Vespa main jet sizing

[thomasvespa]

Hi I am trying to find a start point for jet sizing whilst fitting a sip 2 road exhaust on my Vespa. Currently the 1978 vespa super 150 has a ox engine fitted with a competition crank and mallossi 166 fitted. It runs fine on standard exhaust having just rode to Iow last week. The main jet fitted to 26mm carb is 130. Previously I have replaced fuel tank tap to fast flow but carbs still emptying, nearly had soft cease today so put standard exhaust back on. I was thinking of ordering a 148 push in main jet. But concerned after soft cease. Any advice welcome. Carb in speco deortio. Any advice appreciated. Ton

[gpowell]

130 seems big, never mind 148.A SIP Road 2 or 3 wouldnt need to go up that much from standard

Also dependants at what throttle position you are having problems as it may not be the main jet but mid range.

[thomasvespa]

I was surprised to find the 130 main jet earlier today when I stripped her and have no explanation. The px engine runs ok at lower speeds with a noticeable boost in power on the new exhaust, but soon as I get 4th gear and the last quarter of the throttle she chokes as if the fuel ran out with the new exhaust.  If I coast along she seems to replenish and all ok again.  Its like running out of fuel and switching to reserve and there’s a small delay until the fuel tops up.  What’s strange is I can sit for an hour on the motorway with the standard exhaust box close to full throttle.  Trying to not let it beat me as just brow beaten into accepting the standard exhaust would be frustrating. My thought process was go big (148) and see if I can stop it running dry on fuel then let the plug colour tell me if I need to reduce size again. Tom - I fitted the SIP quick flow tank tap a few days ago eliminating the supply problem, I can only think main jet left to try. Slightly out my depth, just a thought could the main jet be oversized and causing problems? The more I read online I am concerned about fitting a jet size outside the normal jetting range. I have read tonight that 26mm speco delorto carb cannot handle jets above 135. Not sure how true this is? Could it I’m at the limit with carb and it’s time to consider change to a more modern carb?

[jackthekipper]

a 130 jet should be well big enough for your set up...personally I would'nt go higher..problem could be petrol flow....someone will correct me if I'm wrong

[scooterbooter]

Yeah that sound too big. it could just be a dirty carb. You got a dirty carb son? Dirty devil. Need more info. And probably more brains. And yes a too big main will really affect top end performance in a bad way.

[thomasvespa]

The wisdom of experience has spoken on this thread and I think I should listen.  I will remove and clean the carb / jets / new gaskets and see if I can find a reason 130 jet isn’t handling a basic sport exhaust. I appreciate the advice and will come back with more info, other jet sizes etc. she cruises at 55 mh on a standard box which is why I was convinced or fooled into thinking main jet sizing issue.

[scooterbooter]

You have to be careful coming down jet sizes . But I think that's what needed. I say that but if you have a slight air leak the big main jet could be covering it up and you could get in trouble. Check your plug colour as you go down sizes don't just look for it running nice be careful.

[phattanglo]

If my own experience is anything to go by, don't assume the main jet is the culprit. A change of exhaust from standard could have an effect elsewhere in the rev range and you may find increasing your pilot jet and your AC jet will mean you only have to go up one or two sizes on your main jet.
I'm afraid I can't remember the sizes I changed to, but I can say that on a 125 I only had to go from a 96 to a 98 main jet when I got rid of the cat exhaust and all its crap and fitted a Sip road 2.
Effectively I went from 2011 Euro spec to pre cat by fitting a slightly larger idle jet, an earlier model throttle slide and I think a richer air corrector jet.
I appreciate your engine has been tuned so your circumstances are different but it is worth checking your jetting at all rev ranges.
You are probably correct about the petrol flow, but an engine will also struggle to rev out if it's running too rich.
I also experienced a number of soft seizes when I first got my scoot, turns out I had too hot a spark plug. I swapped for the correct plug and all was good.

[adrian]

The easiest way to get it correct is to go to a dyno

[thomasvespa]

Dyno may be last resort but no experience with it.
Just done a strip and clean and the jets are - Air corrector 160, Atomiser BE3, Main jet 130, Idler jet 52/140, choke 60.
Just waiting for the postman prior to re assembly (new gasket set) before test. Carb didn’t look particularly dirty though?
Online there seems to be a lot of threads about drilling out and modifying the carb with oversized float needles, not sure if I want to go down that line.
Currently I have the standard 3.5mm needle fitted (replacing as part of the clean) and I’m running B6hs plug.

[phattanglo]

Adrian is absolutely correct, the Dyno is your friend.

[gpowell]

Im running a tuned 200 top end, increased inlet timing, sip R3, drilled filter, 26mm SI with an larger pilot jet and standard atomiser and air corrector with 125 mj,which is rich.
Using sip fast flow tap

The 26mm SI comes with P200 jetting 160/BE3 and 55/160 pilot, so you are aleady richer on the pilot.

My 26mm carb body has had no modifications, only the banjo and the inlet in the top opened up. The cosa float chamber is an option, but when i used one it made no difference.
Mine is used fast touring long hauls.

Beware the online si carb mods, on the needle seat and fuel chamber. Some of those mods have now been withdrawn, other have more chance of ruining your carb.

The SI 26 in standard form should ableable cope with your set up and 130 should suffice.

I have seen SIP road 2 pipes put on with no change to jetting over standard.

I would strip the petrol tap and check its flowing correctly and that the fuel line is not too long or compromised. Also is it definatly soft seizing or missing under load, could be electrical. Would have thought a B8 plug would be more suitable.
When you rebuild the carb set the air screw at two and a quarter out and see how it feels once warmed up.

[ironsloth]

Where are you in the country? Lincs Lambrettas in Lincolnshire have a dyno saves a lot of agonising pain and a lot of leftover jets…

[thomasvespa]

Been a delay but back on the job today, and not going well. Looking for some advice from the more experienced tinkerer. I have stripped and rebuilt the carburettor of my Vespa super 150 - 1978 with a px engine mallossi 166 fitted. It has a 26mm Speco Dellorto carb after having fuel problems. I did fit after clean and marked improvement 99%. Decided to go for one more clean and On the refit I have found the rear carb retaining bolts is stripped and would not torque. Clear thread damage as a coil of aluminium was on the thread on inspection. Gutted, but how do I deal with it? the female side which is damaged is on the main engine so needs a home repair. I think it’s 5mm thread but not sure witworth etc. can I buy a kit to repair. Advice welcome. A wounded man.

[gpowell]

Its M7, if the inlet hasnt been enlarged you shoukd be able to get a helicoil in. Woukd suggest putting a stud in and using a sleeve bolt

[scooterbooter]

Yes agree with above when I thought about it I reckon helicoil it get a cut, internally and externally threaded sleeve hard core glued into casings then original bits can be used and you'll be tighten up to steel.but that's probably a bit to much drill the holes are quite tight .But the studs are a good idea. I think.

[scooterbooter]

Oh yeah and I'm feeling your pain that's just the sort of shizzle I do all the time. Hang on in there. It gets no better, but hang on in there bro , I got your back

[ironsloth]

Everything on your scoot will be metric, due to when it was built. No UNF, UNC, Whitworth, or any other weirdry..,

[gpowell]

Helicoils are very good if done correctly.
Make sure you get the correct depth, most kits use 1D size (length = 1 times the diameter) which is not long enough

Use threadlocker, BUT make sure it is only between the helicoil and the casing and not between the stud and the helicoil, so as to avoid accidental extraction of the helicoil.

You must have the right installation kit.
After drilling and tapping, blow out all traces of swarf.
Make sure the top surface of the coil sits just below the face surface before breaking off the tang.
Make absolutely sure you have removed the tang remnant from the hole.

If youve never done one have a practice in some ali

Then use the correct stud www.ebay.co.uk/itm/403641488131
And sleeve bolts www.t-a-s-s.co.uk/index.php?route=product/product&product_id=3426 plus you need 9mm flat and spring washer.

[ironsloth]

Always found this company good for machining tools:-

www.tracytools.com/helicoil-thread-repair-kits/metric-thread-repair-kits

Fast and reliable, the stuff they sell is good stuff, that link should take you straight to the helicoil kits.

[scooterbooter]

Here comes the best peice of advice ever.
Stud lock is quite good. Slightly stronger than thread lock.
Your welcome.

[thomasvespa]

Just back after a test drive and I am back on the road. I wish I had read this thread earlier because I did fit a helicoil but getting the bolt to start was a real pressure moment. I have got it fitted and torqued and all is well. I bought a heli coil set initially which included 5,6,8, 10, 12 mm every size but the one I needed. I had to re order m7. If anything goes in the future I’ve probably got it covered haha. The lesson learnt which I would change if I did it again would be to insert a stub permanently during the repair. Getting the bolt started was a real struggle after the coil was fitted. Plus the worry of future carb strips.  I found I had to remove the mixture screw to align the carb anyway which would be the same if I fitted permanently studs. The good news is the advice I received about dirty carb was spot on, I reluctantly stripped the carb although convinced it was a jetting problem  and thoroughly cleaned it using carb cleaner. It resolved my problem and  I now have Vespa with a sport exhaust capable of cruising  high 60s mh.  I do plan to get her dyno test at some stage for peace of mind, May have to. Wait until IOW next year as I live in Amesbury Salisbury Wiltshire. Not a big scooter following unfortunately and dyno facilities (proper scooter guys I can trust) are not locally in my area. Thanks for ever bodies input. Tom Vespa.

[scooterbooter]

Yippee ki yay. I LIKE IT!!! now go show em what you got. Give em hell 😤

[gpowell]

Well done on all fronts, nice when its resolved.

The older models used studs in casing and sleeved bolts to hold the carb in place. These used a smaller air screw so carb could come out without any issue ariund the air screw

The later type used the allen bolts and a longer air screw - again you could remove the carb no issue.
My concern with this a steel bolt into the ali casing, thats why i prefer the studs. I cut down the long air screw so it can stay in carb. I think you can buy a small screw for the newer carb now... As they are different threads.

[gpowell]

Join the vespa club and get to their members rallies.