Tearing my hair out here chaps... Evinirude 4hp twin...

[TedA]

Hi everyone,<br />OK, straight to the p[roblem. I own an Evinrude 4hp twin (s/no: 42B36S B14529) approx 1965.<br />Fault: Engine starts ok. Will run well at 1/3rd throttle range on both cylinders. Starts to miss on bottom cylinder when throttle is moved up the range. Then runs well on both cylinders at full throttle. <br />Cleaned checked everything, good sparks.<br />Would welcome a little help on this one my friends. Ted.

 

[Paul Moir]

Re: Tearing my hair out here chaps... Evinirude 4hp twin...

Ted, Welcome.<br />Your 4hp is a 1973 engine. I have a 1972 Johnson version of that engine myself. It is truely one of the great outboards - perhaps the best small outboard ever built. OMC built them in slightly different variants between the early 50's and the early eighties.<br /><br />I presume your engine also has an adjustable high speed mixture control. Does adjusting this when your engine is at 2/3 throttle get the bottom cylinder to pick up? Which way do you adjust it?<br /><br />A mid-throttle miss suggests two things to me. First, that your carburation is out of synch with your spark advance. This has to do with setting the carburetor's pickup off the cam on the magneto plate, and is called a link and synch.<br />Second, at part-throttle is where the ignition system is working it's hardest, and these engines are well known for ignition problems coming from cracked ignition coils. Compare sparks to see if they can both jump a 3/8" gap. Fortunetly, new ignition coils are relatively inexpensive.<br /><br /><br />Tell us what you find!<br /> Nice 3/4hp tuneup article, click here.<br /><br />One last idea - what kind of condition are the spark plug leads in? Carefully check them for cracks in the insulation, especially where they attach to the magneto plate.

 

[TedA]

Re: Tearing my hair out here chaps... Evinirude 4hp twin...

Paul,<br />Thanks for that. Let me check this info out & I will let you know indeed!<br />Ted.

 

[TedA]

Re: Tearing my hair out here chaps... Evinirude 4hp twin...

Paul,<br />We have checked compression! One cyl 125psi One cyl 85psi! Before we check or change anything else could this be the route cause?<br />Regards, Ted.

 

[alcan]

Re: Tearing my hair out here chaps... Evinirude 4hp twin...

Hi TedA<br />Yes, The compression on this engine is a problem. You need to figure out what is wrong before you procede.

 

[Paul Moir]

Re: Tearing my hair out here chaps... Evinirude 4hp twin...

That's too bad, even compression is essential. Which cylinder is reading low?<br /><br />Try re-testing compression, giving the engine lots of pulls. Make sure everything is even (like number of pulls). Also make sure that the compression tester is screwed in all the way. On my 4hp, those angled plug-holes caused my tester to bottom out on the top hole before it reached the sealing o-ring on it, but not on the bottom cylinder. I had to modify my compression guage for it to fit properly.

 

[TedA]

Re: Tearing my hair out here chaps... Evinirude 4hp twin...

Thanks all,<br />Will do & report back asap.<br />Ted

 

[TedA]

Re: Tearing my hair out here chaps... Evinirude 4hp twin...

Lower cylinder is reading low on this engine.<br />Ted.

 

[Paul Moir]

Re: Tearing my hair out here chaps... Evinirude 4hp twin...

First, get your hands on some decarb spray from your local outboard shop, and follow the instructions in roscoe's Decarb FAQ. Mabey the lower rings are simply stuck from lots of carbon. If that doesn't help you can remove the head easily enough to see what's wrong. A headgasket for this engine sells for around $2 US, so it's worth a peek.<br />Unfortunetly, retorqueing the lower cylinder bolts with the powerhead in place is a problem. But it's simple to remove them.