90cc engine build for atv

[madmike740]

So I got this predator 90 and I'm trying to learn about these minarelli engines and I am curious where to start to get better performance. Fastest I can get it to go is 28mph. I put a new top end on yesterday its reading 100psi, it was 70psi, running a 105 main jet and a uni filter. I changed around some roller weights, shoe springs and main spring but none of that gave any advantage over stock. My boy tried racing this turd against some drrs and got double lapped on an oval flat track..lol..poor kid. It also has an fmf pipe with fresh packing. Would a pipe with an expansion chamber help and racing cdi? I'm really not sure where to start or if I should waste money on this thing some say it'll never be as fast as a drr or apex. Hoping for some honest feedback. Thanks

[aeroxbud]

Expansion chambers really wake a two stroke up. But most of the performance parts for the Minarelli scooters are aimed at the 70cc market.
A few of the 90's have been converted for road use with good results.
A few have been done on here. So I'm sure someone could help.

[190mech]

Cylinder quality is terrible on that type,the market has dried up on the better parts..There are 2 different height cylinders and the porting layout must be carefully checked and adjusted to get more power.Head design is crap also and must be reworked to get proper squish..
Brent has been working with this motor for years,here is a link to his work;
49ccscoot.proboards.com/thread/1712/project-90-detailed-performance-upgrages?page=1
I like these engines as they make a lot of torque,max RPM's will be around 10,500 as the transfer port ducting is undersized due to engine design...

[90GTVert]

Some of the DRR stuff is the top of the line for youth ATVs with Mianrelli engines, so that's tough competition. Polaris stuff is good quality, but my understanding is that the DRR quads are built basically for racers. I wouldn't expect that a basically stock Polaris 90 would hang with them stock for stock, but I don't really follow the ATV scene.

I think 25-30MPH is pretty standard on an unrestricted Polaris 90. I'm guessing their sprocket setup limits speed a lot? I'm 300lb and on a 200-250lb scooter a stock-ish 90cc engine would run about 50MPH. What RPM do you see at top speed?

If you are indeed seeing 100psi, there's power to be found with some work there. I assume your compression tester is reading low though. Check squish clearance and see if it's really wide.

[repherence2]

Maybe exhaust port duration is too low to achieve the results that you are looking for.

[quaddad573]

Did you ever have any luck with this? Just curious if you had any luck lining it out?

[madmike740]

Dec 17, 2020 13:57:09 GMT -5 quaddad573 said:

Did you ever have any luck with this? Just curious if you had any luck lining it out?

I gave up on that turd and bought a drr 90 and wow what a difference. I rode it once and it feels like the bike wants pull out from under me and I go 185.