Johnson 25 hp exhaust crud in crankcase?

laujok

Seaman Apprentice
Joined
Sep 9, 2011
Messages
48
Hey everyone.

I haven't had much luck with this motor as you can see from my previous posts. Still having the same problem. Doesn't want to keep running, has pretty much no idle and pops hard when I try to give some more rev's. I took her apart today and I noticed that the lower connecting rod was covered with black exhaust crud!! When I looked the cylinder head, lower cyl had black crud and upper had none (chrystal clean). Spark, fuel and the usual stuff is checked many times..

Last year when I bought her I tried running in sea, barely got the boat to plane, had low power and anything below 2000rpm would make a hard pop and then stall..

So what do you mechanics think about all this?
Thanks for help!
 

oldcatamount

Lieutenant Commander
Joined
Apr 4, 2010
Messages
1,740
Re: Johnson 25 hp exhaust crud in crankcase?

Have you done a compression test yet? That's where I would start.
 

Bob_VT

Moderator & Unofficial iBoats Historian
Staff member
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May 19, 2001
Messages
26,019
Re: Johnson 25 hp exhaust crud in crankcase?

If the top of the piston is super clean as compared to the other one.......... it usually indicates a headgasket leak and the "cleaning" is done as a result of water getting in there. The water prevents proper firing which will build up crud on the connecting rod.

Clean it out, re-gasket, clean the carbs and you should be good to try again.

Kind of late for a compression test if it is already apart......
 

laujok

Seaman Apprentice
Joined
Sep 9, 2011
Messages
48
Re: Johnson 25 hp exhaust crud in crankcase?

Thanks for info.
I forgot to mention that I did a compression test and it showed about 120 psi on both cylinders. I did the head gasket last year but it had no affect what so ever. I didn't do anything to the head surface or the block surface.
 

laujok

Seaman Apprentice
Joined
Sep 9, 2011
Messages
48
Re: Johnson 25 hp exhaust crud in crankcase?

problems with my computer..
 

laujok

Seaman Apprentice
Joined
Sep 9, 2011
Messages
48
Re: Johnson 25 hp exhaust crud in crankcase?

The head really needs some work, wasn't 100% flat. I did open the head once this year to see what's the story, but I didn't see anything wrong (blind).
I'm waiting for a new top end seal because the original one had few dents in the metal part. I'm also going to replace the lower end seal.

Does anyone have any experience of sanding the head flat? Or should I take it to the shop?

Thanks!
 

gozierdt

Petty Officer 1st Class
Joined
Jun 13, 2010
Messages
364
Re: Johnson 25 hp exhaust crud in crankcase?

I've flattened small heads with a large mill file in the past. I also read while researching
my Evinrude 15 hp, one comment that you can use 220 grip paper on a flat surface-
thick glass, granite, etc.
 
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