Compression Too Low?

[rocksandhammers]

While going through the maintenance recommendations of the manual I came across the section describing compression tests and realized I had never done one in the 40000 miles I’ve had the bike. Went for a good ride, came back and checked. Front compression was 165 psi and rear was 170. These are both above the limit but the manual states if all compression is less than 184 to overhaul the engine. So is this right? Is my engine in need of an overhaul? That would be a real bummer since it’s riding great and it’s the only bike I have. Let me know what you think.

[hillsy]

Are you sure you are reading that right? The manual, that is....

Your numbers are pretty healthy.

[rocksandhammers]

Are you sure you are reading that right? The manual, that is....

Not at all. I’m looking at pg 3-1. It lists standard compression as 184-227 psi, with the limit at 156 and a difference of 28.

Below that it says to overhaul the engine in the following cases:

Compression in 1 cylinder less than 156 psi
-I’m good there since the lowest was 165

Difference in compression of more than 28 psi
-again, I’m good with a difference of only 5

All compression pressure are below 184 psi even when they measure more than 156
-this is what I think I fall under since my compressions were 165 and 170, both more than the 156 limit but less than 184. Am I misreading this?

[navigator]

Did you take compression readings with the throttle held wide open?
They seem sufficient.
If the bike runs....ride.
I wouldn't immediately head to an overhaul.
JMO

[hillsy]

Are you sure you are reading that right? The manual, that is....

Not at all. I’m looking at pg 3-1. It lists standard compression as 184-227 psi, with the limit at 156 and a difference of 28.

Below that it says to overhaul the engine in the following cases:

Compression in 1 cylinder less than 156 psi
-I’m good there since the lowest was 165

Difference in compression of more than 28 psi
-again, I’m good with a difference of only 5

All compression pressure are below 184 psi even when they measure more than 156
-this is what I think I fall under since my compressions were 165 and 170, both more than the 156 limit but less than 184. Am I misreading this?

OK - I see that now.

Thing is - is your compression tester reading accurate? How do you know you don't have over 184 in each cylinder?

Does the bike run poorly / lack power / hard to start, etc?

If the bike is running well then there's really not much point tearing it down.

[Herb]

Did you take compression readings with the throttle held wide open?
They seem sufficient.
If the bike runs....ride.
I wouldn't immediately head to an overhaul.
JMO

With the vacuum controlled slides you either have to pull the slides or you will get as much as 50 psi low readings. My 87 11400 had about 130,000 on it when I did my first compression test. I got readings of 150 - 158 on the cylinders. The bike was starting and running great.

Someone ( on the old purple board) told me to either pull the slides or find a way to hold them open, I pulled the slides and got 195 and 198 psi on the next test. Doing just 3 compression strokes I got 185 and 187.

[rocksandhammers]

Did you take compression readings with the throttle held wide open?

Yes, I did. Throttle was wide open while hitting the starter button.

Thing is - is your compression tester reading accurate? How do you know you don't have over 184 in each cylinder?

Does the bike run poorly / lack power / hard to start, etc?

Not sure about the accuracy of the gauge. I'm assuming it's accurate because I haven't had any reason to believe otherwise, but it's not out of the realm of possibility that it's off. This is the brand I'm using:

As far as the bike performance, I haven't noticed any poor running or lack of power. Put almost 2000 miles on it since the beginning of April and it seems to run great. I did notice when pulling the plugs to do the compression test there was the slightest bit of oil on the threads. The head of the plug were clean and not oil fouled, but just a touch on the threads themselves. Maybe the piston rings are getting worn?

With the vacuum controlled slides you either have to pull the slides or you will get as much as 50 psi low readings. My 87 11400 had about 130,000 on it when I did my first compression test. I got readings of 150 - 158 on the cylinders. The bike was starting and running great.

Someone ( on the old purple board) told me to either pull the slides or find a way to hold them open, I pulled the slides and got 195 and 198 psi on the next test. Doing just 3 compression strokes I got 185 and 187.

What slides are you referring to Herb?

[hillsy]

Carb slides.

[rocksandhammers]

Is there a way to open/remove that without removing and disassembling the carb?

[Herb]

Is there a way to open/remove that without removing and disassembling the carb?

Just pull the top cover off the carb and pull the diaphragm and slide out. This would also be a good time to check the diaphragm for pin holes. if they have tiny holes they can be patched with red RTV smeared on very lightly.

[navigator]

Is there a way to open/remove that without removing and disassembling the carb?

Do as herb says, DO NOT mix the slides and/or springs, they are different front and rear.

[hillsy]

Seems like a lot of mucking around to test an engine that is running well.

Just sayin......

[rocksandhammers]

Is there a way to open/remove that without removing and disassembling the carb?

Just pull the top cover off the carb and pull the diaphragm and slide out. This would also be a good time to check the diaphragm for pin holes. if they have tiny holes they can be patched with red RTV smeared on very lightly.

Awesome. I'll have to try in a week or two and report what my compression readings are.

Is there a way to open/remove that without removing and disassembling the carb?

Do as herb says, DO NOT mix the slides and/or springs, they are different front and rear.

Will do. Thanks for the heads up.

Seems like a lot of mucking around to test an engine that is running well.

Just sayin......

And you would probably be right. I don't really know that it's necessary to do, but now that I'm investigating things it's more for my own curiosity than anything. I like learning how to do this stuff. Plus it will make me look like I know what I'm doing if the next time I go to buy a used bike I pull out a compression tester. Haha. In all honesty though, I don't know if it's just my personality, that it's the beginning of the riding season here or if I'm just still too fresh of a rider/wrencher to have confidence in my bike's performance but I find myself to be a bit paranoid. Like I'm always waiting for something to go wrong or psyching myself out thinking something has a problem. Regardless of the reason it can't hurt to know the ins and outs of the performance of the bike and it makes me feel more connected to it if that makes sense.

[navigator]

IIRC the front slider has a locating hole in one corner of the diaphragm that fits over a pin in the carb.
Also the screws on the cover are soft, don't strip them, if they give you trouble, try using vice grips on the outside of the heads.

Hillsy is right though, if it ain't broke, don't fix it. [emoji2]

[navigator]

If you do it and the compression is much higher....at least you'll know next time that 165 without pulling them is ok.

[Herb]

Is there a way to open/remove that without removing and disassembling the carb?

Do as herb says, DO NOT mix the slides and/or springs, they are different front and rear.

Yeah, Make sure you don't mix up the parts AND make sure you get the slides back in exactly the same way as they came out.

My advice is to do one cylinder at a time.

[rocksandhammers]

Finally got around to pulling the carb slides and testing the compression. It did cause readings to go up a little. Front cylinder went up to 170 and rear went all the way up to 180. Still under the 185 threshold listed in the manual but I'm willing to accept a 5 psi margin of error on the compression gauge.

Naturally I had a head scratcher afterwards when I started it back up and it started idling weird. Somewhere along the way one of the spark plugs got smooshed so the gap was negligible. Regapped to spec and it runs great now. Thanks for the info everyone.

[YoDude]

OMG!!! Spark plug gap got smooched so the gap was negligible. The last time I heard that one was when one of the guys had dropped a small screw and couldn't find it. Turned out it was inside the cylinder bouncing around in there killing the spark plug. He went through hell trying to figure it out. Had pull the engine and the head to get it out.

I certainly hope this hasn't happened to you.

Yo-

[rocksandhammers]

Thankfully that was not me. Found the problem pretty quickly since the only things I had done were take the plugs out, put the compression gauge in and pull the carb slides. Since the compression test says the engine should be warmed up I started it and let it idle between tests, so I knew it had to either be the plugs or the rear carb slide since it ran fine after pulling the front. Definitely thought I was going to have to fuck with the rear carb for a while and because the rubber wasn't sealing somewhere but mercifully that was not the case. Glad there was no actual digging around in the engine either (although I did almost have a heart attack when I almost dropped one of the carb body screws into the spark plug hole....)