3 cyl Evinrude 70 bogging problem solved!

[Normspeed]

I'm starting this new thread hoping this will help others. My 1984 70 hp Evinrude had the fairly common problem of running fine at idle or full throttle but bogging down and stalling when accelerating to get planed out. It would get past the flat spot if I hit the enricher by pushing in the ignition key. Like others in this forum and elsewhere, I suspected a lean condition, probably in the idle circuit, just before the high speed jets took over. Rebuilt all three carbs (twice), rebuilt, then replaced fuel pump, replaced all fuel hoses, checked fuel supply from tank, performed link n sync, checked timing ring for proper movement, and applied grease to the throttle shafts to seal up vacuum leaks. Bought a set of richer idle orifices but before I got them installed I found a reference to an old OMC service bulletin suggesting that venting the prop can help get the boat planed fast and won't alter performance in any other way.

I followed the procedure which involved drilling a small hole near the base of each prop blade. The size and placement are very specific. I used my spare prop (I had the same bogging problem with both props ). Just returned from the lake and the bogging is completely gone! Switched to the unvented prop and problem came back. I'm going to vent that other prop (a brand new Solas) and I'm sure it will also get rid of the bogging. The idea of prop venting is pretty common, but I hadn't run across it in discussions of the 3 cylinder Johnson/Evinrudes with the acceleration bogging problem. I would compare the problem to shifting a standard shift car from 1st to 4th gear without using the gears in between. In my case, the Evinrude was originally on a lighter 16 foot bowrider where it probably worked fine, but my boat is heavier and needed to get rpms up quickly on takeoff, where it would bog. The venting procedure allows some limited controlled cavitation on takeoff, greatly improving hole shot and getting the rpms up quickly. Once you're on plane, the venting stops and exhaust goes out through the hub, not the vent holes. Top end performance is the same. I'm posting a picture of the TSB showing exact hole placement. If you happen to find this procedure in your Seloc manual, propeller section, beware, their illustration is not correct. They got it wrong on where to measure. I hope this helps others with this problem. My props are aluminum, easy to drill, and easy to plug the holes if you don't like the results (JB Weld). Hole shot is now really impressive and no bogging or stalling at all.

 

[lmuss53]

Sounds interesting, but I would ask one question, what is you wide open throttle RPM? I ask because if it is not somewhere in the recommended range, preferably in the high side of that range, you are putting the motor at risk running it there. Trouble getting on plane and a so-so hole shot are an indication of too much pitch in the prop and a prop with too much pitch will not let your engine rev out to the recommended range. I killed a very nice 90 Johnson by pulling it off a 17 foot runabout, putting it on a much heavier pontoon and running it at WOT without the right prop. Don't ask me why, but it creates a lean condition and that allows a cylinder wall to dry out and eat the piston ring. If you reduce the drag on the prop on the hole shot, but don't let it rev out on the top end you are creating the same situation for your motor. I would much rather my OMC motors were at 5600 RPM at WOT than 4400.

 

[ondarvr]

If you needed to hit the primer button to get on plane before, and you didn't solve that issue, you still have a fuel delivery problem. The motor should not bog or begin to die when accelerating even under a very heavy load. It would still be running lean at that RPM no matter what you do with the prop. While you did sort of make the bog issue go away, it wasn't by fixing the actual problem.

 

[oldboat1]

Thanks, Normspeed. Didn't know (or didn't remember) here was an OMC SB on it.

First ran into prop venting discussions years ago at an AOMCI meet. Think there are additional layers -- size of holes, whether to angle them or chamfer the edges. And they are available commercially (Raker and Viper props, Solas).

Might do a posting over in the props forum too and get a bunch of feedback. Lots of opinions and ideas, I'm sure.

 

[flyingscott]

I had the same problem with my 79 70 hp evinrude. The fix I found after cleaning the carbs a lot and replacing fuel pump and gaskets. Was to just change the idle orifices 2 sizes smaller to richen it upon the advice of the dealer.

 

[Normspeed]

WOT is 5300 rpm. Bog issue didn't sort of go away, it is gone. I figure if I got the fix directly from an OMC Service Bulletin, it's likely to be trustworthy. Bottom line, the issue is fixed and motor runs fine now. Thanks oldboat1 for the positive comment.

 

[David Young]

Nice to know this can fix a problem like this. I've been reading about drilling holes in the prop and thought it was a myth of some kind. Seeing a OMC service bulletin it must be something that will work. I don't have any problems like yours with my motor, it planes in 4 seconds

 

[lmuss53]

Normspeed. if you're in the 5300 range at wot, you're where you need to be. I wasn't trying to be negative I was just trying to make sure you had that wot number, I didn't want to see you lunch a motor like I did. I actually have a little stumble in the hole shot on my 70 and an extra prop, my wot is at 5500 so I will be trying the same fix on one of my props, thanks for the post.

 

[ondarvr]

WOT is 5300 rpm. Bog issue didn't sort of go away, it is gone. I figure if I got the fix directly from an OMC Service Bulletin, it's likely to be trustworthy. Bottom line, the issue is fixed and motor runs fine now. Thanks oldboat1 for the positive comment.

Yes, the vent holes have been around for decades, without them it does not cause bogging though, it may create lugging, but they are two very different things. What you did was allow the motor to more easily get passed the RPMs where it was running lean, you didn't fix the lean condition at those RPMs.