Post by wiscootsin on Jul 9, 2023 16:08:38 GMT -5
I've been working on a vespa for awhile now and can't seem to figure out what the problem is. It has 300km on it. It was missing some parts when I bought it (including exhaust) so i put a Tecnigas Silent Pro for Vespa Modern LX 50 4T 4V on it. I put a battery i had laying aroudn in it, which is a 12v 4Ah, maybe half the size that fits in the battery box.
It is a Keihin NCV20 carb stock. It seems to be in working condition.
Carb was jetted as 35 slow jet and 75 main jet (which is slightly bigger than stock 72 but is recommended). It achieves highest idle speed with idle mix/air screw all the way in.
After getting up to speed while doing wide open throttle, if let off the throttle to coast it will backfire (or maybe it's not backfire technically) but keep running fine.
When scooter is cold and first started it operates well. Good throttle from stop to top speed. But once it is warmed up, the 0-10mph is terrible. It doesn't cough or choke out, it just barely gets moving. It's worse if it's held wide open throttle from a stop, but even if trying to start at 1/4 throttle it's slow.
things i've tried:
I put a small washer on the carb needle to raise it, but then scooter ran terribly so put it back to normal.
I tried a new electric autobystarter (autochoke) but that changed nothing.
I checked valves, and they were very tight so i set those at least closer to spec but that didn't change anything (intake side only).
I put a 38 slow jet (cheap one so could be off) and air screw all the way in still provided highest idle. This didn't change any of the 0-10mph acceleration issue, and maybe increased the backfire sound.
I did a full ultrasonic clean and put it back together with 35slow and 75 main. idle speed/air mix screw is back near stock at 1 & 3/4 turns out. still exhibiting the weirdness. I played around with the way the accelerator pump hits and it had some impact but was unable to clear it up.
Checked valves again and set intake and out to spec.
Emptied tank and cleaned petcock. Observed it working and fuel rate appeared good.
I'm not sure where to go next. It always idles fine and once it gets beyond the low RPM range it moves really well up to top speed.
Some of these early scooters with 4v engines had a CDI recall because the timing went bad. Could my symptoms be from that?
It is a Keihin NCV20 carb stock. It seems to be in working condition.
Carb was jetted as 35 slow jet and 75 main jet (which is slightly bigger than stock 72 but is recommended). It achieves highest idle speed with idle mix/air screw all the way in.
After getting up to speed while doing wide open throttle, if let off the throttle to coast it will backfire (or maybe it's not backfire technically) but keep running fine.
When scooter is cold and first started it operates well. Good throttle from stop to top speed. But once it is warmed up, the 0-10mph is terrible. It doesn't cough or choke out, it just barely gets moving. It's worse if it's held wide open throttle from a stop, but even if trying to start at 1/4 throttle it's slow.
things i've tried:
I put a small washer on the carb needle to raise it, but then scooter ran terribly so put it back to normal.
I tried a new electric autobystarter (autochoke) but that changed nothing.
I checked valves, and they were very tight so i set those at least closer to spec but that didn't change anything (intake side only).
I put a 38 slow jet (cheap one so could be off) and air screw all the way in still provided highest idle. This didn't change any of the 0-10mph acceleration issue, and maybe increased the backfire sound.
I did a full ultrasonic clean and put it back together with 35slow and 75 main. idle speed/air mix screw is back near stock at 1 & 3/4 turns out. still exhibiting the weirdness. I played around with the way the accelerator pump hits and it had some impact but was unable to clear it up.
Checked valves again and set intake and out to spec.
Emptied tank and cleaned petcock. Observed it working and fuel rate appeared good.
I'm not sure where to go next. It always idles fine and once it gets beyond the low RPM range it moves really well up to top speed.
Some of these early scooters with 4v engines had a CDI recall because the timing went bad. Could my symptoms be from that?
Anyone have any ideas? think it's carb related (could it be old diaphragm, float height)? think it's timing/electronic related? could the battery size impact something?
I'm putting this away for awhile while I work on getting it registered. So no rush.
