Grubee engine; made for speed (NOT!)

jaguar

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Nov 25, 2010
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Here's the 8 reasons the engine is weak;

1. carb has no true idle jet which causes loss of power at all rpm's
2. head compression is too low
3. intake and exhaust ports are too narrow, and intake is too high
4. transfers are too narrow so that attempts at turning the engine into a high rpm screamer fail
5. crankcase compression ratio on the 48cc is only a pitiful 1.1 which provides only 1.5psi when the pistons uncover the transfer ports. Such a low pressure greatly limits power. 3psi is the bare minimum needed.
6. CDI does not retard the ignition timing at high rpm's which limits power and creates too much piston/cylinder heat. It also produces a very weak spark.
7. piston/piston-pin weight is not correct to match the counterbalancing of the flywheels which causes upper rpm engine vibration which limits power.
8. The exhaust is just a straight pipe with muffler. The lack of a true expansion chamber limits both low rpm grunt and high rpm speed.
 


oldnube

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Sep 25, 2011
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Whats A Person To Do?

What steps can a person take to correct this or how can we get the most from these HT engines safely without causeing damage to them?
 

motorpsycho

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Aug 5, 2009
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What steps can a person take to correct this or how can we get the most from these HT engines safely without causeing damage to them?
ummm, junk the h.t. engine and use a chainsaw engine?
Really, these h.t. engines were not designed to be high performance screamers.
they were built to do a very simple task, and that's to get a bike to move.
if you want true high performance, a h.t. engine would have to be totally re-designed.
if you want to improve the performance of the h.t. engine, it can be done, but don't expect to get a large horsepower increase simply because of all the things that jaguar mentioned.
if you really want a powerful, high revving rpm monster, a 50-80 c.c chansaw engine with reed valves, and an expansion chamber would be the way to go. but then again, with this much power you would be breaking parts on the bike left & right and you would need disk brakes to stop the thing.
 

jaguar

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Nov 25, 2010
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Please click onto my signature link to read about how to correct these problems.
You can greatly improve the performance with my suggested changes. You won't make a 48cc non-reed-valve engine truly HOT but you will be surprised at how much more power you can get from it.
Best thing I ever did is put the 58cc top end on it and custom fit a reed valve to it. People who try it say it's more powerful than any moped of the same engine size. It's still one speed with a 44 tooth sprocket and I have to put the brakes on when I arrive at my house although it is on a steep hill. I have more power now than I thought I would ever have.
 

Cavi Mike

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Jun 5, 2010
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It's a $150 turn-key kit. Did anyone really expect some marvel of engineering here? You could buy 2 of these kits for less than one Robin, engine only.
 

jaguar

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Nov 25, 2010
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It's not a question of engineering or expense (somewhat). They designed it for low power so it would pass regulations in all countries. Now it's our job to make a real engine out of it.
 

HeadSmess

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May 17, 2010
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no se puede pulir la mierda


when the HT was first designed i dont think there were any regulations in any countries, including the state of california.


designed to a budget, built to one.

can get one to scream, and then to blow up dismally.

can leave one the $#^% right alone, it blows up dismally...

they blow up.

no modifications you make can fix the basic design flaws, starting from choice of materials.

(they are still fun though)
 

jaguar

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Nov 25, 2010
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not a jet engine, just something that works like a 2 stroke should.

"no modifications you make can fix the basic design flaws"
I disagree. By changing the flawed parts then it is fixed.
 

geebt48cc

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Feb 2, 2010
Messages
683
Jag, I really like they way you think. Jag, do you mean where the intake is located on jug, or the rake of height of intake to carb? Is the straight intake from jug to carb the best?

Glen
 
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