1978 Mercury 4.5 HP Won't stay running (WITH PICTURES)

[Marcaronio]

I recently acquired another motor for $150. It is a 1978 Mercury 4.5 HP. When I received it the fuel pump (which appears to be a replacement) did not have the vacuum pulse line connected from the block to the pump. Also the vacuum pump itself had been shoved down onto the back of the kill switch and was shorting the switch out. I rerouted the lines for the fuel pump to move it away from the kill switch and hooked up the vacuum line to the block.

The motor started after a few pull and was running great. Plenty of water was coming out of the tell tale and the motor idled well when set to the "shift" position and was able to shift into forward and reverse no problem.

Satisfied that it was working well I went inside for the night. The next day the motor will start no problem but will not stay running without revving the motor up when it sounds like it is going to die (which is once about every 5 seconds). I have an old tank and fuel line and my primer ball seems to have failed as it will not "expand" again after it is pumped. I thought this might be my problem so I purchased a new tank and fuel line. The problem still persists. The primer ball on the fuel line is staying hard even after the motor dies and I took the fuel pump apart and it isn't clogged and the seals and membrane appear to be in good condition.

I also took the spark plug out and rotated the flywheel and there is a nice blue spark clearing a quarter inch gap.

Do you think my old fuel line and tank could have sent some junk through the fuel system and clogged up my carburetor? Is that the next logical step to take? If so, can you help me find the correct carburetor rebuild kit for my motor?

Pictures Below.

Thanks,

Marc

 

[River Boy]

Re: 1978 Mercury 4.5 HP Won't stay running (WITH PICTURES)

I've got the same motor. That's not the original fuel pump for sure. The original was a small cylinder affair that was routed right underneath the carburetor. It had three ports, an in and an out on the ends and the vacumn port in the middle. That looks like too large a hose going from the vacumn port on the block. On my motor the vacumn hose is smaller and doesn't need a hose clamp. It shouldn't need a clamp if it is sized properly.
Well, you got good spark, that's all it can be is fuel delivery. I'd rebuild the carb and while I had the carb off spend 2 dollars and put proper sized NEW hoses on it. Carb kit will run you 20 bucks or so. No telling when it was last done if ever. When those little buzzers are in tune they'll start by spinning the flywheel with your hand.

Carb Kits: http://www.iboats.com/Mercury-Carbu...5562958--**********.349221907--view_id.271586

You might check and make sure your needle and seat and float are in good shape. I don't think they come with the kit but can be ordered seperately .
http://www.iboats.com/Mercury-Needl...5562958--**********.134793991--view_id.271587
http://www.iboats.com/Mercury-Carbu...5562958--**********.134793991--view_id.271591

 

[Marcaronio]

Re: 1978 Mercury 4.5 HP Won't stay running (WITH PICTURES)

Here is a video of how it will run fine until you try to idle:

 

[doberacker]

Re: 1978 Mercury 4.5 HP Won't stay running (WITH PICTURES)

I just rebuilt a 9.9 for my buddy, and it was running great, but then like clockwork something got into the carb and that was the END of the "smooth" running. Carb kits are cheap, and easy to do. Will most likely fix or serverly narrow your issue. D.

 

[fshrgy99]

Re: 1978 Mercury 4.5 HP Won't stay running (WITH PICTURES)

how is the fuel mix? Start with screw out 1 full turn (mine was set at 1.25 turns out when I got it and works great there). What about the idle stop screw. Is it backed completely out, what happens if you turn it in?

 

[sean1]

Re: 1978 Mercury 4.5 HP Won't stay running (WITH PICTURES)

I also have the same motor and I was hoping someone can help me out. I just cleaned my carbs and now it wont start I am aware of the idle screw above the carb. I have that 1 1/4 turned out but is there another screw that is able to be adjusted so I can start the motor??

 

[FishhunterPC]

Re: 1978 Mercury 4.5 HP Won't stay running (WITH PICTURES)

Here is a video of how it will run fine until you try to idle:

I have that EXACT problem with my mercury 45 (4.5hp). Did you fix it? If you did, how so? Thanks!