1979 25 hp Evinrude knocking noise

spacemonkey77

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Nov 16, 2015
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Hey guys, I have a 1979 25hp Evinrude model 25904r. Recently I’ve noticed kind of a knocking noise when I’m running it. I don’t recall if it’s a noise it’s always had and didnt notice until now or if it’s slowly developing! I’ll attach a few videos! The compression on both cylinders was 120 and 121 psi! The motor runs great on the high end. At idle it runs mostly good. A little rough at times and stalls every now and then. The carb is clean and freshly rebuilt!

anyways I was wondering if the sounds it’s making while running is normal or if I should look into tearing apart the power head before something goes bad and causes damage.
 

kbait

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Nov 13, 2007
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Remove recoil and move flywheel by hand like 2nd video. Noise is likely coming from recoil..
 

spacemonkey77

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Ok thanks. I will do that. Won’t be able to get to it until tomorrow morning. Does the sound in the first video sound abnormal though?
 

F_R

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I don't hear any sounds that I would consider abnormal. However, I cannot tell how loud they are, because obviously I can turn the volume up or down.

In the second video you are jiggling it back and forth at TDC and BDC where all the slack is taken up as all the parts reverse direction. You are messing with a two stroke with all roller bearings and pistons with a loose fit, compared for instance with a nice quiet car engine where every thing fits much closer and oil cushioned.

If you want reassurance, check for bearing looseness as follows. Remove spark plugs. Stick a screwdriver or similar instrument into one of the plug holes, touching the piston top. Slowly rotate the flywheel in one direction, following piston movement with your screwdriver. Rotate slowly and smoothly and DO NOT allow flywheel to turn in opposite direction as you do this and DO NOT push on the screwdriver. Turn till the piston is about half way down in it's stroke, then STOP turning. NOW give a push on the screwdriver. You may be able to hear/feel a slight click. That represents the total clearance in the bearings. They may have some barely noticeable clearance, that's normal. But any significant movement is cause for concern. I'm betting it is all OK.
 

spacemonkey77

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Awesome thank you. I will do that in the morning and report back my findings. Yes it’s hard to hear in the video. Perhaps I’m just being overly critical of the ticking noise. I just don’t recall hearing it in the past!
 

oldboat1

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Think you've got a little echo chamber going there. I don't hear anything to be concerned about.

You might tweak the slow speed mix a little to see if you have the smoothest idle you can get. Try adjusting leaner about 1/8th turn (clockwise). If rpm increase, throttle it down and continue adjusting. You should be able to slip it into gear without stalling or hesitation -- idle in gear at about 650 rpm.
 

spacemonkey77

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Thank you for your input. It eases my mind having done the texts you guys mentioned . Today I went ahead and pulled the head because I wanted to inspect the cylinders for wear and clean some of the corrosion and carbon buildup out. It turned out it was all in good shape anyways but now I know for sure! I did the cylinder test while it was open and there seems to be zero play in the pistons. Good news. The noise I was hearing in the second video was in fact just the pull start!

on another note for reassembly. The manual says to never use automotive gasket sealer because it will corrode the aluminum but it says nothing about using a marine gasket sealer. (See attached pictures) Does that mean I should NOT use any gasket sealer at all? I have 3m 847 that I purchased for my lower unit. Is that safe to use?
 

racerone

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Bring top piston to TDC-------Post picture where there appear to be some marks on the piston OD.
 

spacemonkey77

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Ok I’ll be home in about 30. I’ll take a picture. Those marks right in the center are actually from me. I used an old compression gauge a few years ago that was a bit too long. Unfortunately it left those marks because it went too deep in. I went and bought a new gauge with a shorter fitting. Hopefully it did not cause any serious damage!
 
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spacemonkey77

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Ql77jc4. Plugs. I’m certain it was from my compression gauge. When I checked it the gauge was hitting it. I can also see the mark on the gauge where it hit. I’ve never had any issues with the plugs touching and reducing the gap
 

spacemonkey77

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Does anyone have any insight on the head gasket and water cover gaskets? The manual I posted above says not to use automotive gasket sealer because it will corrode the aluminum but it says nothing about what I should use. Does that mean I use no sealer on the gaskets? Or do I need to get a marine gasket sealer? I have 3m 847 right now! The gasket is a Sierra!
 
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oldboat1

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The gaskets will sometimes indicate whether adhesive is to be used. But in any case, you should resurface the mating surfaces for the cylinder head and the water cover.

It's a good idea to have on hand a can of BRP gasket sealer, and a container of marine grease. Unless the gaskets state that no sealant should be used, I would use the BRP sealer. Torque to specs, and work in an inside out pattern done in a couple of passes.
 

F_R

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A highly controversial topic.

1. Gaskets marked Use No Sealer--don't
2 Otherwise I use the OMC gasket sealant. A can will last forever. I think Mercury and Yamaha have similar products (?)
3. Acceptable substitute--Permatex Aviation Gasket Sealant---the one that comes in a can with a brush. It doesn't contain copper and won't corrode your motor. It's nasty black color is messy.
 

spacemonkey77

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Nov 16, 2015
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Thank you all for the information! My gaskets do not say to use no sealer! Quick question. Will the 3m 847 gasket adhesive sealant be acceptable since I already have it for resealing my lower unit, or is it a totally different product?
 
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