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Help with prop and setup on MX 15 Checkmate

M

MX15CM

New member
Howdy Yall,

This is my first post. I have an MX 15' checkmate with a 115 merc. I have been a boater for many years, but have never had a Merc. or a speed boat before. I need some help with setup, and prop selection. I would like to be able to ski and still have a reasonable top speed(i know you can't have your cake and eat it too). Can anyone recommend a good manual and or online resource for an inline 6 merc? I rebuilt the carbs with new floats needles & seats, and gaskets. I think that I need to sync the carbs. I do not know how to set the timing to 21 degrees. There is a loss of power/speed in the last 1/2 inch of throttle. The boat weighs around 550# dry. The tach seems to come and go, so I'm not sure that it is correct. The prop that came on the boat is a 13*19 aluminum 3 blade. Running at wot it's showing 42-4300 rpm at +/-45 mph indicated. Is there a book or online resource that would help me better understand performance setups? I do have a pressure gauge, but it is old. It works on the upper range, 10-15 psi, but not 0-10. Sits on 10. The setup is the problem in my mind. The cavitation plate is almost flush with the bottom of the hull. The motor clamp is about 1" above the transom using a wrap around spacer. At speed, I get a little chine walk. The bushing on the motor side of the steering arm has some play, so I'll be replacing that first. How tight or loose should the motor be in the mounts? I do not know what the "right" set up is, or how to do it.

Any help would be appreciated.

Thanks!
 
Howdy Yall,

This is my first post. I have an MX 15' checkmate with a 115 merc. I have been a boater for many years, but have never had a Merc. or a speed boat before. I need some help with setup, and prop selection. I would like to be able to ski and still have a reasonable top speed(i know you can't have your cake and eat it too). Can anyone recommend a good manual and or online resource for an inline 6 merc? I rebuilt the carbs with new floats needles & seats, and gaskets. I think that I need to sync the carbs. I do not know how to set the timing to 21 degrees. There is a loss of power/speed in the last 1/2 inch of throttle. The boat weighs around 550# dry. The tach seems to come and go, so I'm not sure that it is correct. The prop that came on the boat is a 13*19 aluminum 3 blade. Running at wot it's showing 42-4300 rpm at +/-45 mph indicated. Is there a book or online resource that would help me better understand performance setups? I do have a pressure gauge, but it is old. It works on the upper range, 10-15 psi, but not 0-10. Sits on 10. The setup is the problem in my mind. The cavitation plate is almost flush with the bottom of the hull. The motor clamp is about 1" above the transom using a wrap around spacer. At speed, I get a little chine walk. The bushing on the motor side of the steering arm has some play, so I'll be replacing that first. How tight or loose should the motor be in the mounts? I do not know what the "right" set up is, or how to do it.

Any help would be appreciated.

Thanks!
 
I would love to help you but I am no expert either! What I can do is share with you what my experience with mu 1973 mx-15. Mine has a '75 johnson 135 on it and the prop is a michigan wheel 13-1/4" x 19p aluminium 3 blade. At WOT I get a max of 5000 rpm and my engine's range is 4500-5500. I have not pulled anyone with it but I have no doubt it could. It gets on plane fast and keeps up with other similar boats with V8's. My engine is heavy, about 300 lbs and someone durring the life of the boat switched the seats from the original back to back set-up to 2 front buckets with a bench in the back and it seems this is to much rear weight for my boat. I can trim all the way down and the nose still bounces up in the air, pretty bad too. With passengers in the back seat things get a little scary. I clocked 50mph with 2 people on calm water on a GPS unit and aside from the nose porpusing I love the boat. to see pics go to http://supergenius74.kicks-ass.net and click on boat page. I do know that you need a good tach and need to know what the WOT range of your motor is to determine what the right prop for it is. In my case I am right in the middle of the range wich means that my prop is correct. I could go up to a 21 pitch and loose my take off speed and really load down the motor but probabaly gain a few MPH or i could go down to a 17 pitch and bring up the RPMs and take off faster. 4300 RPM seems a bit low but I don't know your particular motor, I also have a question for you, does your boat twist to the left at all under a good amount of power? mine does, The left side of my boat takes a beating because of it and I am having problems with the deck and the hull coming apart there.

hobbies this expensive and addictive should be illeagle
 
Shane,

I like you boat, it looks good. You did a good job. Mine has a windshield and is still original. The Merc is suposed to turn 55-5800.
My boat does not "twist" to the left, although, I am not exactly sure what you mean. Have you adjusted the trim tab behind the prop. That can cause a boat to turn. My steeting is extra easy(1 finger), not like most boats that I'm used to. My motor is mounted much like yours. The cavitation plate is flush with the bottom of the hull. Do you run the trim tabs on the transom? I was thinking about doing the rear bench, but I guess not. Do you have the factory fuel tank? Is your delamination problem under the dash? Mine had a problem there, but its fixed now. Do you have any other pictures? I'll try to psot some when I can. Mine sits about the same when its in the water, so they must be similar weight wise.

Thanks
 
Shane,
WOW!!! What an awesome boat! It appears you did one hell of a job! You just gave me the itch to find one of those and restore it! As fas as your set-up. Raise the motor to its highest level. Once you do that see what you rpm's are at wide open throttle with the prop you are running. I would recommend you get a Raker to replace the prop that is on there. Mike Combs at Checkmate can help you with the pitch.I would say at least a 22p, but the factory would no better.

You have yourself an awesome ride there, I am envious!

2003 ZT 240 6.2litre/Bravo 320 H.P. 24p Bravo 1
 
Thanks guys, It's nice for someone to appreciate the work I have done. Bill I have thought about a raker prop, kinda pricey and not sure if I wanna go there yet. I leaning towards a jackplate and a stainless higher pitch prop once its lifted. MX15CM what I am talking about when i say "twist" is the tork from the motor and prop turning clockwise makes my boat under good throttle to twist or roll to the left, most boats with one motor do this but mine does it extra bad and the end affect is the left front side of the hull crashes into the waves when the nose bounces up and down, it really takes a beating in rough water and makes my deck loosent from the hull in that area. I took the back seat out this week and im gonna try it tomarrow at the lake to see my nose comes down more without the weight in the rear. If so I may just leave the seat out. My 135 wieghs about 300 lbs, not sure how much your 115 weighs but i would think it would be a bit less. The gas tank is not original it is plastic and 11gal and looks like one you would buy from an online boat store, rectangular in shape. The original was a 6 gal metal tank, not sure how you would fair with such a small tank, I can use up the 11gal on a long day of running. I also thought about moving the battery up front but that will be a major pain to mount. My johnson is old, 1975, and I believe they measured the HP at the crank not the prop back then so our engines may be about equal in power and the best speed i have achieved is 50, thats maxed out at 5000 rpm and only 2 people in the boat and low on gas and no extra stuff. when loaded down with 3-4 people and coolers i only get up to about 4500 rpm or about 45 mph. Alot of people on scream and fly say i should at least be running 55-60 with this boat and motor but I have yet to see it. Maybe my prop is still a bit too much or maybe my engine is to old to really hit its maximum performance, not sure. There is one thing I really love about this boat, the nostalgia, every time i go to the sandbar someone comes up and says "hey is that an old checkmate?" and the appreciate it for what it is and how good it looks for being 30 yrs old. There is a picture on the picture page of a 1973 sales brochure and it shows our boat with the original seats facing back to back, is that how yours is? I think they did that for a reason, helps keep it from being to heavy in the rear. Anyways its good to talk to someone with the same boat, I'd love to see pics of yours.

hobbies this expensive and addictive should be illeagle
 
I had the bow bounce problem with the trimate. The last thing you guys want to do is move weight to the front. I found that with the motor higher and the right prop, (you can't go wrong with a laser 2 for the merc or a raker for the omc) the bounce went away. I can still make it bounce and jump out of the water, but that is the foot throttle doing that, not the boat.

rooster_3.jpg



1975 Checkmate Tri-mate 2, 2.4 200+ / Sky jacker 6" plate / 25p Laser 2 prop
 
ok, so a jackplate is almost a must, I don't mind buying and installing one but what I am most worried about is if it will fit my old johnson. I don't mind drilling holes in the jackplate if I have to but I am worried about drilling more holes in my new transom. Is there any way to confirm my bolt pattern before I purchase one? also I have heard about the raker props before and how they help stabilize the boat but they do this by lifting the bow? my bow is already pretty hight and keeps trying to go higher when it hits waves and bounces up and down. I can trim up and the bow will ride even higher and hence bounce even harder, maybe im wrong but to me it seems like i need to stop lifting the bow so much, get it lower so it rides smoother? Im still new to all this so maybe I'm complely wrong here. Either way a jackplate will come before the new prop and I wanna make sure I can get a jackplate that fits right.

hobbies this expensive and addictive should be illeagle
 
The jackplates can be drilled to match the holes currently in the transom and motor. Also a boat set up well will never have to trim abve 0 deg. This meaning if you trim all the way down and tuck the motor in, this will be negitive trim. When you start to trim the motor up anything past level (0 deg) would be positive. Most checkmates due to there design are unable to lift the bow. This is why a good bow lifting prop works so well. My boat at speed, the motor has about 1 deg of positive trim. Too much positive trim and mine slows down. Lots of trim will give a killer rooster see below picture.

rooster_3.jpg



1975 Checkmate Tri-mate 2, 2.4 200+ / Sky jacker 6" plate / 25p Laser 2 prop
 
You can find Rakers on EBAY reasonable. I picked two up there for 500 bucks for both. I would not buy one until you are sure of the pitch. You can also call the manufacturer of the jackplate you wish to buy to see if modifications are needed. Like gus said, with a jackplate, you will not need to be trimmed out to help carry the bow, this will give you more speed and more stability.

2003 ZT 240 6.2litre/Bravo 320 H.P. 24p Bravo 1
 
Ok so lets say i get one and make it fit, How high is too high? if you look at the picture of the back of my boat you can see that the cavitation plate is about flush with the bottom of the boat and puts my propshaft about 6" below, I noticed that most of the jackplates listed online can lift up to about 5-6" so that would put my propshaft near the bottom of the boat, is that too high? what about the water pickup? How much lift will it take just to see any improvement? This is all new to me so I'm just full of questions hehe. I also noticed a range of offsets from the back, what affect will this have? looks like i can go with 5" or 10" of offset which should I choose and why? thanks for all your help guys I really appreciate it.

hobbies this expensive and addictive should be illeagle
 
Shane,The boat looks good!As far as the twisting to the left on take off I have a couple Ideas.Is there any play anywhere in the steering?If you grab the motor from the back,can you move it a lot without the wheel turning? Or another possible thing is that your hull has a warp to it.Or, under hard acceleration it twists the hull a little and makes it turn left.My brother restored an older boat and had the same problem.He found that from sitting the hull had a hook in one side of it and it made the boat take off in one direction under hard acceleration.
jpshakehead.gif
Hopefully thats NOT it.
 
Let's hope for the best, I think the listing of the boat is being caused by your motor being too deep. In other words, your prop is biteing too much. You have pretty high horsepower for a small light boat, the prop is biting so much, it is wanting to pull the boat over on its side. AS you get the motor higher, your prop will slip more, which you need in this circumstance, and will not lay the boat over. I would only go with a 5 inch setback. If you go more, you are moving the weight back too far. To be safe with height, set your propshaft 3 inches below the bottom. I recommend you get a water pressure guage to monitor water. They are only around 50 bucks and very easy to install.

2003 ZT 240 6.2litre/Bravo 320 H.P. 24p Bravo 1
 
With a jackplate I would start with one that has about 5" of set back. The further back you go the higher you can raise the motor, something about the wave comming off the back of the boat. As far as how high. That depends, there are no 2 boats the same. When you get a jackplate also install a water pressure gauge, this is a must. You don't want to go to high and loose water pressure and cook the powerhead.
Look at the picture of mine. You can still see the outline of the factory splashwell. It used to dip down. When I installed the new transom I made it streight across. This would help slow the water down that likes to flow over the back of the boat when on the river from passing boats. It also allowed me to mount the engine higher to start with. About 3" higher. This is the same height as my other jackplate in the full up position. You want to measure your prop shaft height. You can do this on the trailer. Trim the motor down untill the lower unit is even with the bottom of the boat. Take a ruler put the ruler on the veryfront edge of the lower unit. This is real close to the propshaft center line. Now measure the distance from the ruler to the bottom of the boat. This is how deep the lower unit is running. I am about 3.5" below the bottom. I still have factory water pickup's and have about 25 psi and top speed. This is why I have done the work I have. I need to go higher.
attachment.php


rooster_3.jpg



1975 Checkmate Tri-mate 2, 2.4 200+ / Sky jacker 6" plate / 25p Laser 2 prop
 
ok, back from an exciting weekend, I took out the back seat and moved the battery from the left side to the center in front of the gas tank, taking the seat out and some other things and driving the boat by myself i did some testing. The boat will not go over 50 mph @ 5000 rpm. but with less weight it gets there very fast, so fast that i had to back off a little on the throttle or it cavitates too bad. With the battery in the center it doesn't twist or lean to the left so bad now and maybe I just had to much weight on the left side of the boat. A jackplate and water pressure guage are my 2 next investments, not so much for speed I just can't take this bouncing up and down stuff any longer. A little extra speed would be nice too, since my buddy in his '81 centurion can beat me on top end. I already feel pretty good when i can keep up most of the boats on the lake with my little 15 footer. I will keep my eyes open for a used jackplate for a while and hopefully by next summer everything will be solved. thanks for all your input guys.

hobbies this expensive and addictive should be illeagle
 
HEY GUYS,

i ALSO HAVE THE SAME BOAT AS YOU GUYS AND I AM LOOKING ON IDEAS ON HOW TO RESTORE IT... WHETHER TO DO SOMETHING MORE CUSTOM OR TO TRY THE FACTORY ORIGINAL RESTORATION ROUTE... I AM ASSUMING YOU GUYS ARE BOTH FROM THE US AND I WAS WONDERING IF ANYONE KNEW OF ANYONE AROUND SOUTHERN ONTARIO THAT COULD DO SOME RESTORATION WORK... ALSO THE LINK TO SUPERGENIUS74'S WEBSITE IS DONE ... IT WOULD BE AWESOME IF YOU COULD SEND ME SOME PICS... OR A LINK TO YOUR PICS.

sean_cadieux@westmountstorefront.ca
 
CADS, have you considered taking it to the factory? Just a thought!
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Forums4.jpg

Webmaster
Checkmate-Boats.com-The Fanatics Home!
 
Sorry I haven't posted in a while, the last month has been very crazy for me. My site is down for a while, somone keeps trying to hack it and run scripts on it. I shut it off for a while till I can get better security. Anyways back to the mx-15, When I bought mine almost 3 years ago it was in sad shape, but i only paid 700 bucks for it. I chose the "plumb crazy purple" because I like that color and the boat was originally a dark purple/gold 2-tone metalflake. Everything else had been modified from original, seats, tank, guages, steering, no windshield. So putting it back to factory was going to be more work than it was worth for me. Besides I like that clean, low, drag-boat look so I kind of tried to go that way with it. The engine always had some bogging at about 1/4 throttle to about 3/4 throttle but I thought it would go away as the engine broke in from the re-build. Nope, still there and I have re-built the carbs twice. A couple weeks ago in my frozen garage, a mechanic friend of mine helped me take her apart again and he discovered that my reeds are bad and that is probably causing my engine to run too lean and no build enough horsepower, so there may be more hope this year! I've decided to leave the factory trim tabs off of it and re-do the seats, including the back and I want to put a different gas tank in it that fits better and allows me to move the batter between the tank and the back seat. I have it all figured out now and I am already working on the bucket seats. A jackplate is still in order and the cheapest I have found is a 2 piece el-cheapo style for 130 bucks. I found a mercury speed calculator over at scream and fly and calculated my boat and motor and at my speed my propshaft is only putting out about 75 hp, not good, its a 135. All in all I should be in the sixties so I am getting really excited about all this now. Just have to work hard now and wait for warmer weather.

hobbies this expensive and addictive should be illeagle

[This message was edited by supergenius74 on January 20, 2004 at 12:12 PM.]
 
Glad you found your missing horsepower, hopefully with that fixed and a jack plate, you should be in the sixties for sure! Could definitely use some warm weather, I got the interior gutted on mine, need to rebuild the seats and patch a few spots, but its under a tarp covered with snow and 10 degrees outside, Oh, to have a heated garage!
 
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