinder is still within the original spec, and if so, where can i find them? or do i simply go up to +.25mm in the original bore? do i have to have it bored out, get new pistons or what? im confused
 
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