Compression Test when Buying Bike

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Looking to pickup a new bike between now and spring when the prices get crazy. I've learned a lot since I bought my first one, and I'm going to try and do a more thorough inspection for the next one.

I noticed compression testers are on sale at Princess Auto this month for $12.49 and have been throwing around the idea of getting one, but I'm wondering if thats overkill. The more I've thought about it, it's probably a pain on a lot of bikes to get the plugs out and some people might be worried about their battery while cranking the motor that many times for each cylinder.

So what do you guys think? Would you do a compression test as part of a routine inspection, or only if you suspect there is a problem. Would you let someone who was looking to buy your bike do one?
 
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Make sure if you buy one from princess auto, that it has the right size adapters for the sparkplug threads. Cars are bigger than bikes and I find most princess auto stuff is meant for cars/trucks/farm equipment. I was able to find a compression tester kit at Part Source that had the right size adapter for my application. Without knowing what type of bike its for, its hard to tell what size plugs it has.
 
That's a great point. It says it comes with 14mm and 18mm adapters, which I assumed would fit anything. Now that I look a little more closely, it might be too big for the bikes I'm looking at...

I'm going to be looking for a Japanese 600. Will have to dig a little more.

EDIT: Found this chart and looks like most bikes take a size starting with "C", which is 10mm thread

160763d1261047764-what-spark-plug-alfetta-engine-bp7es-ngk_sym.jpg
 
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I have never done a compression test on a bike I was buying, and I have also never been asked to do one on a bike I was selling...If someone asked me if they could do it on a bike I was selling Id probably say no, especially if they are not a mechanic by trade. There is quite a bit of things that have to come off the bike to do a compression test, and I would not let anyone but myself do that on my own bikes...

What you are asking is pretty rare, unless you are looking at extremely older bikes, I think is overkill
 
Im not too familiar with every model of bikes spark plug size, but 14mm is pretty big (that's 'B' Size). That's standard car size. A 2000 gsxr takes a 'C' size - 10mm where my 1978 CB550 take a 'D' size - 12 mm. You can find out what type of bike it is (im sure you know since your thinking of buying it) then google the plugs it uses. Then you can tell what size adapter you need.
 
I wouldn't let anyone do a compression test on my bike before I sell it. I don't trust them to remove fairings, rads and whatever else is in the way without damaging them
 
Yup I wouldn't let a seller do one on my bike. I'd take it to a shop of my choice and you could pay for the service to have to have it done though.
 
I'd also charge a little extra to cover my time of sending it to the shop and back
 
Nope. Unless you're a licensed mechanic and will be paying me for the extra time to watch you mess with my bike, forget it. I understand the concern when buying, but maintenance records and a simple ride on the bike should tell you enough (cash in hand). Winter prices being what they are, the risk is on the buyer.
 
Fair enough. Wasn't sure if it was a good idea or excessive, but the feedback so far is unanimous.

I also imagine a lot of people would have a problem with test rides at this time of year unless maybe they have access to underground parking. Do you guys have any tricks for checking out the transmission without actually being able to ride it around?
 
Accessing the spark plugs on my ZX10R and putting the bike back together afterward is an 8 hour job. ALL the bodywork has to come off except the tail section. Most modern sport bikes are difficult to work on. Wanna pay shop rate for that??

If it starts easily and runs smoothly and isn't belching blue-tinged exhaust smoke, it's likely OK.

Most sellers of sport bikes that have a shred of common sense are not going to allow any test pilots. Some might be willing to take you for a ride as a passenger and then you can hear that it has all of its gears.
 
Or you can run it on a rear stand but that just shows it shifts through the gears.
 
Brian P said:
Accessing the spark plugs on my ZX10R and putting the bike back together afterward is an 8 hour job. ALL the bodywork has to come off except the tail section. Most modern sport bikes are difficult to work on. Wanna pay shop rate for that??

If it starts easily and runs smoothly and isn't belching blue-tinged exhaust smoke, it's likely OK.

Most sellers of sport bikes that have a shred of common sense are not going to allow any test pilots. Some might be willing to take you for a ride as a passenger and then you can hear that it has all of its gears.
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Just one reason that I prefer my vintage bikes.
 
On the other hand, if somebody asked me if they wanted to do a compression test on my GS500, I'd happily do it *for* them, in front of them. Of course, it would take about 2 minutes (not counting the half hour necessary to get any heat in the motor)
 
TwistedKestrel said:
On the other hand, if somebody asked me if they wanted to do a compression test on my GS500, I'd happily do it *for* them, in front of them. Of course, it would take about 2 minutes (not counting the half hour necessary to get any heat in the motor)
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Big difference between a V-twin and 600cc in-line four...
 
ViperZ said:
Big difference between a V-twin and 600cc in-line four...
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Yup, I am aware, which is why I said it. If somebody had asked to do a compression test on my Ninja 1000, I would have declined
 
Schneller said:
Just one reason that I prefer my vintage bikes.
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Spark plug / compression test access on my cbr125 requires the length of time that it takes to pull off the plug cap and remove the spark plug. It's right there. Nothing else has to be taken off the bike to get access.

Original poster inquired about Japanese 4-cylinder sport bikes ... hence my comment about the ZX10R. All newer sport bikes have a lot of stuff jammed into a very small space, with the usual result that you have to take apart everything in order to do anything.

In fairness ... Modern engines have been designed to reduce maintenance requirements. The ZX10R may be difficult to get access to the spark plugs, but the maintenance schedule only calls for doing it every 24,000 km, at which point you do EVERYthing: spark plugs, valve clearances, air filter, etc. There is no purpose to going through ALL of that to do a compression test unless there is a pressing reason to do so (e.g. other problems that suggest something wrong with the engine). Another point is that it's impossible to do a compression test on this engine with the engine warmed up ... by the time you get in far enough to do the test (3 hours later), it won't be warm any more!

The older bikes may have been easier to fix but my experience has been that you have to fix them a lot more often ...
 
JohnnyP636 said:
I have never done a compression test on a bike I was buying, and I have also never been asked to do one on a bike I was selling...If someone asked me if they could do it on a bike I was selling Id probably say no, especially if they are not a mechanic by trade. There is quite a bit of things that have to come off the bike to do a compression test, and I would not let anyone but myself do that on my own bikes...

What you are asking is pretty rare, unless you are looking at extremely older bikes, I think is overkill
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:occasion5:

I wont even let 99% of potential buyers ride the bike, never mind start taking things apart
 
Paul1000RR said:
:occasion5:

I wont even let 100% of potential buyers ride the bike, never mind start taking things apart
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Fixed that for ya.

A buyer drove an hour to check out a bike i was selling.He poured over the bike for an hour looking at every detail and asking about any and all details before wanting to start it.(it's mid March with a foot of snow) The bike started fine and he ran it for about 5 minutes at not much more than idle.Then to my surprise he asked to look at a plug to see if it was "running rich".Really?
If he had asked to have it looked at by a local tech at his cost i wouldn't have had a problem.People!
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