I might be buying a boat soon, and want to know how to check the compression on the motor?  A member of the bass club Im in is selling a 16 foot Hydro sports Bass Boat with a 110 Evenrude outboard on it, For $1000.  What other tips can you give someone who is purchasing their first bass boat?  I have been informed that the boat needs a little work, but that is fine depending on what it is.  I do reupholstery for a living, so anything regarding the seats or carpet, or any deck/wood work is do-able, and expected.

By the question asked, I would recommend you have some a lot more knowledgeable to check the boat/motor out.

To answer your question, Warm the motor for about five minutes. screw one spark plug at a time out and screw a compression guage in, crank the motor over until it hits six time on the quage.  Do all four plugs that way, making sure it hits the same number of times on each plug.  If you try taking all four plugs out at once, it will hit once on the guage and kick the bendix out.   You didn't say the year model but it should have 90 - 105 (depending on year) and should be withing five percent on all four cylinders. Basically, no cylinder should read more than five pounds lower than the highest reading.  That's why it's very important to make sure each cylinder hits on the quage the same number of times, or you can get wrong readings.   A leak down test is better than a compression test anyway.

Now, with that said, there is a whole lot more to checking out a boat and motor than just checking the compression, a WHOLE LOT more!!

Details, details people. I need details!  Where do I get a compression gauge?How much does one run?  The boat is a 1985 model, not sure about the motor.  What is a leak down test.  Come on, can anyone answer the questions with some detail, and explaination?

Spend a $100 bucks minimum and take the thing to a marine mechanic to get the lowdown on the motor.

but besides the engine

Upholstery, carpet and gelcoat are superfucial things that can be dealt with easy. Wood work on a boat that old is a completely different story and it's the wood you have to worry about on top of whether the motor is a POS or not.  I posted this in another thread on buying an older boat.

Here's the link

http://www.bassresource.com/bass_fishing_forums/YaBB.pl?num=1159235764

I thought my explanation of doing a compression check was fairly detailed.

In no way am I trying to be ugly or non helpful to you but I'm sorry, with your lack of knowledge, you should seriously consider having the boat checked out by a pro or someone very knowledgeable of what to look for in older boats and motors. It's very easy to drop your $1,000 right down the drain. At the same time if may be a great deal, but it takes a lot of boat/motor knowledge to determine that. There is no way I'm going to write you a book on everything you need to look for and go into every little detail on how to do that. It would actually be a book.

Transom needs to be checked thoroughly, floor needs to be checked to ensure it's still bonded and not rotten. The bottom of hull needs to be checked for hook/bow from being trailered wrong. Steering cables need to be checked. Motor needs to checked, not just compression but complete powerhead, hydraulics if it has tilt/trim, lower unit etc. Most seats back then had wood frames, are they rotten? What seems like a $1000 boat can easily turn into a $5000 boat if it needs lots of repairs, or a $1000 boat thats not repairable and is junk. There is just no way to go into all the details for checking this stuff.

Please tell us. What is a leak down test?  I am in the market also! How is it better?

Way2slow, your 1st answer was great, I just don't know what a few of the terms mean that you used.  No big deal anyway.  I talked to the guy and he said the boat is an 85 Hydro bass boat with an 88- 110Evinrude.  The motor runs fine, but could use a tune up.  The wood is soft in a couple spots, and other than that, the boat shows it's age.  I still haven't looked at the boat yet, but for only $1000, it seems like a steal.  I have talked to a couple of other members in our club, and they say they thought about buying it just for the motor.   The guy selling the boat used it in the last tourney, but I missed it, and supposedly it ran good, no problems all day.  

I don't think I'd describe that boat as a steal for $1000. I'd say he's asking about all that boat is worth.  Here's pretty much a breakdown of what the thing would go for if you parted it out on ebay.

1985 16 ft boat, Hydrosport with trailer - $500

1988 110 HP motor - $500

packaged together - $1000 to 1100.

Now, if he was asking $500 and the wood and engine in that thing were in good shape, that'd be a steal.

Approximate costs. On a boat that old that's showing it's age, you're going to spend twice that much or more fixing it back up into decent fishing condition.  I can pretty much promise you, if you spend another $1000 on that boat you'll never get it back.  Old boats, 15 years and older have a habit of breaking alot.  Unless fixing and restoring older boats is a hobby like it is for me,  you may start to get weary of having many fishing trips ruined or inconvienced by stuff breaking at just the wrong time. As for the boat itself, a Hydrosport, Mid 80's models were never a big demand boat.  They were a rather shallow draft hull which sat somewhat low in the water, especially in the stern.  You'll really need to look that transom over carefully.  Also, a "couple" soft spots in the floor can mean a couple spots 6" to a foot across or a couple spots 2ft x 3ft in size.  THAT is not a good thing.

A leakdown test is a method of checking percent of blow-by in the cylinder.  It requires a special guage and a source of compressed air at 100 psi.  The cylinder is brought to top dead center (I mean TDC exactly, if off to either side, when the air hits it, it will blow the piston down to BDC).  The guage you use is basically a small manifold system with a hose that screws into the spark plug hole, with a .040" orfice in the center and calibrated guages on each side (the better ones have two guages, the cheaper only have one guage on the cylinder side) and a small regulator on the compressor side.  You screw it into the plug hole, apply air to it and adjust the regulator so there's 100 psi on the compressor side of the tester.  You then read the pressure on the cylinder side of the manifold and determine the percent of leakdown.  10% is about the max you want to see, with 3 - 5% being what a good motor will show.   This is why you want the dual guage system that the guages are calibrated to read the same.  Without that, you can be off several percent just by differences in guages.

A leak down tester can cost $75 to over $100, just depends on who wants to make the most money and who uses the best guages.  There is a guy (Balzy) on Scream and Fly that make and sells a very good guage for about $80.