Compression Test BEFORE You Buy! Protect Yourself. Save $$

Bike buying season is in full swing.
You might be looking at one to buy, ... right now.

Before you close the deal; get the results of a Compression Test.
Good compression is the sign of a healthy, well maintained machine.

Low compression means the Engine will soon need MAJOR work.

You can pick-up a Compression Tester for under $30.00
Harbor Freight, Sears, K-Mart, WalMart, ... AutoZone.

Throw what you need in a Gym Bag.
Plug Wrench. Spark Plug Tool.
Compression Test Gauge and Hoses/Fittings
Jumper Cables
Duct Tape
New or Old Screwdriver

Hook the Bike up to a Fully Charged Battery
Pull the Plugs
Tape the Throttles WIDE OPEN
Follow the instructions that come with the Gauge and get the Compression on ALL four holes.

Excellent: 145 - 160 psi
Good: 130 - 145 psi
Border line: 125 - 130 psi
Bad News: 120 and below.

(Test used Engines before you buy one, too.)

 

low compression could mean MAJOR WORK, or a valve adjustment.

 

Also, unplug the TCI. It's suck to buy a strong engine, just to have to find a new one because you fried the old one doing the compression test

 

+1, or make sure the plugs are grounded.

 

Just bought an '83 750 Maxim before reading this post but it seems I got lucky. Readings were 150, 145, 140, 150 with a battery that is not quite full strength.

Should I still be checking the valve clearances or does this indicate that they are alright?

 

Re: Compression Test BEFORE You Buy! Protect Yourself. Sav

Rick, I currently have the carbs off a recently purchased bike, can i do the compression test with them off or should I put them back on for this?

 

You should be able to do them with the carbs off... You obviously would not need to hold the throttle open.

 

Massimo:

Makes it easier to do the test.
Unplug the Igniter Box.
Go for it.

 

Orbitor you sure do need to check the valve clearances still.

 

Re: Compression Test BEFORE You Buy! Protect Yourself. Sav

Instead of using the battery to turn the engine, could I use a socket on a drill and turn the engine the same way as I did when checking the valve clearances? Or would that cause damage to the starter?

 

what would the numbers be on a bike that's been sitting in the barn for 15 or 20 years maybe with a few stuck carb slides and a mouse house in the airbox

 

Use WOT & cheese in the snorkel.

 

Geez, that sounds like my 650. Numbers were all within 3 lbs of 150, the main reason I bought it. And there WAS a mouse nest in the airbox.

Massimo33 You could try that, but I don't think the electric drill will spin it fast enough to get good compression readings. You don't need the whole electrical system remember, just the starter, a battery and a couple of suitable jumper wires.

 

they make some kick-ass drills but the nut by the pick-ups would most likely unscrew itself, even if you could find a socket for the square nut

 

I just did a compression test on my bike as I'm worried about really bad gas mileage. I did the test by unplugging the TCI, removed all the spark plugs, cranked the throttle open and ran the test left to right (1-4). I just cranked the starter until the pressure stopped rising on the gauge (is this the correct procedure?).

My results:

150 -- 135 -- 145 -- 152

Obviously #2 is a bit low. I took pictures of the gauge as I read it and also pictures of the spark plugs when I pulled them. #1 was the darkest, and #2 was the whitest. Any chance this tells me something? The "tags" on the pictures indicate which cylinder they belong to.

Here's the link to the gallery with all the pictures

https://picasaweb.google.com/tjquirk/Co ... directlink

Turns out there's a community bike garage in town where I can use a stand and all the tools for $25/hr, so I'm hoping to have some time to pull the valve cover and check and maybe work on the valves this weekend. I'll be reading and re-reading the valve clearance, shimming and synching posts until then. Any extra pointers and tips greatly appreciated.

 

your compression looks ok, the main concern is that all holes have similar values but I believe you're all within spec.

I used the valve cover guide that Mr. Fritz did up:

http://xjbikes.com/Forums/viewtopic/t=14827.html

Very good and one of the first mechanical undertakings I've ever done. Go get yourself a handheld Impact wrench and have some fun

 

No, he's right. His #2 is a tad low, compression-wise; and I'm willing to bet that putting the valves in spec may help.

frantic_vike you used the correct procedure; was the motor warmed up slightly (2-3 minutes per the factory manual) before testing?

 

Still no stories of anyone going to buy a bike with their compression tester and tool box in hand....
where did he tell you to stick your tester?

 

I did the test after commuting home (30 minute ride), so the engine was fully warmed up. (Too warm?)

Since I have no records or indication from the PO that the valves have ever been checked I'm fairly certain they'll be out of spec. Given the decent compression numbers coupled with (as far as I can tell) good performance I'm wondering if there's not something beyond valve clearances that is causing my mileage to be so poor. When I filled up yesterday I had 100km on the trip meter and it took 8.3l to fill up (so 28mpg).

Anyway, hopefully I'll know more after the weekend.

 

Oh, please. I do it every time I look at a bike. Most times the reaction is one of out and out stupidity, they just look at me like I'm from Mars; or worry that I'll somehow "hurt" the bike (like they didn't beat me to it already with years of neglect.)

The knowledgeable sellers (generally) are either fine with it; or somewhat taken aback but consent, regardless.

Of the last four bikes I've showed up to look at with battery and compression gauge in hand, nobody said "no", and I walked away from three of them because of seriously bad numbers; the fourth one was my 650 and I bought it. (GOOD numbers.)