"John Deere Jog" port timing durations

[jellety]

This i a copy of my last post in the build thread, I've posted it here as well as there might be different people in this area of the forum.

49ccscoot.proboards.com/thread/31906/john-deere-49cc-project-question

I've read that the original minarelli's can hold up to 10.000 rpm, correct me if i'm wrong... I've seen a post of 90GTVert where he ported his 49cc to 190 degree opening, and made some adjustments to the piston for the transfers timing, as they are easy to mess up. On this website martysgarage.info/reference/two-stroke-port-duration/ it tells the same story, about 190 degrees open. I'm aware that not all cylinders are the same, but it seemed multiple people porting minarelli's only had to remove about 2 mm of the roof of the exhaust port to get to that duration. But when I measure it out it shows I have to remove 10mm of the exhaust roof (after matching up port heights with piston with base gaskets), and that seems like an awful lot, there's barely any cylinder liner left for the compression part. I must be doing something really wrong with the degree wheel, who knows? I use the light method, so when the port opens and there's the slightest bit of light I zero the wheel and go over the whole port till it closes, and record that degree.

Degrees original, without base gasket adjustment
Exhaust port duration: 163°, opens at 99° ATDC
Transfer port duration: 111°, opens at 129° ATDC

Degrees with 3 additional base gaskets to match ports with piston BDC
Exhaust port duration: 171°, opens at 95° ATDC
transfer port duration: 124°, open at 119° ATDC





Inside the cylinder:



I don't really understand what the boostport does, as it just seems like another transfer port to me. But when i dropped the piston in, the windows match the port, so is that for the 2 stroke oil to cool the bottom of the cylinder and oil the small end bearing?

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Portmap of this cylinder, Writing on the barrel says, Y-2 Yamaha, 4WX00...


degree wheel:


Dial gauge to find TDC:


4 base gaskets to raise the barrel, for the piston at BDC to clear the complete ports:
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Can anyone point me in the right direction? Or see any mistakes i made?

If this all seems correct, any advise about the right port timing?

After the right port map, I will put the information in 2Stroke wizard, and design and build the right pipe for the setup

[broyager]

I'm no expert, but because no one else has replied I'll say my 2c. Everything below is AFAIK only...

Those timing numbers are pretty mild, it's going from very mild to mild by my reckoning.

They possibly were very mild to begin with to match restrictions (under 8k rpm), possibly for longevity also.

190 and 130 are the basis for a mid race setup, again as far as I know only....

Only problem is multiple gaskets get squashed, you might be tightening the head bolts often or more than normal to get a good seal, and then lowering the cylinder.

Can you make a gasket / spacer out of aluminum etc? Then add a gasket on each side.

Also the original head looks good ie it has a squish band, so you might be able to lose the head gasket, use copper gasket spray instead and get a tighter squish clearance. 0.7mm should raise compression and be safe, you may need to run on 95/98 at that point. Anything under 1mm is pretty good in my opinion. Obviously there's more to it etc, but most of this is 2nd hand info from known working configurations etc.

I have this porting calculator, and when I input your port heights it's about the same as your timing wheel measurements. I did have to add 0.5mm to the exhaust port measurement to match your measurements and you can see the comparison between 32 and 32.5mm and how that affects transfer timing.



That's with a 1mm gasket, and assumes there is 0.7mm from TDC between piston to the cylinder deck or top

I'd double check the measurements for the port map you made.

49ccscoot.proboards.com/thread/176/port-maps-post-em?

Possibly the transfer ports might be getting a bit too high with added base gaskets, so it could be helpful to raise the exhaust port roof 1mm or so.

I'm invested in this because I'll be doing the same mods to a couple of stock jog cylinders soon.

Anyone feel free to bounce me if I'm wrong!