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Low compression cb175 hard time figuring out why!!

9151 Views 7 Replies 5 Participants Last post by  DD23
Hi everyone, looking to get some help on 1971 cb175 with low compression. Made the following repairs: rebuilt carbs and set up to initial settings, replaced carb mounting gaskets, Checked valve clearance .002 on all cold, checked point gap ok. Performed compression check 85 p.s.i. on left and 40 on right. needless to say barely ran! Performed wet test, compression came up to about 110 p.s.i. on left and 80 p.s.i. on right. Pulled engine and disassembled top end. Found rings on right piston frozen to the piston, left piston and rings looked ok. Cylinder walls not bad on either. Cleaned up cylinders, ran a quick hone through. No ridges! Removed rings from both pistons. Had to clean the ring grooves a little bit on the right piston, but rings fit good. The ring set I received from Honda part number 13011-439-005 std. have the three piece oil ring and two different compression rings. One is wider than the other, not sure which position they should be in. Test fit rings on bores of cylinders to check end gap, less than .004mm which by the book is within spec. Installed rings and pistons and reassembled with all new gaskets. Re-installed the timing chain with the dot on the cam at 12 o'clock position with the crank at the T' mark pistons up. TDC compression! Re-installed head cover and torqued to spec following the correct sequence. Turned engine 4 revolutions and rechecked, valve timing ok! Re-checked compression, now have 75 p.s.i. both cylinders. Not Sure what's going on here. Disassembled engine and swapped the two compression rings to the other position. reassembled and tried again......No change in compression. rechecked valve timing and clearance still ok...Did notice some blow-by coming out of the head cover vent. performed a wet test again got compression to come up to about 140 p.s.i.. Tried a leak down test, valves seam to be holding fine when each cylinder is on TDC. I'm at a loss!!!!! Was going to disassemble again and check the piston groove clearance between the rings and the groove. Think I need over size rings but not sure. Anyone have any ideas. Frustrated!!!!!
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If the engine hasn't been run and the rings seated your compression will be wacky.
Go ride it. Accel in first to @6K and decel to idle, repeat 3 times. Do that in each gear. Now the rings are starting to seat. Go ride it 20-50 miles using accel and decel thru the gears, after that go ahead and use full rpm's. Now the rings are 95-98% seated and you can run a compression test.
The piston rings have a spec for side clearance to the piston ring gland. That needs to be correct or you'll break the rings or prematurely wear them out.
Thanks for the response Jim! I know the rings need to seat but I would expect to see higher than 75 p.s.i. initially. Since my last post I went and re-checked leak down and discovered that both valves are leaking off. I did not do any work to these heads. Just cleaned so it appears that my valves aren't seating properly. Re-checked with valve adjustment backed off just to be sure the rockers weren't holding the valves from their seat. Going to pull the head off and see if I can re-lap the valves and probably replace the valve seals or maybe the valves and seals. these things aren't know for head problems are they? Cracking seats, cracking heads etc.? This is my first experience with this model. Don't know all the quarks!!!
I don't really know much about these engines so I moved it to the Engine area of the small engines
Ring order:

Oversize rings are for oversize pistons....Are you planning to bore it out?.....
Did the machinest find the cylinders to be out of round?....
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Thanks for the info on the rings. That is how I have them installed. I disassembled the head and found that the right cylinder (the one that had the lowest compression) had very poor looking valve seats on the head and the valves. Going to replace one of the valves and should be able to re-lap the others. Will replace the valve seals on all. Think that should take care of my problem. Thanks for the help
Was this ever fixed I have similar issue
Rings upsidedown ?

He was not sure which ring went where ?

Davegb, start a thread for your particular bike.....maybe you have as davegb sounds familiar...
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