The 139RCV

[motorhead]

Three years ago I started a post on the untapped potential of the 139QMB. CHF50 vs 139QMB. Since that post years ago I learned a lot, acquired the tools and I am ready to return to unleash that 49cc's potential. The goal of this build is to take a stock 49cc Chinese scooter and get it to hit 48-50mph on the 39mm bore! I've dubbed this project the "139RCV" named after the rc214v honda race bike. With that said... Warning, do not apply these mods to your scooter if yours is a daily driver that you need for school or work. Once its built I estimate this motor to last 500 miles tops.

[motorhead]

$200 on a P.O.S 49cc scooter doesn't look like it used to. Before they were in demand I could get one like this $100 all day, three years ago. For two Benjamins I got a descent motor, dead battery, broken seat bucket, broken speed sensor and sun damaged fairings. Anyhow, here it is. Didn't buy it to ride, so the aesthetics don't matter to me all that much.

In the baseline test after airing up the tires I got it on smartphone GPS at 32Mph with Stock jetting and airbox. Pretty good considering I am mainly used to 4T 49cc that top out at 25Mph. So far I am 18Mph away from the goal.

I have a long checklist of mods and the plan is to start with the least invasive mods as possible before tearing into the motor. My first mod was to streamline the butterfly and make that airway as smooth as possible, giving the butterfly leading edge a razor sharp front end and filing down the butterfly screws. I port matched the intake side and did some smoothing. I added a free flow filter and a 85 jet I had on hand. All those mods for grand total gain of 1Mph on the GPS. at 33 it sounded like it was still trying to accelerate but I ran out road. It may be a small gain but its a step in the right direction.

Tried playing with the rollers but going lighter made it sound like a dirtbike stuck in 2nd gear. So far stock weights at 5 grams seem the way to go.

[dude]

Sounds like a lot of trial and error. Mostly a lot of fun. What is your height and weight? I've seen some drag scoots with no 2nd ring and no oil rings. Are you going to a performance cam?

[motorhead]

Not quite sure what I will do about the rings but I am not quite going to be running a performance cam. I rather convert a gy6 cam like oldsman did on this project. I'm not taking any conventional approaches. Also, from my days running a scooter repair shop I learned that not all "performance" parts are what they claim. I saw many racing cams that were just stock cams with a fancy sprocket pressed on. I'll stick with the GY6 cam bc it has measurable difference in duration and lift. Working on it right now as a matter of fact, gotta finish it and take pics before going back to work for the week.

[motorhead]

Using a bad pickup sensor as a reference, I was able to modify this fully operational sensor to adjust the timing. Using Brent's video as a reference I cut into the bracket and to get the sensor even further advanced, I rounded the bolt and cut a slot to tighten it with a Flathead. Using Brent's advice I use a credit card to gauge the distance between the sensor and flywheel magnet. I've advanced the timing to the maximum without cutting into the case or the sensor. Once again, do not put this mod on a street scooter. The intake valve may only last 100miles at this timing setting.

On my speed run I was able to squeeze an other 5mph out of the scooter with the advances timing. This brings it up to 38Mph leaving the project 12Mph away from the goal.



I also finished the GY6 cam mod. I changed up some of the techniques from oldsman's post. I used a spare 49cc cam as reference and for parts. Lacking a propper lathe did not stop me however. While the top bearing will undoubtedly fail from the fit, it will work long enough to give me the performance I need from it. I didn't think it was worth the $40 the local machine shop wanted to lathe the step so I ground away till the bearing fit. Next weekend I'll be tearing into the engine to install this cam and two other top end mods.

[90GTVert]

Did you check the timing by any chance? Hearing any spark knock with that advance? I'm guessing you should be a few degrees more than what I got from a more conservative mod, depending on the CDI and flywheel.

[motorhead]

Didn't run it long enough to develop any ping or detonation that I could notice. I don't have a timing light but my hypothesis was that timing is heavily retarded on these because at my shop I often replaced heads with burned exhaust valves, never once had a burned intake valve in 2 years taking apart QMBs.

[tvnacman]

so how many degree advance do you think your getting ?

John

[motorhead]

I don't have a degree wheel but it seems to advanced 5 to 6 linear mms. Maybe 5 degrees advanced I'd say.

[motorhead]

Finally, the weekend is here and I can get back to my project. During the week I ordered a replacement fairing and muffler from ebay. The front end of this scooter is hideous with the broken fairing and exposed harness. This scoot's gonna look a lot better when I hit the road. Also, picked up a newer stock exhaust. A lot of you will ask, why a stock ehxaust instead of a performance? First the exhaust the bike came with is makes very loud put put noises, that had to go. Second, I am of the opinion that a performance exhaust is adding more noise to your bike than it is adding power. Furthermore without porting the exhaust passage in the head, that bottle neck will nix any gain from the pipe. So... I went stock. Gotta get my hands on some sheet metal and make myself a heat shield.



In terms of performance, I have my GY6 cam adapted and ready. I made myself a lightened slipper piston. And I am also going to run titanium valve springs. Gonna get to work to install all my parts and get the engine bolted back together. Its raining now so maybe tonight or tomorrow I'll get on and post speed results and a display the bike with its new fairing.



Like we discussed in the chf50 vs 139qmb thread, the quality control on scooter parts is horrid. Before modding this brand new piston i slid it into the bore and found a good deal of play side to side on the piston. On the axis with the valves the piston is true but had slop side to side perpendicular to the valves. If it doesn't work well when running, I will mod the piston (I saved) that came from the bike. And at least I was able to get some practice on this piston. After I run it for an hour or so I can't wait to see how it holds up with the reduced skirt. Also, you'd be amazed at how little weight came off this piston after all this lightening. Out the box the piston weighed 39 to 40 grams. Now it weighs 35 to 36.

[motorhead]

So... Got it all together went out for a run. At idle i could hear piston slapping but I went on the run. Got up to a whopping 28Mph then lost speed back to 25~26. Now it wont fire up. Back to the drawing board. I may be able to pin the reduced performance to the ring not seating from the piston rocking. Yes, I said ring. Assembled it with the top compression ring and the oil scrappers. Don't think I'm going to touch it again this weekend but I'll be going back to stock piston and all rings but I will leave all the other mods in place.

[motorhead]

I got some spare time and I went and broke down the top end. Apparently the piston had enough of a lateral gap that crank case oil entered the cylinder and fouled out the spark plug. Looks like I will be going to back to the original piston. Sometimes failures are great sources of data. Looks like the shortened slipper aspect of the piston worked well. Great news for anyone who wanted to run a highly modified piston in a GY6 variant! Next week I will convert the original well fitting piston to a slipper, give it some new rings and try this again.

[motorhead]

Here we go. Last week I had to ditch my 1st slipper piston because of a poor fit in the cylinder bore. I also Had to put my modified 139QMB camshaft asside while I repaired a variable I hadn't accounted for. So this week I took the original piston from the bike, lightened it (not shown) and fitted it with a fresh (and secret) combo of rings. Used a trick I learned from my motorcycle work to raise the compression slightly. By running the new piston and titanium valves I got it up to 37Mph. I took it back home and adjusted the timing until found a sweet spot the bike was happy with. Took it out for a run and got it up to a BBK matching 45Mph! While the acceleration was not close to a BBK the top speed was there. I am now 3 to 5Mph away from the goal.

The last 5Mph. Will very tough to squeeze out. I only have 1 more top end mod to run and it is not guaranteed to increase performance. I am still on the fence about trying to port the exhaust. Unlike most other vehicles its not a simple task to port match the exhaust on Chinese 4T. At this stage of the tune another engine mod could hut not help. The remaining mods involve lightening and modifying various rotating parts. And if nothing else I could always go up a few jet sizes to 90. I'm pretty confident I can hit the bottom end of the goal. 48Mph is in sight.



Got the new fairing and muffler on the scooter. Looks and sounds so much better. Gonna pick up some spray paint when I go groccery shopping and maybe tomorrow I'll repaint the front and the spoiler. You can also see the fan mod, this motor is going to run far hotter than the stock shroud was designed to cool. Also going to mod the fan itself to deliver some more air. Furthermore, looking into oil coolers to help keep the bike from seizing on me. Still looking for some sheet metal to make a heat shield from. So far I am very happy with how this project is turning out.

[90GTVert]

Where do you get titanium valves?

[motorhead]

Jun 7, 2014 18:37:01 GMT -5 90GTVert said:

Where do you get titanium valves?

Oops, I meant the NCY titanium electroplated valves. Not running 100% titanium.