PX Flat Spot from full to 3/4 when backing of with the revs,
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Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Tue Jun 28, 2016 10:43 am quote
Hi All..
For the last year after a total motor rebuild I have the following problem.
When riding full throttle on a nice open country road when I close the throttle to slow down there is a real sudden flat spot around 5500 Revs.
Its almost like a backfire or as if someone has suddely turned the key of with a sudden loss of power, but the motor recovers again after the flat spot.

Motor Set-Up.
Pinasco 177 Alu
Housing ported to match cylinder.
High compression Pinasco (central plug) head.
Mazuchelli Race crankshaft
24SI premix carb, 116 Main with BE3, hole drilled under the air filter.
Sip road 2 exhaust

The plug colour is perfect and I changed from a BE3 to a BE4 air mixture which helped a little but the flat spot is still there.
Timing is @19 degrees.
Running 3 % full synthetic Castrol oil.

Apart from this bloody flat spot when closing the throttle the Vespa runs beautiful...

Any ideas what the hell this could be?

Thanks for any tips of wisdom

Cheers
Robbie
Enthusiast
Px kitted and blinged
Joined: 03 Mar 2016
Posts: 96
Location: Uk West Yorkshire
Wed Jun 29, 2016 7:48 am quote
Put the be3 back in. You ARE going to heat size at higher revs.

Try taking the carb tray cover off. I know it sounds weird but give it a go.
Ossessionato
1964 GS 160 MK II, 1967 Vespa GT, 1968 SS180, 1964 Vespa GL, 1964 Vespa VBB, 2006 Buddy 125, 2013 BMW C650GT
Joined: 07 Apr 2008
Posts: 3032
Location: Boca Raton, Florida
Wed Jun 29, 2016 8:32 am quote
What are your temps?
Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Wed Jun 29, 2016 10:52 am quote
Solo Rider wrote:
Put the be3 back in. You ARE going to heat size at higher revs.

Try taking the carb tray cover off. I know it sounds weird but give it a go.
Hi Solo Rider, If removing the carb cover solves the problem then it means Im not getting enough air to the carb right?
I guess i can try your test and then move to a Venturi set-up if Im not getting enough air..

It just sounds strange though because I know other people with the same set-up and they dont have issues with the carb set-up..

Anyway every Vespa is different, maybe I have a problem child...

Cheers Robbie
Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Wed Jun 29, 2016 10:54 am quote
ScooterRaton wrote:
What are your temps?
I dont have any type of temp gauge so no real idea.
I can only judge by the colour of the plug which looks spot-on..

Cheers Robbie
Enthusiast
Px kitted and blinged
Joined: 03 Mar 2016
Posts: 96
Location: Uk West Yorkshire
Wed Jun 29, 2016 11:56 am quote
Hello Robbie

At work at the moment and using my phone to post so not ideal, it's a long story I had standard T5 with the same issue as you, on mine removing the carb tray cover cured it but that was only found by accident, I thought fuel as at the same throttle position when I came off the revs the engine would stop, I could then count 1 2 3 and it would start again, usually happened approaching junctions.

Anyway after I sold the scooter I was in the local shop buying a Mallosi kit for my new scooter, conversation went to carbs so I told the story.
He said and he is a scooter racer in his opinion it could have been caused by a missing tray that sits in the frame just above the cowl catches, he thought a vacuum was being caused in the carb tray which then starved the carb of air only when the engine overcame this did it fire up. The tray was there to stop this happening.

Now that explanation could be complete rubbish just passing it on.

But do put the be3 back in.
Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Fri Jul 01, 2016 9:43 am quote
Thanks Solo Rider I will try the plastic tray cover as I dont have one fitted as mine was cracked... I will also go back to the BE3.
Thanks for the Tips
Enthusiast
Px kitted and blinged
Joined: 03 Mar 2016
Posts: 96
Location: Uk West Yorkshire
Sat Jul 02, 2016 8:51 pm quote
Hello Robbie

One more point to note, not so much as a cure to your current problems, but it is from my experience as a T5 owner and a few lessons learnt and passed on with good results.

The T5 has a larger air filter this has to be because the folk at Vespa must have recognised a limit to the standard height filter, its worth keeping in the back of your mind, I wouldn't act on it as you say you have good plug colour. On my scooter I recently bought a new oem (sold as) air filter and my plug is now sooty, the conclusion being there must be an air flow restriction going on, which is this mornings challenge to try and sort out, I'll be going up and down the local dual carriageway again! Is it the filter or has the carb tray cover been re-seated in a different way. So my first test is riding without the carb tray cover on and see what happens, if nothing then I will modify the filter to give greater air flow.

On my T5 not only did removing the carb tray cover resolve the cutting out problem, the engine revved more freely, I passed on this tip to others and one bloke who had kitted his cut the lump off the top and was astounded at the increase in acceleration and overall performance. On my T5 I drilled large holes at the frame side of the cover I wanted my engine to look stock, still gave an increase in performance. One of the older Vespa owners I know said that back in the sixties they cut a groove in the carb tray cover to make a wasp type buzzing sound as they drove along, now this was the initial thought provoking comment about air restriction which set me on this particular journey.

The other thing I want to add is about carb slides and the oval hole in the carb body. I at the moment glue a piece of surplus gasket paper from the px gasket kit to block the oval hole on the air filter, which in turn stops the air flow threw the carb body, and therefore UNDER the carb slide. There are various slides who's underneath may or may not have grooves cut in them length ways in order to allow unmixed with petrol air to pass into the engine at various throttle positions, 1/4 to 3/4 in my observations, this on the new engines is emission's based, on older it maybe a tuning aid not sure, but older engines still didn't have the grooves. In my opinion its better to have all the flow mixed with petrol but this may change over time with further experiments. Its worth putting in the memory bank.

I do have the sip speedo which has the temp gauge built in, you do have to insulate the coupler at the spark plug from the cooling air for it to work and be of any great use, and grind a little metal, certainly off a malossi head, for easier sparkplug fitting, but once done it's a valuable aid into what's going on in the engine, not perfect by any means an insight. The cheaper version is a Trail Tech cht gauge.

Good luck with it, please post your result re the plastic tray absolutely amazed you posted you don't have one fitted, super curious to know if the scooter shop owner was right.

Regards
Darren
Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Sat Jul 02, 2016 10:11 pm quote
Thanks Again Solo Rider for the great tips and feedback.
I havent had a chance to buy a plastic cover yet but will order one and fit it when I get a moment.
If that doesnt work and solve the problem then I think I will go with the larger Pinasco Airbox cover and the Venturi set-up from Pinasco. I wont got with the foam Polini filter as I want to have the original standard look as the Police here check everything

I will keep you posted...
Enthusiast
Px kitted and blinged
Joined: 03 Mar 2016
Posts: 96
Location: Uk West Yorkshire
Sun Jul 03, 2016 12:08 am quote
Robbie 11 wrote:
Thanks Again Solo Rider for the great tips and feedback.
I havent had a chance to buy a plastic cover yet but will order one and fit it when I get a moment.
If that doesnt work and solve the problem then I think I will go with the larger Pinasco Airbox cover and the Venturi set-up from Pinasco. I wont got with the foam Polini filter as I want to have the original standard look as the Police here check everything

I will keep you posted...
I wouldn't waste your money to be honest.

I have done my test rides on a 2 mile stretch of dual carriageway, results as per expected regarding air filter, so the original gauze has now been replaced with a piece of my wife's new tights!

I wanted to keep the air filter so I can block the carb body oval hole, a must as far as my road test showed in keeping the temperatures sensible.

Main jetting was a surprise, acceleration never changed from going 119 to 122 to 125 main, have gone for the 125 as it helps keep things cool, pre new air filter I was using a 122 main jet. The spark plug has some chocolate colour but needs more road miles to see if its correct.

I will list my scooter setup do note that I use a 22 tooth clutch.


Px 125 2011, (the engine has no spare metal in order to case match the transfer ports, and has updated stator, flywheel and cdi unit, all retro replaced.)

Malossi 166 kit straight bolt on.
22 tooth Cosa clutch
Mazzucchelli race cut crank
Ducati cdi unit, matching stator+electric start flywheel
Ngk BR8HS
Pinasco exhaust

24 mm si carb (straight bolt on)
Drilled at heart air filter, gauze replaced
Auto lube and adding 1% extra oil to petrol

Main stack
160 Air corrector
BE3 mixer tube
125 main jet

Idle Jet 52/140

Sip fast flow fuel tap
Sip Race bearing flywheel side ( a must in my book)

Sip speedo, Vespa speedo drive
Scooter top speed 73 mph
Cruise at 60 mph
Rev counter matches speed (MPH) as per scooterhelp gearing calculator by +1mph that is my speedo is showing I'm going faster by 1 mph.
Top rev's achieved in third gear 8000 rpm.
Rider weight (me) 170 lbs.

A note on road speed, the police go to a lot of trouble to give accurate actual road speed, you cant rely on speedo or gps to be totally accurate.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7954
Location: Victoria, Australia
Mon Jul 04, 2016 12:35 am quote
What idle jet are you using?
Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Tue Jul 05, 2016 10:45 am quote
Ginch wrote:
What idle jet are you using?
Hi Ginch, Ill have to have a look.
A Vespa workshop made the latest adjustments and he said he tried many different main-jets, idle jets and also the air mixture tubes.
Its a standard 24mm premix carb (spaco).
Do you have any recomendations what size idle jet I should be running?

Cheers
Robbie
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7954
Location: Victoria, Australia
Wed Jul 06, 2016 12:53 am quote
Robbie 11 wrote:
Ginch wrote:
What idle jet are you using?
Hi Ginch, Ill have to have a look.
A Vespa workshop made the latest adjustments and he said he tried many different main-jets, idle jets and also the air mixture tubes.
Its a standard 24mm premix carb (spaco).
Do you have any recomendations what size idle jet I should be running?

Cheers
Robbie
Well it might be something else altogether, but when you are travelling along and close the throttle, the only fuel still going into the engine is via the idle jet. It may be worth trying a richer idle to see if it helps.
Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Wed Jul 06, 2016 9:55 am quote
Thanks Ginch...
Just so you know Im not some Swiss bloke.. Im a Kiwi living here for twenty years. But I have often dreams of riding through NZ or OZ on my modified PX
Although in Switzerland Ive been to some amazing places on the Vespa ....
Its just the weather is crap here

Thanks for your tips
Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Fri Mar 20, 2020 10:44 pm quote
Hi All I hope everyone is well in these crazy times were in at the moment
I start this thread again as the problem I never resolved-

I tried all the Carb recommendations to try and resolve the flat spot on closed throttle but still no luck.
I have currently a 55/160 idle jet
122 HD
160 / BE3 air jet
Im now thinking of moving in another direction to solve this issue.
Is it possible that the CDI or stator could cause similar issues?
I will start with a new plug and cable as a cheap first check....

Can an exhaust also bring a misfire on closed throttle if for example if its pulling in air from an air leak?

Cheers Robbie
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2443
Location: London UK
Sat Mar 21, 2020 1:11 am quote
Not fully sure what the issue is. Is going flat when revs falling at closed throttle? Or as the throttle is being closed from WOT. These are different jetting issues.

New plug is awlays a good idea. CDI, stator and flywheel can do weird things but rare to still be running after so long.
Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Sat Mar 21, 2020 4:10 am quote
Jack221 wrote:
Not fully sure what the issue is. Is going flat when revs falling at closed throttle? Or as the throttle is being closed from WOT. These are different jetting issues.

New plug is awlays a good idea. CDI, stator and flywheel can do weird things but rare to still be running after so long.
Thanks Jack
The flat spot is when your riding Full to 3/4 throttle and back off with the gas..Then there is a sudden flat spot where the motor hesitates (only on closed throttle)...
Cheers...
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2443
Location: London UK
Sat Mar 21, 2020 4:46 am quote
This makes it sound like the main air corrector is too big. Change the AC160 to AC140 to see how much better it gets.

The pilot jet will be too rich but get the 3/4 fixed first.
Molto Verboso
2007 Stella 150
Joined: 02 Nov 2019
Posts: 1114
Location: Rochester, Minnesota
Sat Mar 21, 2020 4:48 am quote
Robbie 11 wrote:
Jack221 wrote:
Not fully sure what the issue is. Is going flat when revs falling at closed throttle? Or as the throttle is being closed from WOT. These are different jetting issues.

New plug is awlays a good idea. CDI, stator and flywheel can do weird things but rare to still be running after so long.
Thanks Jack
The flat spot is when your riding Full to 3/4 throttle and back off with the gas..Then there is a sudden flat spot where the motor hesitates (only on closed throttle)...
Cheers...
Did you ever get a temperature gauge to see how hot the motor is getting? Running WOT gets you about as hot as your going to get, then If you allow the throttle to close your getting no oil, the motor could be on the verge of seizing.
Hooked
Joined: 22 Jul 2018
Posts: 118

Sat Mar 21, 2020 8:15 am quote
Classic sign of the missing tray.

Do that first before you waste any more time. If you want to test you can stuff a rag or sock behind the horn. Does the same thing by blocking air flow through the frame tunnel.
Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Sun Oct 18, 2020 6:03 am quote
After years of trying every carb adjustment and checking all possible electrical faults I finally solved the flatspot at 3/4 throttle when backing off from WOT...

It was the missing plastic drip tray under the petrol tank....
Fitted one today and it now runs beautifully.....

Just by chance I read on the Scootercenter.com website that these drip trays are critical when setting up your carb as the air from the tunnel creates air turbulance which disturbs the air to the carb...

thanks for all the tips...hope this may help someone else with similar carb issues.....

Cheers Robbie
Hooked
PX
Joined: 07 Feb 2016
Posts: 147
Location: Switzerland
Sun Oct 18, 2020 6:06 am quote
BlackT5 wrote:
Classic sign of the missing tray.

Do that first before you waste any more time. If you want to test you can stuff a rag or sock behind the horn. Does the same thing by blocking air flow through the frame tunnel.
Thanks Black T5 you were 100% right....
Hooked
Joined: 22 Jul 2018
Posts: 118

Sun Oct 18, 2020 9:08 am quote
Glad I could help.
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