tune up advice

[sunrisemac]

sure am thanks

[Scooterslag]

I thought I would have a go at a mininal tune to try and improve the performance of the SX150 engine installed in my GP. Thanks to everyone who offered advice, it gave me plenty to chew over and this (if anyone is interested) is what I did.
Made myself a stand to bolt onto the jacking lug and underside gearbox mount so I could get the engine unbolted then lift the frame away from the engine.
Matched the inlet manifold port and gasket to the shape of the hole in the barrel, widened the port into the cylinder, lowered the port by 1mm and evened the shape of the hole to a nice rectangle with 6mm radius in each corner, then dressed out the hole between both ends to make a nice flowing tract.
Bought a slightly damaged Indian big bore exhaust system which needed a bit of attention for £30. I welded the inside of the exhaust flange to the pipe and dressed it out to make a decent shape to match the gasket. Made a new split exhaust clamp with twin bolts to make sure the joint didn't leak and knocked the brackets around make them fit the studs.
Matched up the hole in the barrel to the exhaust and gasket, widened the exhaust port into the cylinder by about 4mm overall and radiused the corners and squared it up as I did to the inlet, dressed the tract to make a nice flowing port shape.
Dressed the transfer ports to get a better finish, put a small radius on all the port edges and corners with a riffle file, bevelled all the port edges by about 1/3mm so that the rings get an easy life as they pass the port edges.
Checked the squish area with a bit of soft solder wire was between 1m and 1.25mm.
Bolted it all together and nailed it back into the scoot, tried a few kicks and it didn't go so checked compresson and it was only 85p.s.i., gave the plug hole 3 squirts of engine oil to bump up the rings seal and tried again, away it went with a lovely cloud of smoke sounding a lot nicer than it did before.
Warmed it up and twiddled the carb to get it to idle then took off down the road with my own cloud following me for half a mile or so, when it cleared I pushed it along a bit harder for a couple of miles then tried a standing start through the gears, it must have pretty rubbish before with the standard replacement cylinder etc., for a standard 150 that has just had a bit of TLC it is a different scoot, it's pretty lively and I can get a speedo reading on the flat with me a 6 foot fifteen stone lightweight of about 52 - 53 (which according to my pal's BMW following me is pretty accurate).
Did a plug chop to see plug colour and looks good so far, lovely and brown with no oily deposits, no jet or slide changes as yet.
Took a couple of hours Sunday and a weeks worth of evenings
Thanks again for the tips

Good stuff, I do think that people seem to forget that with a few small mods a standard engine has a potential to have a bit more go. I remember Stuart Owen explaining on here that all he had on his standard SX150 was a clubman, 22mm carb upjetted breathing through a filter -less airbox, a 16T sprocket and pacemaker gearbox and he really rated how well it went, coming from a bloke who used to sprint at 100mph- that's quite a recommendation

[Kev Fox]

I had this little bit saved from Stuart Owens post.

simple really . I have an original sx 150 and the engine isnt to bad anyway . I polished the ports on my barrell and run through a delorto 22mm carb (italian only though) . I run through the air scoop with it fully cut out like gp and then through the air box but with no filter .Exhaust is a big bore and jetting is a 120 main jet . Gear box wise i ditched the sx150 as its hopeless . I run it on a pacemaker using 16/46 . As for speed it sits happy at 60 mph and still has a bit more left . It also runs on point ignition .Is it good ,well ive run the same set up for 17 years and done 20,000 miles on the same set up . Ive toured Europe on it several times and been all over Britain on it . It has never let me down once in all that time . Along with the fact it does 100 miles to the standard tank it is nice to drive as well . If you want big power then get a 200 lump ect . If you want a bit more use the original set up with these changes im sure you will like it .

[sunrisemac]

I had this little bit saved from Stuart Owens post.

simple really . I have an original sx 150 and the engine isnt to bad anyway . I polished the ports on my barrell and run through a delorto 22mm carb (italian only though) . I run through the air scoop with it fully cut out like gp and then through the air box but with no filter .Exhaust is a big bore and jetting is a 120 main jet . Gear box wise i ditched the sx150 as its hopeless . I run it on a pacemaker using 16/46 . As for speed it sits happy at 60 mph and still has a bit more left . It also runs on point ignition .Is it good ,well ive run the same set up for 17 years and done 20,000 miles on the same set up . Ive toured Europe on it several times and been all over Britain on it . It has never let me down once in all that time . Along with the fact it does 100 miles to the standard tank it is nice to drive as well . If you want big power then get a 200 lump ect . If you want a bit more use the original set up with these changes im sure you will like it .

Interesting bit of info very similar to what I've done, I still run with an air filter and will continue to do so, I'm also running the standard 6v system and points and Italian Dellorto 22mm carb but I can't remember which main jet, the gearing I reckon is where my scoot is losing out as stated in the quote although I'm happy enough with the improvement so far, I have tried a different plug today NGK BP7ECS but only to start and warm up, we use them in race engines, I reckon it will pull the spark a wee bit further from the piston crown and maybe have less chance of a bit of detonation and the possibility of pre-ignition if there is any fuel mixture weakness when I give it a good thrash on test to see what the best top speed I can get.