1986 NB50 Question

[Jdept]

I just bought a NB50 aka "Melody Deluxe" and was wondering what you all thought about what I should do. Whether I should part it out or fix it. It currently isn't running and may need a complete engine teardown and rebuild without knowing anything at this moment. The PO said he had changed the cylinders a few times already and he thought it may need the same. But I'm thinking this guy didn't know what he was doing and I may blow another cylinder and piston if I don't check the crankshaft, crankcase, etc. Would be really interested in what you all think!

~J

[Mystic]

if he changed the cylinder and piston a few times then it definately has a vacuum leak and or a bad oil pump I would say. I dont really have any ideas about checking the oil mump you should be able to download a manual in the documents section of this site, should be under NB50 Aero

[Mystic]

also hard to tell from the pic but it looks like part of the plastic cooling shroud around the engine head is missing that could cause it to wreck cylinders too by overheating. those plastic pieces effectively channel the air around the head to keep it cool by way of the fan pulling air through the fins

[Jdept]

I'll check out the plastic cooling shroud ...I always wondered about that ...I just thought these engines could handle the heat but they're are designs within that allow for cooling. Also, the oil pump is said to have failed and the PO was just pre-mixing the oil 32:1 using STP 2-stroke. The vacuum leak would be easy enough to check. I'll do that too. I know that some integrated oil pumps are designed to use more oil at higher RPM, do you think the fact that he wasn't using the pump contributed to these blown cylinders? Also, I believe he tried a big bore kit at least once. ...I looked at a cylinder he had that was off and it was blown, the sleeve not encased by fins had two big holes on either side of it. ... would going stock keep this scoot cooler or should I need not to worry about it and just big bore it and allow for better cylinder cooling? They're are a number of things I am thinking are possible to keep the cylinder cool. All thoughts and ideas much appreciated!

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[Jdept]

Looked over the scoot yesterday and it looks like the oil tank may not even be with the scoot? That may be the reason why the PO never tried to use the pump or he removed it and never gave me the tank?

~J

[Mystic]

hmmmmm you dont need the pump or the tank but you have to run the premix, just cut the hoses leading to it short and plug them up securely and plug the vaccum oil intake on the carb. its totally up to you whether you do a big bore or stock. stock oem cylinder and bore are generally more expensive than aftermarket but will allow for more errors than aftermarket or bigger bores. You absolutely have to have the cooling shrouds in place to direct the air through the fins on the head. btw if you want to do away with the pump all together you can order a pump plug for around 20 dollars plus shipping from daywot on ebay located in hawaii. a lot of people dont know that generally cylinders and or pistons have a special coating on them that wears off as the rings wear in so you have to go easy on the bore for about 200 to 400 miles to be on the safe side. at break in you should use some good honda 2 stroke, I use the lucas performance 2 stroke myself cause I dont make it to the honda place all that often, the cheaper the oil the more money you are going to spend on your motor you might as well take the carb off and clean it and the intake manifold and check the reed valves while you are messing with it and check for any damaged gaskets that could cause a vaccum leak

btw take some better pics of the engine from both sides so we can get a better look

[Jdept]

Thanks for all the information. I'll see what I can do this afternoon and get some more photos. I'll see about getting some of the pre-mix, as the stuff given to me was STP ....How about using the rest of the STP stuff ......4 bottles ...to break in the new cylinder ...then use the better lube at higher RPMs for everyday use? Does that sound good?

[Wheelman-111]

Greetings:

Quoth Jdept:

Thanks for all the information. I'll see what I can do this afternoon and get some more photos. I'll see about getting some of the pre-mix, as the stuff given to me was STP ....How about using the rest of the STP stuff ......4 bottles ...to break in the new cylinder ...then use the better lube at higher RPMs for everyday use? Does that sound good?

STP 2-stroke is an unknown quantity. (Although it is Scientifically Tested Petroleum...) Why risk it?

What is known is that the cooling shroud is missing and needs to be reinstalled. Running without is a guaranteed Fail, and soon.

Another known thing is that the top end has "blown". What we don't know is if the P.O. bothered to split the cases and sweep out the shrapnel. If he just reinstalled a new top end, the result is predictable.

Complete disassembly is indicated. Follow the Service Manual for the procedure. It can be done in an evening with a harmonic balancer puller and the Tusk crank installer tool. If it's an AF05E block, don't forget the case halves gasket from Honda.

[Mystic]

I wasnt thinking of that but you are probably right Wheelman, better to err on the safe side than pay the consequences down the road...literally

[Jdept]

Yep ...I don't doubt it. I thought it would need to be split. I seen a previous piston which the sleeved portion not in the fins were blown apart and those are probably in the crankcase as well ....LOL Although I feel the kid was smarter than that I can't be sure.

[Wheelman-111]

Greetings:

One other thing:

If the pump is left in situ, geared to the crankshaft, there is a chance that sooner or later it may seize without the lubricating oil that normally flows through it. Pump socket plugs are available for $15 from scooter vendors like VT or DayWOT, and that is the best option. You can remove the shaft, but doing so adds the few cc of volume it occupies to the crankcase volume. Some say that detracts from performance. Another option is to grind off the shaft teeth and re-insert it with the barbs capped off.

What you shouldn't do is just cap off the barbs and run it. The ruined gear teeth will put more shrapnel in the cases sooner or later.