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Advice Plz: '93 Check COnvincor 251

S

seth0687

Member
Heys guys,

Some quick background on the boat, it's a 1993 Checkmate Convincor 251 with a Merc carbed 502, bravo 1 drive, and 5 blade 21p prop.

Some background on me, I'm still just as green behind the ears as I was last year lol, but trying my best to learn my way around the performance boating scene.

So last year when I first purchased the boat towards the end of season I had a lot of issues with porpoising and wasn't able to get much top end performance out of the boat because once I was up on plane the second I started to trim up the drive the boat would start porpoising even worse (at this time the boat had a 24p 4 blade). The prop was then changed out to the five blade 21p and I was able to achieve slightly more performance due to attaining a higher rpm, but the porpoising was still an issue. Fastest speed attained last year was 53mph according to a gps speedometer on a cellphone.

Fast forward to this year. Got the boat out for the first time this past weekend, and while out on the lake the performance was severely lacking. I was only able to see a best of 45mph no matter what I tried. For the record, the boat did have some remaining fuel, with stabil, from last winter, but I also added in 30 gallons of new fuel. I did adjust the idle on the carb down a little as it was running pretty quick. Other than that nothing was changed on the boat.

I do have trim tabs on the boat, and I have tried running with them all the way down and all the way up, and everything in between but they don't really do anything about the porpoising. I am 99% sure I need to fix the gimbal ring on the drive but aside from that I do not know of any other necessary repairs at this time. Also, my altitude is 5400ft above sea level where I run the boat in Colorado.

And just incase it is driver error when I'm trying to run the boat WOT, I get up on plane using the tabs, accelerate to about 20ish, back tabs off, accel up to 35ish, trim engine slightly, then accel all the way out and try and trim up slightly. HOwever, any trim up on the drive causes crazy porposing so I usually have to back the drive all the way down and that is when I hit 43-45mph.

Any help or advice would be much appreciated. Also, if there is anyone on here that owns or has owned this same boat, are there any nuances or specifics to THIS boat or hull that I should know about? Anything, Bueller?

Thanks,

Seth
 
Are you sure the secondaries are opening up on the carb? Check your ignition timing as well to make sure it has not changed. Porpoising is an issue that you will have to resolve with the tabs and drive.
 
Your just another guy with the checkmate bounce, its normal with that hull.
Any more then a touch or two of trim under around 40 and shes gonna start to bounce. Above 40 you should be able to trim up accordingly, more speed the higher you can trim. That hull has also proven to do best with 23 mirage or 24 bravo 1 props.
I find useing my tabs for anything other the leveling the boat is a waste of time. You just need some seat time and figure out where she likes it and how she likes it.
You should be running low 60s but I'm not fimilar with you altitude and what it'll do to your speed.

Just dawned on me, You sure about year and model. 93 is a 253 I'm pretty sure.
 
there is something else going on with your setup other than driver error or the alltitude.....just cant put my fingure on it
 
I'd agree. You got some sort of mechanical issue hurting your performance. As stated above, check that your secondaries are opening up and also check the timing. I have had a dist tightened down but not tight enough. After running it, the dist moved throwing the timing off and killing the performance.

What is your mechanical knowledge?? This will help up to know what we can advise you to do to fix..... or if we should say take to dealer to have them check.....
 
Def something causing the loss of speed from last year, but at that high above sea level, speed is going to be way less than the normal. An increase of 1500ft can cut an easy 5mph. The 2 places I normally boat have a difference of about 800ft and there is close to 3mph difference!
 
Mechanical knowledge is so so. I'm new at working on boats but have done work with cars (cam swaps etc...). I have a buddy coming over soon with a timing light to check timing and we are going to mess with the carb to make sure everything is working with the secondaries. Aside from that I am clueless.

Also, with regard to the checkmate bounce...anymore info on this, does it have anything to do with the x-dim? Just trying to figure out what I can do combat the issue. I don't mind running in the rough but bouncing when the water is like glass lol?

Thanks guys,

Seth
 
seth0687 said:
Mechanical knowledge is so so. I'm new at working on boats but have done work with cars (cam swaps etc...). I have a buddy coming over soon with a timing light to check timing and we are going to mess with the carb to make sure everything is working with the secondaries. Aside from that I am clueless.

Also, with regard to the checkmate bounce...anymore info on this, does it have anything to do with the x-dim? Just trying to figure out what I can do combat the issue. I don't mind running in the rough but bouncing when the water is like glass lol?

Thanks guys,

Seth
Click to expand...

The bounce is caused by the hull riding up on the pad and falling off, its the point where you have to give it a little more throttle to get it out of the water or a little less to put more of the boat back in the water. My 259 does it right around 38-42 and it is annoying.A 4 blade bravo prop rides alot better than any 3 blade on my particular boat. I can adjust the tabs, trim and throttle to minimize the window... I have learned to plane the boat off with the drive down and tabs at 0, run it up to about 45 and start trimming while pulling the throttle back. I cruise at 3600 and 46 mph with a labbed Bravo 26p
 
I would look at the carb considering you said it had a high idle. Im not sure what you have but if the high idle cam is set (when the choke is on ) or stuck after the choke open, it will prevent the secondaries from opening...
 
If its a thunderbolt ignition you have to ground the purple wire to adjust your carb
 
Old Navy said:
Did you mean "timing"?
Click to expand...

Nope carb too the tb 5 will adjust timing to bring idle to preset rpm based on last time it was grounded out been there one too many times screwing around with it a second time cause I forgot even more fun w blower surge I made up an alligator clamp on a solid strand that I keep in the boat toolbox for just that reason plus everything seems to run just a little different on the water
 
Just want to clarify for those that may not understand but I would think its safe to assume grounding this wire is only for setting base timing, idle speed, and idle mixture?

John
 
jazzy said:
Nope carb too the tb 5 will adjust timing to bring idle to preset rpm based on last time it was grounded out been there one too many times screwing around with it a second time cause I forgot even more fun w blower surge I made up an alligator clamp on a solid strand that I keep in the boat toolbox for just that reason plus everything seems to run just a little different on the water
Click to expand...

punctuation would help;)
 
jazzy said:
Nope, carb too. The "Thunder Bolt 5" will adjust timing to bring idle to preset rpm based on last time it was grounded out. Been there one too many times screwing around with it a second time cause I forgot. Even more fun w blower surge!! I made up an alligator clamp on a solid strand that I keep in the boat toolbox for just that reason. Plus, everything seems to run just a little different on the water!
Click to expand...

ross said:
punctuation would help;)
Click to expand...

There you go Ross :thumb:

I am glad I don't run at those elevations! 680-720' for me depending on lake level. :) With as thin as the air and the level, you might not get much more out of her
 
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