1976 70HP evinrude bogging down and dying when throttling up.

[TinyTiffin]

Well on friday i pulled my boat out of the shop, $600 later and the mechanic said it was lake ready. (Fixed a problem it was having last year that 2 cylinders were spitting water) So saturday i take the boat out pumped up to have a nice day on the lake and i get it started up, take it out, got full throttle but it didnt plane out so i stopped for a second to adjust the weight, when i tried going again it would just bog down and die if i tried giving it any real throttle, it would only go at slow speeds and die if i tried going any faster. My brother was the previous owner of the boat and said that he had this problem and had it down to a science that if he used the choke at the right time while putting the throttle down it would take off. My mechanic said he checked the carbs and they were clean, but from my research this seems like its a carb issue to me. As i said i just got it out of the shop and am pretty strapped for cash now and wont be able to get it back in for a while so im wondering if theres things i can check or do to get it running properly myself and save several hundred dollars.
Im an pretty novice mechanic, dont know a whole lot about boat engines. Most i have done so far is change the starter, spark plugs and gear oil.
Oh and it idles perfectly.
So keeping things in simple terms if you could give me any advice on what to check or do that would be great.
Thanks in advance, Tanner

EDIT: Mechanic also said compression and spark is good.

 

[TinyTiffin]

Re: 1976 70HP evinrude bogging down and dying when throttling up.

Update: well i just pulled off the three tubes that are connected to the carbs, they are pretty old and just by touching them a little bit they started spitting out fuel so im gonna replace those today as well as the main fuel line. No idea if these are relevant at all or will do anything but i figure it cant hurt and will be inexpensive.

 

[TinyTiffin]

Re: 1976 70HP evinrude bogging down and dying when throttling up.

Well i changed all the fuel lines, gear oil and cleaned the carb jets as i saw in an old post from somone with the same engine and similar problem. I had it running on the muffs, and when putting it into gear it didnt majorly clunk as it was when i had it on the water and it was going through the full range of throttle fine (i know your not generally supposed to do that just running on muffs but i only did it for maybe 30 seconds at a time to make sure it didnt overheat or anything). I also realize that running on muffs isnt the same as water because of the back pressure and what not but unfortunately closest body of water is half hour away so hard to drag the boat out everytime i change something. Gonna try and get it out in the next couple of days to test it though.
Anyways still looking for some insight from anyone out there so thought i would update and bump the thread up to see if you guys got anything for me yet.
Thanks, Tanner

 

[eavega]

Re: 1976 70HP evinrude bogging down and dying when throttling up.

Hi Tanner
I have the same motor, and had the same problem last year. There is a technical service bulletin #1144 from OMC about the 1976 70 and 75 HP motors having a possible lean condition causing mid-range acceleration problems. In fact, it says that the work-around owners found was to blip the choke momentarily to get through the mid-range flat spot in the acceleration.

If the corrective service was performed, there should be an "X" stamped on the top of the carb body. The other easy way to tell was to pull the idle jet and check the size. The motor came standard with a .030. The TSB fix has you changing that to a .032.

Be absolutely sure that its not something else before doing the work on the carbs. The fix involves reaming out the calibration ports under the core plug as well as the holes in the high-speed nozzle. If you mess this up, you may end up trashing your carburetors. Verify your entire fuel system is in top shape, and that the motor will run WOT at the correct range. If you have verified that the fuel system is working as expected (no air leaks, pump is pumping, etc) then the service may be indicated.

Rgds

 

[durban]

Re: 1976 70HP evinrude bogging down and dying when throttling up.

if every thing is checked & all good , i put it down to one last possible cause , water is getting into the exhaust port .

 

[TinyTiffin]

Re: 1976 70HP evinrude bogging down and dying when throttling up.

Thank you for the reply, only real answer I have been able to get so far. There is no x stamped on top of the carb. Does this fix involve like completely tearing down the carbs or is it mainly outside work. And is it something a real amateur like myself can do or is it best to bring it to a mechanic. If this is a job better for my mechanic are we looking at a 2 hour job or a 7 hour job. If its more around the 7 I'm screwed for this year unfortunately. And about the exhaust por, correct me if I'm wrong but it seems to me that if it were the case it would affect all speeds and not just the one little spot and blipping the choke would not help in that case.
Oh and where might I be able to find this bulletin?
Anyways thanks again for the response.
Tanner

 

[durban]

Re: 1976 70HP evinrude bogging down and dying when throttling up.

it depends when the motor idles there is not so much water pressure , when you throttle up the pressure increases & it seems to me that's the time the water gets in i have had this problem before . jf you can get all the bolts out from the exhaust box . this will only take about 2/3 hours to do the job change all the gaskets , providing the gasket faces are not corroded if this motor was used in sea water , i refaced all my gasket faces , grinding paste & glass top , also check to see if the exhaust cover box & manifold plate is not warped , with a steel rule .

if you gonna do this job , if this motor was used in sea water i can almost garantee you some exhaust box bolts are going to give you **** , the heads of the bolts break off leaveing the stud behind & the studs are ceased in with corrosion . i use my magic nitric acid formula . & they come out sharp shoot ,

 

[King James]

Re: 1976 70HP evinrude bogging down and dying when throttling up.

I had same problem with my motor found it was clogged adjustment screw on carb. All i did was remove screws pumped ball till carbs filled sprayed with carb cleaner in screw hole and set screws 1 1/2 turn out. hope this helps and let us know

 

[MRneatfreak]

Re: 1976 70HP evinrude bogging down and dying when throttling up.

Just like everyone has stated, start with the carbs, they are super easy to work on (off the boat). The hardest part is removing all the hoses. I would check your high and low speed jets. You will need to make your own special tool to remove them, I used a small flat head screwdriver and shaved the sides not the tip to get down in the hole. One set is between the top side screw on both sides, and the other set is behind the fuel bowl drain. I recommend pulling the carbs cause if you drop one you're SOL as they are tiny and non magnetic. I'm thinking they are plugged with slime or debris. So you just removed them and blow air through them, and air through the holes with which they came out of. Once you see light through the jets reinstall. Also verify the number that is stamped on the side of all them, are the correct ones for you engine, there are four per carb. While your at it replace all the fuel lines with ethanol resistant hose. If you do all that might be time to try a new fuel pump. I was having similar problems with my 85 horse. Ultimately tweaking the carbs, and the new fuel pump fixed mine. This is all stuff you can do yourself.