Polini cp idle circuit tuning

[johnnyk]

Hi!
I haven't found much on the internet for this carb, but I am having next to no luck getting it running reliably.
The problem I am currently facing after it has been sitting around all day, it is almost impossible to start without having to fiddle around with the idle screw and/or air screw.

I have tried with a bunch of different pilot jets; 38, 40, 42, 44. 44 was definitely too rich (was boggy), 42 seems like a good fit. 
It seems to be that if I raise the throttle valve by turning in the idle screw, that tends to get it going. But when I do that, the idle rpm is super high (2000+), and it seems to only want to idle at a reasonable rpm if the valve is fully closed, as soon as it gets a little open, rpms jump up.

Usually, once it starts once in the day, it'll start again later on, so say sitting for an hour or more, but more often than not, I need to give it a bit throttle to get it going.
Another bad behaviour I haven't figured out, sometimes the rpm will stay high (around 2600) after giving it some throttle, and letting it go. If I give it another quick throttle, that will usually mellow it out.

Basically, this thing has been driving me completely crazy all summer, and I really have no idea what the issue is at this point.

yamaha zuma 2006
vforce 3 reeds
athena 50cc aluminum cylinder
polini team 4 exhaust

Thanks!

[RyanWiltshire]

if you're hanging high  after releasing, you're running lean. what I would suggest is to remove everything, give the carb a good look over, make sure nothing is broken, lose, debris inside the jets/carb?

strip it, clean it. once you done that, I would then move on to getting the bike to idle correctly, long as you can get it idling it's fine, 38/42 for minarelli is good starting point, most people use these sizes for minarelli.

try not to get to hung up on you're idle, have you looked at putting 38 in working your way up or down and seeing no improvement? if not, look at raising/lowering your needle. how does your bike sound and act moving through each cycle pilot, needle, main? without reading it all, have you done an air leak test to make sure you're not leaking air causing lean symptoms? what are temps like while riding? 

[johnnyk]

Thanks for the reply!
For the carb, I have taken it off enough times in the last few weeks, nothing is broken or dirty. The jets are all new as well.

I tried pilot jet 38 through 44. At 38, it runs best with the screw completely in, so I needed to go up. At 42, it seems pretty good. I can try 44 again to be sure, but last I tried with that size, it was quite boggy at low throttle, so seemed a bit big at that point.

The needle is at its leanest, anything above and it crackles a lot at 1/4 throttle, which I believe is too rich. The bike runs and sounds good past 1/4 throttle. It still crackles a bit at low throttle right now, but nothing like the needle clip.

The bottom end was rebuilt last year at a yamaha garage, I didn't take care of doing the crankshaft install myself, but obviously all new seals were installed etc. At the time, when the engine was completely off, I had done a leak test, it wasn't leaking. Recently, I tried the carb cleaner test around the intake just to be sure nothing was leaking around there.

Finally, I have no idea on temps, I don't have a temp gauge installed.

Thanks for the help

[190mech]

A cold start is controlled by the enricher circuit (choke), If the engine does not start,then it's likely the
fuel orifice within the float bowl is clogged..I have a bit of info on this in Brent's Mina carb rebuild (about 1/2 way down);
49ccscoot.proboards.com/thread/363/carburetor-cleaning

[RyanWiltshire]

Jul 20, 2023 15:11:45 GMT -5 johnnyk said:

Thanks for the reply!
For the carb, I have taken it off enough times in the last few weeks, nothing is broken or dirty. The jets are all new as well.

I tried pilot jet 38 through 44. At 38, it runs best with the screw completely in, so I needed to go up. At 42, it seems pretty good. I can try 44 again to be sure, but last I tried with that size, it was quite boggy at low throttle, so seemed a bit big at that point.

The needle is at its leanest, anything above and it crackles a lot at 1/4 throttle, which I believe is too rich. The bike runs and sounds good past 1/4 throttle. It still crackles a bit at low throttle right now, but nothing like the needle clip.

The bottom end was rebuilt last year at a yamaha garage, I didn't take care of doing the crankshaft install myself, but obviously all new seals were installed etc. At the time, when the engine was completely off, I had done a leak test, it wasn't leaking. Recently, I tried the carb cleaner test around the intake just to be sure nothing was leaking around there.

Finally, I have no idea on temps, I don't have a temp gauge installed.

Thanks for the help

Polini CP carbs for minarelli should be mostly plug and play on 70cc...with some fine adjustment. put the carb screw back to its factory setting and leave it, you don't tune a carb via the A/F, it's done within the circuit(s) and CVT, A/F is just to do slight adjustments when needed. you only need to do fine adjustment if any. I think something is wrong and you need to potentially do what he mentioned above and see what happens. as it seems your getting fuel to the float bowl and into the carter? without being there looking at it, it's so hard to diagnose

[johnnyk]

Will give it a good clean as mentioned. It doesn't seem dirty overall, but maybe some of the tubes are dirty. It is not very old, purchased in 2021.
It is a 50cc however, with an Athena cylinder that has high compression ratio and the MVT ignition that has advanced timing. Could that have an impact on "cold" starts.

Right now, the weather doesn't really drop below 18 degrees C overnight, which is the current outdoor temperature. Today again, does not kick over. But once it gets going, it will usually start fine for the rest of the day.

And yes, fuel distribution is working fine, no issues there.

[Zino]

how soon after you put on the mvt ignition did it become hard to start?

We might be chasing the wrong demon you may have set your ignition so it works best at the higher rpms and it lso makes it harder to start .

[johnnyk]

I was trying to remember this, and its a bit foggy tbh. I want to say it seems related, but I cannot be certain.
That said, I wouldn't be able to explain the reasoning as to why the first start of the day is difficult due to the ignition system :/

[johnnyk]

So the only way that seems to reliably getting it going is by turning in the idle screw so the throttle valve raises a bit.
But the problem with that is as soon as the throttle opens a crack, the rpm will rise to 2500-ish, and won't drop unless the throttle valve closes, at which point, depending on engine temps, it might idle smoothly or stop.

Really not sure what to do with it since a lower pilot wasn't helping, and bigger really felt boggy.

[190mech]

When you cold start it, pull the choke and start it, do not add throttle while the choke is on as this will cancel the chokes function. It should idle high for about 15 seconds,then idle down, you can blip the throttle then and turn off the choke..

[johnnyk]

Choke does not start it either.
I will have to find a bit of time at some point to clean it, but lets just say im not keeping my hopes that this will solve either.
One thing that I have noticed however, every time I take off the carb, there is a bit of gas sitting on the edge of the air intake side. Not sure how normal that is.
Also this carb has this air brake, the instructions aren't entirely clear to me how it works, but from what I understand it ships with the correct setting

[Zino]

Some blow back through the to the air filter is normal on a 2 stroke

[johnnyk]

I cleaned the carb and changed the spark plug since it had fouled due to too many failed kick starts.
It didn't change much the situation, so I decided to try changing the air filter.
I was using a stock airbox type (larger model) and switched to a pod filter to see if that made a difference. This does indeed seem to help. The filter was cleaned and oiled late last year (I use no toil cleaner and oil), and doesn't look too dirty.
The hose connecting the carb to the air box is not a standard intake hose since I could not find the 46mm diameter, so I bought a ribbed hose with the correct diameter. Not sure if that could cause issue. I think Brent doesn't like these due to adding extra turbulence.

[johnnyk]

It has been a few days testing with the cone filter, and it has been starting more reliably.

With the cone filter however, the air screw needs to be fully in with the stock 40 pilot jet. With a 42, the scooter makes some pretty loud popping sounds at low throttle, for example take off, or after closing the throttle and reopening.

I had a similar popping sound when the needle clip was too low, and after I leaned it out (currently at leanest setting), it went away. Any ideas what is causing this?

[johnnyk]

A final update on this, I managed to nail the problem. I had modified the airbox (more air) on my previous 70cc setup, and completely forgot about it until I cleaned the filter.
I closed up the extra air hole with some tape and now things have been going pretty smoothly. I guess it was running super lean trying to start.

Thanks for the help