75 hp johnson vibration

Author Message   Gregorb (Gregorb) Posted on Tuesday, April 16, 2002 - 12:38 pm:    recently purchased a 1983 75 hp johnson i knew it had a vibration. plug in head is stamped rph the vibration was so bad the powerhead mounting bolts were stripped.repaired the bolt holes cranked without foot and still have vibration.also stator has a crack in the outside.all three cylinders are firing. any help would be thankful. gregory   Joereeves (Joereeves) Posted on Tuesday, April 16, 2002 - 05:49 pm:    Greg.... That's a weird one my friend. There's no chance that the flywheel magnets could come loose on that model, and if it was out of time, it simply wouldn't run. It could have a combination of various size oversized pistons and even that wouldn't make it viberate. The "rph" of course stands for "Replacement Powerhead" so we know that the original powerhead had a major problem of some kind, and when one replaces a powerhead or rebuilds it, the carburetors are always dismantled, cleaned and rebuilt. This leads me to the carburetor section. The carburetors on that model have three (3) fixed jets consisting of a "Idle" jet at the top rear starboard section of the carbs, a "Off Idle" jet at the top front portion of the carbs, and a "High Speed" jet at the bottom of the carburetor float chamber (in back of that screw at the front of the float chamber) If one does not pay particular atention, it is possible to mix up the "Idle" jet with the "Off Idle" jet. If that happens, the carbs would be adjusted wrong, and that might result in viberation. Worth looking at. The "Off Idle" jets (top front) should be marked as 67. The "Idle" jets (top rear starboard) should be marked 59. The "High Speed" jets should be marked 56D. Let us know what you find. Joe   Gregorb (Gregorb) Posted on Wednesday, April 17, 2002 - 08:46 am:    Joe i checked the jets the idle jets are the correct size, but there are no off idle jets installed i found where they go but there not there.I am looking for jets today as soon as i get them installed i will let you know back. THANKS FOR THE HELP! GREGORB   Joereeves (Joereeves) Posted on Wednesday, April 17, 2002 - 11:33 am:    Greg.... What was where the "Off Idle" jets would normally be? Was it simply left as a open threaded hole, or was a plug installed there? Any chance you could ask whoever installed the replacement powerhead? I'm very curious about those carbs. Joe   Gregorb (Gregorb) Posted on Wednesday, April 17, 2002 - 01:42 pm:    Joe the off idle hole had a screw in the outside so i removed it and behind it was a smaller threaded hole like where a jet would go.I thought i was in the wrong place so i looked in a repair manual at a type IV carb and it looks like what i have got minus the jets. gregorb   Joereeves (Joereeves) Posted on Wednesday, April 17, 2002 - 06:16 pm:    Greg.... That rang a bell. I dug into the service bulletins and found that many of the 1983 75hp engines had a tendency to hesitate somewhat when throttle was applied. OMC's cure was to remove those "off idle" (intermediate) jets. So apparently, that has nothing to do with the viberation problem. Check the linkages that run from carb to carb to make sure that all of them are adjusted properly ie throttles open and close all at the same time, the throttle butterflies start to open exactly when the cam scribe mark lines up with the carb throttle roller. Many years ago, I did encounter a engine that had a bad viberation at certain rpms which really mystified myself and a few other mechanics. That turned out to be a almost invisible crack in the upper portion of the crankshaft, which eventually resulted in that top portion of the crankshaft breaking off. Hopefully that's not your problem. What are the compression readings of the engine? Also, check the spark to see if it jumps a 7/16" gap on all cylinders. What do the s/plugs look like, and what make and number are they? They should be Champion plugs QL77JC4 or L77JC4 and set at .040. Joe   Gregorb (Gregorb) Posted on Thursday, April 18, 2002 - 08:47 am:    Joe... got in late last night did not get to look at carb. adjustment,or plugs. I will check tonight and get back with you.also if i do not find the vibration what would you suggest run it or part it.do not want to speed a lot of money on older motor would rather upgrade to late model. THANKS AGAIN gregorb

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