Idle jet change
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Member
PX150
Joined: 11 Jul 2020
Posts: 24
Location: Essex
Sat Aug 01, 2020 4:51 am quote
I've got a flat spot on my PX150 with the throttle open, which I cant tune out with the mixture screw but I was just reading that a change of the idle jet might work. With a main jet of 116 I've got a 45-120 in there at the moment, would a 48-160 or a 55-160 help? Do i go up or down?

Cheers
Hooked
Stella 2T 150, Honda C70
Joined: 25 Sep 2019
Posts: 122
Location: Seattle
Sat Aug 01, 2020 3:45 pm quote
Whats the rest of the setup? What exhaust? Stock top end? What do you have for the rest of the main stack?

All that will help us with the diagnosis.

Hal
Member
PX150
Joined: 11 Jul 2020
Posts: 24
Location: Essex
Sat Aug 01, 2020 3:48 pm quote
whereshaldo wrote:
Whats the rest of the setup? What exhaust? Stock top end? What do you have for the rest of the main stack?

All that will help us with the diagnosis.

Hal
Thanks Hal, yes everything else is completely standard and pretty much rebuilt, sitting in a P200E frame. Ive got it as lean as i can with the mixture but i's still flattens on full throttle.

Cheers
Hooked
Stella 2T 150, Honda C70
Joined: 25 Sep 2019
Posts: 122
Location: Seattle
Sat Aug 01, 2020 4:05 pm quote
Unless you're running a performance exhaust, 116 seems a little rich to me actually. Given that you're in the UK altitude shouldn't be a contributing factor.

If I look at the Scooterlounge jetting chart (see http://scooterlounge.com/vespa/reference-library/vespa-stock-jetting-reference-guide.shtml) you should be running something between a 98 and a 102 main. I'm not the expert here on the differences between the 80s and the later PX150 engines or the differences between the European and the US versions but what you're describing to me sounds like you may actually be jetted too rich. I went through all of this with a Honda Passport a few years back and at WOT you can get what feels like flat or possibly bogging, but its just that the jet is contributing too much fuel relative to the air mixture. I know it feels counter-intuitive to reduce the jetting to go faster, but its all about air to fuel ratios and giving more fuel without giving more air isn't going to do anything. Reducing the fuel will bring the mixture into balance and you should get there. The idle jet and the mixture screw should really have no impact on the performance at WOT or even at half throttle.

Maybe start with a 100 and then if that helps then you can start to just play in that area if you want to really try to dial it in. It just means that you need a handful of jets, but once you get that feel for what's right and what's wrong, you may end up slightly adjusting things in the winter months to maintain that level of focus in the tuning. I'm not there yet with my Stella, but on my Honda I have a different setup for wet Seattle winter vs dry summer.

hal
Ossessionato
73 Rally, 74 Rally, 76 ET3, 80 P200, 06 PX150, 65 Li225 Silver Special, 86,95,96 Elite 80s, 2015 HD Road Glide Special, 2011 Ural Tourist
Joined: 18 Jan 2012
Posts: 3658
Location: Oceanside/ SF
Sat Aug 01, 2020 5:01 pm quote
Like others have said, get your jetting correct for your setup. That means, top end, carb and exhaust need to be close before you start messing with things.

After that, use the search button and read about drilling the heart out of your air filter.

Lastly, iíve heard you can swap the blue PX air filter with the P200 wire mesh. The blue air filter mesh is a little fine and restrictive.
Hooked
GTS 300 ABS, 2018
Joined: 07 Jun 2020
Posts: 144
Location: Finland
Sat Aug 01, 2020 11:25 pm quote
whereshaldo wrote:
Unless you're running a performance exhaust, 116 seems a little rich to me actually...
... you should be running something between a 98 and a 102 main
Spot on. My vote to this.
Member
PX150
Joined: 11 Jul 2020
Posts: 24
Location: Essex
Sun Aug 02, 2020 4:31 am quote
[quote="kaukari"]
[quote="whereshaldo"]
[quote="MJRally"]

Thanks all, will work on these suggestions this week. I actually do think it's an air issue as it runs great with the carb lid off!

The reason I've main jetted it at 116 is as the attached guide says to, but I have lower jets to try? But I'm trying to get the bike back to complete standard to run and smooth and quiet as possible, I'm not into speed.

Thanks again

Tony

Capture.JPG

Member
PX150
Joined: 11 Jul 2020
Posts: 24
Location: Essex
Sun Aug 02, 2020 4:33 am quote
MJRally wrote:
Like others have said, get your jetting correct for your setup. That means, top end, carb and exhaust need to be close before you start messing with things.

After that, use the search button and read about drilling the heart out of your air filter.

Lastly, iíve heard you can swap the blue PX air filter with the P200 wire mesh. The blue air filter mesh is a little fine and restrictive.
Yup ive got a wire mesh air filter in there, will look at drilling it out a little more.

Cheers!
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2217
Location: London UK
Sun Aug 02, 2020 4:57 am quote
Tony B wrote:
Thanks all, will work on these suggestions this week. I actually do think it's an air issue as it runs great with the carb lid off!

The reason I've main jetted it at 116 is as the attached guide says to, but I have lower jets to try? But I'm trying to get the bike back to complete standard to run and smooth and quiet as possible, I'm not into speed.

Thanks again

Tony
Mixture screw only works at or near tickover.

This jetting info is wrong. Allstyles made only a slight mistake. 150 jetting is between 96 and 116 depending on the rest of the jet numbers. Over the years the 150 by has had many versions of jetting.
Personal favorite version is AC140 BE5 102 main stack and 45/140 pilot. This is the fastest version with least long term damage.
Main jet will need to be adjusted up or down a little depending on your engine. Biggest jet that revs out clean is the one.
Member
PX150
Joined: 11 Jul 2020
Posts: 24
Location: Essex
Sun Aug 02, 2020 5:42 am quote
Jack221 wrote:
Tony B wrote:
Thanks all, will work on these suggestions this week. I actually do think it's an air issue as it runs great with the carb lid off!

The reason I've main jetted it at 116 is as the attached guide says to, but I have lower jets to try? But I'm trying to get the bike back to complete standard to run and smooth and quiet as possible, I'm not into speed.

Thanks again

Tony
Mixture screw only works at or near tickover.

This jetting info is wrong. Allstyles made only a slight mistake. 150 jetting is between 96 and 116 depending on the rest of the jet numbers. Over the years the 150 by has had many versions of jetting.
Personal favorite version is AC140 BE5 102 main stack and 45/140 pilot. This is the fastest version with least long term damage.
Main jet will need to be adjusted up or down a little depending on your engine. Biggest jet that revs out clean is the one.
Thanks mate, i'll try that. I'm gonna play with it over the coming days, something like a 48/160 idle with a 108 main jet to start with.

Cheers
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