Flat spotin rev range

[Hobbyhorse]

I am sure this has been covered a number of times but my search did not uncover the answer I am looking for.

I have a 1997 PX150 with 5000ks on it which runs well through the higher rev range and will make 95kph on the motorway but has a very annoying flat spot when coming off the idle. I have adjusted the mixture screw and all is well until the engine gets hotter after say 10k, at which time I have to stop and readjust the mixture. It takes almost no movement on the screw and the scoot is then good for a days run but the following day the same procedure has to be repeated. I am now looking at the idle jet which is the standard one at 48/100 and wondering whether a change here would help.

My question is which jet should I buy ... up in the numbers or down?

Rhys

[pxguru]

I assume you have either a BE3 and AC160 or a BE5 and AC150, some even have BE5 and AC140 like a 125 but either way you should never have a 48/100 pilot jet. Don't know how that got in there but its the wrong one. The 100 part makes no differance at tickover but as soon as the carb progression port opens (at 2mm of throttle), then you will be way too rich and it will go flat.

The PX150 has had several different jets over the years, normally 45/140 but sometimes a 48/160 and very occasionally 50/160.

If yours is all standard 1997 spec a 45/140 should be fine and will be the best but you will need to test it well before relying on it. If you are not confident in your testing ability just put in a 48/160, this will run fine but cost you some economy.

[Hobbyhorse]

Thanks for that Pxguru, I appreciate your help.

[Hobbyhorse]

Pxguru I have an apology to make .... I misread the numbers on the present jet and gave you the wrong information. Having taken it outside and into the sunlight I find that the existing jet is a 48/160 ....

[pxguru]

I have to do the same too. Either look in the sunlight or borrow the wifes glasses. Well that explains why the strange jet but doesn't get over the issue.
As you do have a 48/160 already and its running too rich, then the answer is easy, try a 45/140

[Hobbyhorse]

Again, thanks.

[Hobbyhorse]

I am still waiting for delivery of the new idle jet and have spent time reading up on the black art of jetting which has confused rather than cleared my mind.

If the smaller number is the fuel while the larger is the air and the ratio between them is the significant thing presumably by dividing the fuel into the air the resultant figure gives the overall effect 140 ÷ 45 = 3.11 whereas 160 ÷ 48 = 3.33 .... higher richer, lower leaner, or is that too simplistic.

[pxguru]

Searching on here you will find many threads about the dividing thing. It sort of doesn't work like that but has developed from the need to simplify, as a means to understand. This is a secular country and you can believe what you like

To simplify it from the other side of the argument to balance this up, richer and leaner refers to the amount of fuel to air that ends up on top of the piston. Once the flow from the air in this pilot jet enters the main carb vortex the amount of air coming from the jet is near irrelevant when compared with the air in the vortex flowing into the engine. Without the air in the jet to carry the fuel there would be little fuel flow into the vortex but regulating this jet air flow within the jet has little effect on the absolute flow of the fuel at maximum vacuum. As the maximum fuel flow is what defines richer or leaner this relationship tends to me more associated with the pilot jet small number.

Not all engines are identical but from the factory the standard jetting sort of tries to be.

Another thing to be sure of is how you are adjusting your mixture screw, another contentious issue!

[Hobbyhorse]

An update for you PXguru.

The Xmas break has slowed delivery but finally I managed to score a 45/160 .... I ordered a 45/140 but lets not go there at the moment. The main thing is that the problem is now sorted and the scoot is running well. I have just returned home after a 50k run on both motorway and city streets and it ran just fine. I also used your method of setting up the mixture screw .... 4 turns out and then start from there and it worked well for me. 

Again thanks for your help.

Rhys

[pxguru]

How many turns out was the mixture screw when the revs picked up?
45/160 is slightly weaker on progression (the phase between just off tickover and main jet online) but it does sound like you have it sorted. If it feels good and doesn't seize up, then its not far out.
To finally test the pilot jet isn't going to give you any problems, do a 1/4 throttle plug chop. Put a couple of tippex dots on your throttle grip and ride along holding 1/4 throttle in 3rd gear for a good while, 5 miles is what I do. Then stop and take the plug out to see if it still looks good.

[Hobbyhorse]

Surprise surprise, the 45/140 turned up and I have installed it ... it is about 1 1/2 turns out in the optimum position and the I/4 throttle plug chop will be done this weekend.

[RWS74]

Ello Gents, I've been following this thread and managed to find this picture that shows Rich-Lean pilot jettting which I found useful to get my head round.

[Hobbyhorse]

The plug chop will have to wait .... unfortunately.

The scoot was one that had sat in a storage container for something like 15 years and in that time the grease around the needle roller in the front hub had dried out and frozen the needles and as a result the non turning rollers had been sliding on the stub axle and worn it significantly. I took it to my local scooter shop to get the stub axle pressed out and replaced which then became a major when the aluminium suspension yoke cracked and broke. You win some and then you lose some!!

[rab]

sounds silly but i went down from a b8 to a b7 my rev range flat spot disapeard colder weather warmer plug

[Hobbyhorse]

Not silly at all I would suspect. I live in New Zealand where it is supposed to be summer at the moment and a mate of mine with a PX200 went down from a B7HS to a 6 and it is apparently running  smoother and with more power. Strange things, these two stroke engines.

[rab]

air cooled always check plug first

[RWS74]

I run a B6 as I live & commute through London, definitely runs better than before on 8 & 7. Have a spare 7 & 8 for longer distances in my tool bag.

[pxguru]

I had a bit of a rant the other day about plugs. Summary of which was, the lowest number that doesn't overheat is the best. Putting a higher number in when not needed just makes it run less efficiently (ie. slower).

[phattanglo]

True, for some unexplainable reason I had a 5 in my 125 and had a couple of soft seizes. Changed to a 7 and all is well.

[rab]

basic rule of thumb on a px b6 to b7 round town b7 to b8 long runs and rally or kit b7 to b9 every scooter runs different in summer warmer weather one scoot can run a mollosi system on a b9 another will run better on a b8 every change makes a difference to something else on an engine . another rule of thumb ride it till its broke dont fix what aint broke coz you will probebly break it all bow to the px .

[pxguru]

The problem with these rules is everyone tends to ride around with the wrong plug in. The bigger number plug for very long runs is never a bad idea as a precaution.
Once jetted in the plug number can be selected by looking at the plug electrode arm and deciding if its too hot or too cold. Then select the plug to suit, bearing in mind that the bigger number plug will make it go less well.

[Hobbyhorse]

The Vespa PX continued giving trouble and I finally did a leak down test and found a significant leak around the base of the barrel .... some shed time called for.

Bearing in mind that the scooter sat for 15 years in a storage container I presumed that the seals would be the main problem but of course things never go as expected. Firstly I found that there had been a decent heat seize in its short history (6000ks since new) and the piston and bore were bad. The fly side seal appears to have been done fairly recently but the clutch side was bad and had gone very soft. The Cosa clutch was well notched and not doing its job at all well, the gear shift cross and control shaft were poor and the crank was out of true ... well those were the main problems. There has been some very amateurish work done with the wavy washers being used under the head nuts without flat washers which carved into the aluminium causing poor nut seating and bending the head studs, not forgetting the wrist pin circlips being aligned incorrectly .....

I am happy with the power of the scoot and resisted the urge to put on a kit and have gone for a standard Piaggio 150cc barrel and piston, replaced the clutch with a standard one (not a Cosa), new seals etc, shaft trued up, end milled the head stud portion of the AL head to get a true surface to tighten on, and replaced the cross and control shaft.

I had bought the scoot as it was a good price and had only 4800ks on it in preference to one that had 25000ks. It really is a lottery buying second hand and I certainly did not win the lottery with this one. I figure this one is a keeper so hopefully I can recoup something from it all by getting use from it over time. Hopefully the flat spot I began this thread with has disappeared and with the new front end fitted to it several months ago I am looking forward to getting back on it again.

[Gothlife]

Jan 19, 2017 4:10:00 GMT pxguru said:

I had a bit of a rant the other day about plugs. Summary of which was, the lowest number that doesn't overheat is the best. Putting a higher number in when not needed just makes it run less efficiently (ie. slower).

This is probably a huge rookie question but..... Are we talking about the plug over heating or the scoot? How do you tell if it's over heating? At we talking about plug chops/colours/condition here?

[pxguru]

It is unlucky to have such a low mileage with all those issues but at least it is sorted.

There is no issue with some tweaking of it is done right.


Plugs: the number of the plug has no effect on the temperature of the engine. The number is selected after all other work is done to get the plug in its operating range. If the plug gets too hot it will detonate and if too cold it might misfire.

[Gothlife]

Cheers dude. Thought I missing something.
I'm going to stick with the if it ain't broke don't fix it approach and stick with the 7 for now.