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50hp thunderbolt motor.

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bluesalty

Member
I bought a 1975 bomber with a mercury 50 hp 4cyc motor the serial number on the tag reads 4072757 but I can't seem to find any info on it on any of the web sites I go to. I would like to know what year it is and a little about tuning it up . the belt on the magneto is broken i found the number on the belt 20140 but this is about all the informatiom i can seem to get. What is involved in tuning it up, does it have points&condenser ? electronics inside the dist. cap or what. I tried to pull up diagrams on marine parts ,no go nether was there any help on serra ,Anyone that knows please help me out, or should i just junk this thing and stay with johnsons& evenrude which i understand .
 
That would appear to be a 74 model.----The belt is common to all distributor Mercs and every dealer will have it in stock.----------Yes it is electronic ignition , no points or condenser.
 
that motor was one of the best mercury ever built....somewhat of a gas hog by todays standards but will last forever with proper care...if it was mine i would check the compression and if that was good i would do whatever it took to fix it...you will probably find the wiring shot and it will need some attention...there is also a grease fitting under the cowl that allows for timing advance...i dont remember exactly how that works but i do know its not a positively driven advance as on most evinrudes...pull the carbs and kit them and make sure they are linked properly....any deviation from opening and closing together will burn a piston on that motor.....check the lower unit for water and change the oil....if any water find out why and fix it.....the foot on that motor is fairly thin and any freeze will crack it if any water in it... good luck...that would make a good motor to leave on a camp boat.....
 
thank you papyson for your input, The more I listen the more I learn. the boat and motor have been sitting about 3 years, I bought it and the boat on the tennnessee tralier for 200.00 so maybe I didn't get to hurt. I have not started it yet because the belt that truns the magneto is broken . I have ordered one and am waiting on it . I will clean the carbs and change plugs . The wiring seems to be in good shape the only thing I don't know about is it seems to be stuck in gear , the pro will spin free clockwise but won't counterclockwise, I have moved the control lever back and forth and back to netural but it did not free the prop up to trun freely either direction, any ideas ?
 
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There is only one belt made to fit the distributor on Mercury motors -----------1954 4 cylinder to the last motors that used a distributor in the late 70's all used the same belt -3 cylinder / 4 cylinder and 6 cylinder !
 
Hello Salty. As for the stuck in gear problem, disconect the control cable from the shift linkage on the motor and move the motor link to the center (netural pos.) and see if the prop will spin freely. Then place the linkage in fwd. and see if it will spin there by turning the flywheel CW by hand.. Do the same in rev. If all is good, move the shift control to fwd. but not all the way ,just enough to move the cable yet not move the gas cable. This will tel you that the right cables are hooked to the correct linkages. Then adjust the cables for the netural possition and place in the correct linkage. The easy check for shifting problem. The shift cable should move berfore the gas dose.
Have a good new year,
Oldman570
 
bluesalty said:
thank you papyson for your input, The more I listen the more I learn. the boat and motor have been sitting about 3 years, I bought it and the boat on the tennnessee tralier for 200.00 so maybe I didn't get to hurt. I have not started it yet because the belt that truns the magneto is broken . I have ordered one and am waiting on it . I will clean the carbs and change plugs . The wiring seems to be in good shape the only thing I don't know about is it seems to be stuck in gear , the pro will spin free clockwise but won't counterclockwise, I have moved the control lever back and forth and back to netural but it did not free the prop up to trun freely either direction, any ideas ?
Click to expand...
 
Hi I know this a old post but I'm looking for a timing belt for my Mercury 50hp thunderbolt belt number 20140 can't seem to get one here in Europe
Regards Kenneth
 
I have a number of used belts here.--Maybe even a new one.-----But do a complete evaluation of this motor first.-----Post the actual compression values.---Inspect pistons / rings via the transfer ports.----Spin the distributor and see if there is spark.----Pressure test lower unit.----Remove lower unit and inspect water pump.-----There may be a reason for it to sit for a number of years.----Salt water motor ?
 
racerone said:
I have a number of used belts here.--Maybe even a new one.-----But do a complete evaluation of this motor first.-----Post the actual compression values.---Inspect pistons / rings via the transfer ports.----Spin the distributor and see if there is spark.----Pressu
racerone said:
I have a number of used belts here.--Maybe even a new one.-----But do a complete evaluation of this motor first.-----Post the actual compression values.---Inspect pistons / rings via the transfer ports.----Spin the distributor and see if there is spark.----Pressure test lower unit.----Remove lower unit and inspect water pump.-----There may be a reason for it to sit for a number of years.----Salt water motor ?
Click to expand...

re test lower unit.----Remove lower unit and inspect water pump.-----There may be a reason for it to sit for a number of years.----Salt water motor ?
Click to expand...
Hi my Mercury 50hp thunderbolt was running absolutely perfect but while flushing it out the timing belt snapped. It's a toothed belt i have the following information.
Part Number
20140
Or
710-2014
Or
2038711
16" long
10mm wide
7mm teeth
-
Best regards Kenneth Higham
 
??----I know what these belts look like.----Find one on E-bay-----McMaster Carr----Or a website such as ---belts are us.----Sorry.
 
I have more than one !----No shipping to the UK.----Or find one on ---oldmercs.com--- They say that timing belts are ----IN STOCK.
 
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