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Discussion Starter · #1 ·
I recently adjusted the valves on my CL175 and my dad's CL350.

For the 175 I adjusted them to 0.002" for both intake and exhaust.

For the 350 I adjusted them to 0.002" for intake and 0.004" for exhaust.

I adjusted them so that the correct gauge would fit and feel slightly "scratchy," and the next size up would not fit.

Now the thing is that in both cases the compression reading went to down 5-15 psi on both bikes. I would assume that that means that I've made the valves too tight. But what's the point of adjusting them to 0.002" if I can also go ahead and fit 0.003" in there? Should I be measuring more liberally / loosely?
 

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You should stop obsessing over compression readings. There are so many things that will affect compression readings that it is not used as a tuning test/guide. It is for determining an engines condition, or finding a cause of a dead/low power cylinder.
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Also did you take the compression reading after the engine was warm or cold? There would be 5 -15 psi change if the engine was cold.
Not only that but some compression testers can be quite inaccurate depending on the length of the hose and where the check valve is.
 

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Besides the compression difference between hot and cold, the valves should have almost no clearance when hot either way. So testing it while cold should give the same results regardless of how much clearance it has (unless there is no clearance at all).
 

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Discussion Starter · #5 ·
I should probably test it again. I can see that it may just be within the margin of error of the testing device. Both bikes are in progress and waiting for parts, I cannot test them hot at this time. At this time my only option is to test cold.

The CL 175 tested pretty low, about 100-110 psi. This is a cold test. How can I determine if compression is low enough to necessitate a rebuild? If I test a few times (cold) and consistently get 100 psi is that enough? If I were to add oil and get get an extra 10-15 psi that'd put me at 120-125 psi. spec is 142.
 

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Like stated above I would not put too much into compression readings at this point.
I could never get a reading (warm) of over 120 psi (min. spec is 150 psi) for my CB500T and it has been running just fine and with plenty of power for a few years now.
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Discussion Starter · #8 ·
At what point should I begin to worry about it? The CL175 was running on one cylinder. I rode it around and it had pretty low power. I was hoping to get it running on both cylinders, to see if power and idle improved. But after doing the compression test and finding low compression, coupled with the low power when it ran, I figured it needed a rebuild. You can see the details of this particular bike build (including a video of me riding it) here:

http://www.hondatwins.net/forums/45-miscellaneous-discussion/101209-my-attempted-cl175-revival-6.html

I dried out the cold cylinder which had been flooded from the starting attempts. I have reset points gap and timing, and done valve clearances. I haven't tried starting it again because I've got some bad threads on one of the spark plug holes, I am torn between chasing the threads with a back tapping tool ($40) and trying to start it, or just taking it down for rebuild and tapping the threads out when I have the head off.
 

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You were running on ONE cylinder. No matter what the compression reading was, it would still feel under powered.

Figure out why the other cylinder wasn't firing to solve that issue.

There are only 2 reasons why it would not fire since your engine was obviously not seized.

1. No fire. Check ignition (spark plug, coil, point, ignition timing, cam timing, low battery)
2. No fuel. Check the carb for correct float level, clogged jets, etc...
 

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Wait... What?

"bad threads on one of the spark plug holes"

Bad as in how bad? Is it still sealing? If not then you're probably gonna need to heli coil it to fix.
 

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Just because you don't see big numbers, at cranking speed, it doesn't necessarily mean the rings are shot. Idling at 1200 rpm, each cylinder is going through ten combustion cycles per second, not really much time for pressure to leak past the rings. Even less, with the revs up. As long as it starts well, with minimal blow-by and oil consumption the rings still have quite a few miles left in them.
 

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You were running on ONE cylinder. No matter what the compression reading was, it would still feel under powered.

Figure out why the other cylinder wasn't firing to solve that issue.

There are only 2 reasons why it would not fire since your engine was obviously not seized.

1. No fire. Check ignition (spark plug, coil, point, ignition timing, cam timing, low battery)
2. No fuel. Check the carb for correct float level, clogged jets, etc...
Actually there are four:
3. No air -- or too much air. What filters are you running? Leaks?
4. Compression. Is there a significant difference in compression readings between cylinders?
 

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Actually there are four:
3. No air -- or too much air. What filters are you running? Leaks?
4. Compression. Is there a significant difference in compression readings between cylinders?

True although no air can be considered a fuel delivery issue as well. No air, no fuel LOL
 

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Discussion Starter · #14 ·
ok, really appreciating the swift responses today guys. We must all be at work right now.. anyways first to address the spark plug, as I described in the main build thread, I basically messed up the first 10% of the threads. This is on the "good" cylinder. When reinstalling the plugs after doing the compression test, the right plug wouldn't go back in. I can see down in the hole, and the other 90% of the threads are fine. I just need to sort out this little bit so I can thread the plug in. I did some research and the tool i need will allow me to do this (back-tap tool). it's about $40.

For the dead cylinder:

I have spark on both sides. The timing was set but I think I have improved it a little since my last time starting it. I have also set the points gap to spec which I believe improved the strength of the spark while I was setting the timing. I always charge the battery before trying to start.

At first I was not having good fuel delivery, but I've since sorted that out and I am getting fuel in the dead cylinder, evidenced by unspent fuel which leaked out of the exhaust when it was running.

Air, there is sufficient air flow. I am pretty sure there aren't any leaks although its possible. I rebuilt the stock filters using some filter foam and oiled them.

Both cylinders read about 100 psi. The dead cylinder actually read 105, higher than the good cylinder.
 

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Did you try a NEW spark plug in that hole?
 

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Discussion Starter · #16 ·
the first few threads are pretty bad. I can see that they're beyond letting anything pass through. the threads on the plug are clean. the guys here have talked me out of a tear down for now, I will order the back tapping tool in the morning and then I can attempt another start probably this weekend.
 

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Compression gauges are made for automotive engines that are big heavy and expensive to tear down. They are unreliable for small engines. If you are really worried about it, do a leak down test. Or just stop wasting time and tear the engine down. Sounds like there is a reason to remove the head to have the plug hole repaired. You might as well have the valves done and the cylinder bored too. But there is no guarantee this will fix the problem.
 

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Discussion Starter · #18 ·
I had been planning to get right into a top end rebuild, but I wanted to stop in and get a second opinion on whether I should I do it based only on the compression reading. Sounds like the compression reading is not such a tell-all.

Ideally I would like to get it running on both cylinders so I can see how it feels, before deciding on a rebuild. By fix the problem, do you mean the cold cylinder?
 

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I get the impression that you really want to rebuild your engine, and are just looking for the blessing to do so. It is funny, I can sell a total engine rebuild in my shop easier then I can sell a tune up. I have a shed full of engines in box's that were good engines before someone tried to rebuild them. Rebuilding an engine is EXPENSIVE! Are you prepared to put out the money for it?
I myself would just grease up that thread chaser, chase that thread, then blow the cylinder out with compressed air, put in a new plug and fix the running problem. If you really want to rebuild an engine, find an old lawnmower and start with it before tearing into a motorcycle engine. It will be a great learning experience.
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