Low Compression Problems 85 HP Force - Updated with success 8/8/15

[UtahBoating]

I am looking for some help with my power problems. I recently purchased this boat 17' Bayliner Capri with 1989 Force 85HP. The boat starts easily and runs fine. But it is slow and flat on the top end. I posted about prop pitch a week ago thinking that was my issue. The boat currently has a 15 pitch prop on it and my top speed verified by GPS is only 22mph. This is at 5k elevation if that makes any difference. I ordered a 17 pitch prop but I don't want to use it until I am satisfied the engine is good.

Other posts I have read says they get 35-38mph with this size boat and motor. I have verified all 3 cylinders are getting spark. I bought a cheap HF compression test kit and am only getting 92psi on all 3 cylinders, hot or cold, and WOT or neutral. I was convinced maybe the HF tester is junk so I borrowed another cheap one from Oreilly's and am getting slightly above 100psi on all 3 cylinders. I know both of these are cheap chinese made testers but I can't imagine 2 reading almost the same can both be that far off. I also can't imagine if I stick the new prop on I am going to get 10mph more.

Unfortunately I don't have a tachometer on this boat so I can only go off sound. Should I pull the heads and replace the gasket? I was going to pull the carbs and clean/rebuild but if the compression is no good I don't want to waste time on that. The only thing that is throwing me is all 3 cylinders are within 1-2psi of each other. I would think 1 would be significantly different than the others if the rings were bad?

While the boat is off I have put it at WOT throttle and verified the butterfly's are opening in the carbs.

Looking for input please. My choices are to inspect the cylinders and put new rings in (I have read this is not too difficult) or I can sell the boat at a loss and buy a different boat.

Thanks in advance!

 

[gm280]

UtahBoating, I would certainly buy a Tiny Tach and see what your presently turning before reringing anything. Those compression readings really aren't that questionable and it still should rive out well... But until you can verify what your WOT RPMs are, there isn't much more to say...or do! JMHO!

 

[SkiDad]

Hmmmm. You didn't mention your high altitude in the other post. It could just be carbon. Try some carb cleaner like sea foam or Mercury power tune in all 3 carbs while the engine is running until you get it to stall. Then pull the spark plugs and spray some in each plug hole. Let it sit for several hours. Then take it to the lake and run it for a while. You could also try Mercury quickleen in your next few tanks.

 

[UtahBoating]

Thanks for those replies, I didn't even think to mention altitude in my other post. I did go ahead and order the digital tachometer to least give me a baseline for right now. I thought about doing the seafoam thing to try and remove carbon I have not done it yet. This weekend when I get back in town I think I will run the seafoam through it and hook up the tach and take it out for another test run see what happens.

 

[UtahBoating]

OK so here's an update, I adjusted all the throttle linkage according to the manual and the YouTube video on this form. I sea foamed the motor. Installed a digital tach. Boat sounds better and feels better, but we are still only averaging 24 mph according to GPS. This is with the 15 pitch prop at approximately 5000 feet in elevation. My average RPM if the tachometer is accurate is only about 4600-4800 RPM, a couple times I pushed it on glassy water I can get up to 5100. From what I've read this motor should run at 5500 RPM. I did another compression test on it after the seafoam and I am averaging a little over 110 on all of the cylinders. What can I do to get the RPMs up? If anything. Also right I did order three carb rebuild kits that'll be here this week I'm not sure if that will make a difference or not.

 

[SkiDad]

At the 4800 rpm you are turning your should be about 30. Seems like you have a high slip number. Not sure what to tell you. Do you have tilt and trim. If not are you in the 2nd hole ? is your boat bottom clean and strait ? Pics would really help

 

[Frank Acampora]

With 5000 feet of elevation you need to go 2 jet sizes smaller on the high speed jets. Of course you understand that the engine will not be making 85 horsepower at that elevation but understand that currently it is running too rich at wide open throttle also reducing horsepower. Get a manual or have a Mercury dealer tell you what jet sizes to install.

 

[gm280]

Okay finally some RPM numbers to work with. If you verified the RPMs and they are lower then what the manual specifies, you need to drop the pitch to bring up the RPMS. I know that sound counterproductive, but once you get the engine in it's proper RPM range, you may find out it runs faster as well. Why? Because the engine will be putting out the max HP at the proper RPMs. So hence a faster speed. One other thing you really need to check. Is your prop slipping? If your prop is slipping that could be robbing your top speed as well. Scribe a line across the prop hub and the prop itself. The take it out and run it hard. Then see if the line your scribed moved. In other words the prop hub and the prop lines are no longer lined up together. If so, you need to have the prop rehubed, or replaced... JMHO!

 

[UtahBoating]

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The bottom is straight. There is nothing protruding or uneven. The worst part of the bottom is up front where to boat has been beached over the years. I have attached a pic. I have been running the engine in the closest hole to the hull. Is this too close. I did notice today when I was out taking a swim it looks like it angles in too close to the boat? Could it really be something this simple? I didn't have a chance to mess around with the engine position. There were moments I got the boat up to 5300 on a couple of occasions.

 

[Frank Acampora]

UH, YEAH! The hole you have the tilt pin in was made for the tilt/trim unit. As you can see it is much larger than the pin in it. Second, you usually need to set the tilt pin in the hole that places the engine closes to vertical with the anti-ventilation plate parallel to the bottom. Some boats need a little more tilt to get the bow up and out of the water. As it stands no I'll bet the bow is "plowing" with way too much of the bottom in contact with the water.

 

[UtahBoating]

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Ok I will mess with the tilt next time out. Unfortunately won't be until next weekend. Ugh. Somebody brought up maybe the prop has spun. Take a look at the following pic and tell me if the rubber bushing has possibly spun? It is ripped up along the edges as you can see in the pic.

BTW, thanks to this forum and the repair manual I am quickly becoming a marine mechanic!

 

[SkiDad]

It does kinda look spun. But as others said take a sharpie and draw a strait line across the back. Run it see if the line is still matched up.

The trim angle will DEFINITELY slow you down a few mph maybe even more. Put it in the 2nd one (of the group of 4) 1st is closest to the boat. 4th hole is farthest from boat.

 

[UtahBoating]

No Title

Here is an update. Took the boat out this morning for a few hours. Tried the motor angle at 2 holes out then 1 hole out. Feels much better at 2 holes out, that is where I will run it moving forward. Put a new to me used 17 pitch prop on and with the trim adjusted gained 9mph. The boat rides higher in the water and feels much more like it should. I did lose about 5-600 rpm with that prop. Also the water was severe chop today. Lots of wind and large waves. I really don't know how accurate my tach is as it is a chinese made digital but the boat feels so much better. I did lose some hole shot with the prop obviously but the boat still planes rather quickly. I don't think it would pull a slalom skier from a deep water start.

In my other thread I posted how my new Michigan Wheel I ordered didn't fit properly on the splines. They are a great customer service and have shipped me a replacement. I haven't got it yet and didn't want to wait so I lucked out and found this used 17 pitch that fit. I would like to try a 15 pitch again at some point to see the difference.

Thanks for all the help on this forum. I am quickly becoming an novice mechanic on these engines.

Edit to say one more thing. With it trimmed properly this thing is WAY easier to steer almost effortless.