QT50 - MJ50 question

Jimmy Cincinnati /

So i swapped a MJ50 motor onto my QT50 and a thought occured. I used the MJ50 tranny but the QT50 CDI assembly (to make it more plug and play). It runs and idles but lacks any power. Is the timing different between the motors or would the CDI boxes have a different curve? FYI the motor was rebuilt with new oil seals, gaskets, and new top end. Oil injection was deleted. It currently has a stock carb, airbox, and exhaust but a 60cc bore kit. The carb was rejetted from a stock 72 to a 78 main. I used the MJ50 carb as the needle tapers were different.

last question, ive always been under the impression that the arrow on the piston should point down towards the exhaust port. The stock piston in the MJ50 had the arrow pointing up (towards intake) but the motor looks like it might have been tinkered with at one point. Anyone have advice on that?

Re: QT50 - MJ50 question

William Johnson /

is the piston arrow still pointing up? If so, that's your problem.

Re: QT50 - MJ50 question

The arrow always goes in the direction of flow .

Re: QT50 - MJ50 question

Jimmy Cincinnati /

The arrow always goes in the direction of flow .

Thats what i thought. Im running a completely different kit now but the piston is pointed down towards the exhaust port. I guess someone threw the stock piston in the motor upside down.

My next thought is drop the jetting down to like a 74 main (only 2 sizes up to make up for the extra 10cc). I was just wondering if there could be differences in the factory time / curve between the QT50 & the MJ50. I might swap back to the OG MJ50 stator & CDI box if the jetting yeilds no results.

Re: QT50 - MJ50 question

I don't know if the ignitions have a different curve, but I ran a qt50 with a mj50 ignition and it was fine.

Mj50 ignition has a stator, cdi box, and the selector box (for start and run selection). I never found any documentation for it, but I figured out how to remove the key switch and use a 3-way switch instead. My blinker switch: left == start, right == run, center == wizard magic neither. (edited)

Re: QT50 - MJ50 question

Jack Rutherford /

Same part number for CDI on both mj50 and qt50, at least for 1982 -

3L4-85540-M1-00

stock jet on a Towny was 72.5 - at least I'm pretty sure that's what was in each of the three that I've had. 78 could be too big - check the clip on your needle as well. 60cc kit really didn't do much for my Towny. I think 37. My 50cc Towny matched that and may have just beat it. Exhaust is probably limiting factor. (edited)

Re: QT50 - MJ50 question

Jimmy Cincinnati /

Thanks for the info on the part #'s. Sounds like i can rule timing off my list of issues.

stock jet on a Towny was 72.5 - at least I'm pretty sure that's what was in each of the three that I've had. 78 could be too big

So on a 74 main jet last night it ran better but not great. Pulling the air cleaner off seemed to help it alot. I confirmed there is zero air leaks. The stock towny cylinder i have is scored so i would prefer to not resort to stock. I just havent been able to get this thing to run good enough to hold a decent speed to plug chop. Whats good head temps? I have a trail tech, i might put that on it.

Re: QT50 - MJ50 question

Jack Rutherford /

Further research reveals that 70 was also a stock main jet on a towny. Maybe one of my three had a 70 in it. I can't remember.

Two of my Townys were white. I replaced both crank seals on the first one but it was always running around 370F-380F. My 2nd one - I replaced just the left crank seal - I could rarely get that one above 340F. A buddy sent me a Towny engine to put a Malossi top end on with a vm20 carburetor. That one ran in the 340's. Must be that 2nd gear taking the strain off the engine.

That last one was a dream Towny. It did 43-44 with stock exhaust and I could cruise through the neighborhood with the throttle barely open and it's doing 35 in super stealth mode. You might of thought it was an electric scooter or something. I didn't have it long enough to play with the jetting more but maybe it could've eeked out 45.

I've got just one Towny now with 60cc kit, vm18 and stock exhaust. I may try to replicate the Dream Towny set up with a little spacing under the cylinder to raise all the ports.

Re: QT50 - MJ50 question

Jimmy Cincinnati /

Further research reveals that 70 was also a stock main jet on a towny. Maybe one of my three had a 70 in it. I can't remember.

My QT50 had a 70 main while the towny i picked up had a 72.5. The towny had a fatter needle and it appears the choke port is bigger on the towny as well?? Bore looks to be the same (both 12mm's). Both run the same airbox.

Two of my Townys were white. I replaced both crank seals on the first one but it was always running around 370F-380F. My 2nd one - I replaced just the left crank seal - I could rarely get that one above 340F. A buddy sent me a Towny engine to put a Malossi top end on with a vm20 carburetor. That one ran in the 340's. Must be that 2nd gear taking the strain off the engine.

So 340-380 range is what i should probably expect. I replaced all the oil seals on that motor, so its got good cranky seals!

It did 43-44 with stock exhaust and I could cruise through the neighborhood with the throttle barely open and it's doing 35 in super stealth mode.

I would be happy with 35 (40 would be a bonus). The qt50 engine was hitting 29-30 with a MLM better than stock pipe (the one you sent me), stock carb, stock oil injection, and 60cc chinese kit. I know the MJ50 has a higher end gear ratio and ive sinse blocked off the oil injection. I found out yesterday however that the MLM exhaust does not mount up to the MJ50 tranny as the bigger tranny sticks out to far. I need to either mod it fit (will require hacking) or run a stock pipe. Im debating the stock pipe however the one i have has a series of dents in the header that Im sure are hurting performance to a degree. Im either going to make a new header or looking for a nicer stock towny pipe.

Also, a 15/15 Dellorto SHA fits that intake like a dream. I might just ditch the stock carb and airbox.

I may try to replicate the Dream Towny set up with a little spacing under the cylinder to raise all the ports.

https://pwonly.com/pw50-performance/?gclid=Cj0KCQjww47nBRDlARIsAEJ34bmWN_b-3y14WeiaODJWX9wJTE7tebnFS8Vas--g-wWM2DnUqXPD1lwaAjtwEALw_wcB

They offer a "power plate" spacer if you dont want to stack 5 gaskets together. Kind of pricey but the quality looks good.

Re: QT50 - MJ50 question

Jack Rutherford /

Lani at coppergasket63 will make them almost any size for you and may give you a better deal if you buy direct from him instead of through ebay. He's made me anywhere from 0.5mm to 3mm base gaskets. I think there's a limit on copper thickness but he can do thicker ones in aluminum.

I sent him an nc50 base gasket and he now makes them for nc50 which also means nu50, pa50 and nx50 are covered.

yeah pwonly is so outrageously overpriced on that rev plate and other things. I couldn't find it but they also charge big dollar for cylinder milling. I've got a belt sander now with a wide enough belt that I think I can do it myself and finish it off by hand with a piece of glass. (edited)

Re: QT50 - MJ50 question

Jack Rutherford /

More Towny info:

Fowler got an mlm people's pipe on the towny. I'm not sure how he modified it.

The A35 Tomos Biturbo will mount up pretty easily. Just a little Dremel work on the flange bolt holes. I got it to hit with stock cylinder and do low 40s.

When I sent the Malossi Towny engine back to its owner, I told him to try a biturbo. He did and couldn't get it to hit and noticed no difference in top speed over the stock exhaust. He must have had it over-jetted. Hack n welding is a problem with stock header as Yamaha made them so damn long unless you want great low end.

Orange qt50 clutch springs will make it shift at 23-24 mph instead of 18 or so. I used these with the 60cc kit. Don't know if stock engine could get them to shift or you might be able to throttle shift it. You would probably need the Malossi kit to get the black clutch springs to engage. It would be interesting to see where shift point would be with them. Maybe 30mph? (edited)

Re: QT50 - MJ50 question

Jimmy Cincinnati /

The A35 Tomos Biturbo will mount up pretty easily. Just a little Dremel work on the flange bolt holes. I got it to hit with stock cylinder and do low 40s.

Man this is good to know. I really dont want a biturbo on it but i do have one laying around so if i have no luck with my other pipes i can atleast slap a biturbo on it for awhile.

unless you want great low end.

a ripper that hits 35mph sounds nice to me! Im gonna start mocking up homemade header tonight. Im trying me hand at making a "better than stock" mj50 pipe.

Orange qt50 clutch springs will make it shift at 23-24 mph instead of 18 or so.

It did seem like mine was shifting around 18 (again it isnt running right currently) but that orange spring mod might be something i look at doing once i get the carb / exhaust hammered out.

You are a wealth of information on these!

Re: QT50 - MJ50 question

Jack Rutherford /

I like me some Towny. I'm dying to ride/work on mine now!

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