Modern Vespa

Hi guys,
I've got a px 125 which I've just fitted a dr 177 barrel, head,piston and a sip road exhaust with a si 24/24e carb.the problem i got is at a constant speed the engine starts popping as soon has i lift off or slightly accelerate it goes. It rides great apart from that popping. Is this a problem or is it just the characteristics of this set up?

Check your timing

That brings me to another question. After i finished the engine work I attempted to check my timing and found a confusing problem. Based on the marks on the flywheel cover the timing is set to about 60' btdc (using a strobe light)which makes no sense. I checked the position on the stator itself and i seem to be in the area for 19' (it marking)which the manufacturer states should be right.

expand on "popping"..is that a miss or a back fire ???Sounds like points not set..or bad condensor(if you are on points)

Its more like a misfire. But it doesn't feel like its losing power. I'm running standard electronic ignition.

Never trust the marks on the case unless you measured them from TDC yourself.

And is the ignition original or after-market?

If you just fitted the kit, then my first thoughts are cylinder head air leak.

How does the spark plug colour look?

Ignition system is the factory original. Did a plug chop yesterday and looks good. I forgot to mention the popping only happens at low revs.

Sure reads like an air leak but may just be weak.

What are all the jet numbers in your carb?

55/160 pilot
190-be4-128

And the plug runs normal brown?

yea a nice light brown

I'm not keen on the AC190 with the BE4 combination. Do you have an AC140 and BE5?
Still think there might be an air leak though.

ive got a ac 160 and a be5 that I will try.
I will also take the head off tonight and check for a leak.

how about my pilot jet (55/160) does that sound ok?

Biggest pilot jet should be 48/140. The AC160 BE3 you can get away with but often leaves the low/mid too rich if WOT is correct. I now prefer the AC140 BE5 for this kind of set up.

While you have the head off, if you need slightly more power a few extra base gaskets will give a few more rpm. Can go up to 5 gaskets (1.0mm) and still use the same head.

I have ordered those jets you suggested.
thanks for the help.

Hi guys. Just a quick update.
Have check timing using dti and degree wheel found to be 2 degrees out, have but right. Removed head and found no evidence of leaking. Fitting the jets you recommended and took out for a ride had very low power and alot of flat spots refitted my original jets and it was alot better. Still have the popping but it has improved.

Did you reduce the main jet? If you kept the 128 it would have been like riding on choke.

48/140 AC140 BE5 would need a smaller main jet like 116 (but you will need to test and reduce a step at a time).

Jetting these carbs is not easy. I expect the popping to be over rich if you don't have an air leak. If your enigine is ok then this will jet in better.