'75 Mercury 1150 Runs, but cuts out at high speeds

[cslax9]

Hello all, this is my first post here so hear it goes... I have a 1975, 115 hp Mercury outboard. Its in remarkably good shape, and the guy I bought it off of a few years ago said he hardly ever used it. The problem I am having is that at the end of the season last year, and now this year, I can start the thing fine,pull away from the dock, head out at a about half speed speed, and then go to gun it, and itll run fast for a few seconds, and then you can noticably hear, and feel it slacking back off, almost as if it isn't getting enough fuel. If I start it up cold and then just try running it up to half speed it runs fine, its just when you try to go over half that it seems to have trouble. I havnt played around too much, but I've looked at the filter at the top of the fuel pump and I cleaned that out, but it still has the same problem. I also looked quick at the carbs, and they all appear to be opening fine. Anybody that has any ideas on what else it might be, weather a more indepth issue with the carbs or what I would REALLY appreciate the help.
Thanks in advance,
Corey

 

[diaric]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

lots of possibles. probably fuel related. check if the tank vent is working properly. if it can't get air back in the tank fast enough, then it can't run wide open very long. could be a bad fuel hose, they can collapse on them selves inside. also can be bad fuel hose connections. look for bad hoses on your motor, could be getting an air leak from a bad hose. try pumping the primer bulb when it tries to die down and see if it picks up. could also be a weak fuel pump. if your carbs have fuel filters they could be plugged up. since it gets up to speed, it means it will do it, so i doubt its too much of a carb problem, just a fuel delivery problem

 

[cslax9]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

Thanks for the quick response. The carbs don't have a filter in them, but I will check the hoses ASAP when I get home from work.

 

[OldMercsRule]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

Thanks for the quick response. The carbs don't have a filter in them, but I will check the hoses ASAP when I get home from work.

Had the same problem in my 1979 115 last season. It was the fuel pump. I think the filter is at the pump as that vintage is a "black block", (the "silver blocks" were up to 1972), which had the individual filters at each carb. Change both the single filter, (at the pump) and fix the pump at the same time, (much easier job then on the silver blocks). JR

 

[cslax9]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

Thanks OldMercsRule, I figured it probably had something to do with the fuel pump, but I havn't had a chance to look at it yet. The carbs definetly dont have filters in them, as I took them apart to check. Thanks again!

 

[Chris1956]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

The "Back Drag" carbs have a fuel screen under the brass elbow in the bowl cover. Unless you remove the elbow, you will never see it. The non back drag carbs have a round screen under where the fuel goes into the carb bowl cover. A bit easier to find.

 

[OldMercsRule]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

The "Back Drag" carbs have a fuel screen under the brass elbow in the bowl cover. Unless you remove the elbow, you will never see it. The non back drag carbs have a round screen under where the fuel goes into the carb bowl cover. A bit easier to find.

Hey Chris, I know there was a filter at the fuel pump on my 1979 115, (as I replaced it), and the filter housing I mentioned on the earlier motors does not seem to be there, (on the 1979). Are you saying that there is a filter at the pump and filters in each carb on the black block motors from 1973 up? Sorry for the hyjack. JR

 

[Chris1956]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

Yes JR, if you unscrew each brass elbow (or tee) on the carb covers, you will find a tiny round fuel screen. They do not clog ofter, but will sometimes be full of debris.

 

[OldMercsRule]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

Yes JR, if you unscrew each brass elbow (or tee) on the carb covers, you will find a tiny round fuel screen. They do not clog ofter, but will sometimes be full of debris.

Thanks for the heads up Chris!! JR

 

[chuck_050382]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

On an 85 175 merc we had this problem. It was the high speed coil. It would run fin at idle and when you first mashed on it, but then it would fall on its face.

 

[Scaaty]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

The "Back Drag" carbs have a fuel screen under the brass elbow in the bowl cover. Unless you remove the elbow, you will never see it. The non back drag carbs have a round screen under where the fuel goes into the carb bowl cover


Yep, had an XS 150 that did the same thing. Drove me nuts. Merc had a service bulletin to go in there and yank those tiny screens going into each carb...yank'em and trash'em, and it will fly again

 

[Bob_VT]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

Fuel pump rebuild is fairly easy..... I just did mine (along with every other gasket and carb.

 

[Chris1956]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

Corey, Just so you know, your distributor ignition motor does not have high speed windings or a high speed coil.

Your problem had happened to me often. Everything in the fuel delivery system (pump, filters and lines) needs to operate at peak efficiency to keep that motor in fuel at high speed. Do you have the dual fuel pumps? They went to the single large fuel pump with the screen on top, in the newer models. I never thought that pump was anything special, but I suppose they were superior to the dual pumps.

 

[tramp666]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

I have a 78' 1400 that does the same thing. Do you think mine has those screens in the elbows as well?

 

[cslax9]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

Thanks for the info on the coils Chris1956. As far as I can tell it only has one fuel pump. It does have a screen on the top of the pump, right under where the fuel line comes in, but I can't find any screens under the elbows. Im hoping to pick up a pump rebuild kit, and maybe even some new hoses either tonight or tomorrow morning, and I will let you know what happens. Thanks again for all your help!

 

[OldMercsRule]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

Thanks for the info on the coils Chris1956. As far as I can tell it only has one fuel pump. It does have a screen on the top of the pump, right under where the fuel line comes in, but I can't find any screens under the elbows. Im hoping to pick up a pump rebuild kit, and maybe even some new hoses either tonight or tomorrow morning, and I will let you know what happens. Thanks again for all your help!

Probably the reason I did not know about the screens is the directive Scaaty (or someone talked about). If yer motor is over thirty years old, yer carbs may not be virgins, n' the last rebuild of 'em may have eliminated the screens. The Silver block inlines (1972 and before) had the old style twin fuel pumps with antique 5/16 brass fittings that are getting harder to find when you need to replace fule lines that the alcohol has abused. The black blocks all had the single fuel pump with the filter at that spot. My 115 1979 inline has a distributer, so I am going to verify the serial number on the block matches the number on the motor clamp this weekend when I fire up the old girl, as she does have a distributor not he individual coils my 1981 115 has. JR ps: the 1975 should only have one coil and a distributer. That would lead one to cornclude the problem is very likely fuel related.

 

[vulcantim]

Re: '75 Mercury 1150 Runs, but cuts out at high speeds

I had the same problem. I ran the fuel hose without the primer bulb and there was a big difference. Dont know if im leaving it like that but runs great.