76 Merc 7.5 WOT Problems and Shaft

[Axeman]

I have a 76 Merc 7.5 HP.<br /><br />I have been having problems with it all year and have taken it to a dealer who basically told me it is not worth fixing.<br /><br />There is a little gadget that stops you from reving too high when in neutral. Well with the cowl off and in gear I can accelerate until just about where max throttle would be if it was in neutral. Anything past that and it starts acting weird.<br /><br />At WOT it speeds up and slows down and sounds like crap. Eventually it will stall but it is never predicable, sometimes it works like crap for 5-10 mins sometimes it stalls right away.<br /><br />To get it started I usuall have to thottle way up and put the choke on even when it is hot.<br /><br />I thought this might be a fuel / air issue so I tried setting the choke a bit (pulling out about 1/4 - 3/8) and when I did that I can go to WOT and it works great. smooth and steady etc. <br /><br />Do you think this is the fuel pump? That is is being starved for fuel when the choke is all the way in and the plate is perpendicular to the carb body?<br /><br />When it was at the dealer the guy told me that I have a bent drive shaft cause my prop was wobbling a bit when spun. and that there was water in my lower end.<br /><br />Could I just pull the prop off and run the engine to see if the shaft spins without wobble? cause it could just be a messed prop right?<br /><br />Any help / advice is apreciated.

 

[joblo33]

Re: 76 Merc 7.5 WOT Problems and Shaft

First of all, these engines are notorious for the little neutral stop getting siezed up and staying on in forwards. You just need to look at the bottom of the vertical throttle shaft and you'll see a little stop that fits in a groove in a plate on the bottom of that shaft. Loosen off the bolt holding on the stop and it should come unstuck. Your other problem sounds like a dirty carb. Go to a merc dealer and get a carb kit, then dissasemble this carb and clean every passage with compressed air and/or carb cleaner then replace the parts you get in the kit. A bent shaft is a serious problem. If you can find a marine recycler it's your best bet. I rebuild A LOT of these motors and the local recycler knows me and I get legs for them for $20 each. You probably won't find a deal like that but maybe $50 or $75 for a good used leg.<br />Eric

 

[Axeman]

Re: 76 Merc 7.5 WOT Problems and Shaft

Well First off there is no problem with the shifting on my engine. I was giving a reference to what throttle position it is in when it starts to fail. <br /><br />Second the carb was cleaned and overhauled except the pump which is part of the carb on this model. So it is not a dirty carb.<br /><br />I have given a specific set of circumtances and was asking if my deduction about the fuel pump was accurate / probable given the fact that restricting the air makes it fine. (albeit slower than I remember)<br /><br />As far as the bent shaft goes I was asking if it could just be the prop and not the shaft. The guy at the dealer who looked at it litterally just spun it with his hand and said "you got a bent shaft" so I am asking if he could be mistaken and would it not be advisable to take the prop off and spin the shaft to see if it wobbles. Cause if it doesn't then that would be the prop not the shaft. I have seen this with axles (car) before where it was the backing plate or the studs and not the shaft.

 

[joblo33]

Re: 76 Merc 7.5 WOT Problems and Shaft

If your prop will come off you can take a look. The long 13 spline connection doesn't leave much room for the prop to be bent, but I guess it's possible. A fuel pump diaphram could be the issue. Have you played with the rich lean screw on the carb? Maybe you tuned it too lean which would cause it to run great at low speeds but would result in fuel sarvation at high speed.<br />Eric

 

[Axeman]

Re: 76 Merc 7.5 WOT Problems and Shaft

I tuned it to factory specs and have tried to adjust while on the water and making richer does not help. Same problem.<br /><br />If there is a wobble in the prop it would be the horizontal shaft that is bent right? not the verticale drive shaft?<br /><br />The dealer said it would be a huge job to change the shaft and put in a new seal. it looks like only a coupple of bits to me.<br /><br />Would I not just have to pull out 57 - 66 on this diagram and change the shaft?<br /> http://www.mercruiserparts.com/images/MERMAR/13716/33.png