Advice on performance

#1
Hey guys new here, been minibiking since about 82. Currently have a mm80 with 15 street tires. 53 rear 14 front straight chain. 212 w big valve 14cc head 308 cam 20 longer rod ,billet flywheel, straight pipe. I have tried 22mm,24mm flat slide and 28mm carbs.Also had 12 tooth front. Won't turn much over 6000 rpm with me on it, fastest 68mph. What am I missing? Shouldn't it pull more rpm?
 
#2
I don't think you are missing anything. You could probably put a 9 or 10 tooth on it and turn a whole lot more RPM, but I doubt that you will get it to go much faster. Just like gearing too tall and loose top speed because it can't get into its peak power RPM. 68 mph is pretty darn fast. You have a healthy built engine. You are running a pretty healthy top speed. What were you expecting ?
 
#3
I don't think you are missing anything. You could probably put a 9 or 10 tooth on it and turn a whole lot more RPM, but I doubt that you will get it to go much faster. Just like gearing too tall and loose top speed because it can't get into its peak power RPM. 68 mph is pretty darn fast. You have a healthy built engine. You are running a pretty healthy top speed. What were you expecting ?
Cool,appreciate the reply. I was thinking it would pull 6500-7000,with the cam and head upgrade. First time I have ran a high compression bigger valve head. May tune the 28mm some and see what happens.
 
#4
Hey guys new here, been minibiking since about 82. Currently have a mm80 with 15 street tires. 53 rear 14 front straight chain. 212 w big valve 14cc head 308 cam 20 longer rod ,billet flywheel, straight pipe. I have tried 22mm,24mm flat slide and 28mm carbs.Also had 12 tooth front. Won't turn much over 6000 rpm with me on it, fastest 68mph. What am I missing? Shouldn't it pull more rpm?
 
#5
What changed when you changed the front sprocket from 14 to 12 as far as max rpm? Also define big valves head, what size are valves, what valve springs, what seat pressure/ install height, and is the head ported. What rocker arms.
 
#6
What changed when you changed the front sprocket from 14 to 12 as far as max rpm? Also define big valves head, what size are valves, what valve springs, what seat pressure/ install height, and is the head ported. What rocker arms.
Actually went from 12 to 14, picked up 3mph lost couple hundred rpm.ec carb 32/28 head dual 32 springs, ec billet retainers. champion 1-1 rockers. Idk installed height just put them on.Did small amount of blending and guide boss shaping nothing crazy.
 
#7
So your engine build should run good and rev higher. When you add a longer duration cam and larger valves you move the torque peak and hp peak higher in the rpm range. Your gearing should be at least 5 to 1. A 24mm carb should work fine,
 
#8
If you lost max rpm but picked up 3 mph switching from 12 to 14 tooth sprocket it sounds like the next move would be to a 15 or 16 tooth. At some point you will over gear it and it won't be able to reach its power peak.
I agree with ole4, on paper your peak power should be pretty high on the
RPM band. But the speedometer and tachometer are the real proof. As stated in my first post, 68 might be all she has. FWIW I have a MB Trailmaster 200 with a mildly modified 196 clone engine ( basically a stage one with a mild 265 cam, 26 lb springs, billet rod and flywheel ) running a torque converter and it tops out at 44 mph. I could go for a little more but would be scared sh#tless at 68.
 

SAS289

Well-Known Member
#9
It's already over geared. Less than 4:1 ratio with a 15 inch tire? In theory the difference between the 12 and 14 should be more than a couple hundred RPM.
 
#10
Thanks for the input guys. May lower the gear a bit and play with it. Kinda chasing mph more than rpm, just figured it would pull harder up top. Can't get it to not leak out the overflow on the 28 and 24 carbs, may have to swap needle. So back with the vm22 which is supposed to be 26mm? Will say with current gear cruises at 55 pretty nicely even up decent grades.
 

GustoGuy

Well-Known Member
#13
Hey guys new here, been minibiking since about 82. Currently have a mm80 with 15 street tires. 53 rear 14 front straight chain. 212 w big valve 14cc head 308 cam 20 longer rod ,billet flywheel, straight pipe. I have tried 22mm,24mm flat slide and 28mm carbs.Also had 12 tooth front. Won't turn much over 6000 rpm with me on it, fastest 68mph. What am I missing? Shouldn't it pull more rpm?
Post a picture of your bike. I have a resto-mod Rupp Roadster 2 with a Suzuki K10 Motorcycle fork with predator 212cc with billet rod and Billet fly wheel with Mukuni VM 22mm and during a 3.8 mile ride according to GPS app max speed 57 mph.
 
#15
The fuel pump may be the reason you are dripping out the overflow on some of your carbs. Mikuni makes a needle and seat for using fuel pumps. It has a smaller seat hole.
 
#17
The fuel pump may be the reason you are dripping out the overflow on some of your carbs. Mikuni makes a needle and seat for using fuel pumps. It has a smaller seat hole.
I kinda wondered about that going to look into it. If I lower the float down it quit but then would run out of fuel. Wish I could get a needle that wasn't spring loaded.
 
#18
68 mph on that, not questioning the speed ! But the one time when I can comfortably state "Yours is bigger, than mine"
Lol I have a newer road glide I ride, highway speed is quite different on the mini. The other day it threw the chain and locked back tire at about 45 skidded to a stop with a truck coming at me.(2lane hwy). It was interesting. Lol
 
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