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.)

 

well considering i don't know you from Adam, you don't ride the bike and you don't lay a wrench on it. You can sit on it, hear it run,sit on it while it's running, squeeze the brakes,kick the tires or check the oil but if you want a compression test, i'll do it while you watch and it's reflected in the price of course. your honestly going to tell me you would let a complete stranger do a compression test on one of your 550's with his gauge that reads 20 lbs low, gimme a break

 

If the bike was for sale, and a prospective buyer wanted to run a compression test, I would certainly let him do it; with me supervising of course. Or better yet, as you suggested, I'd run the test and he could watch. In most cases, the guy has no clue what I'm even talking about, so he's willing to watch and learn.

On a couple of the bikes, just finding and unplugging the TCI proved to be a genuine learning experience for the prospective seller, as did the procedure for actually doing the test.

The guy I bought the 650 from knew his stuff, and we pretty much "did it together" with him holding the throttles open and mashing the button; and both of us watching the gauge, me recording the numbers, etc. It was my battery and gauge; his spark plug wrench.

I thought one dude was gonna cry when his 400 Max blew 106 in one pot and 135+ in the other. (That was one I passed on.)

Nobody's ever said "no" yet, and nobody's gotten super upset at the results.

 

I stopped by the shop yesterday to see if they have the gasket in stock. Turns out they can't even get it from their regular supplier and would have to get it from Yamaha (which takes a while). Since that's the case I'm thinking I'll just order the gasket and a set of donuts from Chacal.

My question now is whether it's worth it for me to pop the cover off this weekend to measure the valve clearances and just put the old gasket back on. I'm guessing since I already have a slight oil leak from (I think) the valve gasket that I should wait until I get the replacement parts and then measure the valves and replace any shims all in one go.

Thoughts?

 

How ya gonna "measure and replace" all in one go? Unless you already know what shims are installed in each position already.

You'll need to measure the clearances, then lift out and read the currently installed shims on any tight valves, in order to know what shims you need.

You could probably get away with popping the cover for step one; and then reinstalling it and replacing all the stuff (gasket, donuts) when you go back in to actually swap shims.

 

Fitz,

I'm not sure I'm understanding you fully, but here's my interpretation. (Also, should I move this discussion to a separate thread? This seems to be getting away from compression talk)

I can pop the valve cover and check my clearances, but I should leave the cover off until I get the replacement gasket and donuts. Once I've measured the clearances I pop out the tight shims, check the numbers, order/find the new shims and replace. Once the new shims are in and the new gasket and donuts are available I put it all back together.

This sounds like I'll have to leave the bike at the garage for a while (not a great option as I'll have to pay for storage and I'm heading back to school in a few weeks.)

So, I'm thinking the better option is to order the gasket and donuts. Once they arrive I open up the cover to measure clearances. Then I pop the tight shims and order replacements. -- I feel like I'm missing something here though. Anyway, once I've got the replacements on order can I bolt the cover back on while I wait for the shims, or should I really only put the new gasket and donuts on and bolt it back together once I've got all the shims in and remeasured?

Sorry if I'm making this more confusing than it needs to be. I've never done work this involved.

 

You can throw the cover back on with the old gasket and donuts until you get all the new stuff unless it crumbles too badly on you when you take it apart. Usually they're crispy/cracked in only a couple of spots (like where it's leaking now.) No need to leave the top of the motor open for an extended period.

Don't be shy about asking questions; ask however many you need to until it's solid in your head; it's OK.

 

When I sold my previous bike I stated in my CL ad: No test rides without endorsement and cash in hand.
As for compression testing, maybe you would be more comfortable with making it the last step to seal the deal. For example: buyer comes over, sees the bike, hears it run, maybe even rides it, etc. If he says "I'm interested but I want to do a compression test." You say "Let's agree on a price first. If the bike passes the compression test at or above XXX pounds, you buy it. If bike fails, you don't buy it. I won't lower the price based on compression test results."

 

Sure. But like I said, nobody's ever given me grief; most of them didn't even know what a compression test IS. Knowledgeable folks never have a problem with it.

 

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

If a seller won't let me perform a compression test I would have to assume there's something he doesn't want me finding out!

No problem, there's lots of other bikes out there!

 

it's like this, price agreed on, money changes hands, keys change hands,they ride it about 13 miles to get the title changed, if they change their minds on the way there, no problem.
they want it, they rode it they still liked it, if that's not enough they should just run along.
if they insist on a compression test i don't think i want to sell to them anyway, they may be the kind of person that comes back next week and says the tires flat

 

"but if you want a compression test, i'll do it while you watch "

Agree 110%....No one takes a tool to MY running bike, or rides it while I still own it...my feeling is that as long as I'm willing to return someones money for my unscathed bike back, I have every perogitive to limit a buyer......but with as many bikes as I buy and sell.....I never had a buyer that took issue nor would I ever cheat anyone. Lifes too short for either.

Since nearly all my deals are vintage two strokes, I can feel & hear both primary and secondary compression as long as it's quiet enough as I slowly kick them through. The only bike besides my turbo that I have that doesn't have a kicker, I bought new. If it's a "barn find, I don't care since I'm tearing it to the crank anyway. I'm the guy who calls and talks with you for 20 minutes first, then shows up to see your bike and goes through his little list of things he checks, alignment, head bearings, shifting etc....if everything checks out, you see money and you help him load it up or he makes arrangements to pick it up sometime in the next month or so.....all for the price we agreed on the phone....if not, a handshake says "it was nice meeting you" and I leave...no hurt feelings.

When selling, I really try to only sell to guys that are going to appreciate the bike....I can usually tell them on the phone. Sometimes we'll get the deal done, then BS for another hour or so. I cannot stand and don't put up with nitpickers though. You know, the guy who think it's cute to find "issues". Most guys who know their stuff don't try it, they understand what they can expect to find. Usually, it's the newer guys to riding or the VERY tight guys<LOL> I've got a great "system" for them. Once I decide that I'm not going to do business with someone, I press the button on my phone while it's in my pocket causing it to ring....then excuse myself to "answer" it. I quickly return and tell the nitpicker that the bike is sold but "thanks for your interest". I know it's not 100% honest and that actually does bother me but I've told people to go pound sand before and havealso tried being 100% honest and just flat out told them that I wouldn't buy or sell to them too and have decided that the fake phone call is nicer then being honest. I suppose that if I really needed the money, I might sell to anyone with cash but thankfully, I don't have to endure them anymore.

jeff