1976 Mercury 7.5 will not run.

Insanity889

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May 2, 2015
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8
Hi guys!
so I got this motor from my land lord and it is new looking. Just old.
I changed the impeller.
Tried starting and it hit on the second pull but wouldn't run. Then it got worse. It would fire and run wide open in foward gear. But if you decrease the trottle to about half way it dies. No stumble or miss it just dies like you hit the kill switch.
So I cleaned the carb and changed the fuel pump. Same thing happened again. So I changed the plugs. Still nothing new. Other then it runs higher rpms.
Any ideas what to check next.
I sure don't have the money to take it to be fixed rite now.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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You need to blow out all the passages. You have a chunk of crud in the idle circuit
 

Insanity889

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May 2, 2015
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You need to blow out all the passages. You have a chunk of crud in the idle circuit

I blew carb cleaner and air threw everything. Except the brass plug above the idel mixture screw. I couldn't get it out with the carb off. But I did get it out after I put it back on the boat. ( more leverage) But I cant see in it only blew air threw it with the idel mixture screw out. Think I need to remove the carb and so I can see in there?
The only thing in the carb was a couple of grain of dirt in the float bowl. It was this cleanest carb I've ever seen.
But I agree it sounds like the carb. Man this will make the forth time I've had it off. Lol
 

Insanity889

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May 2, 2015
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Ok I took it back off and cleaned the emulsion tube and idel air screw holes. It's spotless. Still the same result. Man I'm stumped.
 

gm280

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Jun 26, 2011
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Take a compression test and see what the cylinders are doing. An engine that has low compression will run easier towards the upper RPMs and not so much on the low end. So see what you have as compression first... JMHO!
 

Insanity889

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May 2, 2015
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Take a compression test and see what the cylinders are doing. An engine that has low compression will run easier towards the upper RPMs and not so much on the low end. So see what you have as compression first... JMHO!

95 on the top 94 on the bottom.
 

Mi duckdown

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Apr 14, 2007
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2,575
insanity. run a piece of 25 lb of mono fishing line thru all carb orifices in carb. sometimes air does not work. been done that. must see line come thru.
 

Silvertip

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Sep 22, 2003
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28,758
It sounds like the throttle and ignition advance system are not synchronized (called link & sync). Unlike a car, most of the throttle control is done with spark advance on a two stroke engine. The carb is obviously in the mix but it and spark synchronization are critical for slow speed operation. With the engine cover removed, work the throttle control. Note that there is a linkage to the spark advance mechanism and that mechanism is then tied to the carb throttle lever. It may be as simple as one of the linkage rods has popped off. Link & sync is covered in the service manual for your engine. Do not by guess & by gosh this process or you may turn an otherwise good engine into a very good boat anchor.
 

Insanity889

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May 2, 2015
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Il bet your rite Silvertip. That makes the most sence.
Thanks for the help guys!
I don't have a Manuel yet. My funds are limited rite now . So I can't afford to buy a Manuel and not be able to fix it and still have to carry it to the shop. So I guess il just carry it to them.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Google boatinfo manuals. No need to buy a manual
 

Insanity889

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May 2, 2015
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Ok someone sent the Manuel. According to it the dang thing should run.
Cleaned the carb three times. Fully striped it even the main nossel that the Manuel says not to remove. Guess why. The Venturi is a pain to get back in place if it falls out.
You can see threw every needle and jet. And carb cleaner and air passes threw everything with ease.
But it does hit and try to run if you spray gas threw the throat of the carb. So it has to be the carb. hopefully the shop can figure it out next week.
 

Insanity889

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May 2, 2015
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8
Hey guys I talked to the shop today was going to drop it off but they said it would be two weeks before they could look at it. Heck I could rebuild the thing quicker.
So I ran it again when i got it home. And I'm thinking it's got an air leak. Every so often it will scream way high rpms. High enough to be concerning. When it's done it in the past I thought maybe the prop hit air. Because it's running in a barrel. But today it was apparent there was a problem.
It still only starts in gear wide open. But gets worse when it warms up and thats when it decides to run to high rpms also.
Think it could be an air leak?
The previous owner said its only been run about ten times. But it is a 1976 model. You think the seals dry rotted? Is that common?
Can I vacuum test it or maybe blow air into the block and look for soap bubbles like a car tire? Or do you think I'd blow a seal or gasket?
If the Manuel tells how to test for an air leak I can't find it. It's 500 pages. Lol.
 
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