Matching transfers

maniac57

maniac57

Old, Fat, and still faster than you
Gasket matching the case to cylinder helps smooth out flow in your transfer ports and is extremely easy to do. It's also safe since you're not changing port timing so there's no risk of screwing it up (unless you go crazy with the grinder)

Start by tearing down the engine and splitting the cases.

Then use a fresh gasket to scribe your ports. (I use marker on the metal to make the line show easily)


Then use your dremel to remove the metal to fit the scribed line. Blend it down into the case a bit and be sure it's all nice and smooth


That's all there is to it! I also blend the blunt edge of the cylinder casting to a sharp edge which also improves flow.(pic next post)
 
Blunt edges are bad in a port. Sharpen this edge.

Once you're done you can slip the jug onto the case and see how much better the transfer seam looks!



 
Boost porting is a totally different subject and slightly more advanced, it can be done at home but it takes a bit more skill and a steady hand since you'll need to cut it to the same hight as the transfers and it needs to be angled correctly so it doesn't short circuit, but instead, help push out the spent exhaust gases, while keeping the intake charge from escaping too much out the exhaust port. An expansion chamber can fix that problem tho once the engine is running at the rpm range the pipe is tuned for.
 
What is your take on porting the intake side of the piston? I plan on doing the usual porting but I am going to run a short intake and an 18 mm Mikuni carb with either a Slant plug Fred Head 6.occ or Puch 5.9cc 70cc Hi Hi Comp Head I want this to be a Bad Ass! engine and I am kinda in a friendly ( Engine Build) competition with a friend.
 
If you're staying piston port, just cut out the bottom 5mm from the intake side of the piston skirt for better port duration but not so much that it becomes boggy at low or midrange rpm's. Short intakes do work but best at higher rpm, best to tune the length so it'll be strong in the rpm range you plan on riding it most. Mine has 5mm cut from the lower skirt and it easily hits 9k rpm with 225 pounds of me on the bike. Of course, the intake and exhaust ports have been widened 1mm on each side, the exhaust port roof has been raised 1mm, and the piston crown ramped 1mm at the exhaust and transfer openings, transfers matched to the case, casting ridge removed from transfer openings, and all ports chamfered to prevent ring snags... Even with a stock NT carb I'm hitting these RPM's with really nice acceleration to go with it, the 18mm Mikuni will give you more midrange on up punch and more rpm as long as your exhaust is free flowing enough and the rest of the port work is done to match... Proper jetting after break in and setting the squish clearance nice and tight are also part of the recipe for a really fast China Doll engine.

Others will have similar stories but may have done theirs a little differently to fit their own needs, but that's the basic recipe to get ya up to 40mph or so...

The 6cc Fred head is a very nice choice, and the Puch Hi Hi works great too, just costs less and sticks out a bit, make sure you got the frame clearance if going the Puch route.

Now if you decide to go with a reed valve later on you can cut a nice sized hole into the intake side of the piston skirt and run the full duration of the up stroke, but this don't work so well on a piston port.
 
I recently swapped out My Pig motor for a customer test motor, and when the testing was done, I just happened to have a ported 66cc to try out.
Started with a bone stock engine, widened both intake and exhaust about 3-4mm, ramped the piston for better transfer flow and aim, removed the casting ridge in transfers, knife edged the bottom of the jug, and trimmed the intake side of skirt for full duration.
Start

finish

Transfers


exhaust
 
Motor was re-assembled and installed.
Today I finally got out in the freezing cold and fired it up!




She runs like a champ! Pulls harder than any stocker and runs out farther than I'm willing to rev with under 10 min run time!
I'm VERY happy with the throttle response and overall power increase over the usual chinadoll!
 
No port timing changes, just wider intake and exhaust, and ramped piston for better transfer flow with the ridge in transfers removed.
It's a safe and easy way to get a MUCH better running chinadoll....
The key is not to go nuts with the Dremel.
 
I was a bit nervous since it's been decades since I did any porting, but it turned out great!
The engine runs better than the old one with less than 10 minutes run time so far!
INSTANT power increase throughout the band!
And like I said, it's fairly safe as long as you don't change port top or bottom dimensions! Widening does not affect powerband. Changing port timing is what makes them peaky and hard to live with. This is more of a "torque" porting. Think mid 80's enduro two stroke rather than MX racer two stroke.
 
Here's a video.
Still on the first tank and it already pulls hard right past my old top speed with a stock engine.
I shut off because it was trying to speed wobble and because it's still on the first tank of fuel. It was still pulling hard and clean!
Revving a chinadoll this hard is very scary! I think this engine will be plenty fast for me...
and if it's not, the wide port 66cc sitting in a box under my bench is ready to go with a bit more radical port job!
 
That does sound nice, especially for a fresh engine.. mine 4 strokes a bit as it climbs but that's my own doing, keeping it extra rich at least until this tank is finished.

Yeah, I saw that wobble starting, that can get real scary real fast... What type fork are you running with this bike? You could try a different front end to try to match up a better trail for your rake angle, or try some longer forks to add a little more rake. Those ebay tripletree forks are fairly inexpensive and can be bought in just about any length needed, and some of them have clamp on fork tubes so ya just buy one longer than you think you need, then adjust to the length that it rides nice and stable, then cut off the excess that's sticking up past the top clamps. the tripletree type are also really stable by default since they can add a small amount of trail just due to their design.

Nice job on the engine and it does sound and pull pretty good.
 
It's from the additional weight of the front rack out over the axle. Never had a problem with both hands on the bars, but it pendulums one handed and gets hairy about 30.
And it's never been an issue since 29.7 was my old max engine driven speed since I got the speedo.
I'll slow down before I give up the utility of my rack though...
 
Yup... that'll do it too...
Maybe a steering dampener? probably able to score one off ebay for fairly cheap... It won't fully cure the problem but it could raise your max safe speed by a few more mph, it sounds like you got anough engine to do it.
 
Two hands is all it needs. Just don't have the torque to hold it one handed and it goes into a feedback loop. Take my hands off the bars completely and it stops.
 
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