1992 40HP Johnson Out Board problems

[yesmaamsergeantham]

I have a 1992 40HP Johnson. The compression on top was 110 and bottom was 135. What is normal range for this motor and what would be causing the difference in compression. Also, it runs rough on low speed idle, but when RPMS are increased it smooths out. Thanks, Kathy

 

[OBJ]

Re: 1992 40HP Johnson Out Board problems

Welcome to iboats Kathy....<br /><br />There is no real "range" when testing compression. General rule is if the compression on all cylinders is 90+ and with in 10 to 15% of each other, thats' good. The higher the compression the better.<br /><br />Your lack of compression on one cylinder may be an indicator of trouble and why your engine idles rough.<br /><br />Try this....do a decarb on the engine and then take another compression test with the engine still warm. Decarbing agents can be found at any dealership or where ever marine supplies are sold. Just follow the instructions on the can. <br /><br />Keep us posted as to your progress. Thanks.

 

[yesmaamsergeantham]

Re: 1992 40HP Johnson Out Board problems

Thanks! We will try that. Kathy

 

[yesmaamsergeantham]

Re: 1992 40HP Johnson Out Board problems

Well, bad news, the decarb didn't work. My husband pulled the heads and found that there is scoring on wall of the top cylinder. Does that mean the motor is shot???

 

[fishingdeal007]

Re: 1992 40HP Johnson Out Board problems

Hi Kathy, I've been working on boats for 20 years<br />and yes, if your husband found score marks on the<br />bad cylinder, get it bored, new pistons and you will be ready. Make sure you install a water <br />seperater after the rebuild, thats the best 35.00<br />you can spend.

 

[yesmaamsergeantham]

Re: 1992 40HP Johnson Out Board problems

Thanks, Dwayne!

 

[rickdb1boat]

Re: 1992 40HP Johnson Out Board problems

In addition to that good advice, rebuild the carbs while you have the rebuild done. Major cause of engine failure is blocked up passages in the carbs that lead to a lean condition which cooks the pistons and cylinders.

 

[OBJ]

Re: 1992 40HP Johnson Out Board problems

And a little more to add to Dwaynes and Ricks good advice, if the engine is equipped with VRO, make sure the alarms for the VRO are working. These alarms are very important. They tell you off trouble with the engine temp., VRO oil supply, and oil reservoir low level.<br /><br />Also mention to the better half that if he (and you) are going to tackle this job, a service manual is a must have item. It will guide you through the process. OEM manuals are prefferred as they are more engine specific than the generics. OEM manuals can be ordered through any Johnny/Rude dealer.<br /><br />Good Luck....If you'all have any more questions, please don't hesitate to post.<br /><br />Jim

 

[yesmaamsergeantham]

Re: 1992 40HP Johnson Out Board problems

Thanks to everyone for all the help. We have a 35 Evinrude on our boat now and will just stick with that. The Johnson was too much to tackle, so we sold it. I really appreaciate all the great advice. You guys are great! Kathy