87 SE50S engine compression

[buchholz56]

I have a couple of questions about my two 87 SE50S scoots, first is what is the engine pressure suposed to be. Both run/start/idle fine, is was just curious the other day as to why one is 5mph faster than the other. The faster one has 90psi and the slower on is 83psi. I am using a snap-on gage and am sure it is accurate as my chainsaw had 155psi. I am running larger tires (90/90-10) and am using GPS to verify speed. After about 1 mile at WOT one will hit 46mph and the other struggles to hit 40mph. the only difference between the two is that one is a true 50S with the factory 12 tooth drive shaft (slow one) and the other is a 50P with the factory 13 tooth drive shaft (fast one) and a 50S muffler. I have a hard time believing that one tooth difference would make that big of a difference, and am wondering if one or both scoots need to have the top end done, both have over 6000 miles and have never been done.

thanks.

[PimpinSpree]

compression sounds low but that top speed is not. 40mph is still really fast unless youve done some aftermarket stuff. 46 must have aftermarket parts

[teeoce]

What mods have you done?

[buchholz56]

I have very lightly opened up the intake and exhaust with a dremel grinder bit, other than that, stock carb, jets, SE two step stock mufflers. The larger tires added 4-5mph over the stock tires as they are over an inch taller in diameter. The 90/90-10 tires did completly kill my take off though, it dosen't bog but there was a VERY noticeable difference in take off. The tires are designated as tubless (heavy) but I am using tubes and the overall weight of the new tires is roughly double what the stock 1.75-10 tires were. I have to imagine that the take off was killed by the increased tire weight and increased tire diameter, but it also increased the top speed. One other downfall of the larger tires is that my kick stand no longer touches the ground, I have been too lazy to weld on some spacers and have been using a 2x4 under the stand to keep it from tipping over. FYI, the front tire clears everything easily, the back on the other hand only clears the back of the motor by a little over 1/8" and can not be inflated until after it is attached to the scoot, its a bear to squeeze it past the muffler mount. I ahve been running these tires for a little over 1000 miles thus far and have not had any problems. Does anyone know how to calculate what the true final drive is with the tire diameter increase?

[Bear45-70]

Tire weight probably has next to zero to do with the change. The increased diameter, when you figure the increase in overall gear ratio change this is a major difference. The same apllys to a one tooth change on the primary when compared to a one tooth change on the secondary.