What should compressoin PSI numbers be for an 1988 XR4 150 ?

[Sleek Jr]

Sorry if I'm not posting in the right place, but I'm at my wits end trying to find the answer to this, and since it's 20 plus years old, perhaps I need to seek those with "vintage" knowledge.

I have both the factory Mercury manuals for this, the one published in 1988 which Merc says is the correct on, is crap, it has very little info on the XR4. The other one is published in 1989 and is better, but BOTH of them will only say "it should be even".

Now we should all have enough sense to know this is complete BS Merc published. So they are saying, if all six cylinders are 50 PSI plus minus 5 or 10, then I have a good motor?

And that motor is in just as fine a shape as another one which reads 160 psi on all six? What a bunch of crap, and most all mechanics I see posting, and people selling outboards, use the same criteria: "It's even!" LOL...

Now my background is in automotive and motorcycles, so am I missing something here, or is this some cultic outboard community believe that everyone takes on blind faith?

We should all know an aluminum piston with rings in a cylinder should not be magically exempt from needing to produce a specific PSI range to run as the manufactuer intended. So why doesn't Mercury post the numbers?

My opinion: Outboards with low numbers still run decent, and they sound normal, will get you home, so Merc doesn't want the warranty claims and arguments when the motor drops below a certain PSI.

Sort of like GM claiming piston slap and burning a quart of oil every 1000 miles is "normal" for their aberrant engines when other identical engines of the same year make and model do not have that problem.

Ideas?

[2us70]

Do a leakdown test. A fresh motor will usually have low leakdown numbers and gradually increase over time. I think up to about 15% is ok as long as they are even. Just as with compression a high leakdown number on one or more cylinders indicates trouble. I recently helped rebuild a Honda motor and after break-in it was running 2% on all four cylinders. My own 250 Optimax runs about 6% last time I checked.

[Sleek Jr]

Do a leakdown test. A fresh motor will usually have low leakdown numbers and gradually increase over time. I think up to about 15% is ok as long as they are even. Just as with compression a high leakdown number on one or more cylinders indicates trouble. I recently helped rebuild a Honda motor and after break-in it was running 2% on all four cylinders. My own 250 Optimax runs about 6% last time I checked.

Thanks, that's helpful, but does that mean you never do a compression test? I'm just curious if you ever checked your motors or one like mine in the past and remember the numbers. I know and agree that a leakdown is a better test, although a local mechanic who says he's built several Merc V6's disagrees... (he's wrong IMO). I still think that the comp check PSI number is valid test, especially where a leakdown tester is not available.

Can I ask what year and how many hours are on your Opti, and if it's been run hard or not? I'm thinking of getting a used Opti to replace my 33 year old Merc V200. Any advice, things to watch out for, or avoid, etc? I'd like to find a 225, 250, 300 to replace it with.

[willabee]

.....I still think that the comp check PSI number is valid test, especially where a leakdown tester is not available. ... When I was at a race site and troubleshooting a malcontent inline, I had a quick way to see if it was a lost cylinder (compression) problem. I'd pull the plugs and grab the block so that I could put some pressure on my thumb as it covered the spark plug hole. Crank the powerhead and if it blew my thumb off of the hole, that cylinder was good to go. :thumbsup:

[Capt.Insane-o]

Anything pre 90-91 should be at least 120-125. Some inlines were 140-150. When they introduced the relief slots in the exhaust ports cranking compression usually comes in at 105-110, some 91 up 135/150 v6's have 85 psi because they came with 2.5 heads on a 2 liter for whatever reason.

[Sleek Jr]

Anything pre 90-91 should be at least 120-125. Some inlines were 140-150. When they introduced the relief slots in the exhaust ports cranking compression usually comes in at 105-110, some 91 up 135/150 v6's have 85 psi because they came with 2.5 heads on a 2 liter for whatever reason.

THANKS! That's some accurate and specific information, exactly what I was looking for! Also very interesting, that as low as 85 psi on certain models can be considered normal.

It looks like my 89 XR4 is near your borderline date of 90-91, but I think the XR4 spans 1989 to 92 or 93. Which puts them both before and after that date.

Do you know if the later XR4's had relief slots, and the earlier ones didn't? How can one tell?

Also, I had a local Merc shop mechanic tell me on the phone that at 100 psi "the motor is shot" and "he's never seen a Merc (in good running condition) with 100 psi." I found that hard to believe, so looking for others with more experience, there really aren't that many running outboards here.

[2us70]

My Optimax is a 2001 and it has at least 1000 hrs. It is on a 21' Triton Fish&Ski. The motor has not been run too hard since it burns a lot of gas art WOT. I have never done a compression test only leakdown tests. The Capt is right about the old inlines. My 44ci 50 race motors would pull 145 on a fresh rebuild and that was on a recoil starter. I never leak tested one of those 50s, wonder how they would do?

[Sleek Jr]

My Optimax is a 2001 and it has at least 1000 hrs. It is on a 21' Triton Fish&Ski. The motor has not been run too hard since it burns a lot of gas art WOT. I have never done a compression test only leakdown tests. The Capt is right about the old inlines. My 44ci 50 race motors would pull 145 on a fresh rebuild and that was on a recoil starter. I never leak tested one of those 50s, wonder how they would do?

How does your Opti do on it's last leakdown test? Did you put all 1000 hours on it? Any problems?

[2us70]

Haven't done a leakdown lately but the last one was about 6% a few years ago. I suspect that it would not be that low now because I have seen a dropoff in top speed the last couple of years. I have recently been running fuel system cleaner through the motor and we will see if that restores some of the top end otherwise I will have to assume that ring wear is the reason. We have had this motor since new and have had few problems. I have changed the plugs several times as well as a couple of plug wires. I have also changed three water pumps and one poppet valve. I have been using synthetic gear lube since the warranty ran out and the lower unit still has no leaks and holds required pressure.

[Sleek Jr]

Haven't done a leakdown lately but the last one was about 6% a few years ago. I suspect that it would not be that low now because I have seen a dropoff in top speed the last couple of years. I have recently been running fuel system cleaner through the motor and we will see if that restores some of the top end otherwise I will have to assume that ring wear is the reason. We have had this motor since new and have had few problems. I have changed the plugs several times as well as a couple of plug wires. I have also changed three water pumps and one poppet valve. I have been using synthetic gear lube since the warranty ran out and the lower unit still has no leaks and holds required pressure.

That's very interesting. I'm still learning about these since since hardly anyone runs outboards out here (but I prefer them).

I was considering getting a used Opti and found some around 99-2001 that were reasonably priced. But then I started reading all the "opti bashing" comments especially about the early ones (pre-2005) and the law suit comments. But you have had a good experience with yours, can I assume you never had the issues or repairs done that were part of the lawsuit? And what was that all about, anyway?

I still don't know whether to take a chance on an early opti or not, do you have any recommendation about this, or know of any differences between early and later Optis?

BTW, I'm curious, what was your top speed before, and now?

[2us70]

This motor is the only Optimax I have any experience with. My motor is a 2001 225. I am not familiar with the facts of any lawsuit but the only repairs done to this motor have been done by myself with the exception of the first water pump.
The best speed I ever saw was 65mph gps . This was about 10 years ago when I finally found a good prop for this boat. Lately if I get the jackplate just right I can get a little over 63mph gps. I am hoping cleaning the fuel system will restore some top end. Last time out I noticed that the intermittent high speed miss I had been having seemed to be gone.

[Sleek Jr]

My Optimax is a 2001 and it has at least 1000 hrs. It is on a 21' Triton Fish&Ski. The motor has not been run too hard since it burns a lot of gas art WOT. I have never done a compression test only leakdown tests. The Capt is right about the old inlines. My 44ci 50 race motors would pull 145 on a fresh rebuild and that was on a recoil starter. I never leak tested one of those 50s, wonder how they would do?

I don't know, but I saw a late model 250 or 300 Opti (maybe a ProMax or an XS, I can't recall) but he posted 1% leakdown on some cylinders, so I guess they can be that good when they're newer or low hours. (?).

[Sleek Jr]

This motor is the only Optimax I have any experience with. My motor is a 2001 225. I am not familiar with the facts of any lawsuit but the only repairs done to this motor have been done by myself with the exception of the first water pump.
The best speed I ever saw was 65mph gps . This was about 10 years ago when I finally found a good prop for this boat. Lately if I get the jackplate just right I can get a little over 63mph gps. I am hoping cleaning the fuel system will restore some top end. Last time out I noticed that the intermittent high speed miss I had been having seemed to be gone.

I guess your speed sounds about right. I used to get around 58, sometimes 60, out of my 1980 Sleekcraft 21' mini daycruiser (closed bow), with the same year 200 Merc (premix). On one unusual day, a little choppy, into the wind, I saw 62 at 6200, the best it ever did.

From what little I read, Merc decided to settle the lawsuit. I don't know when, I think around 2004? If I recall it was about excessive failures, or short life. Anyone else care to chime in about this and the first few years of Optis?