'78 70 hp Loses Power

[MESS147415]

Hello all. Still having probs with my new toy. Engine will start ok on muffs and appears to run OK in my yard. Once on the water she will run ok on the first trip untioll I dock and allow her to idle. Then after a while tickover speed slowly drop s and she will stall when engaging gear. Shes then a pig to get started again - sometimes she will run at a slow speed and stall again, others she will very slightly spit back. Using the choke kills her cmpletely. The only way to get her going is using the fast idle control and 'coaxing' her as she cranks on the starter - once shes caught she very sluggish to rev for a second or to, but then she away and runs fine - I towed mt daughter on sat afternoon for about an hour once the engine was running so i know there is nothing major wrong - however after mooring up for five mins while she changed the same problem recurred!<br />ive changed plugs and had the float bowls off the carbs and everything is clean as a whistle.<br />i'm convinced its a fuel problem. i've tried the priming bulb when the problem occurs and it will take about half a pump!<br />One thing that does concern me is venting of the fuel tank - i noticed when i stripped my carbs that the tank had pressurised with temperature - is this normal? help please - steve

 

[rickdb1boat]

Re: '78 70 hp Loses Power

Check out the fuel tank vent first and see if it's clear. You can leave the fuel cap slightly loose to prove this out. Do you have a built-it tank or portables? Also, you may want to change out the primer bulb. Also, make sure the low speed screws are adjusted correctly, as it sounds like it's running lean at Idle. 2- 2 1/2 turns out from lightly seated is the norm.

 

[MESS147415]

Re: '78 70 hp Loses Power

Thanks Rick - I was suspicious of the tank vent - there isnt one - my local yard assured me the tank was self venting thro the quick release connection? - i have been leaving the cap loose to eliminate this - tank is portable. Primer bulb and all hoses have been replaced recently by previous owner. My carbs appear to be fixed jets - no obvious external adjustments - can anyone recommend a good service manual available in UK.<br /><br />thanks steve

 

[rickdb1boat]

Re: '78 70 hp Loses Power

In that case, I would have to say you need to clean and rebuild the carbs. Even though you took the bowls off does not mean there is not a restriction in one of the passages in the carbs. Could also be that the float level is off in one or more of the carbs or a float needle is worn or sticking. A lean condition on these 3 cylinder motors can be a quick death to the motor. Either find out yourself what's going on or have a Mechanic take a look. I don't know who will ship a Manual to you, but you may try here at I-boats or at the link below for a Factory Manual.<br /><br /> www.marineengine.com

 

[MESS147415]

Re: '78 70 hp Loses Power

I'll strip them again this week and have a more thoro look inside. Most logical explanation would be sticking floats as once its up and running again its fine! Are sticking floats common?<br /> steve