I have an 85/86 LT250R and I was thinking about raising the compression level since my buddie works at a machine shop so he can re-face my head edges for free. How many thousands can/should I remove from the metal surface where the gasket goes? Is there anything else that needs to be done when raising the compression level/ratio??

PinkFloydEffect, on 17 May 2012 - 07:21 PM, said:

You need to get a measurement of the distance between the piston and the head. You can do this with modeling clay or lead solder taped across (over the pin) the piston. Find the smallest measurement, which should be at the very edges of the cylinder. You will want to cut this down to 1mm or about 0.040" (40 thou). This distance is important. Too much compression is bad in a 2 stroke. Gives mid-range but kills top end. Puts heat into the piston. If you have problems with too much compression, machine the chamber a bit larger at the roof and side radius. Don't make the chamber diameter any larger in overall diameter.

Also you can raise or lower the cylinder for compression or power band movement. Raising the cylinder (with more base gaskets) will raise the powerband rpm, Lowering the cylinder (thinner base gaskets, sealer alone or cutting) will give more low end torque. You must maintain the 1mm head to piston distance when you do this.

  Before you get any machining done, try lowering the cylinder with home made paper (0.006") basegaskets to get that 0.040" piston to head distance. You will be amazed at the power gain, and if you get detonation, you will know you have to machine the chamber bowl larger too.

Take the chamber bowl from the shape on the left to the shape on the right without increasing the overall bowl diameter. (not an LT250R but same idea)



Steve

Edited by sbest, 19 May 2012 - 06:45 AM.

Sounds like it could get a bit complicated, I will toy with the bike and see where I lack power. So the bowl shape inside the head is milled out more to make the head deeper? (by the looks) the two on the right appear to be deep dish style compared to the salad bowl looking head on the left.

If I do not do this on my 250R I am DEFF doing this to my 1984 KDX80 engine that is in a miniature drag racing lawn mower, I do general riding with my LT so maybe changing the head is not a good idea but all I do is go in a straight line up a hill on the tractor so I'm sure there is more performance to be gained (it's geared really high now so I lost a lot of power which I can gain back with compression modifications since I can not fit a larger rear sprocket under the rear of the tractor).

Old Photos:

Edited by PinkFloydEffect, 20 May 2012 - 05:20 AM.

To deal with high gearing you want the motor to make more power at lower rpm.
You do this by lowering the cylinder by leaving out the base gasket and just sealing it with tri-bond or Hylomar or Yamabond or what ever you normally use to seal cases. Extent the exhaust about an inch (if you are still using a tuned pipe) will help with low end power too. She will idle better too. Again, you have to make sure the piston to head distance is more than 1mm or 0.040".

No, don't deepen the head, just sharpen the radius. It is not as complicated as 4 stroke stuff, but you have to have an idea of what you are doing. It is all about numbers and experimentation. You can shave the head 0.020" and it may work better than ever or it may blow up in a week. You have to know what you are doing. The manufacturers put them together with a wide margin for error. You tighten up that magin and you can go fast.

Just add more oil to your premix fuel to get more compression.

Add more oil? Then what change my ratio screw on the carb to compensate for all the oil?

Extend what part of the exhaust? Middle of the expansion chamber? I planned on making a custom straight style expansion pipe that comes out the side. I'm guna have to do some more homework on this squish test before I start experimenting (these motors are hard to find parts for).

PinkFloydEffect, on 21 May 2012 - 04:09 PM, said:

The middle of the expansion chamber will work, or the head pipe from the cylinder. I'd prefer the head pipe for better low rpm.
Cut some thinner basegaskets from paper and try dropping the cylinder until you have the 0.040" piston/head gap.
Run it until hot to see if you have detonation.
If you do, cut the bowl as I outlined.
If you don't, shave the head by the amount you took off the base gasket or keep as is, if you like the lower rpm power.
Simple no risk stuff.

I ride a built 86 lt250r by FTZ. Dont add more oil to raise compression, it will just lead to oil fouling problems. Here is a picture of my head in its last teardown. FTZ milled the head and cut their dome into it. The head isn't milled as much as they wanted, I didnt want to have to worry about running strait 110 all the time, I ususally do a 50:50 premium avgas mix, with 40-50:1 oil mix (redline). Never had any problems with detonation or overhating. You will be able to immediately tell if it is detonating. happened once because I accidentally put bad gas into my tank and immediately turned around and changed my fuel. My bike runs at about 160. its hard to tell and the gasket kinda came apart in the picture but the head is milled to about half to 3/4 of the distance than the stock head is to the little tabs in the water jacket area (i have a stock head and this head to compare).



I also have midrange porting and run a airstryker 38 carb with a 158 main and 38 pilot. after the porting the bike went from a 190-220 main (depending on where I was) to a 158 main. Don't remember how much the pilot changed but it was even more dramatic than the main. The increased vaccume literally sucked the fuel out of the carb faster than the stock porting and made it necessary to drastically lean. Wasnt really expecting that but I guess its common at FTZ. Also had to get a quadzilla pedcock (larger diameter) and larger fuel line because I was draining the carb bowl on hill climbs.

Edited by EDJY, 01 July 2012 - 06:58 AM.

Wow thanks for the post packed with info! Your head scoop is deeper which I would think would lower your compression?

Edited by PinkFloydEffect, 01 July 2012 - 08:58 AM.

PinkFloydEffect, on 01 July 2012 - 08:56 AM, said:

They really cut the head down. Not sure the exact cc of the dome but there when comparing it my spare stock head it looks like the volume is much less. Also the picture is a little deceiving because there the picture is shot strait on without perspective and the carbon build up makes it look more shadowed and deeper than it really is