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High Compression

Ribrickulous

Ribrickulous

Veteran Member
Joined
Jun 8, 2020
Total Posts
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Location
Brooklyn, NY, USA
This is more of a technical question than guidance, I'm going to reset the valves this weekend, but out of curiosity - I know a tight valve can lead to low compression readings, but can a loose valve give high compression readings?

I took cold compression on my twin yesterday just out of curiosity, and picked up 165PSI on the right side, 195PSI on the left side (!). Same pistons/rings since the rebuild ~3,000 miles ago, and this is about 7-900 miles after I swapped the head. Both plugs read fine, right side maybe a bit lean, so I didn't think it was carbon buildup on the valves.

The left cylinder may have an occasional miss, as the bike is idling a bit rough, and the bottom pipe is a bit cooler than the top.

I plan on running through the paces - cam chain, valves, timing, carb clean - this weekend, but curiosity led me to post here.
 
Fifteen pounds difference between cylinders indicates a problem and necessitates a tear-down.....Yours are TWICE that!....

ASSUMING your gauge uses a 10 to 14" hose connecting it to the cylinder means those readings are 15 to 20 PSI LOWER than the actual pressures within the engine when the spark-plug is in place...... Since 180 is the norm for this engine, the 195 cold reading is also possibly problematic as it WILL rise in the hot engine, and above 215 indicates it should be de-carboned.....

Some of your "miss" problem is due to the cylinders "fighting" as they TRY to run at different speeds.....
Feel free to phone......
 
Thanks Steve... I'll try to come up with a shorter compression tester and see if that impacts it, as well as checking hot compression. If anything, it seems to idle better warm.

I'll stick my borescope down there too and see if I can come up with some pictures of the piston heads and the intake valves to check for carbon buildup.

Not sure if this means anything but when I first reassembled the engine (3k miles ago) my readings were 165L/170R using the same gauge, with no run-time on the engine cold compression. After swapping this head 7-900miles ago my readings came up near the same after a seafoam treatment. About 165L/160R
 
Maybe somebody skimmed that head and/or the cylinder you put on to clean up the gasket surface, smaller combustion chambers mean more compression. Stock compression pistons or 12 to 1?
 
on a CB350 I had 170 and 195. the service manual said high compression was caused by carbon build up. so, I ran a good bit of engine tuner through the cartb and did a basic decarb treatment. a couple treatments and when I rechecked the compression each side was spot on 170, so - for what it is worth, that is my one and only story about high compression on a honda twin.
 
mike in idaho said:
Maybe somebody skimmed that head and/or the cylinder you put on to clean up the gasket surface, smaller combustion chambers mean more compression. Stock compression pistons or 12 to 1?
Click to expand...

I believe they’re stock compression. The readings I took originally when I swapped this head in were pretty favorable. About 165 on the left if I recall.

I spoke with Steve. Going to try and patch it without taking the head off, basically run a bunch of carb cleaner, sea foam, etc, through it and see if that’ll take away enough of the carbon to make it normal again.

In the very short term, the Italian tuneup seems to be helping somewhat.


Ed
1972 Honda CL350
 
mike in idaho said:
Maybe somebody skimmed that head and/or the cylinder you put on to clean up the gasket surface, smaller combustion chambers mean more compression. Stock compression pistons or 12 to 1?
Click to expand...

Mike, thinking out loud - how would I tell if I’m a little shy on either the block or the head? I don’t recall seeing a dimension in the manual.


Ed
1972 Honda CL350
 
For the Cylinder Jugs you would need a FRESH FACTORY Set and MIGHT/MAYBE you could accurately measure from the bottom of the Cylinder Liner to the top, then use that as a comparison.
But Skimming that surface for minor scratches or even gouges that might make a Gasket not seal would be only between .004 - .015 at the most. Maybe .020 if they were intentionally looking to increase the compression.
That would be a tough measurement to make and might not tell you anything at all because the lower edge of the liners could vary by as much as .075 or more and make Zero difference in the compression / performance.
That lower edge just needs to NOT interfere with the motion of the Crank so who knows if they were consistent.

For the Head you would need to MEASURE "CC" the Combustion Chamber.
By leveling it pouring in a liquid until it fill the combustion chamber than measuring the Volume of that liquid.
 
Yendor said:
For the Head you would need to MEASURE "CC" the Combustion Chamber.
By leveling it pouring in a liquid until it fill the combustion chamber than measuring the Volume of that liquid.
Click to expand...

Right, that much I had figured, and point well taken about the jugs. I can’t imagine that much was taken off skimming the thing on a flat stone with some sandpaper. Even with the pretty minimal displacement.


Ed
1972 Honda CL350
 
165L/160R today with the same gauge when hot [emoji848]

Will take a cold compression reading on Sunday and take a peek around the cylinders and intake valves.

I’m pretty set on seafoaming the thing at this point, because it’s still not idling great. Will post a bit more then.


Ed
1972 Honda CL350
 
Back to about 195L/165R when cold... going to adjust valves and cam chain just for good measure. It’s been ~1,000 miles anyway.


Ed
1972 Honda CL350
 
Ribrickulous said:
Back to about 195L/165R when cold... going to adjust valves and cam chain just for good measure. It’s been ~1,000 miles anyway.


Ed
1972 Honda CL350
Click to expand...

Wait - so the post before you said it was 165L/160R, and now the left side is up to 195 again?
 
I have the same reaction....Almost seems that you are getting some liquid volume (oil or fuel) in the cold engine that is increasing the compression.....
IS the bike on the center-stand for these tests? ... (thinking fuel overflow from leaning left if on sidestand/jiffy-stand, or oil through worn valve guides or seals)
 
Always on center stand.

When I park it, it’s usually on a slight downhill just because of the slope of the streets and needing to be perpendicular to the curb. Almost seems like it would flood into the air box if it were leaking (though I’m religious about the petcock being off).


Ed
1972 Honda CL350
 
Ribrickulous said:
Always on center stand.

When I park it, it’s usually on a slight downhill just because of the slope of the streets and needing to be perpendicular to the curb. Almost seems like it would flood into the air box if it were leaking (though I’m religious about the petcock being off).


Ed
1972 Honda CL350
Click to expand...

But are you sure your petcock doesn't leak past even when shut off?
 
For further diagnosis, It might be better if you separate the carbs from the engine and re-run the compression tests......
 
It’s off right now, and off the bike... haven’t seen a drop come through.... will keep an eye on it over the next hour or so.

Pic of the right intake valve:

dbe49bc56abdbfde48f3f225918d2d38.jpg


Pic of the left intake valve:

4a9f4a9cdf4c5843ecd43b89a8300e8b.jpg


I’m going to do cam chain and valve adjustment and run compression again. If it’s still high I’ll run it for a few seconds and retry if it clears I suppose it’s gas. Certainly looks like something accumulating on the back of the valve...
 
Right valves were spot on, didn’t touch them.

Left valves were a little tight, tweaked slightly. Cold compression (carbs on) still the same.

Popping the carbs off now...


Ed
1972 Honda CL350
 
195L/180R with the carbs off. Pics of left intake valve/port:

4c199a3e2ef4d3b0b2b91457600718e9.jpg

55c882323a2f8feaa43b7ab03bc13d07.jpg


Right intake valve/port:

2f91efaa9c2d1fc043ee1a3082e9b5df.jpg


a7c40d0766f352d13989e4b3d4fd0dd7.jpg


I’m feeling the takeaway here is that the left intake is carboned up more than the right, but that isn’t saying the right is clean (it obviously has issues also).

Right now my jetting is 35/70/105. I may replace the primary with a 68 in lieu of what’s there now. This fouled up a little too quick for that to be right. And it’s been mostly city street riding, not too heavy on the throttle...


Ed
1972 Honda CL350
 
Lots crap build up on the valves, were they cleaned before it went together? Looks like oil has running down the stems for a while.
 
And with my buddy’s shorter TEC compression gauge (14” from tip to schraeder vs my OTC ~38” from top of thread to schraeder) I’m coming up 150L/135R [emoji15]

Now I’m just confused.


Ed
1972 Honda CL350
 
Had a good talk with Steve and Tom.

Came to the conclusion that oil is likely making its way past the valve guides, which is where that buildup is coming from.

Also ran a leak down test afterward. came out to ~3% on each cylinder, with the left side hissing from the crankcase (probably the valve guide) and the right hissing out through the intake, meaning bad valve seating. Hopeful numbers, but given the performance I’ll be taking this one apart again at some point in the future. Basically just going to get from A to B with it until my second engine is set and ready.

Going to rejet a tad lower on the primary to see if I can at least discourage the carbon buildup without running too lean, as well as seafoam the heck out of it, hopefully break some sludge loose and then change the oil. Will see if it makes any visual difference with the borescope.


Ed
1972 Honda CL350
 
What about using a hotter plug?
just around town and no long pulls on the highway?
 
Realized I never came back to this thread.

I didn't wind up running any kind of carbon removal process for it. Ran it for ~40 minutes on the highway and reset the idle fuel mix on the left carb, which was running lean.

Commute in today it was still a little fussy at startup (~28degrees outside) but came to life easier than it has in the past. Idle is a bit high (~1,500 when warm, ~1,600 when realllll warm), but smoother than before, so I'll run through that again this afternoon when I pop it back in the garage.
 
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