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You can't tune a scooter, but you can tune a fish

Started by mr72, October 08, 2023, 04:35:48 AM

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Is there a carb guru here that would like to offer a little assistance?

I have some odd running issues with my oddball scooter. This is on my Stella Automatic, which for those who care, is a sort of factory resto-mod of a Vespa PX. It's 125cc air cooled with a Dellorto "Electronic Carburetor System" (ECS) carb, which is essentially a rough copy of a Keihin CVK 24mm carb. Dellorto has stopped making this carb or any parts for it, and servicing it in the USA is pretty much impossible.

The ECS system uses an ECU with sensors like normal EFI, O2 sensor, temp sensor, TPS, to do closed-loop mixture adjustment on the fly by allowing air into what would be a vacuum port on a CVK carb using an air solenoid valve. The scooter runs mighty crappily with this air solenoid hooked up, so I have plugged the vacuum port on the carb and I'm letting it run free. Seems to be kind of rich on the main jet which is to be expected since it must have been jetted rich to enable automatic mixture adjustment, but I haven't been able to identify the type of jet to get to replace it, so for now I'm going to work on it with the jet that's in it. FYI regular Mikuni or Keihin jets don't fit.

One other wrinkle is the fuel delivery arrangement. Fuel gets pumped from the bottom of the tank up to a little reservoir that's about 6 oz that sits above the carb under the seat. Then it gravity-feeds from this reservoir down to the carb. The reservoir has an overflow tube that lets fuel drain back into the fuel filler neck if it gets too high. I recently had to replace this vacuum fuel pump with one that's made for a lawn tractor but is basically the same part, just one I can buy in the USA.

OK, so the problems I am trying to solve, which are over my head from a carb tuning perspective, in order of importance. Oh, and they might all be related, same cause.

1. If I sit and idle at a stoplight for a long time, the scooter runs bad when I take off. Takes it a few minutes of running at WOT to clear up and run like it's supposed to. Low power, surging/hunting, etc.

2. It has a pretty rough idle and if adjusted low enough that it doesn't try to engage the clutch, it can stall when stopping once the engine is hot.

3. It's sometimes hard to start when the engine is hot and it's only been shut off for a few minutes.

My current theory is that the float level might be contributing to these issues, related to the fuel pump/reservoir arrangement. Either it could be that the float is too high, and when it sits and idles it allows the fuel level to get too high and then it's mega rich when I take off, takes a few minutes at WOT to bleed off the fuel faster than the fuel pump can replace it. Or, it could be that the fuel pump can't supply enough fuel at idle, so it allows the float bowl to get low and then it takes a few minutes running at higher revs for the fuel pump to catch up and refill the reservoir.

I guess to test my theory I could get some clear tubing and hook it to the carb drain, go ride and get it good and warm, then come back to the house, let it sit and idle on the centerstand, and open the fuel drain and watch the fuel level for a few minutes, see if it goes up or down. Since the carb is buried up under the monocoque, this is non-trivial, so I am looking for some other things to check from the collective before I try this.

Any other ideas on what could be wrong?

FYI I have seriously considered getting an aftermarket EFI kit and swapping this over, but there is very limited space to work with and I don't think I could cram everything in between the cylinder head and the fuel tank without some very fancy intake manifold fabrication. So that's like a long-term idea, maybe during the one month of winter while I can't ride. But right now, I need to get this scooter working properly because it's peak scooter season in Central TX and mrs72 just got a new one, so we have to go ride!


It's going to run too rich without the air solenoid. I wonder if the ECU has a default run mode
if the O2 sensor fails. If so, you might be better off to unplug the sensor and reconnect the solenoid. If the signal for the O2 sensor is removed the system may default to a simple TPS vs RPM mapping. Of course this assumes that the O2 sensor is the problem and not the solenoid.

Another workaround solution could be tuning the air volume entering the port with something like a jet in a piece of tubing.


Sounds similar to the issues I'm having with my GS500. My bike runs well enough to ride around, but sometimes acts like its rich and sometimes acts like its lean. I was also thinking that my float height might be too high but now chris900f's post makes me wonder if me removing the diaphram vacuum system on top of the carbs has something to do with it. I always assumed that system was added purely for emission control reasons, since the '01/'02 carbs are so similar but don't have that system.

Either way, it's worth checking the float height and comparing to the factory spec. That's just part of tuning a carb...can't work right if the fuel level isn't right. But I also know the feeling of not wanting to mess with the carbs again. I just rode my bike 400 miles this weekend with the carb not fully dialed in because I would rather ride than work on the bike anymore.


Thanks for the discussion, guys. A lot has transpired while I continued fiddling with this, but suffice to say I have put it back to stock condition with the air solenoid hooked up and the OEM air filter on its inlet which might slow down the air coming in. Reading the plugs tells me that even in OEM configuration, it's still running very rich. MPG is also pretty bad. But it's way better than before. It was like 30mpg running in open loop with the air inlet plugged, now more like 50mpg with everything hooked up right. But it should be more like 85mpg and the plug is still black.

I might try unplugging the O2 sensor, that's a really good idea. Probably only runs closed-loop at or near idle anyway, so this might stabilize the idle with no other bad effects. At this point I think the surging/variable idle might be because I have adjusted the pilot mixture the old fashioned way which makes it slightly rich, but the ECU wants it to be really lean so it cycles on the air solenoid to allow in air which has the effect of like doubling the amount of air in the carb and is like cranking up the idle speed.

Current plan is: hook up my digital voltmeter to the O2 sensor somehow while it's still connected and running, and lash it to the handlebars. Go ride around the block for 10 minutes until it gets good and hot, and the idle starts its surging thing. Note the O2 sensor voltage when the idle is surging, on the theory that this is the level the ECU is targeting for idle.

Then go home and take it apart to adjust, run it on the centerstand and adjust the pilot mixture while the idle is not surging until the O2 sensor reads the noted target voltage, likely very lean, but hopefully lean enough that the ECU won't try to adjust it and it'll stop surging.

If it keeps surging after that, then I'll probably disconnect the O2 sensor and try that.

So that's the surging idle situation, and my current plan.

Problem #2 is running real bad after long running. On the theory that this was caused by poor fuel pump efficiency and fuel starvation, I put larger vacuum hose on the fuel pump, going from 3mm to 5mm. Hard to say if this has improved anything, since I made other changes at the same time. But now I am thinking the very rich idle and surging is probably the core problem. To keep the surging from being out of control, I have to set the nominal idle speed very low. But if I fix the cause of the surging, then I can set the idle where it's supposed to be, which will run the fuel pump faster at idle and may solve the theoretical fuel starvation after idling, or at least potentially rule that out as a cause.

I recently discovered Honda used an electric fuel pump on the 49cc Ruckus and I can get replacement parts like this for like $15 on Amazon, so I may eventually swap the vacuum fuel pump for one of these electric pumps if I still suspect fuel starvation at idle. I guess I would also need a tip-over switch eventually.

The reason I am focused on this fuel pump thing is that the scooter didn't do this bad running after idling problem before I had to change the fuel pump. It could be a totally wrong assumption. It may actually be opposite, it could be that the overflow from the little fuel reservoir is clogged on one end or the other, and when it's idling it's burning fuel much slower and it allows the fuel pump to pressurize the fuel line, so it's flooding. Who knows? I hate carbs!

No kidding I'm low-key looking for a wrecked or broken EFI 125-150cc scooter or mini bike to buy so I can just rob the entire EFI wholesale and transplant it. A wrecked 1st gen Grom would be perfect.


I read a little about these carbs, they were designed to meet the Euro3 requirements without having to go full EFI; so it's likely that the system is designed to run super lean at idle.

I'm thinking that if your idle mixture requires so much correction (according to the ECU anyway) that it affects rpm, you are simply too far out of spec. If you lean out the (hot)idle with the fuel screw can you get the ECU to behave?

Also, O2 sensors don't last forever and they can get wonky.

If your system has an ambient air intake temperature sensor, you can trick the ECU into "thinking" it's colder outside (usually by adding resistor in line with the IAT sensor) so it will supply more fuel.


Yeah, that's the basis of my current theory. Idle rpm is dependent almost entirely on airflow, and intaking air through that vacuum port will introduce a whole lot of air at idle, which is causing the rpm to jump. So yeah, I thought maybe I could lean the idle so much that the ECU won't try to correct it.

This scooter has just 1K miles on it, so I doubt the O2 sensor is bad. It doesn't have an IAT, it has a cylinder head temp sensor, which was bad when I got the scooter and I have since replaced it, so maybe the good temp sensor might be what has created this idle correction condition. One thing I was trying to fix by replacing the temp sensor was hard starting when cold, since it has an electric enricher ("choke") that's controlled by the ECU, and I think it was never coming on because of the bad temp sensor. That problem is fixed now, but a new one came with it.

I think I'll try unplugging the O2 sensor and see what happens.


Once I had a loose connector on my CHT sensor. It lost contact with the Megasquirt while I was riding. The bike started running really badly: stalling, no power--I barely made it home. It turns out that unplugged-equals-infinite-ohms and since the system is set-up for a var-resistor(sic) that loses resistance as temperature increases, the computer kicked in the enrichment as if the cylinder head temp was -40F. :icon_eek:


I know this scooter will run with the TPS and temp sensor disconnected. It will probably run with the O2 sensor disconnected. I am tempted to unhook them all and try riding it.

In other news, I removed the tiny cat and replaced with a straight pipe. It was leaking so I put some Permatex muffler repair putty on the leaking joint and did the thing they recommend, let the cold engine idle for 10 minutes to set the putty. After doing that, the scooter wouldn't start up again. I pulled the spark plug and it was black and covered in soot. I can smell fuel when trying to start it. So I think it's just wicked rich at idle, probably partly due to it being cold and running on auto-choke. But it seems to be fouling the spark plug or maybe just not enough juice to get a hot spark.

So right now I'll be happy enough if I can just get it to run again. Then I'm inclined to run it with the temp sensor and the O2 sensor both disconnected. I can at least monitor the O2 sensor voltage this way.


FYI, now I am leaning towards the problem being that the spark plug is getting fouled when it's idling. I think it is running rich at idle, plug is still black, so I have pretty good evidence. I think as it idles, carbon builds up on the spark plug, then eventually too much piles up so it doesn't fire well and runs like crud until it burns the carbon off.

I have been trying to set it leaner at idle to try to solve this. So far I'm at 4 1/2 turns out of the "air screw", under the assumption that this is indeed out=lean unlike a GS500 carb. However, it seems like there is no position I can put this screw in where it actually improves the situation. Sitting on the center stand running, I can turn this screw all the way in and it doesn't affect idle at all, and I can back it all the way to 4.5 turns out, no change in idle at all, except that while it sits and idles it screws up the spark plug and eventually idle gets slower and slower until it stalls no matter what you do.

This thing is just plain old un-tuneable. At this point I am nearly convinced that the real solution is to switch to a carb with support. Maybe a non-CV Nibbi carb (Mikuni VM clone) or the brand new PD24J I have in my garage right now, but I will have to basically fabricate a throttle cable stop/linkage from scratch as well as build an intake manifold.


For the two of you who might care, you can read the saga on my latest blog post:

Summary is this: I think the problem was actually a loose spark plug cap coupled with running a resistor spark plug when it's supposed to have a non-resistor type, all resulting in a very weak spark at idle. This allowed the plug to foul, when then takes some few minutes of running hot to clear it up.

Still need to test thoroughly, and I have a non-resistor spark plug coming in the mail. If I get any clear weather today I'll verify my conclusions.


FYI, for anyone who cares, the spark plug thing was contributing to the problem, but not the root cause. Finally the scooter died while running the other day and I discovered it still had loads of rust, coming off in flakes, in the tank. So I redid the on-bike tank cleaning job and I'm hoping it'll limp through springtime well enough that I can wait until it's 100+ degrees to tear it down further and do a proper tank cleaning.

In the meantime, I bought another basket case scooter, this time a '08 Vespa 250GTS. It was a salvage scooter with no key. I had to get a new lock cylinder and have the ECU re-mated to the new keyset (immobilizer) to get it to even start to turn on. Then with a bit of random debugging including discovering a spark plug that came off in two pieces, I got it running. It has been wrecked, so it needs bodywork, and also I found the front wheel was bent, so I have a new one ready to go on. And it's leaking coolant into the oil, so I have a water pump rebuild kit ready to go in, hopefully this weekend. But it starts instantly and runs pretty much perfectly, once I can get the cosmetics and random stuff sorted.

Now we have 4 scooters. Ugh!

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