motor rebuild

[digtech]

I have a 1990 40horse Johnson that died on me. Two different marina's says the motor will need rebuilding. Has 50# compression on 1cyl and 110# on #2. The second mech. pulled the head to show me the scoring on the cyl walls. I suspect that the VRO failed as oil consumption has been very little in retrospect. My question, Do I only need to bore cylindrs and replace pistons? How will I know if crank or rods, etc need to be rplcd? I plan to bypass VRO after fix and premix gas/oil from now on. I will probably go ahead and rebuild carbs while I'm at it. Thanks for any help

 

[Waterbugtoo]

Re: motor rebuild

Bearings are cheap enough on a 2 cylinder....replace em and forget em. Dont cut corners if you want a good chunk of motor to last. If crank journals are discolored or scored (they doubtfully are), then get the crank redone as well.<br /><br />

 

[rickdb1boat]

Re: motor rebuild

I doubt that it was your VRO, as both cylinders would have been damaged, along with the bearing from lack of lubrication. VRO is not selective. If it goes, so does ALL your inside parts, unless you shut it down in an instant. You really need to find out for sure what went wrong to cause your problem, before you proceed. It could be that the one carb was running lean and fried the piston(Common), in which case the bearings and such should be OK. But you need to determine the cause, before you treat the effect.

 

[Hooty]

Re: motor rebuild

...and if you find only one piston and cylinder is damaged, that's the only one you've gotta bore and replace piston in. I'd replace rings on both pistons. By all means, figger out what caused the failure. I'll bet rickdb1boat has the hot hand with a lean carb.<br /><br />c/6<br /><br />Hooty

 

[digtech]

Re: motor rebuild

Thanks for the advice guys, but what would make carb(s)all of a sudden start running too lean? And whats the best procedure or trick to verifying this as the problem? I'm going to dealership for an OEM repair manual tommorrow, will it contain troubleshooting techniques? By the way, the pistons look okay, on top anyway, but both cylinder walls are slighty scored

 

[bonitoman]

Re: motor rebuild

If you rebore the block and go to a bigger piston don't forget to increase the jet size. It is not as noticeable on a 50 compared to a 200 but this way you are sure the engine is going to get a rich mixture.

 

[Hooty]

Re: motor rebuild

Might have picked a little trash and partially stopped up a jet.<br /><br />c/6<br /><br />Hooty

 

[Waterbugtoo]

Re: motor rebuild

*SIGH*<br /><br />It baffles me why someone would advise replacing only one piston during an overhaul on a 12 year old motor.<br /><br />

 

[Joe Reeves]

Re: motor rebuild

Dave... The reason for replacing one piston on a two cylinder engine would be simple economics... replace what is damaged or worn but do not replace if replacement is not needed.<br /><br />A eight (8) cylinder Evinrude for example, if one carb clogged and caused damage to one cylinder wall and piston due to being run lean, the other pistons, which were not damaged, are not replaced simply for the sake of being replaced, they're cleaned to new condition, inspected, and if found to be okay, new piston rings are installed. At approximaterly $100.00 bucks per piston, random replacement of those items can be prohibitive.<br /><br />Also, there is no need to rejet the carburetor when installing a larger bore piston. OMC and other manufacturers have taken into consideration the jet sizes, allowing the initial factory size installed jet to accommodate the recommended O/S pistons.

 

[Waterbugtoo]

Re: motor rebuild

JR,<br /><br />So you're saying that if you scored one cylinder on your 300 Jonnyrude, and you decided to replace the powerhead with a rebuilt unit, paying $4000 to $4500, you would accept one that only had 1 new piston and 7 used ones, right?<br /><br />Why would you rebuild one yourself using the same principle? To save a few bucks? If you could afford a V8, you could afford a COMPLETE overhaul. Gamblers are why there are so many boat repair shops in business.<br /><br />If you advise someone to replace one piston in an overhaul and they went out only having a skirt come apart off one of the old pistons, would you take responsibility? Shops do not have that luxury, but ones that do that kind of work take big risks that it wont happen, and those are ones that you wouldnt want to associate with. Put one of those 12 year old CAST pistons under an x-ray and comparing it to a new one, tell me you would put that 12 year old piston back in the motor.<br /><br />There are a lot of good people with good advise in these forums, but there are also many that cause others undue time and expense towards failed attempted repairs with heeded off the wall suggestions. Some questions are hard to iterpret and diagnose on a BB and very hard to be 100% accurate in advice, but some are straight to the fact.

 

[bonitoman]

Re: motor rebuild

There is no reason to replace a good piston. I do not care if it is a two, four or six. I rebuild them all the time. I always put new rings on every piston but if a piston is in good shape there is no reason to replace it. An outboard is not like a 4- stroke auto engine where if you bore one cylinder that you have to do all. This is whats great about outboards. If you want to put new pistons in your outboard go right ahead. I feel and have been doing it for years where one changes the bad piston and bores it but all good pistons get new rings.

 

[Joe Reeves]

Re: motor rebuild

WaterBugToo.... I said nothing about replacing a powerhead, I was speaking of the method OMC recommends in rebuilding a powerhead, one that I and many other technicians have incorporated in our work for years.<br /><br />I speak from experience that started in 1961, and since that time have had only one powerhead rebuild fail. That failure was due to having a removable crankshaft counterbalance retaining bolt break.<br /><br />We do not replace parts simply because they are old. If such was the case, that line of thought would pertain to bearings, crankshaft, rods, etc etc. Although retired, I still do some work for my friends and all of them refer to me as a honest and knowledgeable technician, marine mechanic, and friend.... but never a parts replacer.<br /><br />I have no intention of carrying on a debate on this issue and this will be my last post pertaining to original versus new components.

 

[G DANE]

Re: motor rebuild

Good Words Joe ! - Ditto

 

[digtech]

Re: motor rebuild

Thanks a million for the advice guys! This was my first post to this forum and I certainly appreciate yall taking time to give a fellow boater some good insight. Even the mild debate on changing parts makes for healthy discussion and I will take all into consideration. This will be my first attempt at a rebuild, and while I am mechanically inclined to some degree, all of the controls and linkeages,etc., on the motor look a little intimidating, so any future advice would be great. Hope everyone has a merry Christmas and a happy new year

 

[Waterbugtoo]

Re: motor rebuild

JR,<br /><br />That line of thought pertains to ((((****12 year old CAST ALUMINUM pistons****)))), not STEEL rods and crank. If the pistons are quality FORGED ALUMINUM (which OMC doesnt offer), then maybe reuse them, depending on egg of cylinder.<br /><br />The difference between quality and just "so so with a risk" has clearly been defined in this thread.<br /><br />Happy Holidays