tune up advice

[sunrisemac]

I have a standard 1969 sx150 engine fitted to my 1971 gp150 (I guess as a replacement at some time in the past).

I know there is a wealth of knowledge regarding these engines on this forum so if anyone can give me any pointers as to what would be good for consideration to do to said engine to improve performance I would greatly appreciate it.

I have the capability to do most engineering tasks although I am more used to dealing with engines that have considerably more cylinders........

[Scooterlam]

Budget, standard looks? , expectations, budget

[Scooter Paul]

These were sweet engines. You could do far worse than to build it as good as you can but keep it standard.

Paul

[Storkfoot]

You always lust after what you don't have

When all I had was as sweet as a nut Series 3 li50 engine with standard exhaust, I wanted a TS1. Now I've had a TS1 for several years, I'd really love a standard engine to pootle about and get to work on.

I wouldn't get rid of the TS1 though

[MickYork]

Upgrade the exhaust and carb'. Two bolt-ons that significantly improve performance (not mpg !!).

Or, as said above, are you wanting to "home tune" or go down the kit route. Cheap alloy 175 kits give a marked improvement for reasonably little cost, alternatively start getting into the bigger HP kits and you're talking new crank, clutch, bearings, etc.

It's all about budget and expectations as previously posted.

[dave411]

I am just about to put a 150cylinder and head on my li 125.The exhaust port on the cylinder looks tiny compared to the manifold on a 42mm clubman.Anyn ideas,ie,open it up?

[Captain Pugwash]

I am just about to put a 150cylinder and head on my li 125.The exhaust port on the cylinder looks tiny compared to the manifold on a 42mm clubman.Anyn ideas,ie,open it up?

Maybe don't fit the 42mm clubman and fit the smaller Indian 38mm instead.

[dave411]

Allready bought it

[Scooterslag]

Just in the process of building up a GP150 engine myself, was going to go the ally 175/190 kit option but my standard SX150 road bike goes so well and its such a reliable little beast so all I'm going to do is match and polish a 22mm carb manifold and 38mm exhaust down pipe to it. Slight upjet and run it as.
I do think that (motorway and fast duel carriageway aside) the SX/GP 150 engine is still quick enough for modern usage.

[sunrisemac]

Thanks for the replies, I am enjoying having the scoot to ride as I have never owned or ridden one before I restored this one so I am not looking to change the looks or to spend a load of cash on the engine but more spend a bit of effort in the way of porting and optimising gas flow as much as possible, maybe a new exhaust. I still intend to keep the 6v system and points that I have with the GP setup.

When I get into it I will post results good or bad.

[Andyf]

I have a standard 1969 sx150 engine fitted to my 1971 gp150 (I guess as a replacement at some time in the past).

just out of interest, I think the early GP150's were actually built with SX150 engines, it might not be a replacement

[CHRIS in MARGATE]

Casings were stamped SX150.

[rossclark]

GP 200 casings are stamped SX200, what's the first digit of the serial?

[Captain Pugwash]

Casings were stamped SX150.

thats how they are supposed to be. If I remember right the engine number should start 22

[sunrisemac]

SX 150: Frame: SX150* followed by numbers starting at 750,001 - Engine: SX150* followed by numbers starting at 750,001
SX 200: Frame: SX200* followed by numbers starting at 830,001 - Engine: SX200* followed by numbers starting at 830,001
GP 125: Frame: 22/1* followed by numbers starting at 001,001 - Engine: 125LiS* followed by numbers starting at 001,001
GP 150: Frame: 22/0* followed by numbers starting at 200,001 - Engine: SX150* followed by numbers starting at 200,001
GP 200: Frame: 22/2* followed by numbers starting at 250,001 - Engine: SX200* followed by numbers starting at 250,001

mine starts 76

[Captain Pugwash]

SX 150: Frame: SX150* followed by numbers starting at 750,001 - Engine: SX150* followed by numbers starting at 750,001
SX 200: Frame: SX200* followed by numbers starting at 830,001 - Engine: SX200* followed by numbers starting at 830,001
GP 125: Frame: 22/1* followed by numbers starting at 001,001 - Engine: 125LiS* followed by numbers starting at 001,001
GP 150: Frame: 22/0* followed by numbers starting at 200,001 - Engine: SX150* followed by numbers starting at 200,001
GP 200: Frame: 22/2* followed by numbers starting at 250,001 - Engine: SX200* followed by numbers starting at 250,001

mine starts 76

Haha looks like I did'nt remember it right and I really should have because I've owned at least a dozen to date

[sunrisemac]

In my business there's nothing wrong with forgetting the insignificant things but always remember to tighten your nuts.....

[lofty]

maybe..
bore it to 175 and modify/change the head.
fit a clubman exhaust and open the exhaust port a tad to match
open the inlet port a little..
change the jetting/slide in the carb (I think GP150's had a 22mm carb??) to match..

most of the above info is available on this site (other scooter related tuning sites are available )

you will get a useful performance boost...

[sunrisemac]

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

[dave411]

Excellent,you must be a happy chappy?