Power loss on Envinrude 40hp, 1970

[correspondant2003]

Hello,<br /><br />I'm having a problem with my Evinrude twin-barrel, 40hp outboard. Basically, any new spark plugs that I put in provide good power for about only 1 hour of operation. After that, I have trouble planning the boat. Any suggestions why?<br /><br />Here are some hints:<br /><br />- A new set of plugs will give me good power for only 1 hour<br />- After the 1 hour, if I remove the plugs, the top one seems to be clean but wet with fuel<br />- The bottom plug is dark grey but dry.<br />- I tried with 2 new sets. When both gave me the same result after one hour, I tried mixing the 2 sets: if I put the 2 clean/but wet plugs together, I can't even start the engine. If I put the 2 dark-grey/dry plugs together, the engine runs (but eventually loses power).<br />- The fuel/oil mixture that I use is good: 50 to 1.<br />- It's new fuel.<br /><br />'Appreciate in input.

 

[Paul Moir]

Re: Power loss on Envinrude 40hp, 1970

Correspondant - It sounds to me like the top cylinder is fouling out on you. Does the power loss seem to coencide with extended idling periods, or will it happen when running hard? <br />3 other questions:<br />How does it idle?<br />What spark plugs and gap are you using?<br />What's the compression like?<br /><br />I belive on your 1970 engine the fuel pump is attached to the top cylinder's crankcase chamber. Try unbolting it from the engine (do not disassemble it!) and pump the primer bulb up hard. Does any gasoline come out from the port on the back of the fuel pump?<br /><br />EDIT: Another and probably better thought - rig up some way to get spark to jump a 3/8" gap. You can get cheap ignition testers at automotive tool stores. Compare both sparks.

 

[correspondant2003]

Re: Power loss on Envinrude 40hp, 1970

Hi Paul Moir, here are some answers to your questions.<br /><br />"Correspondant - It sounds to me like the top cylinder is fouling out on you. Does the power loss seem to coencide with extended idling periods, or will it happen when running hard? "<br /><br />- There is no particular pattern. I always try, an one time or another of an outing, to run hard (there are significant river currents in my area) but at one point it won't run hard anymore<br /><br />"How does it idle?"<br /><br />- Not bad. I find that it runs pretty rough at 1/2 speed, a lot of shaking of the engine. I've just had this boat for a couple of years, I can't tell if it's normal.<br /><br />"What spark plugs and gap are you using?"<br /><br />- NGK B6S, at .030 inches. I tried with Champions also, with no better result<br /><br />"What's the compression like?"<br /><br />- I measured it last year and it was fine in both barrels.<br /><br />"I belive on your 1970 engine the fuel pump is attached to the top cylinder's crankcase chamber. Try unbolting it from the engine (do not disassemble it!) and pump the primer bulb up hard. Does any gasoline come out from the port on the back of the fuel pump?<br /><br />EDIT: Another and probably better thought - rig up some way to get spark to jump a 3/8" gap. You can get cheap ignition testers at automotive tool stores. Compare both sparks. "<br /><br />- I too have the impression it is an ignition problem, not a fuel problem, which perhaps would explain why the motor runs great when new plugs are installed. Since the ignition is not 100%, as soon as the plugs get a bit dirty from running, I lose a barrel. I will try the spark test. Do you think that a bad spark could be caused by a fouled coil?<br /><br />Thanks for your ideas.

 

[ebnf1969]

Re: Power loss on Envinrude 40hp, 1970

i am sorry to say, but it sounds like the fuel gasket on the inside of the motor might be leaking (slighlty) allowing too much fuel to get into that one cylinder and flooding it. That is the wetness you are experiencing on the plug.<br />the other one being grey and dry , is normal

 

[Paul Moir]

Re: Power loss on Envinrude 40hp, 1970

Ok, great, thanks for the information.<br />Like ebnf1969 said, we should rule out that fuel pump first. Just do that test above to make sure it's not leaking and we can move on.<br />As for bad coils, they are very common on your engine. Fortuently,they are very inexpensive to replace. The failure is that the insulation on the coil will crack due to age, and it arcs over as the rpms pick up.<br />But it's important to first properly diagnose the problem before throwing parts at it.<br />Tell us what you find!