1973 mercury 650 3 cyl thunderbolt

nrosseau

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Mar 30, 2017
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I need some help, I am having a hard time trying to identify the issue with my lower unit. When at 3/4 wot I turn about half to full turn and when I do this three quarters through the turn the motor revs up and the boat slow down I pull back the throttle the first time to 1/4 and then the second time to 1/2 the motor then starts to accelerate again to full power. This only has happened on turns always at the same point in turn my dad was with me both time and thought that the prop came out of the water but the prop is a good 6in below bottom of the boat (top of prop) and it is a extra long motor. It has only happen 2/20 times I took it out. Do you guys have any ideas and could you tell me how to check for that.
I found out last fall that there was water in the gearbox only about a tablespoon and milky oil so I drained and put away. Could the water be the problem??
 

racerone

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How high is the transom of the boat ?-----Not sure if they marketed an extra long 73 model 65 hp.
 

ezmobee

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Water won't cause slipping. It's either ventilating in the turns or perhaps you have a spun prop.
 

nrosseau

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Mar 30, 2017
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23 inches from where the motor bolts on to the bottom of the boat, the motor is 33 inches from underneath power head (which is the closest measurement I could get from motor mounts) to bottom of skeg
 
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racerone

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Boat needs an extra long motor for a 25" transom.-----Is there a 5" spacer anywhere on the motor "------Any pictures showing cavitation plate relative to the bottom ?
 

nrosseau

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I don't see a spacer and no I do not have a picture of the cavitation plate while the motor is down. But the plate is pretty much even with the bottom if I remember correctly. I do have the serial number which is 3664569. I can check and see what the measurements between the plate and motor mount is.
 
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racerone

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I do not believe any 25" model 65 hp motors left the factory in 1973.
 

nrosseau

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Mar 30, 2017
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Okay so it is a long shaft then what can I do to get less ventilation?
 
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jimmbo

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May 24, 2004
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Pictures would be a great asset. Does the boat have a Keel running down the centerline? What hole is the tilt adjustment bar in? What condition is the prop in?
 

nrosseau

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Mar 30, 2017
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Here is think, I cannot put outboard down at the moment because my battery was dead. Thank you guys for helping me figure this out this is my first boat and I have had it less than a year.
 

jimmbo

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May 24, 2004
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Well your prop looks like it doesn't have any cupping which would make more prone to losing its grip or bite on the water, especially if the engine is trimmed out. How are you operating the power trim? While over trimming can be good for top speed, any deviation from straight ahead almost always results in the prop losing its bite. I will assume you are removing the aluminium thing with the bow roller before you go boating. Your boat does have a keel but it ends a foot or two in front of the transom. Keels have been known to create turbulence, a prop with cupping may, and usually helps.

On a slight sidetrack, your boat needs to be further forward on the trailer. The rearmost rollers need to be directly under the transom so as to support the weight of the engine. Looks like a late 70s EZLoader
 

nrosseau

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Yes the aluminum thing is a transom saver I built it takes the bounce out of the engine going down the road I take it out before putting it in water. I trim down in turns just a little though. What prop do you recommend for this old girl. And yes I know the idiot I bought the boat from had it to far back and I need to move it forward. One question should the back of the transom be flush with the beam going across back? Anymore hints or tip would be helpful thanks for your help. Ps do I have enough room to move forward it needs 9in forward. How much room needs to be between truck and boat?
 

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jimmbo

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I'm partial to Mercury Props, but a lot of people use Solas props, and Turning Points' Hustler. You need to know what RPM your engine is running at WOT, and you need to know what pitch your current prop is. I don't have much experience with the 3 cylinder Mercs to be recommending a prop off the top of my head.

As for the trailer. The rear most rollers need to be under the transom, not 1 or 2 inches in front of it, but under it. How much clearance? Well I guess the bow has to be clear of the tailgate when turning. You need to extend the ball out further using a longer tow bar in the hitch reciever
 

nrosseau

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Mar 30, 2017
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Haha that is easier said then done the guy that had it before cut all the wires and put the 60hp in and now I don't know how to rehook up the tac. Thanks for the info on moving the boat forward. I do have the 3 blade plug on the controller but not the plug it self do you happen to know where I could get the plug for the tec??
 

nrosseau

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Mar 30, 2017
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Yeah that looks like it but mine only accepts the 3blade kind that is the 5 pin... I moved to boat forward today it looks a lot better and down not bounce as much thanks again for that info. One last thing last year while running the motor low going about 10mph for about 30min it seemed like it flooded it everytime what can I do to fix it if I can at all. It idles great and runs great at wot at wot is runs 29mph on a 18.5 ft sylvan with 2 people and a 3gallon tank and a 13 pitch prop
Estimated weight being 1200 pounds or so maybe heavier
Thanks and anymore tip don't hurt :)
 

merc850

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Jul 7, 2010
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The 3 blade adapter is hard to find and was originally for early Merc tachs, today you can get an alternator driven tach that hooks up to the rectifier, which tach do you have? Post a pic if you're not sure.
 

jimmbo

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I was looking in the Mercury Accessary book at the Dealer on Saturday, there were several tach leads. I have to go near there today, I'll get some part #s
 

merc850

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Jul 7, 2010
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Dealer might have some used ones; new one $ 60-80 US (part Nrs. might not help). Post some picture on this site of the dash and the wiring on the motor.
 
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