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Re did the top end.. absolutely zero compression on the Left side

SamN350

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Joined
Oct 3, 2020
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Location
Salt Lake City UT United States of America
So after doing my time cert on the right side i got the whole top end back together and torqued down. I go to start turning the engine over to begin timing it and I've got ZERO compression on the left side. I'm testing with my thumb. The engine is all sealed up. When doing the right side, it's blowing my thumb off. Left side there is nothing.

I just pulled it all apart again. all the rings are intact. and visually the piston meeting the cylinder wall looks fine. Here are pictures of the measurements i took about 30 minutes ago

https://imgur.com/a/KJJQOv6

The top two are snap gauge readings of the cylinder of the right side walls. measured on X and Y. The other pics are self explanatory. I will also say that before I dropped the engine to do this job I was getting like 80 psi on the left side, but could still feel some pressure with the thumb test.

Did i just cross the threshold of it going bad entirely somewhere during this rebuild?
 
Personally a total loss of compression would have me looking at the valves. possibly stuck or bent valve stem. Possibly adjuster holding one open.
 
Okay. So about the valves. I had valve seals show up with the top end gasket kit I put in. I've never had valve seals on this bike before. Could those be throwing it off?

This is is a 1969' cb350 K1
 
Did you set your valve clearance yet?
recheck your non compression side.
With the plug in, can you hear the engine breathing in during the intake stroke?

Edit: no valve seals on the stock setup on this bike
 
The purpose of checking clearance is if you can't achieve minimum clearance then something is preventing the valve from closing
 
The measuring tools you are using are not precise enough for that job. You would need a hole in a piston to completely dump all the compression. One lifter/rocker being improperly adjusted or one valve sticking will only affect the compression on that cylinder. The valve stem seals have zero effect on compression.
 
Ok so maybe this is the question to ask: should I not even bother checking compression until I have set the valve clearances?

Further more what valve would be sticking open going into the compression stroke on that side? Intake?
 
Is you cam timing off?
what work or cleaning was done with the head?
did you feel any resistance on assembly?

edit: if you pull the head you can liquid test the valves for leaks
 
Liquid tested the valves before reassembly. All good there. all the flat mating surfaces were cleaned and flat sanded. There was a time cert put in the right side(which was the reason for the tear down in the first place) everything was reassembled with fresh gaskets.

I went off the shop manual for the cam timing. LT lined up with the hash on the stator which is lined up with the L line on the cam sprocket.
 
I have to question the measurements. A piston that measures the same or larger than the bore is not going to fit.
Standard bore spec is 64.01-64.02mm
Standard piston spec is 63.97-63.99mm
You mention liquid testing the valve sealing, what liquid was used?
 
A little off topic, I just realized you took the pistons off (to measure them?). When you reinstall you used new wrist pin clips, not reused the ones that have been removed. Or are you using Teflon buttons?

As for the compression issue, I might be tempted to put the left cyl at TDC and shove a bunch of rags or towels in the combustion chamber and put some fluid (gas, marvel mystery oil, engine oil or other?) in the ports and see if anything shows on the towels? Or pump air and see if you can hear leaks?
did you confirm your rotor is indexed correctly and hasn't slipped? It's a quick and easy visual check
 
SamN350 said:
Ok so maybe this is the question to ask: should I not even bother checking compression until I have set the valve clearances? Setting valve clearance should be part of the re-assembly process.

Further more what valve would be sticking open going into the compression stroke on that side? Intake?
Click to expand...
Either valve could be the culprit if the adjustment is not allowing the valve to close completely.
 
Depends on what fluid you're using for the test and how fast the leakage drains away the entire amount. Mere seepage can just mean the valve needs to be lapped or the seat needs to be cut, but if it flows out steadily and fairly quickly the valve could be slightly bent from tapping a piston if the cam timing was off a tooth or two (not an indictment of your rebuild, it's pretty easy to be off a tooth or two the first time through on an engine you aren't familiar with). If you have the head off again (and that's really the best way to check it), take and post a video of the fluid test. Also, LDR asked what fluid you're using which is important
 
Update, I just now took the valve in question apart wiped down the mating surfaces on both the valve and head. took out the valve seal put it all back together. filled the intake port with carb cleaner and it no longer weeps!

Should I just take all those valve seals out of head while Im here. or are they really not an issue even though this bike wasn't built with them?
 
Not necessarily, may just have a particle of carbon or dirt that got caught in the seat while it was being disassembled/reassembled.
 
Ok all. So Im going to put this back together, I'm certain I found the issue also Im going to wait to test compression until after I get everything timed and set the valve clearances properly.

I've found the method to check compression with the motor off the frame; hooking up the positive lead to the starter and grounding it to the case. My remaining curiosity is, do I need to have the points installed to do this? Im just getting the starter to turn the engine over a couple times just like I would be doing with a wrench on the rotor? The points are only needed to keep it running?
 
No need for any ignition parts to be involved, no. Just make the engine turn over as if it were in the frame. Just be completely sure your cam timing is right before turning it over.
 
Got it. And to be sure since ive got all this apart. How do I make sure the rotor is timed or aligned properly with the crank shaft so the hash mark on the stator is accurate going to into aligning the cam sprocket?
 
I like to do it before installing the head, and only if it's a rebuild where I didn't remove the rotor

i once had an engine where the rotor had slipped position and thought it was timed fine but turns out was several degrees off and the piston was hitting the valve.

Since you are where you are, just peek trough the plug hole and watch the piston go up and down. If it doesn't travel any further up or down at tdc then you're fine
 
You do have the FSM, right? I'm not immediately familiar with the 350 cam timing, haven't done one in decades but the FSM should show the marks to align. I believe one mistake that can be easily made is having the cam sprocket backwards. IIRC the flat on the molded plastic has to sit horizontal when the crankshaft is on LT, with the flat parallel to the cam case
 
Yeah that im definitely familiar with L facing left with the line on the sprocket parallel to the top of the rocker box. Ill check the FSM to make sure the crank shaft is lined up. Ive NEVER taken it apart. But I just want to be sure since Im this far into the top end
 
Sorry, what I should written above is, if the piston doesn't move any further(up or down) at LT then you're fine

You can also use a wooden stick in the plug hole to feel the movement.
I wouldn't do anything metal that can potentially get stuck and damage the piston

just curious, what is the L facing left?
did you use assembly lube?
did you locktite your cam sprocket bolts?
 
Just so you know, you can use the "BB Code" link for pics from Imgur and paste it directly into the text window here, it will pull a full sized pic from your Imgur storage.
 
Hmmm, I can't see or recall any L left for this part, but the sprocket fits the cam in a specific way and is shown in the FSM. I think it's the diagram showing which shouldered and non shoulder bolt placement
 
With the left piston at TDC, the LT marked aligned with the stator mark, the flat on the cam gear should be parallel with the top of the cam box. Looks right in you picture. IIRC the shoulder bolt fits the lower bolt hole in that particular alignment.
 
Morning All, so I got everything back together, torqued it all down, set the valves, topped it off with oil and jumped it. Got 180 on both sides :) feels damn good. Im again reminded that work ethic on bikes can sometimes reflect the ways we're dealing with things in life. It's just been a bad couple months riddled with rushing through things and overall anxiety. I just need to slow down.
 
Good to hear, especially that no valves were bent in the process of rushing through the job. It should run well... and yes, it always pays to take your time and be completely sure of the results. It's not like you're working in a Honda shop trying to beat flat rate time to make a decent paycheck (y)
 
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