Compression check procedure

[jamesbond]

I’m Wanting to do a compression check on my engine. Boat shops around here act like they’re too good for little work like this or they say they’re out 3 months. I know it’s simple procedure but I’ve never done one on an inboard.

I would assume I would get the boat to the lake and run it for a while, I do not have a fake lake set up yet. Once ready I’d probably pull up to the dock to do the test... pull all spark plugs and test cylinders, one by one. If cylinder is low squirt some oil in that cylinder. Do I need to shut off fuel pump or unplug it?


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[freeheel4life]

Wont have to disable fuel or ignition if you use a jumper button to the starter B+ and S terminal. If you have a Merc you can jump the starter relay a lot easier. This is all assuming key is in off position. You just need to run until its warm. Also, be sure throttle plate is open while doing the compression check. We do compression checks regularly (on every presale inspection). Surprised you are having trouble finding a shop to do it for you. \


https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds

Get you one of these guys ^^^

[UNSTUCK]

Basically the same procedure as a car. Just as you said it. I'd charge my battery to full before doing it. Get at least 3 full revolutions from the engine for each test. Pull the fuse or relay for the fuel pump. Or unplug the injector wires. I guess the real question is what do you consider a low cylinder? Under 100, or maybe 10-20% lower than the average?

What has you wanting to do a compression test?

[jamesbond]

Wont have to disable fuel or ignition if you use a jumper button to the starter B+ and S terminal. If you have a Merc you can jump the starter relay a lot easier. This is all assuming key is in off position. You just need to run until its warm. Also, be sure throttle plate is open while doing the compression check. We do compression checks regularly (on every presale inspection). Surprised you are having trouble finding a shop to do it for you. \


https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds

Get you one of these guys ^^^

Ok wow sounds a lot simpler that way, curious what is reason for throttle being wide open?


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[jamesbond]

Basically the same procedure as a car. Just as you said it. I'd charge my battery to full before doing it. Get at least 3 full revolutions from the engine for each test. Pull the fuse or relay for the fuel pump. Or unplug the injector wires. I guess the real question is what do you consider a low cylinder? Under 100, or maybe 10-20% lower than the average?

What has you wanting to do a compression test?

I just bought the boat and I’m very concerned how it has trouble getting out of the hole. It’s a riders edition 21v with an oj 13x11.5 prop. With just two people no ballast it seems ok but when I had about 10 people on no ballast I could barely get it to plane out. My last boat was a 21v limited with 1235 prop, with around 2000lb ballast and 4-6 people I didn’t have any problems. Guy said engine was replaced with a reman block. So did some digging around and I personally don’t think it’s even been pulled out. It all looks oe to me


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[freeheel4life]

I just bought the boat and I’m very concerned how it has trouble getting out of the hole. It’s a riders edition 21v with an oj 13x11.5 prop. With just two people no ballast it seems ok but when I had about 10 people on no ballast I could barely get it to plane out. My last boat was a 21v limited with 1235 prop, with around 2000lb ballast and 4-6 people I didn’t have any problems. Guy said engine was replaced with a reman block. So did some digging around and I personally don’t think it’s even been pulled out. It all looks oe to me


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If its OEM Merc it will have a tag riveted to the block near the starter heading towards the bell housing right on the cast portion of the block. Dont know of anyone that swaps that tag when they do a block swap, but it is possible I guess.

Throttle open so you arent sucking air thru the IAC. Its hard to compress air when the intake manifold cant get any. Opening the throttle will pull more air into the cylinder and compress it faster (it will take less engine rotations to get the results/data you want)

https://www.dbelectrical.com/product...BoC04kQAvD_BwE

^^ You should find this sitting on top of your engine. Jump the posts with the starter button to crank with key off. Injectors wont pulse without ECM energized, and fuel pump and ignition will be disabled as well WITH KEY OFF. If you key on to IGN position then ECM will be energized.

I feel like most the times those Merc 350s test out at around 155PSI but it all runs together.

[jamesbond]

If its OEM Merc it will have a tag riveted to the block near the starter heading towards the bell housing right on the cast portion of the block. Dont know of anyone that swaps that tag when they do a block swap, but it is possible I guess.

Throttle open so you arent sucking air thru the IAC. Its hard to compress air when the intake manifold cant get any. Opening the throttle will pull more air into the cylinder and compress it faster (it will take less engine rotations to get the results/data you want)

https://www.dbelectrical.com/product...BoC04kQAvD_BwE

^^ You should find this sitting on top of your engine. Jump the posts with the starter button to crank with key off. Injectors wont pulse without ECM energized, and fuel pump and ignition will be disabled as well WITH KEY OFF. If you key on to IGN position then ECM will be energized.

I feel like most the times those Merc 350s test out at around 155PSI but it all runs together.

Free and unstuck I appreciate it greatly.

I do recall that tag on last boat as someone asked me to take a picture of it. This boat doesn’t have it nor the holes to put a tag...?

As far as the solenoid post does it matter how I connect the starter switch? Or just connect the clips to it? One red one yellow with red stripe.

Also for the spark plugs I did research some people said manifolds got in the way but that was for other boats. Looking at the plugs here it looks like it might be the case, will I have to modify spark plug socket?

Here are pics of the starter location. I’m sure this was all me getting worked up over nothing. But just seems this engine is in a worse looking condition than I’d imagine.

As for rough starting when warm I’m thinking iac valve? Also fuel pressure reads 40 but shoots up when I accelerate to max. I assume gauge or sending unit issue?

IMG_9304.jpg
IMG_9303.jpg


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[freeheel4life]

Connect one lead of starter button to big red wire. Other lead to the yellow with red stripe. When you push the button it should crank. Like I put in the link above, hooking to the starter relay at the top of the motor is a lot easier to get to tho.

Usually its Ford engines that a PITA to get to because the plugs are angled into the heads(The Indmar 6.0 is a pain too but thats not your boat ). If this is a Merc then just a socket and extension should do it.

[boatwakes]

Or pull the ignition and fuel pump fuses and turn the key. Theres a black plastic cap with 3 fuses underneath it located next to the relays. There are 6 red, pink and red with blue striped wires going into the bottom of the fuel holder. Ignition is middle and fuel is outside 10 amp. With the fuel pump fuse pulled you won't hear the fuel pump prime when you turn the key to on. Pull the bullet connectors for the white with green and white with red stripe wires coming out of the bottom of the distributor. Now you have no ignition and no fuel so no run. Turn the key and compress to your hearts delight.

An easy way to tell if you've got low compression in one or more cylinders is during the start up sequence. Leave all the spark plugs in and disconnect ignition and fuel pump fuses. Turn the key and you should hear an even and steady rotation as the engine cycles. If there is a high spot or multiple high spots in the sound of the starter as the engine rotates, that means one or more cylinders have low or no compression. If the rotations sounds even for every cycle of the motor, you're not dealing with a compression issue.

That prop sounds undersized as mine was a 13 X 13 4 blade and had no problems at all. Keep us posted.

[freeheel4life]

I got to thinking about it and wondering if his previous 21v had the blue Walter on this one has a black one...

Also I dont know the OJ 13x11.5 but is that a 3 blade or 4 blade??

[Jetdriver]

The Acme is a 4 blade 14.5 x 14.25 with .105 cup. ThaT OJ is a 3 blade I believe, even if it is a 4 blade there really is a world of difference in the sizes.
I’m guessing it’s your prop man. That OJ is nowhere close to the Acme 1235.

[chpthril]

curious what is reason for throttle being wide open?

I did not read all the replies word for word, so forgive if this got answered.

WOT allows the engine take a full gulp of air, creating the greatest and most accurate compression results. If measuring for a spec, this is the most accurate. If looking for consistency, it does not matter.

[jamesbond]

I got to thinking about it and wondering if his previous 21v had the blue Walter on this one has a black one...

Also I dont know the OJ 13x11.5 but is that a 3 blade or 4 blade??

Correct my last 21v had blue Walter current one has black housing. I was just comparing the old one to this one and just seemed better getting out of the hole. Wakemakers prop finder says acme 525 which is 13x11.5 is best for wakeboarding and wakesurfing. Prop on it now is oj 13x11.5. I purchased an acme 911 which is a 13x10.5 prop, which according to wakemakers it’s a heavily weight/high altitude prop.


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