Trouble jetting my Polini 177

[KTy]

Ok guys,

So I have freshly rebuilt a PX engine, with Polini 177, matched cases, cut crank, Leo pipe and, ... a PHBH 28 carb ...

Let start by saying that I'm a bit suspicious about this carb (I couldn't tune my Polini 133 with it either but I thought I might have been quite impatient), but anyway...

Idle jet is 60 and main jet is 120, needle is an X2 and circlip position is first from the top ... and I can't sort out what is going on at low/mid-rpm!! The best I could get was screwing in a lot the micture-screw...

Engine is ratling quite violently, from time to time, low rpm is smooth and the second after the engine is boging. The only think that seems to be ok is high-rpm, but I'm aware that 120 is too small.

I check the spark plug (new), but it was clearly dry/lean.

So, maybe I'm too rich at low-rpm and too lean at high-rpm, and/or maybe I have some issue with the idle/low-rpm circuit inside the carb...

Any idea reading my story ?

Oh, and btw, the scooter is a VNB5T in 8" wheels with 22/68 primary, so running the Leo pipe is, for the first time, quite nice !

KTy

Bearbeitet 3. Mai 2006 von KTy

[John Rambo]

Are you running your engine vane or membrane? What happens if you pull the choke? Are the seals OK?

As far as I know, the main jet is merely responsible for the upper rpm band/top speed (slider opening in the last 1/4er), so it should be a problem with your clip and/or idle jet, but could also concern the float gage (does it work proper?).

You should get your main jet a good deal bigger, until your engine starts to choke (like a four-stroke sound) in the upper rpm level. After that reduce the size jet by jet until your engine revs up smooth and without choking.

You should try another idle jet in either case.

Hope I could help

[KTy]

Engine is rebuilt *as new*, that is, new bearings, new seals, and the rest checked and/or replaced

Of course, I could have done something wrong...

The trouble is really at Low/Mid-RPM, exactly like if the chocke was engaged *from time to time* !! I checked and setup correctly the floater as mentioned by the Dellorto tuning guide.

More detail about my config is: AV266 diffusor no air filter (rubber connection to the frame), no fuel pompe and original rotative valve (cut crank own made).

Thanks

Bearbeitet 3. Mai 2006 von KTy

[KTy]

I just realize something, could it be that the diffusor (AV266) is too large for the needle I use (X2) ??

[morbin]

Hi KTy,

I'm no PHBH expert but I'd start carburation with circlip in middle position...

Matt

Ps. going to send you a PM

[Rietzebuh]

I had in my PHB a 264 calibrator with a X3 needle. It worked well with a 211 Malossi.

The needle is very important at a PHB, in my case more important than the main-jet :plemplem:

[John Rambo]

Agree with that 'cause the main jet is, as yet said, only important for the upper rpm level...

[Marc Werner]

Ok guys,

So I have freshly rebuilt a PX engine, with Polini 177, matched cases, cut crank, Leo pipe and, ... a PHBH 28 carb ...

Let start by saying that I'm a bit suspicious about this carb (I couldn't tune my Polini 133 with it either but I thought I might have been quite impatient), but anyway...

Idle jet is 60 and main jet is 120, needle is an X2 and circlip position is first from the top ... and I can't sort out what is going on at low/mid-rpm!! The best I could get was screwing in a lot the micture-screw...

Engine is ratling quite violently, from time to time, low rpm is smooth and the second after the engine is boging. The only think that seems to be ok is high-rpm, but I'm aware that 120 is too small.

I check the spark plug (new), but it was clearly dry/lean.

So, maybe I'm too rich at low-rpm and too lean at high-rpm, and/or maybe I have some issue with the idle/low-rpm circuit inside the carb...

Any idea reading my story ?

Oh, and btw, the scooter is a VNB5T in 8" wheels with 22/68 primary, so running the Leo pipe is, for the first time, quite nice !

KTy

Hey there KTy,

maybe this configuration works a bit better:

Idle Jet = 52 - 55

Needle X 2, X3 or X7 second position from top, matches most times

Main Jet = 125 - 135

Diffusor 266 is too rich ! take a av 262 oder 264

Bearbeitet 3. Mai 2006 von marc74

[morbin]

Hi my friend's engine works well with:

Idle 52

Needle X2 second position from top

main 122 (with Leo exhaust you'll need more than this)

AV 264

gas valve 40

Greets,

Matt

[KTy]

Hey there KTy,

maybe this configuration works a bit better:

Idle Jet = 52 - 55

Needle X 2, X3 or X7 second position from top, matches most times

Main Jet = 125 - 135

Diffusor 266 is too rich ! take a av 262 oder 264

Maybe the diffusor is my problem since the begining !

Where can I read about the differents between diffusors ? Can I use a diffusor from a PHBL 24mm ??

Thanks !

Bearbeitet 3. Mai 2006 von KTy

[KTy]

Can I use a diffusor from a PHBL 24mm ??

Auto-reply after verification:-D : No, they are totally different !

[KTy]

So,

I only manage to find a CF262 (from a PHBH28BD) that looks very very similar to an AV262...

I put a 55 idle (I tryed 60, too much) and 130 main. Needle is still X2 and clip in second position from top.

What I can say now:

* Low to High rpm is smooth, but there is still something a bit anoying: If I maintain the throttle in the same position (for instance 1/3) for some times (few seconds), the engine revs smoothly then it starts to "rattle" (too rich or too lean ??) a little to bit, i.e, I cannot ride smoothly-all-the-time with the throttle opened between 1/3 and 3/4. During "transition", the engine is smooth, but when it is "stabilized" it can momentary do "Rrraararraraa" (4 stroke like ?!) . Atomiser too small ? Should I put the jet needle upper ?

* I have the Leovince pipe and I think 130 is still too lean, the engine is reving a lot ! At full throttle in 3rd and 4th gear, the engine "rattle" also, but it's not like when it's too rich "Boooooahhh" , it's more like when the spark is misfiring... Possible ??

Bearbeitet 4. Mai 2006 von KTy

[John Rambo]

Seems your ignition preset ain't correct. On which degree did you set it?

And what happens if you change the air-fuel-ratio?

Bearbeitet 4. Mai 2006 von John Rambo

[KTy]

It's more probably a trouble with the atomiser/needle...

I'm running with 18° of timing advance...

Btw, the cutaway of the round slide is 40.

Thanks,

EDIT: Engine was smoother on all rpm range when it was cold... Too lean ?

Bearbeitet 5. Mai 2006 von KTy

[Marc Werner]

hey there KTy,

let's have a look at http://www.setup-largeframe.de.vu/ (look at the polini 177 configurations with dellorto 28 carb). maybe it helps a bit.

4 stroking is a sign of a to rich carb adjustment. i would take the X 3 needle. i don't know exactly where are the differences between the CF and AV atomizers. sorry. i believe the CF atomizer is one for 4 stroke engines ? so it is possible to make a photo of it ?

kind regards

marc

Bearbeitet 5. Mai 2006 von marc74

[KTy]

I already had a look a this website, it's a little bit helpful for a starting point, not really for fine tuning the engine ;)

I will first try to set the neddle upper and see if it's change the pb at mid-rpm.

I will also compare the engine agrreement with a bigger main jet, as I suspect the 262 atomiser to be just a little bit too small !

[KTy]

Some more information,

Without any change to the carb, the engine rattle more easily at full-throttle when the load is increased (me + a friend of mine :D)...

Main jet too big or too small ? Not enough kilometers to read the spark plug colour !

[Marc Werner]

Some more information,

Without any change to the carb, the engine rattle more easily at full-throttle when the load is increased (me + a friend of mine :D)...

Main jet too big or too small ? Not enough kilometers to read the spark plug colour !

I think the Main Jet is to rich. Please take a picture of the atomizer. Maybe it is one for a four stroke carb.

[KTy]

I'll take a picture of it, but as far as I can see, it is pretty much exactly the same as the AV 262...

I have a 120 main jet, I'll see if it helps... But I am affraid to size as the Leo pipe really push me in the high-revs !!

Bearbeitet 5. Mai 2006 von KTy

[Juan Kerr]

Okay mate,this is nothing to do with carburation but.........

........what is the crank?You said it is cut but is this a modified standard item or have you used a a Mazzachelli?I'm not a huge expert but my friend Oz,he of Rotax 50 Special fame,tells me the duration is far too long on a Mazz or a standard modified to that spec. on a Polini.

I don't want to complicate matters but as always with tuning everything has a knock on effect.

[KTy]

Okay mates

Of course, everything is connected (like we all are :love: ),

My crank is the standard one, I removed something like 10-15mm to it, and I also openened the case admission, something like 4+4mm...

As for my jetting,

I put the clip at the top of the needle, engine was smoother at mid-rpm

I tried a 140 main jet :plemplem: , everything fine except full throttle still giving a freaking metallic sound, so I switch to 120, and that was it ! Much better under full throttle ! I will try the 125 tomorrow and I will probably keep it for reliabiliy reason

Idle jet is 55, I unscrew the mixture screw 3-4 turns... I could probably switch to 60 idle jet, again, for reliability reason

For now, I'm keeping the CF262 atomiser, I will try to take a picture of it, just to compare to an AV26x...

Running on 8" wheels with 22/68, it is very funny !

Thanks to all for your help

Bearbeitet 5. Mai 2006 von KTy

[KTy]

Back on track

First, as previously discussed, you'll find below 2 pictures of a CF262 (left) and AV264 (right) atomisers...

http://ktylife.free.fr/vespa/AtomiseurCF262.2.jpg

As one can see, well, maybe not, but I can tell you , except the needle hole, both atomisers are very very very similar, almost identical ! Anyway, it doesn't help us to understand the difference...

As for my jetting, I'd like to ask again your opinion...

Setting: Polini 177, cases matched, polini intake enlarged, rotative admission (enlarged 4+4), crank manually cut (15-20mm), PHBH28 and Leovince pipe !

I have the feeling that I have a very nice and smooth engine, with some regulation at full throttle (main jet too large for reliability).

I just tried an AV264, engine was ok, except 25-50% throttle when it can "rararaaraaaaaa" a little bit

So I switch back to CF262 !

I ended up with:

Idle jet: 55

Mixture screw: (only) 1 turn

Atomiser: CF262 (similar to AV262 ?!)

Needle: X2

Needle Clip: Top position

Main jet: 125

I have the feeling that it could also work with an AV264 with a larger needle (X3 ?), what should I do ??

Any comments on the rest of the config ? No worry with the idle jet / mixture screw setting ?

KTy

Bearbeitet 30. Mai 2006 von KTy

[KTy]

:wasntme:

[ric_two]

It's more probably a trouble with the atomiser/needle...

I'm running with 18° of timing advance...

Btw, the cutaway of the round slide is 40.

Thanks,

EDIT: Engine was smoother on all rpm range when it was cold... Too lean ?

cutaway 40 is standard on the phbl's I use the 30's cutaway with AV264 and X2 needle. Mainjet is 128(a little bit too rich) lowjet is 48

Bearbeitet 1. Juni 2006 von ric_two

[KTy]

cutaway 40 is standard on the phbl's I use the 30's cutaway with AV264 and X2 needle. Mainjet is 128(a little bit too rich) lowjet is 48

What cylinder and what exhaust ?

Isn't your idle jet a bit small ?? Does the 30's cutaway make a sensible difference ??

Cheers,

[ric_two]

What cylinder and what exhaust ?

Isn't your idle jet a bit small ?? Does the 30's cutaway make a sensible difference ??

Cheers,

Exhaust is an FAR Pitone..... The barrel is 1,5mm higher without any porting except piston ut to outlet clearence. Is use an standard rotating Valve Inlet but with 16mm longer inlet timing puuuh

[KTy]

Back on this one :)

I just have a quick question in my head...

After this Polini 177 broke (the skirt broke !!), I bougth a new one along with a new mazzu crankshaft.

I have now the feeling that I have to run richer, I have this metallic sound (Leo pipe ?) sometimes around mid-rpm and high-rpm.

I put the needle clip on the 2nd position from the top, and the engine is not rattling or 4Ting at any rpm as it was doing before.

Question is, considering enriching at mid-RPM should I switch back to an AV264 atomiser or only play with the needle ?

[Brosi]

Might be tipical ringing due to lean mixture. Do you know if the timing of the new crank differs from the former one? If so, and I assume it´s longer, you should first readjust the main jet (bigger). As the main jet also influences the mid-rpm-range further adjustment of your jetting depends on the results you´ll recieve...

[KTy]

Indeed, the previous crank was an original PX125E reworked (1.5cm if I remember)...

You're right, usually a mettalic sound under heavy load is usually the sign of a lean mixture... We'll see

I will first enlarge the main to 130, isn't that big ? :)

[Lucifer]

Usually the X2 on Position 2 or 3 within AV264 is a good choice to start with on Polini 177!

The more you tune the timings and go for an exhaust with more volume and power-potential, you have to change your settings accordingly.

If you want to gain really more revs, probably a X3 within a AV260 or AV262 will do the job. (X2 is 2.5mm at start, X3 only 2.46mm, therfore much richer and even with a quite steeper taper).

Usually the differences between Needle-positions in DOs are quite large, one might be far too leen, the next far to rich! Just in these cases you have to swich to different needle and emulsifier combinations, eg. X13, X11,.., AV260-266!

Your mechanical defekt may even be due to another problem you probably still haven't fixed: Squisch, lubrication,....., just a guess, I don`t know...