I ordered a big bore kit,gaskets,seals and new reed valves for my '78 NC50 Express. A good thing too. While I wait for the parts to show up, I pulled down the engine ( off bike). First thing I noticed was the piston skirt and rings were gouged and scuffed at the exhaust port side ( 6 o'clock). I'm the third owner of the bike. The original owner drove it for 500 miles and wrecked it bending the forks a little and breaking the throttle and brake lever. They gave the bike to my neighbor who parked it under a tree 25 years ago. He gave it to me telling me he tried to start it when it was given to him and it would not run but for a second or so. Dry cylinder. not good. Sitting for all those years, I pulled the spark plug and lubed the cylinder. After 24 hour soak,the engine turned freely with no plug. I got it running within a couple days. Card was shot. Insides were corroded. I put on a new Delerto SHA 15.15 carb. Bike ran very well, four stroked a little at first,gutless 25 MPH max out. Compression was only 80 PSI. I could not remove the bad right seal that was causing the 4-stroking. I had to split the engine crankcase in half. No a big deal if you know how to do it. The seal had shed shards into the crankcase. Bearing are great. Crank has very little play in it. Neddle bearing s are like new. Bike only had 533 miles on the clock when I first got it. Jump forward to this week. The parts should be here soon. A little more compression, torque perhaps and a couple of MPH faster, I hope. I'll keep you posted on the work.
Great!,,,post pics of the build I want to see how it comes out,,,,every once in a while one of those comes up on CL,,,seems here in California anything with the honda name on it the seller wants an arm and a leg no matter the condition ,,,been kinda looking for an s90 ,,,way into the thousands even for beaters that need everything
I did the conversion yesterday. I did not take photo's as there are just hundreds of them on the web and Youtube being done. What I did notice is that no one ever talks about is engine break in. I'm not a 2-stroke expert by any means but I have rebuilt many 4-stroke engines. So what I found was: The new cylinder head and piston ( DR Racing) came with no markings what-so-ever as to how the piston was to be placed in the cylinder. No instructions other than a flier in Italian! Not much help. You had a 50/50 chance of putting the piston in upside down! Not good. I had to email TreatlandTV and ask how to install the piston. The answer was cryptic to say the least. Not much help. I did my best guess and must have done it right. All the gaskets and seals were replaced. I did have to separate the engine half's to replace the seals that were dry rot.( There is a well done Youtube Vid of a NU50 being converted that is over an hour long. Lots off good ideas to try.) The cylinder does not fit in between the Honda Express foot peg brackets. You have to cut off 1/4" on both side of the top two cooling fins of the cylinder. You also have to cut 75% of one cooling fin off in order to get to the Oil Fill Plug for the transmission. One of the online service manuals show you can fill the transmission with the bike on its right side and the left transmission cover off. Good luck with that. The vent and rear bearing ( no pressure on the drive shaft from the left transmission cover while off) will leak oil like a sieve.I wont try that again. To brake in the new rings and cylinder I purposely ran a rich pre-mix fuel/oil mix. Lots of smoke but it did not foul the plug at all. I followed the instructions from a dirt bike site on how to break in the new parts. I asked around and found that on my 1978 NC50 the oil pump is set to a ratio of 25/1. Far richer than needed for the conversion kit larger bore. I ran the engine today for a few miles varying the RPMs and staying off max throttle. The jet I used was a #69 in my Delorto SHA 15.15 and it was way too rich for my 7,000 ASL altitude. The engine bogged down badly. I put in a #61 and the engine really came to life. Very smooth idle and good throttle response from the stand still. I re-torques the head bolts when thee engine was cool again and double checked the spark plug. All are A-O.K I took the bike out again and did a quick burst of speed with my stock muffle still on the engine. The bike did 35 MPH. I did not hold this speed.I want to break in the engine a little at a time to increase life of the parts. I was given two 2-stroke muffles and may try to make one of them work on my bike. I would need to make the front pipe connecting to the cylinder block which is going to take some doing.There is a flange plate that pushes the pipe up against the copper doughnut that is recessed in the head rather than bolt up to a flat flange as there is on the CHI-COM bikes. I'd love to find a used Honda NC50 muffler and do the "guts conversion" but even the used mufflers are expensive. The tuned pipes you see are nice and the sellers charge a lot for them. Yet you can get a really nice "tuned exhaust" from China for 50 bucks for newer bikes and scooters. Oh well. LED brake light ( running light on) [img src="s1.postimg.org/5h9hmmkaun/BB8.jpg" src="" alt=" "]
Last Edit: Oct 18, 2017 14:35:18 GMT -6 by rapidrob
I'll try again. Front view. Walmart basket ( 3 bucks) to hold my mail.
cylinder cooling fins shaved to fit between foot pegs.1/4" had to be removed on each side. Treatland Delerto adapter. Delorto SHA 15.15 carb Head on view-Head light bulb is replaceable Rear view Speed-o-meter with LED back light Tail light is also LED. All battery powerr goes to headlight which is good. It does not go bright/dim via engine speed.
Last Edit: Oct 18, 2017 17:14:37 GMT -6 by rapidrob
After reading many post from all over the web, I'm torn as to change out my exhaust system. The fellow who posted the Modification of the original muffler vid has since recanted. He says it does not help performance and makes the bike really loud. I've also seen post where some of the guys who installed the "side bleed" type tuned exhaust have also removed them for poor low end performance and excessive noise. I'm on the fence as to do anything to my bike's exhaust?
If the original is good / solid run it,,, the only thing that’s going to change the engine characteristics is an expansion chamber,,, even with a stinger silencer it’s going to be loud,,, then you’ll have to play with the carb again
I drove down to my mail box and let the bike idle while I got the mail. When I pulled away from the mail box the bike acted like the plug was fouled a little. It stumbled for the first 100 feet or so and then it will catch and take off. Is this a jet problem or is something else going on. The engine had been run for about four minutes when this happened. The spark plug has a light coating of black carbon on the ceramic rather than a brown coating. I'm running a 68 jet (Delerto). Straight fuel and the bikes automatic oilier. Any ideas?