Down on compression...

OhioBackRoad

OhioBackRoad

Veteran Member
Joined
Jun 20, 2025
Total Posts
190
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Location
Ohio, USA
Here's where I'm at...

When I got the bike, (less than 7000 miles on the clock), it started and idled well but ran like crap. Looking at the carbs they were in dire need of a good cleaning and the manifold and carb-to-manifold o-rings needed to be replaced.

With that done, I did a compression check and it was not the best. As the bike had been sitting for years I took a look in the cylinders and all seemed well so I thought a bit of exercise might free up the rings and bring back some compression.

It took a bit of adjustment to the carbs to get the bike running well enough to put some miles on it, and it currently starts well and runs well up to about 7000 rpm or 55 mph in 5th gear.

I haven't done a leak down test, but as the bike doesn't smoke and isn't using a noticeable amount of oil, I'm thinking perhaps it's an issue with the valves. With that said, I'll most likely do the test before getting into the engine. I'll likely continue to ride the bike until bad weather hits and then pull the engine and go through it.

My question is, is there a right way as opposed to a wrong way to remove the head to do the valves followed by a hone of the cylinders and new ring installation? Any and all pointers and suggestions would be most welcome.
 
OhioBackRoad said:
My question is, is there a right way as opposed to a wrong way to remove the head to do the valves followed by a hone of the cylinders and new ring installation? Any and all pointers and suggestions would be most welcome.
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Well, there's really only a right way... :giggle: but lots of people go off without reading the FSM and do it one of many possible wrong ways. With these engines you can get away with a light valve lapping but if the valves have any significant wear they have to be replaced, you can't grind Honda valves as they have a thin stellite coating for hardness. And by this point in their lives they generally have dished valves, so unless you're not planning to keep it then new valves would be the way to go. As to the cylinders, it's best to wait and get them measured before planning a course of action in case it's near or at the service limit. In your case, a ring or two might be partially stuck from it sitting a long time and more riding might loosen it up. But these engines are really simple so a top end rebuild wouldn't be overly difficult.

BTW, I knew you were talking about your SL175 but it's always best to remember to mention the bike in a new thread.
 
ancientdad said:
BTW, I knew you were talking about your SL175 but it's always best to remember to mention the bike in a new thread.
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Cool, thanks Tom, point taken. :)

I generally work from the FSM but since I haven't worked on a SL175 engine I didn't know if there were any tricks of the trade so to speak that were not mentioned in the FSM.

I'm probably a couple months from starting this adventure and thought I'd start collecting any pointers now.
 
OhioBackRoad said:
I generally work from the FSM but since I haven't worked on a SL175 engine I didn't know if there were any tricks of the trade so to speak that were not mentioned in the FSM.

I'm probably a couple months from starting this adventure and thought I'd start collecting any pointers now.
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If you've done a 175 at all, they're the same. Richard Pitman is one of the most active 175 owners here at VHT, he has 3 of them and has done most everything that would need to be done to them. His experience will be very helpful to you when the time comes.
 
OhioBackRoad said:
With that done, I did a compression check and it was not the best. As the bike had been sitting for years I took a look in the cylinders and all seemed well so I thought a bit of exercise might free up the rings and bring back some compression.

It took a bit of adjustment to the carbs to get the bike running well enough to put some miles on it, and it currently starts well and runs well up to about 7000 rpm or 55 mph in 5th gear.
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How bad was it? Fix everything, fuel, exhaust, valves/timing, ignition, then ride it hard as many miles as you can before snow. Then see.
 
ballbearian said:
How bad was it?
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#1 cylinder 85psi - #2 cylinder 75psi

Adding a bit of oil does not increase numbers. While doing a low pressure leak down test it appears that the problem is with the intake valves. Increasing the pressure may expose other areas of leakage.

ballbearian said:
Fix everything, fuel, exhaust, valves/timing, ignition, then ride it hard as many miles as you can before snow. Then see.
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Sounds like a good plan. I'll probably just freshen up the top end over the winter. In the mean time, I may readjust the carbs every so often to see if I'm gaining any ground.

The fact that it runs as well as it does is a testament to how tough these bikes are. ;)
 
OhioBackRoad said:
#1 cylinder 85psi - #2 cylinder 75psi
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Yeah that's low for sure.
OhioBackRoad said:
Adding a bit of oil does not increase numbers. While doing a low pressure leak down test it appears that the problem is with the intake valves. Increasing the pressure may expose other areas of leakage.
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You could try adjusting them a thousandth or two looser than spec for a few miles to see if they hammer off any carbon buildup from possibly being a bit too tight somewhere along the way, just to see how it responds.
 
ancientdad said:
You could try adjusting them a thousandth or two looser than spec for a few miles to see if they hammer off any carbon buildup
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Sounds like a plan. Not sure how many miles I'll get as I hate the sound of those tappets clacking away when they are out of adjustment. :oops:
 
On small engines on a leak down I normally only use about 50 PSI for the test. I don't think you will get a much different reading using more PSI. I am amazed the bike runs and rides with such low numbers below 100 PSI.
Not to be insulting here, yet I have to ask. You had the throttle wide open when you did the compression test? Those kind of numbers are more common when the throttle is left in a closed position when testing the compression.
As AD noted adjust the valves a little looser. It has been proven by Mike Nixon (respected tech) that on various Honda engines compression will improve slightly by setting the valves a little looser than the Honda spec.
 
Flyin900 said:
Not to be insulting here, yet I have to ask. You had the throttle wide open when you did the compression test?
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No worries, yep throttle wide open. First time I tested, I used the screw in hose on my gauge and was only getting like 60 psi. Realized the long hose was causing false reading and switched to the short straight shaft but had to use my cheap set of gauges. So in reality, the numbers are probably higher, but not where they should be.

Flyin900 said:
I am amazed the bike runs and rides with such low numbers below 100 PSI.
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Agreed, it runs surprisingly well through the gears up to just shy of 7000rpm. Above that it just runs out of breath.

ballbearian said:
Better too loose than too tight. "Slappy valves are happy valves", says my wise but aggravating friend.
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I'll readjust the valves in the morning. Need to make about a 40 mile trip tomorrow and will give it a shot. I'll dig out my best ear plugs... :rolleyes:
 
That PSI is so low I can't imagine that is correct. Does it start easy? What kind of compression gauge? If it was some cheap Amazon one or a harbor freight it's probably not very accurate. Got a picture of the gauge?

If it's starting easy I'd guess the compression is probably better than you think it is. If RPM is limited have you verified the timing?
 
Maraakate said:
That PSI is so low I can't imagine that is correct. Does it start easy?
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Yes, starts with just a kick or two.

Maraakate said:
What kind of compression gauge? If it was some cheap Amazon one or a harbor freight it's probably not very accurate.
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Lol, actually I've got one of each and I would agree they are probably not very accurate.

Richard Pitman said:
You say it runs up to 7K rpm in 5th gear. Can you redline it in the lower gears ?
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Hum, that's a good question. I've been trying to ease it back into service and haven't really tried high rpm in the lower gears. I'll give it a try and get back with an answer...
 
OhioBackRoad said:
Yes, starts with just a kick or two.


Lol, actually I've got one of each and I would agree they are probably not very accurate.


Hum, that's a good question. I've been trying to ease it back into service and haven't really tried high rpm in the lower gears. I'll give it a try and get back with an answer...
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Having both of each as your only gauges is probably not helpful. Those numbers are so ridiculously low that there's almost no way it would start and start easy. I say almost because it's possible I can be proven wrong. Are you able to borrow a quality gauge from a mechanic buddy? If not, I know autozone and others have a tool rental program.
 
After ordering a cheap Amazon/Made in China compression tester I found it was not totally worthless as I was able to use the hose and a spark plug adapter from the kit with my old Made in USA compression tester so I could check the compression on my S90. Ordering a separate adapter to fit my old gauge would have cost nearly the same. Take the win where you can. :giggle:
 
Maraakate said:
Having both of each as your only gauges is probably not helpful. Those numbers are so ridiculously low that there's almost no way it would start and start easy. I say almost because it's possible I can be proven wrong. Are you able to borrow a quality gauge from a mechanic buddy? If not, I know autozone and others have a tool rental program.
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Not trying to be a pain in the neck, contrarian.
About 5 years ago a person brought me a gas powered golf cart that had many issues. It ran but would die once it warmed up. But even then it was inconsistent. Might go 15 minutes, might go 30 minutes before it died. Then it had to cool down.. a lot.. and then it would start right up again.
My compression test was shocking. One cylinder was in the 40's for PSI, the other around 65. o_O I would have thought that to be impossible. So.. there's my one instance where stupid low compression actually ran. I even test my lawn mower engine just to be sure my gauge was working.. lawn mower was around 140 PSI. Gauge seemed to be working.
 
Richard Pitman said:
You say it runs up to 7K rpm in 5th gear. Can you redline it in the lower gears ?
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It will go just a bit higher in 1st & 2nd, but 7k is tops in all the other gears. It doesn't start to miss but just seems to run out of power. It will rev higher when going down a grade. I just assume this is from being down on power from having leaky valves???

When I first put it back on the road, it had a bit of blow by and was blowing a bit of oil out the breather, but that has cleared up and it's not using a drop of oil so I'm thinking the rings have settled back into a proper working condition.

When I pull the head to do the valves what should I have on hand? New gaskets, new valves???, new valve seals??? I'm hoping I can lap the valves and fix the problem, but if not, I'd like to be prepared for something more...
 
Lapping valves does not fix the problem, might even make it worse. If the valves are pitted, receded, or damaged the seats must be cut and new valves installed. This is not something for the home mechanic.

Before you start ripping things apart a leakdown tester will tell you if the valves are leaky: https://www.amazon.com/OTC-5609-Cylinder-Leakage-Tester/dp/B0030EVL60

I have that brand, works great. Parts store probably has one they can loan you.
 
Maraakate said:
Before you start ripping things apart a leakdown tester will tell you if the valves are leaky
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Yes, a leak down test has already shown the valves to be leaking.

Maraakate said:
This is not something for the home mechanic.
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I'm pretty confident in my abilities, this isn't my first rodeo Lol
 
Was out on the bike today running some errands. I've put a little over 400 miles and the bike and have been riding it a bit harder as the mileage goes up. Had it spinning up to 9K in 3rd & 4th and a bit over 7K in 5th. That has it topping out at around 62 mph. I might pull the carbs and check the float height, but I guess if it will pull the rpm in lower gears it probably not a fuel issue???

At any rate, it seems to run better with every ride. The old girl has just been sitting in a collection for way too long.
 
OhioBackRoad said:
At any rate, it seems to run better with every ride. The old girl has just been sitting in a collection for way too long.
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If it ain't broke, don't fix it. I've got a couple like that, a small puff of smoke here or there but they run great and fast.
My 62 150 with 15K miles got the head pulled to de-carbonize the chamber and ports, that's it. It'll do 65-70mph.
 
Hi. I'm new to the group and this thread seems appropriate. CB175 with 90psi in both cylinders. Throttle open. Drop of oil didn't seem to make much difference but when I do a leak down tester I get a considerable hissing out of the oil filler cap hole. I cant hear anything discernable from the valves.

I think I'm looking at pistons or a head gasket failure which on a recently restored bike by the PO is dissapointing.

Engine out I guess......

Any thoughts anyone?

Ian
 
Ian, might be better to start your own thread on this problem. However, regardless of apparent compression readings, how does the bike run ? It ought to get up to an indicated 70mph on a still day on the flat, average sized rider in a slight crouch. ( UK bike with flat bars ? ), redlined in 3rd and 4th before 5th.

( Obviously, correct ignition timing and valve clearances etc. Fact that both cylinders have equal PSI is a good sign. )
 
Richard Pitman said:
Ian, might be better to start your own thread on this problem. However, regardless of apparent compression readings, how does the bike run ? It ought to get up to an indicated 70mph on a still day on the flat, average sized rider in a slight crouch. ( UK bike with flat bars ? ), redlined in 3rd and 4th before 5th.

( Obviously, correct ignition timing and valve clearances etc. Fact that both cylinders have equal PSI is a good sign. )
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Richard, his new thread is here

 
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