Modern Vespa

So I have a Vespa gtr 125 which is identical to a sprint veloce apart from that being a 150cc. Both scoots run a SI 20/17 carb. I have just fit a 150 cylinder and head and also a 20/20 carb. I also run a sip road 2.
So my question is, is there any point in the 20/20 carb or should I replace the 20/17 which is stock. Will I actually gain any power with it or just end up chasing jetting issues to get no real benefit?
I keep thinking if mr Vespa used that carb then perhaps he knew what he was doing regarding power.

I believe Sprint Veloces came from the factory with a 20/20 carb.

Veloces had 3-port engines. Since your GTR is also a 3-port, I'd run a 20/20 with a 150 barrel.

Well if thats the case then I would use the 20/20 but I got my info from here:-

http://scooterlounge.com/vespa/reference-library/vespa-stock-jetting-reference-guide.shtml

I'm not saying that is correct but I had nothing else to go on.

Have you any idea what the D stands for on the carb?

I did that ..bolted on a 20 20.(and up jet)to a stock standard 150 super .and instant gain in acceleration and a wee bit of top end(5km/hr more )..then went and took a Px 24 24 and bolted it on with jet changes..even more acceleration and about 10km more (that 15 now) top end

all my 150cc and 177c scoots have a 24 24 and sip road pipe and forget all the peps that say jetting is difficult..its not...they were prob trying to re jet and engine with an air leak

24 24 all day long

I used to have a 71 GTR that had a 3 port 150 cylinder on it (and maybe a 20/20 carb?). I think i also had a Sito+ on it as well? But that scooter had a true cruising speed of 55mph, top speed of just over 60mph on any flat road, and TONS of low end torque. The reason being that it had so much torque was that the transmission was still stock. It would pull super steep hills at low speeds in 4th gear, while i would have to drop to 3rd gear in any of my P200's, 210's, and 215's.

My Sprint V had a 150 3-port and a 20/20 carb on it stock. My VBB had a 150 2-port and a 20/17 stock.

If you're not going to spend the time and effort to open up the airbox into the case, you might as well stick with the 20/20.

Thanks for the replies people, I will persevere with the 20/20 carb and see how I get on. I did consider opening up the casings to match the carb but when I looked the difference was so small that I decided it wasnt worth the time and effort, but may change my view on that in time. I'm pretty sure mine is a 2 port as it only has two transfers from the engine casing and a matching two on the cylinder.

I will play with the jetting and see how I get on.

Cheers

If, down the road, you still feel underpowered (and have some cash to part with...) I'd skip the 24/24 and look at the Pinasco 177 for 2 port Vespas. More expensive upgrade but worth the coin.

Will definitely give you some good grunt and a more top end speed.

I didn't get that much more top end from the Pinasco on my VBB, but it definitely gave it a lot more power, especially down low.

I was also runninng it with the piston it came with, so two big ol' 2mm rings keeping max RPM's down.

And port timings on it are pretty abysmal out of the box, too. It's not a revvy cylinder by any means, but you could get away with upgearing, to get a tiny bit more top speed.

I second this!!!

So it's not going well, must have soft seized about 7 times now. Ran it in for about 80mile before giving it a bit more. Soft seized, but I thought it had ran out of petrol so I changed the tap. Went again, soft seized. Main 102, pilot 42, 160 and an E1. So swapped to a BE3 as i think it is richer and still soft seized. Took cylinder off and there is evidence of scarring around ring pin area. So I opened up the rings from 0.25 to 0.5. Still soft seizing.

What do we think chaps?

Have you already tried a bigger main jet like a 105?

My first thought is that you have an air leak, so you'll never get your jetting fixed until you sort that out.

The E1 is a fair amount richer than a BE3, so you leaned out your mid-range and made the problem worse.

At this point, it's probably time to have a conversation about the things that need to happen before you're going to be successful dialing this in, starting with a pressure test.

These motors are simple, but also unforgiving, especially if you're doing more than just around town stoplight-to-stoplight type riding. It's when you push them that the "little things" like air leaks or head leaks pop up and ruin your day.

Bloody hell I thought I read the BE3 was richer than the E1. I bought a compression tester a year or so ago, so just been out to test compression, sodding cheap nasty thing doesnt work. I'll need to borrow one. Head torqued to 13 lb ft. Copper gasket at the head and alluminium at the bottom. Didn't replace carb base gasket when I went from 20/17 to 20/20. Been up as high as 104 main.
Proper upset at this as I thought it would be plug and play. I can see me going back to the 125 which was super reliable.

This won't be your issue, but you don't need the copper head gasket. Was the 150 cylinder and piston a working pair or sourced separately? Could there be insufficient bore clearance?

Yeah my original kit didn't have the copper gasket. I'll remove that and lap the surfaces. The head and piston was bought privately but came brand new in a moulded polystyrene box so I have no reason to think they were not matched. I sourced the 150cc head separately. It starts and runs fine, the top end seems less, and any prolonged WOT and you can feel it coming on, a soft seize. Bloody thing.

The copper gasket will increase the squish clearance too much, which makes the piston run hotter. Closing the clearance too much makes everything hotter. Somewhere between the two is great.

What type of ignition and where is the timing set?

20/20 carb would be better suited to an AC140 BE5 for this.

Thanks Jack. The squish with the gasket is at 3.2mm so I imagine it will be 3.1 with it taken off. I'll look to get those jets for next week.

Also it is 6V ignition and the timing was set to standard (what ever that is) about 4 years ago by a friend. I dont know how to time points.

Why is the squish so big even without the gasket? Think we need some pictures of the piston at TDC and the head.
About 1mm is the usual target for a 150.

6V and points will need more advance than electronic does, as slower and weaker but they do work fine, so unlikely to be anything to do with this issue if at stock setting.

Jetting probably the biggest issue though.

The piston at TDC is very good, around 1mm. The cylinder head is the one shown with a large lip on it that makes the squish 3.2 mm. I tested it with solder.

Oh, you have one of those heads. Don't worry too much about the squish. Unless you know someone with a lathe, then you could take 2mm off.

If not the piston/bore clearance, the seizing up is the jetting. AC140 BE5 105 45/140 is where I would start.

You basically have a 150 Super/Sprint top end, given that you have only two ports. Millions of Vespa 150 scooters had that big old squish, so that's not your issue. Jack's suggested jetting is the same as a 'modern' PX150, albeit with a bigger main jet to account for your SIP Road 2. It 'should' be more than enough. It's what I was running on a PX150 with three ports and a Scorpion expansion. Totally pointless on that top end, but it looked good

Ok Jack I went with your jetting advice but started at a 106 main as that is what I had. Four stroking badly at open throttle. ( i think that's what it was doing ). Went down to 104, better but not great, then 102 and now 100. Ragged it about all over and no seizing and seems to rev out nicely. Only thing is I'm a bit disappointed with the supposed extra grunt. Doesn't seem to have any more at all and I'm still changing down gear at the same point on hills as I did with the original 125. Do I now need to tweak the jets to release some power, and if so which ones?

Thanks people.

Great it's not seizing. Nothing more annoying when overtaking.

Now the easy bit. More torque. First thing is get the squish clearance down to 1mm or less. Unlikely the compression will go too high if you skim that head.

If you raise the cylinder a bit and skim the head on a lathe to drop into the cylinder BGM style, it will go even better.

Anyone just a quick question, my carb currently has an 07 slide in. Is this the correct one for the above jet setting or do I need to change it?

Thanks

Alright Jack, I have had the head skimmed down to 1mm and have changed the rings as I think I might have gapped them too big. It feels like it pulls better already low end whilst running in the rings. Do I need to alter the jetting due to skimming the head?

Thanks
Steve

What did the squish clearance come out at with 1mm head?

Jetting will need to be a bit richer but maybe not a whole jet jump. Try the bigger main until WOT splutter and walk it down. Do the same with the pilot jet.

The squish is now at 1.2mm.

I'll do as suggested with the jets thanks.