Can't get my PX 177 running right
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Member
PX150E
Joined: 23 Dec 2018
Posts: 11
Location: Vietnam
Sun Dec 23, 2018 6:53 am quote
Hello everyone, I'm new to this two stroke scooters and currently need helps.

my bike is a 1995 PX150 with:
- Pinasco 177 + Racing Crank
- Pinasco SI 24/24 (160/116/BE3 and 55/160)
- Polini Original Box Exhaust

Previously, I broke in the engine with the stock exhaust and SI20/20 carb with no problems. After 1000km, I decided to put in the brand new SI24/24 and the polini box but could not get the bike running correctly.

I'm having those problems below:

- Throttle hesitation, flat spot around 5-10% throttle
- Always backfire after full throttle
- Sparkplug tip is black (meaning it is running rich ? should I increase the fuel ratio at the idle jet because it feels like it is hungry for fuel at low throttle )

Please advice

Molto Verboso
Kymco AK550
Joined: 09 Feb 2008
Posts: 1071
Location: UK
Mon Dec 24, 2018 12:58 am quote
I'm no expert on this particular engine but two observations:-
1) Backfire may be because the joints in the exhaust leak and allow air to get sucked in.
2) A change of choke size this great is bound to cause difficulty. The area of the intake has been increased by about 44% ( 12 squared / 10 squared = 144/100 ).

When you have got the main jet size correct check the top speed in a repeatable way. Then try with the smaller carb. If you find there is no difference you might as well stick with the smaller one but if you like the improvement maybe someone can advise on jetting or slide changes.
Member
Mongrel 187
Joined: 21 Oct 2018
Posts: 9
Location: Belfast
Wed Dec 26, 2018 4:51 am quote
Throttle hesitation could be air screw related. The SPACO delorto carbs with the brass air screw have a finer thread and need to be two to three turns out to get the idle circuit rich enough. Try that?
Backfire could be related to this?
Member
PX150E
Joined: 23 Dec 2018
Posts: 11
Location: Vietnam
Wed Jan 02, 2019 8:46 am quote
Thanks Johnd and roadster,

Got the bike on a dyno so I can tune the carb correctly. Turned out that the idle jet on the stock 24/24 carb (55-160) was way to small. AFR on low revs was dangerously high at 17.2





After a few runs, I put in the biggest jet I got which is an 65-160 and got the low revs AFR to 15, idle at 14.2 which is good enough for me. All the problems went away and I also gained 1hp






Also tried the smaller 20/20 carb with 38/120 idle, 116/160/BE3 main and polini venturi kit. The bike ran butter smooth but rev a lot slower than the 24/24 carb, although they have same gas mileages and top speed.
Member
Mongrel 187
Joined: 21 Oct 2018
Posts: 9
Location: Belfast
Thu Jan 03, 2019 3:51 am quote
What was your mixture screw set to? Unusual that a 55/160 would be dangerously weak, they can be a little weak at part throttle when slowing from high rpm, usually the next richest jet fixes that. But all engines are different I suppose.
Member
PX150E
Joined: 23 Dec 2018
Posts: 11
Location: Vietnam
Thu Jan 03, 2019 8:21 am quote
Johnd wrote:
What was your mixture screw set to? Unusual that a 55/160 would be dangerously weak, they can be a little weak at part throttle when slowing from high rpm, usually the next richest jet fixes that. But all engines are different I suppose.
I put it at 2 full turn, but the screw only help with the idle. I think it is something to do with the pinasco 24/24 carb. I putted a spaco 20/20 carb on with 38/120 jet and it ran fine. eventhough 38/120 is alot leaner than 55/160
Member
Mongrel 187
Joined: 21 Oct 2018
Posts: 9
Location: Belfast
Fri Jan 04, 2019 3:28 am quote
jamessybuizz wrote:
Johnd wrote:
What was your mixture screw set to? Unusual that a 55/160 would be dangerously weak, they can be a little weak at part throttle when slowing from high rpm, usually the next richest jet fixes that. But all engines are different I suppose.
I put it at 2 full turn, but the screw only help with the idle. I think it is something to do with the pinasco 24/24 carb. I putted a spaco 20/20 carb on with 38/120 jet and it ran fine. eventhough 38/120 is alot leaner than 55/160
The air screw on an Si carb helps with throttle position up to 1/4 throttle, there is also overlap as the slide and main jet stack takes over. If it's a SPACO carb then 2 turns out is still on the weak side in my opinion. Try it at 2.5-3.5 turns out and see if it improves. I know all engines can be different when it comes to set up but a pinasco 177 is a pretty straight forward kind of cylinder to need drastic idle jet changes. Just thinking out loud and from a distance 👍
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1202
Location: London UK
Fri Jan 04, 2019 10:47 am quote
65/160 pilot Wonder where that size came from? Would have a better result with a 50/140
Member
PX150E
Joined: 23 Dec 2018
Posts: 11
Location: Vietnam
Sat Jan 05, 2019 5:20 am quote
Jack221 wrote:
65/160 pilot Wonder where that size came from? Would have a better result with a 50/140
I bought the pinasco 56-66/160 idle jet set. As I always start with the biggest jet and work my way down to prevent engine failure, I started with the 65/160 and the AFR sit at 15 already.

Wouldn't the 50/140 be leaner than the 65/160 ?
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1202
Location: London UK
Sat Jan 05, 2019 9:45 am quote
How many turns out is the mixture screw with that 65/160? Higher performance engines should run a pilot jet with 140 or less.
Member
PX150E
Joined: 23 Dec 2018
Posts: 11
Location: Vietnam
Wed Jan 09, 2019 5:52 am quote
I will try the 50/140 then, will put it back on the dyno to see how it works out.

The mixture screw is 1.5 turns out with the 65/160 jet. But i'm having crazy carb backfire going on recently after WOT ? I wonder how could it happen ?

Member
Mongrel 187
Joined: 21 Oct 2018
Posts: 9
Location: Belfast
Wed Jan 09, 2019 12:02 pm quote
Have you tried the air screw any further out than two turns? If it's a brass SPACO type air screw then 1.5 turns is NOT enough. Go out to 2.5/3 turns and see how it runs then?
Member
Mongrel 187
Joined: 21 Oct 2018
Posts: 9
Location: Belfast
Wed Jan 09, 2019 12:03 pm quote
Johnd wrote:
Throttle hesitation could be air screw related. The SPACO delorto carbs with the brass air screw have a finer thread and need to be two to three turns out to get the idle circuit rich enough. Try that?
Backfire could be related to this?
At the risk of repeating myself
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1202
Location: London UK
Thu Jan 10, 2019 1:04 am quote
If that is fire (actual flames ) down the inlet after running at WOT, then your engine has mechanical issues with the crankcases, seals and/or rings. Would explain why a 177 needs a 65 pilot jet to run too.
Member
PX150E
Joined: 23 Dec 2018
Posts: 11
Location: Vietnam
Thu Feb 07, 2019 6:19 am quote
Updates:

Finally the new idle jets arrived after 1.5 months of shipping

The bike runs great with a 58/160, with the idle jet screw at 4.5 turns. It runs fast and smooth with zero flat spot, rev drop quickly back when I pull the clutch in. Performance is stable at all engine temperatures.

However, it still idle pretty rough and fast, it will die out if i adjust the idle screw to a lower speed. The spark plug look black but not oily (carbon fouled). I've already replaced the carb gasket. If it could idle smoothly then it would be perfect because it ride so well.
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