Compression Test Results

Srawl

Srawl

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Ok, I went to harbor freight and purchased the compression and leak down testers. I haven’t had a chance to do the leak down test but did a couple compression tests on each side, each with the same result. (Didn’t count kicks the first time)

These tests were done cold due to me not being able to start the bike (by either kicking or electric). The caps are in bad shape and the right side is barely sparking if at all so I ordered new caps, wires, and plugs. I also just purchased a new battery and charged and installed that.

I’m still debating what to do about coils. I haven’t looked too much into it but my thoughts on options as of now are NGK splicers, new coils, or just go with the pamco and dual tower... feel free to drop your opinion in.

Compression test was done only kicking.

Compression results were as follows,

Right side:

10 kicks- 90psi
20-110
30-115
40-115

Left side:

10- 60
20- 90
30- 110
40- 112
50- 115
60- 115

In all honesty between my lack of expertise kick starting (sure learning quick) and my tired leg, I may be the reason it took longer to stop building pressure on the left side.

I also kept track of kicks because I’ve read anything from “a few” to “a dozen” to “until it stops building pressure.” I did the last one... I don’t know if the number of kicks matters but the end result was the same both cylinders stopped at 115.

What does this tell you about the engine?

Thanks
 
Were you holding the throttle wide open during the tests?
Were you kicking vigorously?

Been so long, seem to recall that "slow to come up" means leaking rings.

Try again with the "wet test", and report back.

CompressionTestWet.jpg
 
Srawl said:
Ok, I went to harbor freight and purchased the compression and leak down testers. I haven’t had a chance to do the leak down test but did a couple compression tests on each side, each with the same result. (Didn’t count kicks the first time)

These tests were done cold due to me not being able to start the bike (by either kicking or electric). The caps are in bad shape and the right side is barely sparking if at all so I ordered new caps, wires, and plugs. I also just purchased a new battery and charged and installed that.

I’m still debating what to do about coils. I haven’t looked too much into it but my thoughts on options as of now are NGK splicers, new coils, or just go with the pamco and dual tower... feel free to drop your opinion in.

Compression test was done only kicking.

Compression results were as follows,

Right side:

10 kicks- 90psi
20-110
30-115
40-115

Left side:

10- 60
20- 90
30- 110
40- 112
50- 115
60- 115

In all honesty between my lack of expertise kick starting (sure learning quick) and my tired leg, I may be the reason it took longer to stop building pressure on the left side.

I also kept track of kicks because I’ve read anything from “a few” to “a dozen” to “until it stops building pressure.” I did the last one... I don’t know if the number of kicks matters but the end result was the same both cylinders stopped at 115.

What does this tell you about the engine?

Thanks
Click to expand...

Hi Srawls,
the GOOD number is 150psi each side. Anything better than 100psi is OK to run with.
To save your aching leg, boost from a car battery and spin the engine with the E-start.
I'd advise removing both sparkplugs and pulling the carbs to let the engine turn easier
and breathe better before you start spinning the engine.
And it's never wrong to pull the carbs and thoroughly clean 'em, as most ignition problems
are really carb problems although the reverse is equally true.
 
The results are the same on both cylinders which is a good thing. The low max reading may be because you did it on a cold engine. If you weren't holding the throttle wide open during the tests, test again while doing so. The results may not change but it shouldn't take nearly as many kicks to reach the max that way.
 
Do the test again with the EStart and throttle wide open. Don’t worry about a cold engine. It will only make a difference of 5PSI or so. Make sure you pull both plugs and put them back in the boots and ground them to the head. I just zip tie them to the head and then check the ground to make sure. Just get a good dry cold test first then if there is a problem you can look at a wet test or hot test. First thing is to make sure your numbers are correct.
 
Throttle was at max and both plugs were pulled and grounded.

I will try it again tonight dry, then I will try to get the leak down test done.

So assuming the dry comes back the same should I do the wet?

Also assuming it comes back the same, what does 115 tell you?

The checklist I was recommended on here says <100 Bad, 100-125 Good, and > 125 excellent. The good number is 150?
 
The real good number is around 150. Some claim to get even a bit higher than that. If this bike was sitting several years, putting some miles on it will most likely bring your numbers up some. The rings will re-seat in the bores and the compression should get better. Sure, you can try testing "wet" but what that will tell you if the numbers come up is that the rings aren't sealing real good. Like I said, if it's been sitting, that's probably true. Even though not ideal, your 115 reading does fall in the range where it will run, so I'd put some miles on it and hope for an improvement. And also as I said, having both cylinders read the same is a good sign.

Also, when testing with the kicker, if the ignition isn't on, and it shouldn't be, there's no need to ground the plugs out.
 
Another thing to consider is the quality of the gauge. It isn't uncommon for cheaper gauges to have the Schrader valve located in the gauge body itself, instead of at the end of the hose, just as it screws into the cylinder head.

What does this do? It then incorrectly adds the hose volume to the combustion chamber, as part of the test. The swept volume of the cylinder remains the same, but now the space it's compressing into is several cc's larger. This can lead to up to a 30psi discrepancy in the final reading. I've personally seen it, when comparing a no-name gauge to my Snap-On.

Never mind the fact that Harbor Freight gauges are suspect to begin with...
 
If you’re getting 115 Just kicking it, that’s not too horrible. The engine should certainly run OK.

The 150 PSI number comes from the fact that atmospheric pressure is about 14.7 PSI (absolute) and the compression ratio of an XS650 engine is just under 10:1 ....sooooo...

~14.7 x about 10 = 150 (for large values of 10).

There are several gross approximations there - but it’s good enough for the girls we go with.

Anyhow - a brand new XS650 engine in mint condition might give 150-155 PSI, but most would be a bit lower and a somewhat “previously enjoyed” engine will give as much as 20-30% lower (like down to 100 PSI or even less) and still run quite well enough to start and ride reliably.
 
Last edited:
delagem said:
The swept volume of the cylinder remains the same, but now the space it's compressing into is several cc's larger. This can lead to up to a 30psi discrepancy in the final reading. I've personally seen it, when comparing a no-name gauge to my Snap-On.

Never mind the fact that Harbor Freight gauges are suspect to begin with...
Click to expand...

I don’t disagree, harbor freight is hit and miss but hoped it was better than nothing...

Does that 30psi discrepancy usually add to or take away from the final number?
 
Srawl said:
I don’t disagree, harbor freight is hit and miss but hoped it was better than nothing...

Does that 30psi discrepancy usually add to or take away from the final number?
Click to expand...

I'm sorry I wasn't being clear. The gauges with the Schrader valve in the body and not at the end of the hose typically read lower. So your 115 could be significantly higher.

You've got decent numbers. I wouldn't worry about it much.

I bought that Harbor Fright Leakdown Tester too. Basically useless. I would get readings varying by 20% back to back, on the same cylinder. The Snap On gauge I bought to replace it with was rediculously expensive, but it never varies in its' readings.
 
The valve on my no-name gauge is on the body and it consistently reads 150lbs on both sides. If it means I really have that plus 30% that's pretty cool. Sometimes it only gets to 140 if the hose blows off the adapter first :D

Your compression ratio is 8.5:1 if anybody wants to figure out the theoretical maximum compression.

P.S. I found mine on the shelf in a Goodwill store for $1. I've left the sticker on it for 20 years, so nobody will think I have poor taste in tools...
 
Last edited:
Doubtful on 30% over 150, unless you've got some serious buildup in your combustion chamber. More likely it's out of calibration in that range.

If you really care, have it calibrated. Easier to do than it looks. Once you disassemble most compression testers (or leakdown or any other, for that matter), you'll find a 1/4" NPT connection. Tee that in with a certified gauge, apply pressure in a range using a regulator, and compare/adjust. (Better gauges have adjustability, most of what we're talking about do not).

I calibrate mine every couple years, mostly out of boredom and access to a free calibrated gauge at work. Certainly a bit much for our needs, however.
 
It could be out of calibration, but I doubt it because it goes up to 150 and won't go any further. And since there's no such thing as coincidence.
 
Ok so I just went and bought a different compression tester and did another compression test, twice on each side, still cold (and dry) due to the whole not starting thing, on both sides the compression was 160 within 10 kicks.

I made sure to get one with a valve in the threads too, not only at the gauge.

So that changes things a bit.

I don’t think I’m even going to open the leak down tester from HF...

Should I still do a leak down test with these results?
 
Last edited:

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