porting of SX150 barrel

[johnnyXS]

I have a standard SX150 barrel kindly given to me by Steve .
I am planning on getting it bored out to 185mm and have already purchased a suitable 185cc piston kit.

My question is this..... will the standard porting of an SX150 barrel be to ok to use as-is ? or will I need to modify the porting to suit this piston ?

I am planning to fit the following
reworked 175cc head
42mm AF Rayspeed clubman
22mm inlet manifold
22mm Jetex carb (might upgrade to 25mm later)
standard Li 150 pacemaker gears

All I'm hoping to achieve here is a little more low and mid range torque and a slight increase in cruising speed say 60mph
Please don't suggest a cylinder kit as I simply cannot afford it

[EddieStone]

If you're going to the trouble of getting it bored out, I'd get it ported as well to make the most of the extra capacity.

[johnnyXS]

how much difference is porting likely to make to a SX150 barrel bored to 185cc ?
I couldn't get the cylinder boring engineers to port the barrel as they wouldn't have a clue how to .
if I sent the barrel to a Scooter specialist for rebore, hone, porting etc I'd end up with a huge bill that would probably exceed the price of a brand new 185cc cylinder kit !

I wouldn't mind having a go myself but what I am asking is

Is it worth it ?

How much extra power or torque would I actually achieve over the standard SX150 porting.?

My guess is very little...... given the other components that I will be using with this engine.
Also there is also always the risk of me screwing up the barrel and creating running problems if the porting work isn't carried out correctly

[Knowledge]

Before we get to porting "is it worth it?", can we have a little more about the 185 piston?

I guess it is 64mm diameter, but is the compression height (the distance from the gudgeon pin to the crown) the same as the original piston? This has quite a bearing on whether the barrel needs shortening or the con-rod changed. It is an easy calculation, but if you are worried about the cost of porting, then you may/may not need to factor in these other costs.

Don't get your machine shop to port it unless they know what they are doing. I've seen barrels tuned by two-stroke motorcycle tuners who have ported the inlet and exhaust for 6000rpm, and the transfers for 11,000rpm. Not a good combination. It won't hurt to contact a tuner and ask for a price. I would think a stage 3 tune would work well with a 22mm + Clubman, and the bigger holes in the barrel will breath a little happier than a re-bored 150.

(I remember a mate who had a Dave Webster tuned 185cc, and it was a pocket rocket, but used a bigger carb, expansion and big holes in the barrel than you need)

[johnnyXS]

yes theres some s good question but I'm not convinced the answers will help us answer the question much .

I have the barrel original 150cc piston and the new unbranded 185cc piston so i can measure all of them tomorrow .
The 185cc 64.0mm Std piston like most standard upgrade pistons is shorter overall with an open skirt and an identical pin centre to piston top measurement to the 150cc piston for which it is a direct replacement . it is therefore unlikely to flow less efficiently than the original 150cc piston with the existing barrel and case porting .

I acknowledge your advice about recommended porting specialists doing porting work and agree wholeheartedly. i'd probably do any mild porting work myself if I decided to go that route but i'd need to be convinced it would be worth the cost

[Phil D]

Hi Johnny XS.
Just out of interest have you been give a quote for the re-bore, also I was under the impression you could only take a 150 out to 175 as the cylinder wall thickness was compromised .

[johnnyXS]

no you can take a 150cc cylinder out to 190cc but yes it can be risky if you use a poor casting or the boring work is not done accurately.
I have had a rebore quote of £30 which is very reasonable

[Phil D]

£30 is very good .They usually charge per pass.
What head are you going to use

[johnnyXS]

I only have a 175cc head but I reckon its close enough with a bit of reworking to the squish area.
I have both a 175cc and 185cc pistons and I can barely see the difference between the two to be honest.
Have you built or used a 185 engine Phil ?

[Phil D]

No Johnny I haven't built a 185 but in a previous life I rubbed along with a couple of guys who dabbled in racing MZ 's .
Not dissimilar to lambrettas - cheap utilitarian two stroke transport for the masses
I found that the 150 ran better with a 125 head - that and a Mikuni carb!
I'm with you on the cost side, and as somebody on here so eloquently put don't see the point on spunking £500
on a top end.
Horses for courses and all that.

[dscscotty]

My experience with 186 conversions on cast cylinders is that they can benefit from extended port durations, particularly the inlet and exhaust port. The exhaust port width in relation to bore size is also a good way of improving performance and is useful in keeping the heat down, as this is something that you will encounter. Any internal transfer work can only be carried out with specialist tools, however will give good performance gains.£ 30 for a conversion rebore is about right as taking the cylinder from 57mm bore to 64mm bore is quite frankly a ball ache, my own boring bar cuts about 0.2mm per run, so do the maths.
Any way if you like a good read this may be useful to you? Full of old school, relatively easy/cheap methods for tuning cylinders.



Good luck and keep yer had over that clutch!

[johnnyXS]

thanks scotty I have the webster tuning guide which is very interesting .
I'm not really interested in that level of tuning I am just looking to increase the cubic capacity and in the process hopefully improve the torque a little.
I guess it was just the sort of idle question that passes through your head when upgrading your engine

[dscscotty]

This book maybe more suitable, it has good info of early stage tuning and cylinder conversions etc., its an easy read and although quite dated has good advice on simple performance gains.

[Scooterslag]

I'd be inclined to leave it as it is, maybe got for a quality 22mm carb , a decent clubman pipe and a port polish/match , the SX/GP150 is such a sweet running topend . Paul

[CHRIS in MARGATE]

Rebore it, blueprint everything and run it. You will feel a difference. By the way many tuning houses took 150 barrels to 200 back in the day, true 66mm. I gave one away to a pal a couple of years ago and it still runs. The walls are very thin !

[johnnyXS]

I'd be inclined to leave it as it is, maybe got for a quality 22mm carb , a decent clubman pipe and a port polish/match , the SX/GP150 is such a sweet running topend . Paul

You can't get much better than a AF Rayspeed clubman for low and mid range torque Paul. Its not top end I'm after its low and middle end grunt mate.

As for the carb I cannot fault the 22mm Jetex carb.( I have 5x ) , It is an identical clone to the Dellorto 22mm. I have one fitted to my LiS 150 special and it starts and goes like stink on a 52 year old engine.

jetex get an undeserved bad press often from those that have never even used one ! I have always maintained that I could get any jetex carb running perfectly . The problem usually is incorrect timing, crankcase leaks ,inlet air leaks, over-jetted carbs , bad porting and other faults..... not the Jetecx carb.

Good point about matching inlet and exhaust ports to manifolds though and I definitely plan on doing this as both manifolds are considerably larger than the GP150 cylinder ports of course.

[Scooterslag]

I'd be inclined to leave it as it is, maybe got for a quality 22mm carb , a decent clubman pipe and a port polish/match , the SX/GP150 is such a sweet running topend . Paul

You can't get much better than a AF Rayspeed clubman for low and mid range torque Paul. Its not top end I'm after its low and middle end grunt mate.

As for the carb I cannot fault the 22mm Jetex carb.( I have 5x ) , It is an identical clone to the Dellorto 22mm. I have one fitted to my LiS 150 special and it starts and goes like stink on a 52 year old engine.

jetex get an undeserved bad press often from those that have never even used one ! I have always maintained that I could get any jetex carb running perfectly . The problem usually is incorrect timing, crankcase leaks ,inlet air leaks, over-jetted carbs , bad porting and other faults..... not the Jetecx carb.

Good point about matching inlet and exhaust ports to manifolds though and I definitely plan on doing this as both manifolds are considerably larger than the GP150 cylinder ports of course.

the jettex is a bit hit and miss in my experience but I know some people do ok by them with not sounding like a advert for Scootopia but their 22mm carb and their clubman are both excellent , the exhaust in general not only perfoms well (good rev's and mid range) but actually fits! good luck with the porting/upgrading . Paul

[EddieStone]

I'd send it to pro porting and ask Harry to bore it and port it and sort the head, it'll still be cheaper than a kit. It'll make a definite difference to jus boring it, which will make a bit of a difference. imo.

[johnnyXS]

Rebore it, blueprint everything and run it. You will feel a difference. By the way many tuning houses took 150 barrels to 200 back in the day, true 66mm. I gave one away to a pal a couple of years ago and it still runs. The walls are very thin !

yes Chris I'm definitely going to bore it ...I already have the piston kits grinning at me from the shelf
I've still not decided yet whether 175 or 185cc though.
The question that has still not been answered though is is it worth porting the GP150 barrel at the same time or not . I'm kinda keen to try it as there are several good guides around but would hate to make a doorstop out of it for the sake of 1/2 BHP increase


Not sure about blueprinting though I've never been entirely sure about what people mean about 'blueprinting' an engine. This has got to be a cheapy DIY rebuild cos my pension won't stretch too far

[johnnyXS]

I'd send it to pro porting and ask Harry to bore it and port it and sort the head, it'll still be cheaper than a kit. It'll make a definite difference to jus boring it, which will make a bit of a difference. imo.

this has got to be a budget rebuild Eddie

By budget I mean way way way under £100 ......you guys have just got way too much pocket money by crikey
What does Harry charge for said work ?