Stuck at 37mph

Feeling a bit down...

The bike is a Puch Magnum MKII. Ran stock at 30mph.

Added a 70cc euro-kit (Amal carb.) Once all the parts were on and made to work, it tops out at 37-40 mph. The bike pulled strong to that speed and just leveled off. The engine is turning some pretty high rpms at this speed. It's a 2-speed and it is shifting properly.

Pulled the 70cc kit because it was loud and vibrated like hell.

Went back to a stock 50cc head, 15mm BING carb with a 14mm Puch intake bored to 15mm. Added some holes to the baffle of the stock exhaust...probably doubled + the flow. Added a low restriction air cleaner. Rejetted the carb from a 60 to a 64. Runs nice, but only hits 35 to 37.

Added the Bi-turbo to the mix...no improvement, just louder.

Is this all there is? The timing seems good..plug looks good... I thought I should see around 45mph at 70cc and 40mph with 50cc.

I've heard these 2-speed Puchs don't produce much speed. Any truth in that?

Re: Stuck at 37mph

XBrandon EdgeX /

I had a two speed puch that didn't do too well top speed-wise, but it has bad main bearings. You could always re-gear it, put a taller sprocket on the front or a shorter one on the back. That'll give you some more top end. 37 mph is actually really good for a 50cc moped.

Re: Stuck at 37mph

I forgot to mention that...I went from a 18 - 40 sprocket set to a 18 - 36 sprocket. How are these others...Tomos and such...hitting 45+ without nuking themselves apart?

I could always diet...

Re: Stuck at 37mph

XBrandon EdgeX /

wow! that's a pretty high gear ratio. If you gear it too high, you'll probably lose speed, because it can't get up into the powerband (rpm range where it produces the most power) before wind and road friction start slowing you down. Maybe you need to adjust your throttle cable or put more air in the tires or something. I'd also go to a lower gear ratio.

Re: Stuck at 37mph

Well, I'm only 4 teeth off from stock. It came 18 - 40 from the factory. I gained 2 to 3 mph going to the 36 tooth gear. The engine is still churnin' some high rpms. It's flat here, so I don't really miss the "hill ablility" any. My feeling is that they changed the ratio inside the transmission on these for that bizzare gear set. You'd think that it wouldn't pull high rpms, but it really is buzzing pretty good. When it was 70cc, I thought the engine would bust apart if it went any faster than 40.

Re: Stuck at 37mph

going close to 40mph is awesome for a moped (esp. 50cc). don't push it too far beyond that, since the brakes may no longer hold under the pressure.

Re: Stuck at 37mph

Hi! What I think I would have done is geared the bike up higher with the 70cc kit.Probably wouldn't vibrate too much if you did that.

Re: Stuck at 37mph

I have a Magnum MKII too. I have messed with just about everythng and can't seem to get it over 35. It would only do about 27 when I got it. It only had 600 miles on it so I think the engine was in good shape. I rejetted about five different jets, pulled the air cleaner, and tried a different intake. It runs the best now with a biturbo, 48 main jet, and a 12mm intake. It still isn't as fast as my Maxi. It does look faster though. the only thing I did to the Maxi is pull off the corrugated restrictor tube. Did you try pulling off the restrictor tube from the inside of your air cleaner? When I did this to mine, It ran worse. I will continue to mess around with it to get a liitle more top end. If I figure something out, I will post back. I am probably going to stay away from the 70cc kit though. Especially now. Good Luck! Joel

Re: Stuck at 37mph

Hi! Listen,Joel.A #48 jet is not usually going to give you much speed and power.My engines have nothing under #52 and up to #56.So increase your jet size and play with the air supply,doing a plug chop to make sure your mixture is not too lean.

Re: Stuck at 37mph

That's so odd how there is such a big difference in main jets for essentially the same bike. The stock jet on mine was a 60. If I were you, I'd get one of those 15mm BING replacements (they sell them as stock replacement carbs almost everywhere) and go with the 14mm tall intake. I once tried a #54 jet by accident and couldn't get out of the driveway.

I guess I need to be satisfied with my results according to all the follow-up messages. I still remember that Swedish kid from high school who's Maxi would do 40 when he drill out his carb...

Re: Stuck at 37mph

50cc motor will pull a small motorcycle to 65mph , you have made the wrong modifications if your only going 37mph .......rog

Re: Stuck at 37mph

I wouldn't say wrong...it was a 27mph bike. It just hit the ceiling of power at that speed. It's like it's being held back by the ignition or something. Don't forget, they did not design the intake ports and heads for speed on a moped. A dirt bike engine has much bigger and less restrictive ports along with high compression.

I tried different air filter /jet combinations today. It still is best with it's stock 60 jet. Running a foam filter with a wide open air box and an 80 jet made it run exactly the same...just very loud. I tried just about everything in between (62,64,66,68) with different filter combos and restrictors. 35-37mph is it...

Re: Stuck at 37mph

XBrandon EdgeX /

So, what you're saying is if a moped doesn't go 65 mph, it wasn't modified right? Do you know why a 50cc engine can push a motorcycle 65 mph? Because it has FIVE GEARS. A moped has one or two, so going at least half of 65 (32.5 mph) with two speeds is a very effective use of power. Also, it's not a case of not being modified right, just not enough :)

Re: Stuck at 37mph

Dead on, X-Brand! Gears are everything!

Re: Stuck at 37mph

your weight also matters. I am only 115 pounds and push 42 MPH out of my stock geared targa with bi-turbo. I pushed an easy 53 out of my friend's highly modded, geared and 70cc targa. However my friend (who is 180 pounds) only finds his moped going 43-46, and mine going a bit over 35.

just something to keep in mind.

-Wisky

Re: Stuck at 37mph

Not only that,Wisky,but think of the wind resistance us big broad-bodies have.Just like havin' a big sail holdin' you back.If I ever figure out how to turn my torso at a 90 degree angle and still be able to use the opposite mirror I know I'll gain 14 mph.Ha!Of course my golf would improve dramatically too!

Re: Stuck at 37mph

Matt Wilson /

Question/Comment:

About the gear sizes and how a 50cc mini bike will go 60mph+ but a moped will top out at 40.

It seems to me that if having 5 gears was all that mattered mopeds would be able to reach 60mph if they had one really big gear. Theorhetically it would take a really long time, and they would have horrible acceleration, but they should be able to reach that speed. However, we all know they dont. If you put a gear on a moped that is too big it just slows the ped down because the engine cant turn it over. So, maybe peds are slower partially due to gears and partially due to a more poorly designed engine and carb. Does this make sense?

Re: Stuck at 37mph

In a way,but remember the `torque curve'principle.A moped can wind out at 40,but AT 40 is not where the torque is at maximum,but rather down around 20-24 mph,or a little higher.Mopeds are generally not set up for max.torque at 30+ mph.Hence,they'll quickly drop off in power when the wind resistance starts to get high.Now,with gears,you control the rpm to stay in your` torquey' area.Make sense?

Re: Stuck at 37mph

Matt Wilson /

Thanks Don, great answer, it makes sense too!

Re: Stuck at 37mph

Well,I'm glad if it helped.There's a lot of other factors that can come into play,but remember,TORQUE=POWER;+GEARS=SPEED.

Re: Stuck at 37mph

I had my fun with the 70cc conversion on 1983 Magnum II. What is the trick to the needle adjustments on the Amal and what does it do exactly? The only problem I have is that the ped sometimes bogs at top speed because of too much airflow into the carb. If I put my hand in front of the air intake side of the carb I get ~5 more mph out if it. So, before I re-jet....what's this needle adjustment and would it remedy this?

I am also running the 18-36 setup. I increased to ~40mph with my 200lb frame on it with just the 70cc kit. Then I took the back wheel apart during the sprocket change, lubed the bearings, filled the tires up to higher pressure and put a small fairing on the front. I get ~53mph out of it. (Had a car follow me so take it for what it's worth) I think the needle adjustment might get me that extra 5mph it still has in it.

Nick

Re: Stuck at 37mph

Ron Brown /

Nick,

At full throttle, only a larger main jet will help. the needle is only significant at 1/4 to 3/4 throttle.

Ron

Re: Stuck at 37mph

Geesh...53!

I thought these kits came pre-metered, but I guess not. After the GA fun run, I'll have to try it again!

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