Modern Vespa

The main thing that's thrown me is some old posts I've read which say the motor shouldn't run with the adjuster screw all of the way in, I think they're correct, but mine with run with it fully saved in.

It's a fresh build that I haven't ridden yet due to the roads being covered in salt, the other setup step that I haven't done yet is double checking the timing with a strobe, it has passed a leak test.

BGM crank and 177 kit. Jetting as kit instructions 55/160, 160 BE3 130 to start with.
Starts easy, sounds about right, responds to throttle well.

Some details.....
Min idle is at 2.5 turns from slide starting to move, also pictured.
Mixture screw best at 1/2 turn, doesn't cut out if fully in. Tip is clean and identical to the other one I have.
There is some slack on the choke cable, pressing on the end of the plunger to ensure fully in makes no difference, rubber tip is hard so it could leak a little.
Blow back misting seems excessive, airfilter is wet at one end (pictured), sprays onto seat with a throttle blip.

Questions

Is the adjusters behaviour normal, perhaps indicating that I need a leaner idle jet, or are the forum posts that I've read correct when they say the motor shouldn't run at all with the screw all of the way in?

Do they normally have quite a bit of blow back? I haven't worked on a rotary valve 2 stroke before.

Time it first. 90% of carburetor problems are electrical is a saying for a reason. I'm sure you are aware that the idle jet is missing in your last picture.

Blow back is normal and expected. How much is the only variation.

The all the way in thing is for an airscrew. This carb has a mixture screw, so is the opposite. Shouldn't run if already at idle, then turned over 4 turns out.

The correct point on the mixture screw is where the mixture starts at 4 turns out and most of the idle screw in to maintain the idle. Should be smoking too much and splutters when revved.
From this stage, turn the mixture in until it smokes less and speeds up. If it gets in too far either the idle jet is too small or you missed the point where it sped up. Should go from rich to faster within half a turn.

Is the idle jet missing? Is that not the 55/160 thing recessed down into the hole next to the main jet stack?

Timing should be at least close according to the vape instructions and degree wheel, but I'm not trusting it to be right until I double check it with a strobe.

It does bog down if the adjuster screw is wound out a few turns, I'll need to go back out and count the number of turns but it'll be around 4. It's subtle but fastest idle is around 1/2 a turn from fully in.

Pilot jets for that carb look like this.

To verify we are on the same page, the idle adjust screw comes out the top of air box cover and holds the throttle slide open a small adjustable amount.

The mixture screw is accessed at the rear of the air box behind a rubber plug. The number of turns recommended depends on if you have original old Del Orto (coarse thread) or newer Spaco (fine thread). More here

Mixture screw adjustment, got a good link?

Yes on the same page idle adjuster screw that pokes out the top of the cover, wound inn until it contacts the slide then a further 2.5 turns.

Mixture screw at rear pretty sure it's a Del Orto (coarse thread), 0.5 turns out from fully seated seems best. Fully seating it drops revs slightly.

From memory the idle jet looks like the one pictured but I'll remove it to check, I also have a 24G T5 carb so there's a chance that a T5 jet could be in this carb and I think the plain bit at the top of those jets is shorter.

Yep.

But that won't cause excessive blowback, though you'll have to remove it with a toothpick or idk.

Did you check the condition of the rotary pad?

If you take the carb off, you can measure the gap. A .1mm feeler gauge should jamb between the crank web and the pad.
I inherited a bike that a PO went a bit much on the inlet, widening it
JB Welded it. A LOT less blowback. This was 2 years ago, and the repair still looks as when I did it, even with the E10 bath it gets.

Older threads, 1.5 turns out as a starting point.
If you use the method of mixture screw adjustment often mentioned, ie you open the slide a lot, you can partially open the progression hole and it will not die fully closed.

Too much ignition advance can also do this, but in my anecdotal experiences you have to be way off. If you were certain about TDC, it's difficult to be way off with the Vape. You can check the timing with this method if you don't want to wake the neighbors or fill your garage with smoke.
*This is 18 degrees advance, the "22" is from when I marked it with the original points stator. Middle of the 2's is 18.

https://www.youtube.com/shorts/z1_iLmhIsoo

I used the stopper in the spark plug hole rotate to it in each direction and split the difference method to get TDC, and I'll redo it to check.

Rotary pad is good, it had two thin gouges which I filled with JB weld.

I did lengthen the inlet but left the width alone, and I didn't go as far as the examples I found online. Measurements are from memory but I think it's now 35mm long, I think I extended it towards the rear by 6mm. The examples I saw online went to 40mm.
I really hope it's not this, that's a lot of steps back when I thought it was ready for the road.

It's looks like there's excess fuel around, it's pooling on the slide, that might be why the blow back looks excessive.

What size carb is this?

If you pull the carb, in addition to measuring the gap you can also measure the opening and closing intake angles using your degree wheel and a flashlight, which will now include your new crank's geometry. I'm not saying this is your issue, but it's worth noting.

Carb is a 24.24E, I also have a 24.24G that I could try.

I'm going to start with the ignition timing then work through the other suggestions. I'll probably get the time to do it on Wed or Thurs.

Thanks for all of the help so far.

I think I've solved it, and it was much simpler than expected, it was just flooded.

I ran through the sip vape instructions again this time with a helper double checking every step, not because it was in any way difficult, but if I'd made a mistake last time then I'd probably repeat it. Timing was already spot on and had also been confirmed by strobe.

Tried to start it and fuel came out the exhaust. Once all of the excess fuel was pumped out the blow back through the carb stopped.

I don't know why it didn't clear the flooding when I had it running last week, or why it flooded in the first place, but it's an easier and less concerning problem to deal with so I'm calling it a win.

The flooding is sorted along with the clutch problem that was covered in another thread. I'm now starting to look at jetting. I've read and bookmarked the jetting guide.

It runs best with the mixture screw fully seated to 1/2 turn out, it's still ok at 3/4 but any further than that and to keep it running the idle needs to be turned up.

I'd like to know if running well with the adjuster fully seated really does indicate some kind of fault as I've read in a few places or if it simply indicates that the idle jet is too rich?

Response just off idle is good, return to idle is quick, tickover is a bit lumpy but for all I know they might all be like that or it might be the light vape flywheel.
Mid throttle is ok.
I've avoided full throttle so far. I don't have a good feel for this I've only had any success holding bikes flat out and seeing if a change makes it faster or not which I'm reluctant to do until after the specified 1 hour running in.

Focus more on the mixture screw and less on the idle screw. Just adjust the idle screw to a fast idle and then follow Jack's instructions for the mixture screw—back it out 4 turns and then turn it in slowly until you hear the revs increase, indicating the mixture is going lean. Then back it out to the point just before the revs increase and call it done. Now, you can return to the idle screw. There is no set number of turns. If you have a tach, 900 rpms is a good target. Otherwise, just set the idle as low as it will go without stalling.

^^
Only things I'd add is this has to be done at operating temperature, and in my experiences 900rpm at idle is close to impossible if you want to avoid stalling once hot with idle set rich enough for gear braking and a Vape flywheel.

The position it ends up at from that approach is 3/4 out from fully seated. The sustainable minimum revs is around 1100, 1300 seems better.
That all seems to be ok other than the fact that it'll run ok fully seated and I'm not sure it should, as well as on every other carb I've set up being well outside of the expected range on an adjuster generally means a jet change is needed.

Something wrong here. Although it runs and rides 3/4 out is not enough.
Is the jetting still as the first post? If so, main jet is way too big, this is flooding the pilot.

This makes sense to me.

Yes it's still as the first post. I've got 130, 128 and 125 mains and intend to step down them once it's ran in a little, it's had 40 min of the specified 1 hour. Sounds like I need to just set the idle mixture on rich side to be safe and focus on getting the main right before coming back to it.

The safest thing to do is change the AC to 120. This should be rich enough for your set up.

What range would the main jet be in with a 120 AC?

I've seen posts from other people running 125/126 with a 160 AC
I'm working through some searches looking for for what people are running with a 120 AC, the first one I've come across is120AC-BE4-128MJ

What carb is yours? And what box/filter/autolube?
Looking back up the thread What's that T5 pilot jet doing in there.

24.24E
Premix (no oil tank)
Auto lube carb box with oil pump blanking plate
Genuine drilled filter should arrive on Friday, an undrilled lml with blue layer removed is currently fitted
T5 pilot jet is because I bought the wrong one, if it makes a difference I'll add the right one to this jet order

I think we're closing in on your problem. It appears a T5 uses a 50/100 idle jet. If, as you said, BGM calls for a 55/160 for this kit, a 50/100 would be way too rich. Definitely add a 55/160 to your order and let us know how that works out.

I think Kowalski is right about the pilot jet. Probably a stock T5 one. I would still go richer on the AC to start with. If the AC is too big, it can lean out in mid range, even while being rich low down, feels great but rapidly increases piston wear.

AC120/BE3/125 52/120 would be where I would start on your carb and cylinder.

The idle jet is a 55/160, it's just T5 length.

AC120 and 52/120 ordered, I've already got a 125 main.

I have seen there is a firm in the UK selling T5 length idle jets in various sizes for people who want to use a 24.24G on something other than a T5. You also could cut a normal one down yourself. Either way, fluid dynamics is complicated and there is more to it than the size of your orifice. I don't know for sure but, in this case, I suspect a short tube 55/160 flows less air than a long tube 55/160, effectively making the short version richer. If it was me, I would still want to get my hands on one of the BGM recommended standard 55/160s.

125 is likely the biggest the MJ will be. If you had all the jets from 115 to 125, you'd have it covered.

As said T5 jet in an E carb is going to give weird results. Stick to the correct jets.
KMT do a set of 120 air pilots. Expensive for needing just one.
https://www.sip-scootershop.com/en/product/idle-jet-set-set-kmt-120-38-52_KM120400

I probably should have ordered more mains, but since I didn't I'll see how the 125 feels before putting another order in.

I normally use KMT jets, that's also where the T5 55/160 came from, I've got their jets in three other bikes at the min. You can buy them individually from their website or eBay store.

125 main fouled the plug and wouldn't rev out on the stand so it didn't get a proper road test.

122 main
Max temp 305F
Top speed up test hill 65mph
In second and third it feels like it's not revving out cleanly but it's at 8000rpm by that point and I've never had a good feel for fuelling so I'm not sure if I should risk trying a 120.

It feels pretty close throughout the throttle and rev range, close enough that I'll need more time on it to work out where it could be improved.

If 125 was so rich it fouled, then 122 is not the correct jet for you. 3 sizes down, like 118, then maybe.

3rd gear is more difficult because of the wind noise. Hold it out in 2nd and listen to where it hits a blubbery limit.

Whats all the jetting now?

52/120 120/BE3/122

The plug had been used with the previous jetting so the foul may not have been entirely due to the 125 main, but it was definitely too rich to rev out. I need to pickup a couple more plugs, it's on the last fresh one now.

In both 2nd and 3rd it felt the same, maybe blubbery, like an extra soft rev limiter at 8000rpm. In 4th it didn't reach 8000 but I only had it pinned up hill. I can't tell much from the sound, it did change at this point but I couldn't describe it. I've got some mild heading loss and I struggle to make out anything subtle when there's background noise.

I've got a 120 and 118 ready to try, I'm just being cautious after soft seizing the previous motor twice, and the performance now is more than adequate.

With that jetting and what you described, it will be ok to go straight to 118.

Thanks, I'll try it at the weekend

Went to work on it today, to much traffic on the way there but I got to open it up through the gears a couple of times on the way home and it seemed to rev out cleanly every time. No noise change or sensation, max revs was the same. I'm going to have a few more rides to try to get a feel for it before making any more changes.