Low throttle stutter PX150
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GS/PX150/P200/VBA/90/Li150s1
Joined: 11 Jun 2019
Posts: 6
Location: Squamish
Tue Jun 11, 2019 12:12 pm quote
Last year I bought a stock PX150 05í, still in the crate after 14 years. Had all the stock jetting and catalyzed exhaust. Replaced those with the SIP Road 2 exhaust and usual suggest jetting (102 main, etc). The first 1/8th of throttle has a Ďcoughí/rough running, missing sort of thing going on, but the mid and upper throttle are beautiful and torquey. I canít figure out whatís up with the lower end of the throttle. I donít think it is a jetting issue, iíve swapped the stock 50/160 and 55/160 back and forth, tried dialing the mixture in. Starts fine, idles fine, just struggles as I start off or lower the throttle to 1/8th or so.

Any ideas? Is this just a byproduct of having the added backpressure of the road pipe? Or maybe timing, or air leak? Any help appreciated.
Addicted
'64 Motovespa 150S (177) , '64 V90
Joined: 27 Mar 2013
Posts: 570
Location: S.Salem, NY
Tue Jun 11, 2019 12:32 pm quote
idle mix or jet. sounds like when your main comes in it's fine.
Member
GS/PX150/P200/VBA/90/Li150s1
Joined: 11 Jun 2019
Posts: 6
Location: Squamish
Tue Jun 11, 2019 12:37 pm quote
Moto64 wrote:
idle mix or jet. sounds like when your main comes in it's fine.
Does it sound rich or lean?
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4907
Location: So Cal
Tue Jun 11, 2019 12:41 pm quote
Rich
Member
GS/PX150/P200/VBA/90/Li150s1
Joined: 11 Jun 2019
Posts: 6
Location: Squamish
Tue Jun 11, 2019 12:48 pm quote
SoCalGuy wrote:
Rich
Thatís what I would have thought, but it doesnít make sense that I would have to put a leaner than stock idle jet in there, would it?
Hooked
1974 Vespa 125 Primavera, 1980 Bajaj Chetak, 1962 Lambretta 175TV3, 2006 Yamaha Vino
Joined: 07 Jul 2017
Posts: 434
Location: Boston, MA
Tue Jun 11, 2019 12:48 pm quote
following. I have the same problem with my 74 Primavera, but for about the first half throttle. Weird thing is that it ran fine for several hundred miles before giving me any problems.
Ossessionato
73 Rally, 74 Rally, 76 ET3, 80 P200, 63 Li125, 65 Li225 Silver Special, 86 Elite 80, 2015 HD Road Glide Special, 2011 Ural Tourist
Joined: 18 Jan 2012
Posts: 3306
Location: Oceanside/ SF
Tue Jun 11, 2019 1:18 pm quote
First quarter twist of your throttle is the idle circuit/ air fuel screw. Have you cleaned the carb well since uncrating it? How many turns out is your air fuel screw?

Once youíve adjusted it correctly it should rev up as clean as you can get it from idle to half throttle. Itís not going to rev up like a race car since you need to get the flywheel/ engine mass spinning, but it should dip as little as possible before revving up cleanly.
Addicted
'64 Motovespa 150S (177) , '64 V90
Joined: 27 Mar 2013
Posts: 570
Location: S.Salem, NY
Tue Jun 11, 2019 3:38 pm quote
Have you tried just turning in the mixture screw 1/4 turn at a time to see how it responds ?
Member
GS/PX150/P200/VBA/90/Li150s1
Joined: 11 Jun 2019
Posts: 6
Location: Squamish
Tue Jun 11, 2019 10:21 pm quote
Moto64 wrote:
Have you tried just turning in the mixture screw 1/4 turn at a time to see how it responds ?
Yes, I've been through this a few times, setting the mixture screw where it seems optimal, then doing micro adjustments both in and out.

I haven't cleaned the carb since I got it, I guess that it the next step. Could be something in the passageway that the idle jet screws into.

One thing to make clear, the rev up from idle is very clean if I'm not in gear, this problem arises when I try to move the bike, and when I'm going fast and I drop the throttle below 1/8th.
Ossessionato
LXV 150 3v ie. Midnight Blue (Sold)
Joined: 06 Dec 2010
Posts: 3003
Location: Bangkok
Tue Jun 11, 2019 11:05 pm quote
It's the rarefied atmosphere in Squamish?

What speed is the idle set to. It think it is supposed to be quite high around 1800-1900 rpm. then adjust mixture screw until idle is maximum then lean 1/4 turn, then adjust idle screw back to 1900.

Bung some fuel system cleaner into the tank.
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1581
Location: London UK
Tue Jun 11, 2019 11:32 pm quote
Are you getting good WOT plug colour? WOT affects the pilot on an SI carb.

55/160 should really work (should be too much). And you are describing exactly what a weak pilot circuit does.
Hooked
PX 150
Joined: 06 Aug 2007
Posts: 327
Location: Dublin, Ohio
Wed Jun 12, 2019 7:49 am quote
A couple of months ago I found a 2005 PX 150 that also was brand new. I put a SIP Road 2 exhaust on it and it is running well. I did switch to a "drilled" air filter which is an original Piaggio part that is less that $20 from scootermercato. I am running a 102 main, a BE1 mixer (OEM was a BE5), a 160 air jet and the OEM 50-160 idle jet. My mixture screw is about 1/3 turn out from stock. Also check that your carb mount bolts are tight--they have a habit of loosening with the new bolt style. They only need 10 to 11 ft/lbs of torque, but they are often loose and tend not to stay tight. When cold, mine idles fine but won't take throttle for about 30 seconds until it starts to warm up a bit. After that it runs great. I do have an ongoing nagging issue with what feels like fuel starvation at high speeds on an incline when the tank drops below 1/8, and I have changed the fuel tap and fuel line but it persists. My guess is that it just the fuel being away from the tap when the level drops low enough and I am heading up any incline in the road. Stock I was getting a top speed on the speedo of maybe 50 mph, which is under 45 mph real speed. Now I am seeing about 64 mph, which is just shy of 55 mph on the gps.
Member
PX 125/Polini 177
Joined: 07 Jun 2019
Posts: 22
Location: Gloucester UK
Wed Jun 12, 2019 8:34 am quote
Mixture screw
Canít help with this, but I am trying to set my carb at present. The mixture screw is impossible to adjust, it would have been easy on the pre electric start version, but the starter gets right in the way. Iíve tried heating and bending a 7 mm spanner, but thereís not enough room to get enough torque to turn the screw more than about a tenth of a turn. Iíve ordered a 7mm ratchet ring spanner, any other tips?
Moderator
PX150 and 1 drowned P200,
Joined: 28 May 2008
Posts: 3880
Location: Hustletown, TX
Wed Jun 12, 2019 8:54 am quote
Clean your carb and jets thoroughly.
Quote:
Mixture screw
Canít help with this, but I am trying to set my carb at present. The mixture screw is impossible to adjust, it would have been easy on the pre electric start version, but the starter gets right in the way. Iíve tried heating and bending a 7 mm spanner, but thereís not enough room to get enough torque to turn the screw more than about a tenth of a turn. Iíve ordered a 7mm ratchet ring spanner, any other tips?
I once had slotted my mix screw that someone had done with a grinder. Allowing me to just use a screw driver like with the old delorto carbs. Worked a charm.

I suspect you would have to be very careful not to fuck up the threads when holding it for the grinder. (and be very careful not to slot your thumb and forefinger in the process.)
Member
PX 125/Polini 177
Joined: 07 Jun 2019
Posts: 22
Location: Gloucester UK
Wed Jun 12, 2019 9:28 am quote
Mixture screw
Birds nest, my mixture screw is already slotted, the problem is that the starter motor is in front of and close to the screw. It is impossible to get a screwdriver into the slot without removing the entire starter motor. Spanner is the only option I can think of, but there isnít room to manipulate one enough to turn the screw. Iíd forgotten how awkward and frustrating scooters are to work on!
Ossessionato
73 Rally, 74 Rally, 76 ET3, 80 P200, 63 Li125, 65 Li225 Silver Special, 86 Elite 80, 2015 HD Road Glide Special, 2011 Ural Tourist
Joined: 18 Jan 2012
Posts: 3306
Location: Oceanside/ SF
Wed Jun 12, 2019 9:58 am quote
Re: Mixture screw
Beedude wrote:
Birds nest, my mixture screw is already slotted, the problem is that the starter motor is in front of and close to the screw. It is impossible to get a screwdriver into the slot without removing the entire starter motor. Spanner is the only option I can think of, but there isnít room to manipulate one enough to turn the screw. Iíd forgotten how awkward and frustrating scooters are to work on!
Bee- Do you own a dremel tool? A hack saw and a vice? If you trim the mixture screw short enough that the screw is inside the air box itíll be much better in the long run. Not only will you be able to rest a screwdriver in the air box hole and adjust a little easier (providing you have a long enough one to get around the starter) but youíll also be able to remove your carb in seconds, not hours. Be sure to add the slot after youíve done your cutting.

I had a long air fuel screw and fucked up the fine pitch threads using the bent spanner method trying to re insert it after a cleaning.
Hooked
PX 150
Joined: 06 Aug 2007
Posts: 327
Location: Dublin, Ohio
Wed Jun 12, 2019 11:03 am quote
Re: Mixture screw
Beedude wrote:
Canít help with this, but I am trying to set my carb at present. The mixture screw is impossible to adjust, it would have been easy on the pre electric start version, but the starter gets right in the way. Iíve tried heating and bending a 7 mm spanner, but thereís not enough room to get enough torque to turn the screw more than about a tenth of a turn. Iíve ordered a 7mm ratchet ring spanner, any other tips?
I use needle nose pliers that are 10 inches long and that have the tip bent at about a 45 degree angle. It really is pretty easy to reach the screw behind the starter with them. Hold them horizontally, grab the screw, and turn it. They look like this and are available pretty much anywhere:

https://www.homedepot.com/p/TEKTON-11-in-Long-Reach-45-Degree-Bent-Nose-Pliers-34404/206200670?MERCH=REC-_-PIPHorizontal2_rr-_-207096548-_-206200670-_-N
Member
PX 125/Polini 177
Joined: 07 Jun 2019
Posts: 22
Location: Gloucester UK
Wed Jun 12, 2019 12:41 pm quote
Mixture screw
Thanks guys, Iíll let you know how I get on. I have a dremel, hacksaw and vice, will travel.Graham
Hooked
2005 Vespa PX 150
Joined: 23 Feb 2010
Posts: 241
Location: Houston, Texas USA
Wed Jun 12, 2019 7:29 pm quote
Re: Mixture screw
Beedude wrote:
Canít help with this, but I am trying to set my carb at present. The mixture screw is impossible to adjust, it would have been easy on the pre electric start version, but the starter gets right in the way. Iíve tried heating and bending a 7 mm spanner, but thereís not enough room to get enough torque to turn the screw more than about a tenth of a turn. Iíve ordered a 7mm ratchet ring spanner, any other tips?
You need one of these... works great!
See this link:
2005 PX150 carb air mix screw

idle mix screw wrench.jpg

Member
GS/PX150/P200/VBA/90/Li150s1
Joined: 11 Jun 2019
Posts: 6
Location: Squamish
Wed Jun 12, 2019 9:59 pm quote
apex wrote:
A couple of months ago I found a 2005 PX 150 that also was brand new. I put a SIP Road 2 exhaust on it and it is running well. I did switch to a "drilled" air filter which is an original Piaggio part that is less that $20 from scootermercato. I am running a 102 main, a BE1 mixer (OEM was a BE5), a 160 air jet and the OEM 50-160 idle jet. My mixture screw is about 1/3 turn out from stock. Also check that your carb mount bolts are tight--they have a habit of loosening with the new bolt style. They only need 10 to 11 ft/lbs of torque, but they are often loose and tend not to stay tight. When cold, mine idles fine but won't take throttle for about 30 seconds until it starts to warm up a bit. After that it runs great. I do have an ongoing nagging issue with what feels like fuel starvation at high speeds on an incline when the tank drops below 1/8, and I have changed the fuel tap and fuel line but it persists. My guess is that it just the fuel being away from the tap when the level drops low enough and I am heading up any incline in the road. Stock I was getting a top speed on the speedo of maybe 50 mph, which is under 45 mph real speed. Now I am seeing about 64 mph, which is just shy of 55 mph on the gps.
Thanks! Looks like I need to go through the carb and make sure everything is clean, and ensure the carb is on tight. My jetting is the same as your save for the mixer, I am running a BE3. If my carb work doesn't solve the issue I will try a BE1. Cheers!
Molto Verboso
2005 PX150 In a Love-Hate Relationship with a 2-Stroke Vespa Since 2007
Joined: 01 Sep 2007
Posts: 1813

Thu Jun 13, 2019 11:10 am quote
This is how you fix the starter being in the way, you remove it and plug the big hole with a rubber grommet.

http://scooter-speed.com/vespa-electric-start-rubber-plug-piaggio-8008-7823j000/

I never use mine anyways except maybe if the engine somehow gets flooded just to save from having to kick it a dozen or two times. Even when cold it should start on one two kicks, so IMHO itís really not worth having in the way.
Hooked
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
Joined: 06 May 2016
Posts: 286
Location: Central Ohio
Thu Jun 13, 2019 11:42 am quote
I too have a US 2005 PX150. It had just 368 miles on it when it got it. I added the SIP Road 2 muffler and re-jetted it (102/E1/150). I am using the leaner 50/160 slow idle jet instead of the 55/160. It ran great except for an off idle bog. What solved this problem was turning the idle mixture screw out (CCW) about a half turn from stock making that part of the fuel circuit a little richer. I took Apexís advise and got a pair of 11 inch pliers with the tip bent at 45 degrees. This tool allowed me to reach the idle mixture screw and turn it with the electric starter in place. I like the electric start and have no intention of removing the starter. Check the two carb installation bolts for the proper torque as suggested by Apex and keep tweaking the jets. You will find the optimal settings for your bike and it will run great.
Molto Verboso
2005 PX150 In a Love-Hate Relationship with a 2-Stroke Vespa Since 2007
Joined: 01 Sep 2007
Posts: 1813

Thu Jun 13, 2019 6:35 pm quote
@RB
So, you went with an E1 mixer? My 05 came with a BE5, which I later replaced with a BE3 just because the older PX150s came with a BE3, but Iíve basically struggled with the same issue and thatís the one component I havenít revisited, only because I have the darndest time wrapping my head around how to even select a different one.
Hooked
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
Joined: 06 May 2016
Posts: 286
Location: Central Ohio
Thu Jun 13, 2019 8:18 pm quote
Pdxjim

I ended up using the E1 since it gave me the best performance. The E1 is similar to the BE5 and it is leaner than the BE3. The E1 just seem to work better with the 102 main and the 150 air corrector which I kept constant during the atomizer (emulsifier) testing. I also am using the 50/160 slow idle jet which is leaner than the usual 55/150. There are other factors as well including using the drilled air cleaner and turning out the idle mixture screw about a half turn from the original setting. All in all, this combo seems to work the best on my 2005 PX150. I also have the SIP Road 2 muffler installed.

The chart below shows how these different atomizers compare. The BE5 (similar to my E1) gives a correct mixture (5) in the low to medium throttle range and is slightly lean (4) at WOT. The BE3 is leaner in the low range (3) but richer in the upper range (6). Since I usually do not ride at WOT, the E1 (BE5) works best for me. Also note that the BE1 is similar to the BE5 in behavior across the throttle range but is slightly richer with more surface area of holes in square mm (10.1 for BE1 compared to 8.3 for BE5). However, the BE1 is richer in the upper section (38% vs 24% for the BE5) and leaner in the lower (62% vs 76% for the BE5). The main jet and air corrector also factor into this. Charts are great and can point you in the right direction but putting the various atomizers to the test is the best way to get the one best suited for your bike and riding style.

emulsifiers_3_10418.jpg
Atomizer chart.

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