HT motor modifications

bigtrucker2007

New Member
Local time
8:30 PM
Joined
Aug 17, 2010
Messages
27
Location
pa
I and looking to modify my 66cc HT motor. I was hoping someone could help me on how to do some port mods and what are good upgrades. thanks
 
if your going to port and polish the intake and exast your going to need to buy the cylinder /crank case gasket to replace when you put the top end back toether , its in best intrest to buy the race carb becuse the porting is gona suck alot of fuel and needs alot of air to mix it. if you go with the cn carb get a few bigger jets. as far as the actual porting and polishing goes once you have the cylinder off the crank case and the intake manifold and exaust is off you will notice there is a lip on the inside of the port on the cylinder wall. I use a series of small rattail and flat files to get rid of this lip then a series of emery cloth to start the polising of the port itself. get a fair amount of emery cloth the aluminum is realy "sticky" it gets it the grit of the papper very easly. I then finish up with some brilow pad the trick is to make the ports as smooth as posble and try to make it look like chrome. also get a slant head for it and polish it as close to a chrome finish as possible. I use a wire wheel then work down in emery and brilow when working with the head If you havent done this before try to find a vid on youtube or something to get a good visual. GOOD LUCK!!
 
Last edited:
illl try to get you some pics of the process when time premits. I have that stupid job thing I have to do its a real buzz kill
 
Do not make the intake ports mirror smooth, a "brushed" finish will actually run better.
The little bit of surface irregularity helps create turbulence which aids in atomizing the fuel in the intake charge. The exhaust port and cylinder head both could benefit from a mirror polish.
When working on the cylinder, be careful to only cut the ports when drawing the file or sand paper away from the bore. If you cut while going towards the bore you risk flaking the Nikasil plating off around the port edges.

Even after doing all this work, you will not notice a tremendous improvement, but there will be one none the less. Notching the piston about 1/4 inch on the intake side and just cleaning up the ports a little bit seems to do more.
 
I dont mean to highjack this thread. But i followed another thread on porting the so called "80cc" on this forum. I matched the ports along with the gaskets so that intake and exhaust manifolds flow freely, I took about 2mm of the top of the exhaust port so that its 1.1" form the top of the port to the top of the cylinder and i opened it up laterally from 3/4" to 15/16". I was told the exhaust port can be as wide as up to 70% the diameter of the piston . I lowered the intake port by 1/4". But i think i messed up when i also notched the bottom of the piston skirt 1/4" on the intake side.

I lost alottt of midrange torque yet the bike revs like crazy. You also see alot of fuel coming out of the carb through the intake when you free rev it.

Should i order a new piston? or a new motor?



Do not make the intake ports mirror smooth, a "brushed" finish will actually run better.
The little bit of surface irregularity helps create turbulence which aids in atomizing the fuel in the intake charge. The exhaust port and cylinder head both could benefit from a mirror polish.
When working on the cylinder, be careful to only cut the ports when drawing the file or sand paper away from the bore. If you cut while going towards the bore you risk flaking the Nikasil plating off around the port edges.

Even after doing all this work, you will not notice a tremendous improvement, but there will be one none the less. Notching the piston about 1/4 inch on the intake side and just cleaning up the ports a little bit seems to do more.
 
Porting increases the power in a certain rpm range, but narrows the whole power band.
You could try a new piston with a stock length skirt. That will open the intake port later and close it sooner. Maby it will help, maby not?.

Correctly porting a 2-stroke requires a bit of science, geometry knowledge, and careful planning.
It is not "cut and dry" like making a sandwich.
 
yes i know its not cut and dry, i didnt mean to undermine the science of two strokes. I've seen entire books dedicated to the optimization of these engines.

What is cut and dry is that based on the spec i typed earlier people get indredible power gains on these engines.

I just need to know if you increase an intake port by lowering it 1/4" if your supposed to also cut the skirt off the piston.

I think its either cutting the port lower or cutting off the skirt because obviusly the combination of the two yiled really bad results.

Thanks
 
If it was me, I would start by getting the thing breathing good first. dump the stock exhaust,air filter,and intake manifold. I went with a SBP EXP exhaust, and one of their "performance" air filters, and a Manic Mechanic machined intake, take time to match the ports clean up the exhaust manifold of welding splatter. You got to go there anyway, might as well be sooner than later,you'll be surprised just how much that little helps without cracking the case. Then go from there.
Cheers
 
Cool man thanks,

The bike ran good before i was just looking for more power, I did match the ports. And last week i ordered the billet machined intake from justin at pirate cycles, but they are on back order. So I am waiting on it.

The notching of of the piston plus the lowering of the intake port must have prolonged the intake cycle way too far.
 
expansion pipe, air cleaner, plug and wire, and slant head if you are running a regular head and good enough mods for most...the single best mod is a shift kit, really lets you keep the engine in the range. And the roller clutch bracket is one of the best mods I have on my bikes!
 
Back
Top