Modern Vespa

Jack, pretty sure the genuine Keihin PWK Airstrikers have fixed atomisers, so cannot be changed. Certainly not listed as a part that I can buy. The explanation is that Keihin carbs have a larger selection of needles than is typical, to do the fine tuning, rather than the ability to change atomisers.

Does that sound plausible to you?

You're right, the Atomiser on a PWK Airstriker is not like others but its not fixed and can be changed. They call it the 'metering block' instead of atomiser but it is essentially the same thing, although much, much more expensive.

You may be able to get it jetted as it is but it will be tight. BGN is already the fattest diameter, so you might need a weaker slide to get 1/4 in.

It is very unlikly that BGN will be the last needle you buy. The reason to start on the clip is to know where to go next. Now you are on clip 3. Are you happy that 1/2 throttle is running perfect. Forget 1/4 and WOT for now. How is 1/2? Rich, weak or spot on?

BGN at clip 3 was just my starting point, as most people with 'similar' setups are using this needle. I realise that no two setups are the same. My motor has an old BigBox rather than an expansion chamber. Would this make it sufficiently different from others? Using Kyajet, I'm thinking that BHN or BJN needles may be a worthwhile investment, although they do appear to lean off 1/4 throttle as well, which seems pretty good at the moment. Beyond that, I'm tempted to get the no.6 slide and start again. I cannot find any evidence on GSF that the 4.5 slide suits an MHR setup.

I'll need to mark up the headset with tipex to show me exactly where 1/2 throttle is. I thought the splutter was at 3/4, but it could well be closer to 1/2.

To get this done quickest, you need to forget about anything except 1/2 throttle for now. If you can't drop the needle enough to get 1/2 correct, then you need a new needle.

As these needles have 5 clips. And you can add a few 0.5 washers to get a sixth. I would go for a bigger needle jump like BLN. increasing the length will make 1/4 weaker, so you will need to reduce diameter at the same time, so maybe BLK. While selecting a new needle would be good to take a guess on the taper too. If clip 1 gets 1/2 near, how is 3/4 with a mild splutter at WOT? If 3/4 is better than 1/2, then the taper is too shallow.

I managed to get a swap out for for incorrect slide, so I now have the correct no.6 slide installed. I think it will prove a much better starting point. It's hunting a bit now at idle, so I'll need to sort that out later. Half throttle is now much cleaner, but still needs a tweak. Dropping the BGN to clip 2 is my next move, which will have the desired effect. I'll still have clip 1 if I need it, but after that it's the BLN needle, most likely starting at clip 4.

Well done swapping the slide. Gets expensive when you have to buy them to test.
before deciding on BLN check that 1/4 is really good on the new slide at clip 3. if its too weak it will need a bigger diameter. You shouln't need the weakest diameter for this.

We've had some lovely Vespa weather here in the south, but alas, I've not been out much on the scoot. I'm finding that it runs really well on the flat, but as soon as I go up a hill, it's spluttering. This means I have to drop the throttle, which in turn means I lose momentum and revs. Very annoying. Not sure if the splutter is the main jet being too big or if the float isn't being filled quick enough (unlikely but perhaps the carb angle on the hills is a problem). It's happening at 1/2 throttle, as I'm climbing in 3rd gear. Changing down to 2nd and opening the throttle has the same effect, so I just crawl to the top and then accelerate away on the flat.

I have a Mikuni pump installed, which is working well, but I'm still using the stock PX fuel tap. I have a BGM mk2 tap ready to fit, but was waiting for the tank level to go down beforehand. Should I need a fast flow tap as well as the pump?

Current setup is:

Carb: PWK35AS
Pilot: 48
Main: 160
Needle: BGN clip 3 (tempted to try clip 2)
Slide: no.6

On reflection, the clip position and/or needle length is my next area to focus on and I'll leave the main jet alone for now. After a long ride on clip 3, the plug looks like I'm running rich in the midrange. I can disregard the main jet for now as I've not managed to get beyond 3/4 throttle and definitely not WOT.

If BGN clip 2 is too lean in the mid range, then a BHN needle at clip 3 will be my next move.

Wow, what a difference a clip position makes! Moving the clip up one (from pos 3 to 2) has completely cleaned up the mid range. Just been for a long ride, filled up with 97 strength petrol (BP Ultimate) and it flew through 2nd and 3rd gears. I don't have a rev counter, but I'm getting to at least 60mph in 3rd. Judging by the gearing calculator, I'm almost touching 8000 RPM at 60 (96KMPH).

As I approach WOT, I'm getting some splutter, so my next move is a smaller main. I have a 160 in there right now and my next smallest is 155. I've ordered a 158 to try first, rather than jump down too far. The other possibility is needle taper, but going leaner would mean a needle starting with an 'A' and I haven't come across any Vespa or Lambretta setup with an 'A' taper.

The idle seems good with a 48 pilot, and it returns to idle really quickly after opening up the throttle. However, the mixture screw is less than one turn out, so I'm going to pop a 50 pilot in there and hopefully end up at 1 1/2 turns out.

I followed this excellent web link as well as Jack's advice:
http://www.shiny-red.net/guides/keihin-carburetor-jetting


I'm expecting V2 to be my optimum setup

Great news SWA, well done!

Solid progress. You'll need a needle that does the same but on 3rd or 4th clip.

Airscrew on this set up should be at least 1.5 turns. 2 turns max. A bigger pilot will get this up. But first check the diameter on the needle. If you do a long 1/4 throttle cruise, firstly it should feel good at powerful at 1/4 but it should also run cooler, about 280F and steady while held at 1/4.

Cheers Jack. I'm under no illusions that it's absolutely spot on at this stage. That would be too easy It's just great to have full throttle control and be able to feel the full potential. It is quicker than I had expected and the gearing seems to be up to the job. I'm going to concentrate on plug colour and temperatures now to get it fine tuned. The plug is looking slightly darker than I'd like after a good mixed ride, but I have a perfect gentle, straight and long incline near me, so I can ride it hard in 3rd at WOT, then look at the plug again.

For the 1/4 throttle long cruise, is that in 4th gear?

Yes, get up to about 65mph in 4th and ride about 5 miles holding 1/4 throttle or slightly less, 3/16. This will focus directly on the diameter and slide. Slide you did, so just looking at diameter. If over 280F at this 1/4 cruise it means the diameter is too big on the current needle. How much is the question. But too weak here and there is a big seize risk when you least expect it. Also acceleration and hill pulling are not as good. If you touch the clip again this part is obviously null and back to the beginning.
I imaging its feeling quite exciting already.

I've not fitted my CHT yet, so checking the temps is not possible. It'll be ready for next weekend with any luck. What I did do was try the next diameter down, a BGM needle. At clip 3 it was horrible, spluttering like mad from 1/4 to 1/2 throttle. Clip 2 with this needle was only marginally better. This confirmed to me that BGN clip 2 is the place to be for my setup. I did run at 1/4 throttle for a few miles on a dual carriageway and then checked the plug after killing the engine. The plug was most certainly chocolate brown, and probably a tad darker than it should be, but not lean by the looks of it.

Feeling that my needle is definitely in the right ball park, I put in a 50 pilot jet and a 158 main and adjusted my mixture screw to 1 1/2 turns out. Very, very good with just a little splutter as I pull away in 1st, but once past that, it absolutely flies in 2nd and 3rd. The splutter was still there from 3/4 throttle onwards, so I put in a 155 main jet. Better still, but some splutter with it almost wide open in 3rd on a fairly steep hill.

What next? WOT on a fast dual carriageway with the 155, or drop to a 152 ??

You make it sound like you're getting there and faster, so must be close. From what you are saying BJN would be the correct needle. If this ends up on clip 4, then even better. Every good set up should end on clip 3 or 4. This means that the needle starts pulling sooner, which is ideal.

If you still have splutter on the main jet it is more than one jet over. This said, I would leave this splutter until right at the very end of the set up. Once fully hot it is easy to lose one jet size and on a 10 mile WOT run, maybe even two. Two sizes down from optimal will get you into the seize zone. And we don't want to go there.

The splutter as you pull away is probably that the airscrew is too far in. They are tough to adjust sometimes. Try it another 1/2 a turn out and see if its any better.

I agree that BJN clip 4 looks good on paper (identical to BGN clip 2), but I don't think you can get the 48-BJN needle, at least not from my normal supplier (Allens Performance). I'll do some more searching. In the meantime, I'm going to stay at 50 PJ / BGN clip 2 / 155 MJ and play with the mixture screw. See if I can clean up the take off in 1st gear.

I'll try BGN clip 3 again, just in case other settings can clean up the 1/4 to 1/2 throttle. Otherwise I'll have to stay at clip 2.

[Edit] Just a thought. If my original (2013) BigBox was too restrictive and/or a bit clogged up with age, would it be harder to fine tune? I'm wondering if a nice new BigBox Sport or PD S-Box would help? I really want to keep the spare wheel, as a PX doesn't look right without one IMHO, and I'm on a 120/70-11 rear tyre, so a decent box pipe is my preference. I was planning to wait for the elusive SIP Viper (Nordspeed collaboration).

Small update. With the mixture screw out to 2 turns, take-off is crisper, but it's now hunting slightly once warm and at idle. I'll try 1 3/4 turns for the next run.

I've also re-visited clip 3 with the BGN needle. It's actually pretty good, and a picture of the plug after a decent ride in the rain is below this post. Bit blurred, but it's a good colour. The smaller main jet (currently 155 down from 160 via 158) has clearly had some impact. I can live with it in the absence of a BJN needle, and will now focus on the main. I have 152, 150 and 148 lined up and ready to go!!

Still spluttering when I'm north of 1/2 throttle, despite dropping the main jet to 152. It'll sit nicely on a fast dual carriageway at ~60mph and it gets there very quickly and cleanly. This is using < 1/2 throttle, but when I open it up, it's going nowhere due to the splutter. but I don't think that reducing the main down further is a good idea, so it must be something else that I need to look at. Other similar setups are at least 152, and I've seen a few with > 160 mains.

My rotary 221 is running a 162 right now. But it has been running a 175 on a different needle. It doesn't mean a great deal. The main jet size depends on the needle taper, clip/length and atomiser. Change any of these and the main jet will need to change also.
152 is a smaller jet but higher float with a pump and on an Airstriker which are known for being rich, it could be normal. While there is still WOT splutter it is still over. Reducing the taper to A will help. Especially as the main jet is having a heavy influence on the 1/2 throttle.
Probably as said a while back you need a different metering block (atomiser) but while you still have WOT splutter and a dark plug it is still safe to reduce.

Thanks Jack. I've started to wonder if the airflow into the carb is restricted due to the bend in the Dumbo between the carb and frame. I had similar issues on a 177 rotary with a PHBH 28 and a foam filter. I sorted it by using Malossi bellows to the frame. The foam filter must have been restricting it.

In the photo, the red arrow shows the angle of the carb but not the perspective. The green shows the direction of the metal Dumbo connector which is fixed to the frame. The beginning of the green arrow marks where the metal connector ends and the flex hose takes over. There is a pronounced bend to compensate for misalignment between Dumbo and carb.

The second picture is my feeble attempt at trying to show how misaligned the two things are, with green arrow showing the natural direction of air flow from the frame void. The red shows the actual path taken due to the bend.

If you remove the dumbo tube and ride with the carb open does it still splutter at WOT?

Gonna try that today, weather permitting. I'll need something over it's big mouth temporarily, so will look for some gauze. There are cheap 'K&N style' filters on eBay with 60mm connection (PWK35 AS is 59mm). Tempted to have one on hand. Like I've said above, there are very few needles available that make any sense, given the symptoms. If air flow restriction does turn out to be a thing, there'll be a few more needles that come into play. Other 'similar' setups are using 'C' needles with a richer taper for example.

I took the Dumbo hose off and clamped some fine gauze over the carb mouth. Quite a nice improvement, albeit with much more intake noise!! Took it on my usual stretch of dual carriageway once it had warmed up. I managed to get beyond 3/4 throttle and it was still clean (no splutter), but I bottled it before I hit WOT. It was dark and there was still a lot of commuter traffic about, and I didn't want this to be the time that it blew up. That said, the plug colour was nice when I got home, but I'd ridden on slower roads in the meantime.

I'll go back to a bigger main jet and work down again and perhaps I'll end up at 155 or 158 this time. As for air filtration, I'll probably get a RamAir MR-011 from a local bike shop, or better still, I may be able to modify the Piaggio bellows to fit the PWK. One thing is certain though, the MRP Dumbo is not a good match for the PWK35 when it's mounted on an S&S CNC reed block/manifold. Maybe the alignment is better with an MRP manifold?

I'm down to a 150 main jet now and it's just beginning to feel clean at almost full throttle. I suspect a 148 main is going to improve it further. Acceleration in 2nd. 3rd and 4th is amazing, however it seems to get stuck at ~7000 in 4th. I don't think it's over geared, so I can only assume that the exhaust has become the bottleneck, which is presumably why the bigger main jets were bogging it down. SCK have said that my original BigBox (purchased in late 2013) is somewhere between the current BB Touring and Sport, and mine is likely clogged to some extent by now.

The following 'similar' setup uses a 148 main with an S&S pipe (presume a Newline expansion), so I won't be the only one below 150:

http://lfs.alexander-hepp.de/body/singlesetup.php?id=1177&language=0

Marco from Pipe Design has suggested the S-Box 220 III Plus and I'm considering this alongside the BigBox Sport. The release of the SIP/Nordspeed Viper may just be an urban myth. As said previously, a box is my preference, in conjunction with the spare wheel and wide rear tyre, and the S-Box looks better quality on paper. It's derived from a genuine Piaggio exhaust, so wins on construction, but performance-wise, the BB Sport is very well regarded.

Gearing:
Stock PX125 box
68T primary
22T clutch
120/70-11 rear tyre


Trying my best to avoid a big money expansion chamber, but maybe resistance is futile.

Getting better by the sounds of it.

Even with the big tyre on a 125 gearbox it is probably very slightly low, not high. Either way should be faster than 7000rpm in 4th.

Something is up. Yours should be at least in the same ballpark as my rotary inlet with a 30mm carb. Mine will always go over 8000rpm in 4th on the flat and if shes in a good mood and calm wind (prone to tank slapping), a little over 8500rpm is possible.

Your carb jetting is getting close. 148 is still a big jet. It has to be the exhaust. Dropping the packer might help. Some porting surely would. As would an expansion, something nice and fat.

Remind me where your ignition timing is. And what the port durations are.

I have a 1.0mm base packer, giving me 129/185/28. The stator position is at 26, and I'm at curve 9 with an original Kytronik. Last week it was on curve 8, but I'm not feeling much difference between the two.

Just about to get a ride in with 148 MJ before the next deluge of rain !!

Probably is the exhaust. Ignition timing should be good enough for 8000 in 4th. If wanting to try another curve, curve 7 would be next but will need a slightly bigger main jet.
I was hoping this would work out but porting is on the dull side. With the 35mm and big reed inlet the outlet has to match up, or the power dies early. With a good fat expansion it would liven up but as you are on a box there is a much smaller baffle wave, so dull plus dull equals lightning acceleration but little over 7000 rpm in top. Some days will be better than others, you will see 8000rpm at some point on this set up but to get it everytime something needs to change.

From here you have quite a few choices. Try another exhaust, anything you have to check the 2013 box is not clogged up. Run slightly weak on the main to get the exhaust hotter. Drop the packer, split the 1.0 and put 0.5 on the base and another 0.5 on the head. Port the cylinder. Talk to Marco and get a proper exhaust.

Cheers Jack. Roughly translated, I have three options:

1. Leave the porting as is, buy a better box exhaust and see what happens. This maybe moves the RPM in 4th nearer to 8000

2. Leave the TD at 129, increase the ED (to what value?) and buy an expansion chamber. This is all out street racer, making good use of the 35mm carb.

3. Drop the cylinder by 0.5mm (or lower), changing the timings to 126.6/183.7/28.6 (or lower) and buy a better box exhaust. Would this be a waste of the MHR and 35mm carb, as it starts to look more like touring spec.?

Part of me thinks go the whole hog, whilst the other part thinks should have bought the 210 Sport.

I would say get the jetting done first. Try curve 7. And run it for a while. If the exhaust is a bit clogged it might improve on its own.
If you are taking the cylinder off dropping the packer to 0.5mm will be better.

See what the exhaust options are. There are many expansions that work with a centre stand and spare wheel.

To get the MHR really working on a 60 they need porting. Something you can do? Or need me to come and have a go?

With the MHR or Sport and the way they are ported, it seems to me that the MHR is made for reeds and the Sport is made for rotary. Obviously either will run on either but I think that was the idea.

Been doing a bit more research. A good state of tune for up to 9000 RPM is 126/183/28.5. This is achieved with my 57 MHR, a 60mm crank, a 0.5mm base packer and no extra Dremel work. The PD S-Box is recommended for ED up to 185 but TD only up to 124. To keep within these recommendations would mean using the 0.2mm base gasket and nothing more, with 1.8mm head gasket to give me the same 0.85mm SBC that I have right now.

If I stay at 1.0mm packer and 129 TD, looks like I'll need an ED north of 190 (192/3?) and a serious expansion chamber. I wouldn't do this until I transfer the engine to the proper PX200e scoot (spring next year). My understanding is that I could have centre stand and spare wheel, but not wide tyre with a left hand, or I could have wide tyre but not spare wheel. I could possibly have all three with a right hand pipe if they are more robust than they used to be.

[Edit] Re: porting of nicasil, I have a Pinasco Magny Cours cylinder that I can practice on. I've read that you should cut in a downward direction from nicasil through to the aluminum, which means grinding the exhaust port from the inside top of the cylinder rather than going in via the exhaust stub. Is that correct? That means having a right angled grinder which I do not currently have.

swa45 this sure has been interesting to follow along.

Interested to learn more on the nicasil porting. I have been doing some reading in the A Graham Bell tuning book pertaining to the exhaust port and then looked again at mine. Seems some more material could be removed at the bottom mine is not totally flush yet at the piston BDC. I had used sanding drums before. I remember Jack recommends diamond cutters however I was getting good results with a 180 grit sanding drum. Also I went through the exit but whenever I was near the cylinder the drum was always past cylinder wall grinding up or down on both surfaces, the cylinder wall and inside the port never pushing out that may result in flaking. Looking forward to more insight.

The Germans mostly run the non ported 57MHR with a 60 on a 0.2 gasket and 2mm head. We've talked about this before somewhere. This does go well and makes happy Europeans. I think you'll get away with 0.5mm. So, sounds like the best option for now is just what we said. Change the packer to 0.5mm, curve 7 and get the jetting done.

Thanks Jack. Going to drop down to 0.5mm packer, with 1.5mm head / 0.85mm squish at weekend, so 126.6/183.7/28.6. If the exhaust still feels like a bottleneck, I'll most likely go with a BigBox Sport.

Will get a new 0.2mm base gasket and 2.0mm head just in case I decide to go lower, giving a slightly bigger squish of ~1.05mm and 125.1/182.55/28.73

One of my objectives is to make good use of the 35mm carb, so let's hope it doesn't just empty the fuel tank without giving me the benefits!!

[Edit] Before the weekend comes, I've spent the grand total of 7 quid on 145, 142 and 140 main jets. I'm curious to know how much smaller I need to go before it stops spluttering from 3/4 throttle.

Just reading up on SDJohn's thread and particularly the latest bit about the pump 'over fuelling' the carb. Could I be suffering this issue? Would the symptoms be similar to an oversized main jet ie. spluttering and bogging? It's most noticeable when trying to accelerate up a hill.

The PWK35 sits pretty level as the S&S reed block and manifold exits at ~90 degrees. The float chamber is at a similar level to an SI carb, so I'm wondering if I really need the pump. With the fast flow tap, I've possibly got sufficient flow, even at a low tank level. The only issue I see is the fuel entry point being vertical and near the top of the carb. This means the routing of the hose goes high, with a significant bend down to the carb.

There's only one way to find out.........

If a hill (up or down) affects the running and goes rich , then the float level is too high. If a pump was over pressuring the float it would do it on the flat too.

I had one a few years ago that kept pissing itself at traffic lights. Lucky it never caught fire. Found the float was wobbly on the pivot and sometimes it touched the side. Since fixed still ok as far as I know. Sensitive things floats. If the height or function is wrong the jetting can be confusing.

It is happening on the flat, but it's not so obvious as I'm rarely able to give it a good WOT thrash, and I have been putting it down to the main jet being too big. I'll look at the float level, as I'm getting suspicious that I'm going too low on the main. Was just about to try 145, then 142, then 140 this weekend. I did adjust the float as per a YouTube vid, but I'll take another look before any more jet changes.

I was able to get the MHR head and cylinder off with minimal fuss, due to the engine being an extra 1cm over to the right. This meant I could get the top end off without dropping the engine, removing any studs or even taking the carb off. Unrelated, I was able to swing the BigBox down and off the cylinder stub just by loosening the bolt through the swingarm. Phew, cause getting the big rear wheel off in order to completely remove the exhaust bolt would not have been easy.

I'll now replace the 1.0mm base packer and 1.0mm head gasket with a 0.5mm base and 1.5mm head, get it all torqued up and back on the road in the morning. Looking for a more favourable power range to suit the BigBox.

126/183 feels better, without quite the kick that it had at 129/185 when it hit the RPM sweet spot. I'm down at 142 MJ and it does seem clean beyond 3/4 throttle. That said, the only opportunity I had to ride it was after the light had gone and with lots of commuter traffic around. The roads are are also a bit greasy from all the rain. Getting a clear, fast run was impossible.

The hill problem still persists, and I've checked the float height (YT vid) and set the Kytronik at 7. The hill that I use has two bends on the incline. Having got a got run at it in 4th, I cog down to 3rd for the first bend, then 2nd for the second bend. When coming out of the second bend (still climbing), I open it up and it splutters quite badly and won't pull well, something that I didn't have on the flat prior to the hill. Once at the top of the hill, it continues to splutter on the flat during acceleration, clearing a few hundred yards later. Maybe my float setting skills are flawed or something else is wrong.

If pulling better at 126/183, then you were a bit over. Once set up better you will want to fiddle with it again but that’s for another day.

Ok. Float height. If you are getting rich issues, up or down hill. Running a main jet that should be bigger. Struggling with tiny pilot jets. Then it’s the fuel level in the float bowl that is the issue. There is nothing else that is the root cause. Loads of contributing factors and associated reasons but if the level of fuel in the float wasn't too much we wouldn't be talking about it. Now, how to get the level down. If nothing is worn out or damaged, then its set up wrong. As you are pumped and with a vacuum pump that is big enough to feed 4 carbs, when running just one, there is a certain amount of overpressure to deal with. A spark plug top in the bypass line works, it reduces the pressure enough so the float can handle it. Your level is high, as the float isn't handling it. First check the needle valve is a 250. If it is reduce the float height a few mm. If not better after that, reduce it a few more. What you have seen on YouTube is probably for gravity feed and maybe not for your float weight and/or jet size either.

Stick with it. So close now. Reducing float height will weaken all the jetting across the whole range. Up jet will be required.

I discovered that the Malossi bellows fit perfectly inside the MRP 'Dumbo' hose (or any 60mm flexi hose). I cut it back, to shorten it and to get rid of the angled bit on the end, and then 'screwed' it into the 60mm hose. Perfect fit, unlike when I tried with a stock Piaggio bellow. Fitted between carb and frame, it works a treat and so much better than with the Dumbo metal frame connector.


Before I shortened the Malossi bellows

More of the hose and less of the Malossi bellows = more flexibility