Basics of Boosting Engine Power

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These Grubee engines are anemic. period. Anyone who denies that has never ridden a 50cc motocross bike which has some good ponies. If you are ready to really make yours a fun machine without destroying the engine then these are the steps I recommend:
1. Increase the compression to no more than 150psi. (It's highly recommended you get a compression tester for around $20.) A 4.5% increase in horsepower results from increasing the compression ratio from 6:1 to 8:1 according to this paper. If you have a 66/80cc engine then do so by removing 1mm or more from the head. You can do it yourself with a piece of thick glass and #60 sandpaper. Press the head mating surface lightly down on the sandpaper on glass and move it about in a circular motion. (do the same to the top of the cylinder to make sure it is truly flat.) If you have a 48cc you can do the same or if you also want more torque and less top speed then take the cylinder down to a machine shop and have them lathe 1.5mm from the base of the cylinder (the surface that mates to the case via the base gasket). This will lower the cylinder ports for better low rpm power (which the 66cc already has). Some people advocate only removing the base gasket but that would only lower it around .5mm. If you don't want lower rpm power with the 48cc then you can lower the head and lower the intake port at the bottom edge by 1.5mm. That brings it into the "sweet zone" for both low and high rpm. The weak link of the engine which higher compression affects is the top connecting rod needle bearing. So order one next chance you get and replace the stock one.

2. Replace the stock CDI/coil with a motorcycle CDI modified to work with these engines. That, in combination with a motorcycle ignition coil, will boost your engine power around 10%. Doesn't sound like much but it is a noticeable difference in power and also makes it run smoother (more consistent combustion) all thru the rev range. Click on my signature link to find out more.

3. Experiment with carburetor jetting so that it runs right all thru the rev range. First play with different hi/low settings of the needle. You can easily change the position of the needle clip so that it rests further down (making jetting leaner) or further up (making it richer). Change its setting for better idling and acceleration from a slow speed. How it runs at top speed is affected by the main jet. With a candle and solder (not the leadless type) and micro drill bits You can experiment with main jet sizes. The main jet is the small brass piece that is surrounded by the plastic float. Just unscrew the main jet and use the shafts of the micro bits to manually determine its current size. Write it down. Then pull out the next size of drill bit to use. If the engine ran
consistently but weak then you need a larger hole size. If it ran OK but combustion was inconsistent (occasionally missing) then you need a smaller hole size. Next heat up the jet with the flame from the candle and melt solder into the center hole. Then use the micro drill bit to drill a new hole size. Try it out and if it's improved then try the next size drill bit to see if it makes it even better. If you want a great replacement carb that only requires selecting one jet for your altitude then get the Dellorto SHA 14mm. It has an idle circuit (which your stock carb don't have) which will greatly improve low rpm power because it atomizes the gasoline properly. More info on it is at my site.

4. Match the ports in your engine. You will need a Dremel type rotary tool with the little cutting discs.
Better yet is some rotary bits such as the #114 high speed cutter. Make sure the transfer ports (left and right) are the same distance from the top of the cylinder. Use the rotary tool to remove metal from the top surface of the port which is lowest to match the other one. Widen the intake port 2mm on each side for better intake breathing. Then use a mini file or sandpaper to round off the sharp edges so that they don't catch the piston rings. A 30 degree angle from vertical is a good angle. Use an exacto knife to trim the base gasket so that none of it sticks out into the flow path of the transfer ports. The metal that extends down into the cases from the cylinder should not obstruct the flow of air/fuel into the bottom of the transfer ports. Cut them to match if they interfere. Same goes for the piston skirt. The intake and exhaust should not have any ridges sticking out into the flow path. Reduce them with the rotary tool. Use an exacto knife to trim the gaskets to match also.

5. If you want more low rpm power (if you're overweight or have hills to climb) then you can create your own intake extension tube which takes advantage of intake resonance to cram 15% more air/fuel into the engine for more power. (more info on my site)

6. If you want more high rpm power you can open up the exhaust port with your rotary tool. First just widen it a couple millimeters on each side. (Maximum allowable port width (straight line) for ring safety is 28mm.) That gives better high rpm power without ruining the low rpm power. The next two suggestions will give you more top power at the expense of low rpm power. Use your rotary to raise the exhaust port. Do so by only 1mm at a time. Don't forget to round off the edges for ring safety. Use your rotary to lower the bottom edge of the intake port .5mm or raise the piston intake skirt by .5mm. It's advantageous to modify the piston skirt instead of the cylinder because they are cheaper to replace in case you went too far removing metal. In the old days the piton port road racers used to have to be push started very fast because the intake port was so low it wouldn't hardly run at all at idle speeds. But they ran like a bat out of **** at high speeds!
 
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@Jaguar: Do you recommend sanding down the head on the Grubee 66cc? As you stated it already has low RPM torque. Any Benefits of increasing the compression? I'm definitely ordering your CDI/coil setup, porting the exhaust wider, modifying intake transfer port angle, and extending the intake manifold.

The goal is to get to top speed quicker as all my friends ride Puch, Garelli etc...They have more top end but not as much bottom end.
I weigh 160lbs. just to give you an idea of what my motor is pushing around.
 
All of his suggestions are right on. More compression is a part of the entire HP making package. It's not about any one mod, it's the combined total that's important. Because mods mainly add percentages not specific HP numbers so you can't say, "it adds X.X HP" because that's not how it works.

Too much compression can be a killer, too. Really not much more than a 9:1 ratio can be used due to the difficulty in getting a clutch to grab and overcome the compression resistance. I have seen 11:1 heads that have a compression release. You hold it open, get the piston moving then release the compression control so it will start. But I think the theory is the best is 9:1, you sure don't find MX cycles with much more than that and if it helped to have more compression you KNOW they'd have it at the max.:helmet:
 
I agree with everything that was said except for a few things. When have these engines ever had good low end ? I've owned 2 of these engines and the only way to get anything that resembled take off was to rev the snot out of them. One way to help offset that was to go with a smaller diameter wheel, and a larger rear sprocket. I'd also tend to think these engines would be a little better if the tolerances were kept a little tighter. The pistons i had in these engines flopped around in the cylinder like a beached carp. They do need better bearings in them yes, also better seals are also a good idea. Is there any ignition units that anyone knows of that are a decent fit without the need to re-engineer the whole engine ?
 
jaguar's set-up is the only one I know of for a MB. For $51 shipped here for a hand built CDI and $17 for a Honda replacement coil I thought it was a pretty good deal. I might put micro switches in where he has jumpers to change the rate of timeing change in two of the ranges, a simple on-off switch would effect the change to give you some external influence on timeing without having to open the box(this is my un-authorized modification :) ) Only other thing I'm doing is siliconeing the holes for the wires. This will also serve to help shock isolate the circuit board from the box.

There does seem to be a lack of choice as far as ignition setups go. This was all I can find but I'm sure there's more out there but I'd guess it would cost more than what jag's got going.

He bought himself a short vacation(1 week) but he'll be back, I talked to him and he said so. If you want to order one of his CDI units go to the link in his siggie. I'm sure he'll be happy to hear from anyone ordering ;)
 
What's the piece of thick glass for while sanding down the head?
I am thinking about only taking a mm or so off.
i only want to take a very little off, but you said 1mm or more.
how much is the 'or more' that others are getting away with?
 
The thick glass is perfectly flat. No ridges or wavy surface to bugger up your sanding job.
The mm thing I have no answer for.
 
1 mm or more = more compression or better bottom end. You may or may not be able to get that much taken off without the piston hitting the head or spark plug.
 
The thick glass is perfectly flat. No ridges or wavy surface to bugger up your sanding job.
The mm thing I have no answer for.

The only reason that I mill my head is not to increase the compression ratio, but to make a good head to gasket surface. In doing so this increases the compression ratio , so much the better. I NEVER put a engine together on a bike out of the box. I haven't found ONE head that didn't need to be milled.
 
Put the glass on the floor. Put the sandpaper on the glass. Put the head on the sandpaper. Put your foot on one corner edge of the sandpaper. Hold the opposite corner down with your hand. With your free hand move the head in a circular motion while pressing down. Taking off 1mm is the minimum necessary. You really need a compression gauge so you can keep reducing the head until you get up to 135psi. It's not easy to do by hand, by the way. It takes perseverance. You should do the same to the cylinder top to make sure it is flat. That way it won't be hard to get a good seal by using high temp silicone sealant.
 
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