Carb adjust changing - cowl + box top off vs. on
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Hooked
'79 P200e
Joined: 29 Apr 2017
Posts: 116
Location: Northern California
Mon Dec 31, 2018 2:48 pm quote
All -
It's been a while since I've posted. My 79 P200e has been running like a champ. I have the SIP Road 2 on there and upped the main jet to a 118. Otherwise the bike is stock, stock, stock. Del 24/24 with mixer - stock jets except main for SIP road 2.

The weather has changed here - cooler, more humid and I was thinking the carb could use a slight adjust so I popped the cowl off, opened the carb box - checked the filter etc. I wanted to make sure there weren't any unexpected leaks etc. Everything checks out.

I fire the bike up - with carb box open and cowl off and go zipping down the street and feel like I'm flying. Bike sounds great, super smooth through all the gears and just responsive as hell - super stoked.

As I button it all up I have a smile on my face and think about how great my Vespa is and then go for another quick ride. Now, however, it's not quite the same - not as smooth etc. Not 4 stroking, backfiring or anything - just not as amazing as when all that air was getting in there.

I've spent the last hour searching "carb tune" trying to find mention of this situation but came up empty.

Can anyone confirm what I'm experiencing and make a recommendation? I know there are mods to carb air intakes and all sorts of things - but I'm leery of changing stuff on my mostly stock bike.

Of course I could lean stuff out a bit more but I'm not sure that's the same effect. I feel like I'm super close to the right tune on the carb- and it's very adequate the way it is - but I'm a touch OCD.

Thanks in advance and happy new year all!
Hooked
68sprint,65super,64Allstate,55 Allstate
Joined: 20 Jul 2017
Posts: 241
Location: Central california
Mon Dec 31, 2018 4:31 pm quote
Carb adjust
Well not sure what to tell you on the carb adjust. I have a 68 sprint with a 1983 P200 engine. Mostly all stock with the exception of a modified muffler. The jets in my 24/24 si are
55/160 pilot
125 main
BE3 mixer
160 air bleed
Plug is a bit dark but certainly not black. It runs awesome. Like you Iím almost afraid to mess with it.
Remember the ole saying ď if it ainít broke donít fix itĒ
Just ride....scott
Molto Verboso
Joined: 26 Oct 2015
Posts: 1252

Mon Dec 31, 2018 11:11 pm quote
A 118 main is on the lean side. If you haven't done it yet, I'd start out with a 125 main to be safe for doing some plug chops, and then work your way down from there.
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1032
Location: London UK
Tue Jan 01, 2019 2:56 am quote
The main jet mostly affects wot. Riding with the box open, I expect is leaning out the much more sensitive pilot jet. The stock pilot jet 55/160 is nearer to rich on most stock scooters. Running a weaker pilot like 48/140 and a bigger main like 122 usually works out better.
Hooked
'79 P200e
Joined: 29 Apr 2017
Posts: 116
Location: Northern California
Tue Jan 01, 2019 2:47 pm quote
Hey Jack221 - I'm a little confused by your comment about the 48x140 and I hope you can explain.

For the pilot jets - I'd read somewhere that you divide the larger number by the smaller and got a number that you can use to compare to other jets - a richness rating.

Stock pilot of 55x160 would have 2.90 vs the 48x140 having a 2.91 - those seem pretty similar. This makes me think I don't understand something about the mixing and it's not as simple as comparing one number to another.

I've had a 50x160 in there with my 118 main jet but the lower 3rd throttle felt rough. I assumed it was a little too rich - but the top 2/3 were perfect. In this setup - I was thinking I could adjust either the main jet or the pilot and then adjust the entire thing with the needle.

I do have a box of main jet options as well as the following pilots:
50x120, 55x160, 45x160, 50x160

Aside from the stock combination with the 55x160 pilot - should I give another combo a try?
Ossessionato
1979 P200e
Joined: 18 Mar 2013
Posts: 2297
Location: West of Boston
Tue Jan 01, 2019 3:03 pm quote
You want advice? Don't mess with what already seems to work fine for you.

A few years ago, I rode all spring just fine, then went to get my yearly inspection. Pulled in, went through the routine with lights and horn, and went to get lunch next door at a sub place. Started the scoot and rode to the hardware store, then home. Somewhere between the sub shop and the hardware store, the scoot started to run funny (big dead spot after idle, and idle very high), and it took me three tries to clean the jets to get it to run right again.

So I would say, clean your air filter, clean both of the jets, and put it all back together (with carb box cover on). If it runs fine, then scoot your heart out. If it doesn't run fine, go back to cleaning the jets again.

As for differences in weather... unless you're riding rage goes from freezing to 110+ F, then you probably don't need to adjust the jets. And unless you're riding from sea level way up to significant altitude, then you probably don't need to adjust the jets.

Part of the problem, as I see it, is that it's so easy to change the jets, and there are so many choices and combinations that will balance out correctly, that you start to play a little too much, and you can get all sideways with your jetting if you're not careful.

Think of it like blackjack. It's easy to get 21. Jack and Ace are usually shown as the ultimate hand, but you can match an ace with a 10 or any face card... or 10+3+8, or 10+5+6, or 8+7+6... you get the idea.
Hooked
'79 P200e
Joined: 29 Apr 2017
Posts: 116
Location: Northern California
Tue Jan 01, 2019 3:11 pm quote
Hi Larry -
Yeah, I totally get it with the different ways to make blackjack - which is what sort of confuses me about jets and this carb.

I'm fine to leave it as is - I was just surprised that I could notice a difference with the carb box off and wit it on and covered - no adjustments to the needle.

Again - I don't think there's anything "wrong" it's just that it was more smooth with the carb open.
Hooked
68sprint,65super,64Allstate,55 Allstate
Joined: 20 Jul 2017
Posts: 241
Location: Central california
Tue Jan 01, 2019 8:18 pm quote
Carb adj.
Just a FYI....theses carbs donít have needles like I think your referring to. They do have a needle and seat, but do not have a needle like a flat slide motorcycle carb would have. If your going to make a few changes donít forget where you started from....you will most likely end up close to where you started. Scott
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1032
Location: London UK
Wed Jan 02, 2019 5:02 am quote
Tend to agree. If you think this carb has a slide needle, then maybe this isn't a carb you should be fiddling with. All the jets and air jets in an SI carb have major overlap. More than any other carb I ever worked on. Can get really confusing and damage can occur, if you don't quickly recognise the signature sounds of weak jetting.

The 48/140 is 2.92 and the 55/160 is 2.91 but the dividing thing, although not incorrect, is far from the whole story. As with any emulsion tube the fuel mixture is delivered on a curve. The dividing gives an indication of the total fuel flow at full vacuum pressure but says little about the delivery curve, which is equally important. Being quite OCD I set up my carbs to run totally smooth from zero to WOT and am very happy to spend time setting up each scooter.

That said the AC160 BE3 MJ122 and 48/140 does usually perform better than stock on most stock scooters.
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4395
Location: So Cal
Wed Jan 02, 2019 7:54 am quote
118 main is fine with your setup. Try a 48/140 idle jet, as Jack suggests.
Hooked
'79 P200e
Joined: 29 Apr 2017
Posts: 116
Location: Northern California
Wed Jan 02, 2019 6:36 pm quote
Hi All - sorry no, I realize the term needle is wrong for this carb - I was refering to the mixture screw to lean or richen the carb. It is pointy and delicate but not the same as a PHBH or anything like that.

Jack!! THAT was the piece of the puzzle I was looking for but couldn't seem to find. Thank you for that explanation.

Aside from trial and error - is there a way to measure if the carb is working better or worse with the changes I'm making besides: Feel, Plug Check or Dyno?

I'm going on feel mostly - plug checks are sort of a hassle and I have no idea where I can find a dyno, let alone someone who works on Vespa's up here near Sacramento.

Thanks all... ordering a new jet and a renewed sense of hope!!

PT
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