Li150 Strange Starting Issue - Vaccuum Fuel Seal? Compression Seal?

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kurtminus
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Hello,

I've got an Li150, bored out to 175cc, with a SIP electronic ignition, SH2/22 carb, and Casa exhaust. It starts and runs great, but it has a weird intermittent starting issue that I can't figure out.

So normally, you turn the fuel on, turn the choke on, and kick start it typically with the throttle closed. It fires up, you idle with the choke, rev a bit, turn off the choke and ride away. It usually starts on the 2nd or 3rd kick every time.

But sometimes it starts and runs for a second and then bogs out. Then when you go to kick it over again, it's almost like the engine doesn't want to turn over, it's like there's vacuum or compression build up. In these instances, you can kick and kick and kick and nothing happens. Sometimes if you wait a while, and then kick it t'll fire up. Sometimes if you opened the throttle all the way for a few seconds, then kick, it will fire up. Sometimes if you take off the gas tank cap, wait a few seconds and kick, it'll fire up. One thing that works every time (but is kind of a pain) is taking the spark plug out and putting it back in, that works instantly. Kick it once and BAM it fires up. It feels like that "relieves the pressure" so to speak.

It's not a spark or timing problem because I've got spark the whole time and it runs fine. It's not a kickstart problem because with the plug out it kicks over great. It could be a fuel vacuum problem since I've removed the plug and it doesn't look quite wet enough for an engine that's been kicked over a dozen times. But the fact that re-installing the plug seems to fix it points to some kind of pressure build up. I've never heard of a bike not starting for a reason like this. Is it even possible?

Thanks!
Daggs
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Well that is quite a minestrone of a question. But deep within it I think I MAY have the answer.
You said : Sometimes if you take off the gas tank cap, wait a few seconds and kick, it'll fire up.
I think the vent in the petrol tank cap is blocked or non-existent.
kurtminus
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Daggs wrote: Thu Jun 10, 2021 7:04 pm Well that is quite a minestrone of a question. But deep within it I think I MAY have the answer.
You said : Sometimes if you take off the gas tank cap, wait a few seconds and kick, it'll fire up.
I think the vent in the petrol tank cap is blocked or non-existent.
Ya I mean it is kind of a kitchen-sink question, but that's partially because it's so novel to me. I can't figure out the cause.

In my experience with tank cap vents on other bikes, if you remove the cap it pretty much starts right up again, but it seems to not have the same effect here. The immediate fix is to take out the spark plug and put it back in. Is it possible for a gas cap vacuum seal to build up enough pressure through repeated attempts to kickstart the bike, that the only way to really relieve it is by removing the spark plug?
kurtminus
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OH, one reason I don't suspect the fuel cap breather is I don't starve for fuel once the bike is running, it's more of a weird pressure build up trying to start it. With other bikes, the lack of a fuel vent will stop the bike from getting any fuel at all, even when running.
Warkton Tornado No.1
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is the SH2/22 a genuine Dell'Orto or a copy?

I know very little about the copies, nor am I implying that they are neither good or bad, but I'd guess the float is sticking a little to cause fuel starvation.

A little vibration (as with all super smooth Lambrettas....) is enough to stop sticking once the engine is running.

I'd look inside the carburettor before doing anything else. Amal concentric Mk 1's had a tickler to push down the float. A very reliable system & far better than their owb choke systems.
kurtminus
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Hi Folks,

Wanted to just issue an update. So the issue is still happening but I feel like it's not a timing issue or a problem with the carb, it might just be tuning or oil mixture, not really sure.

Usually it goes like this: Scooter fires up super strong on the 2nd kick, but SOMETIMES it'll fire up and putter out real quick. Then if you wanna start it you basically have to remove and reinstall the spark plug. Starts immediately after you do that. It happened again this morning, I kicked like 15 times, then removed the plug. The plug was BLACK and wet. So it's definitely getting fuel. It also always has spark. I reinstalled the plug and it fired up instantly.

Lambrettas are new to me so I'm not sure how easy it is to foul plugs. Maybe I'm running to rich somewhere and fouling the plug?

I was super lean in low rpms (almost un-rideable) with a 50 pilot jet and 5899-4 atomizer. I upjetted to a 55 pilot jet and a 5899-2 atomizer, which got rid of the lean bog, but feels and sounds a tiny bit rich at low rpms. My main jet was too rich at 115, so I downjetted to 110 and it feels decent, maybe on the rich side. I don't ride at WOT much though, so any rich fouling would probably be coming from the pilot/atomizer combo. Most of my riding is short distance city riding. Lots of idling and low rpms from stop-light to stop-light. I'm running a 7 plug, so not a cold plug. I have a 52 pilot and a 100 and 105 main I could try too.

My oil mixture is bout 4.5oz per gallon, which is roughly 33:1 I think. Is that too much oil perhaps? It's a stock jug bored out to 175cc. Has one of the Casa exhausts, not a clubman. The engine isn't a super tuned aluminum kit with crazy exhaust or anything. Does that mean it could take less oil?

Any advice would be great. At this point I am suspecting fouled plug, and the "intermittent" nature might relate to how long the last ride was. Perhaps after longer rides the plug burns off the oil/fuel and therefore starts more easily, whereas after my quick 2 mile ride to work, it doesn't. Next thing I'm gonna do is go on varying lengths of rides and check the plug after coming to a stop, see if that seems to have any affect.

Thanks!!
Lambretta67
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You sure it's not flooding. Turn the petrol off before you stop & start it with the petrol turned off
Warkton Tornado No.1
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Just to add to my earlier suggestion (still relevant) I think your carburettor settings are not causing the issue

Float sticking aside, personally, I have had issues with NGK plugs & reverted to Champion. My understanding is that NGK get counterfeited. Perhaps an N4 might be worth a try.

The other thing you could do, as you are now so familiar with the process, is to whip out the plug even before trying the engine, rather than after. That may just confirm the state of play & give you a better idea of where the problem is. If the plug looks OKeh & yet (after reassembly!) the starting issue remains, it's likely to be fuel delivery.

Of course, the earth connections & wiring are all rock solid, presumably?

As I keep banging on about the unreliability of electronic ignitions, it is worth spinning the motor up via a good electric drill on the flywheel nut with plug out but earthed & a strobe connected. You'd be amazed @ how much the timing can dance around with brand new SIL ignition kits.....
kurtminus
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Warkton Tornado No.1 wrote: Wed Jun 16, 2021 1:34 pm Just to add to my earlier suggestion (still relevant) I think your carburettor settings are not causing the issue

Float sticking aside, personally, I have had issues with NGK plugs & reverted to Champion. My understanding is that NGK get counterfeited. Perhaps an N4 might be worth a try.

The other thing you could do, as you are now so familiar with the process, is to whip out the plug even before trying the engine, rather than after. That may just confirm the state of play & give you a better idea of where the problem is. If the plug looks OKeh & yet (after reassembly!) the starting issue remains, it's likely to be fuel delivery.

Of course, the earth connections & wiring are all rock solid, presumably?

As I keep banging on about the unreliability of electronic ignitions, it is worth spinning the motor up via a good electric drill on the flywheel nut with plug out but earthed & a strobe connected. You'd be amazed @ how much the timing can dance around with brand new SIL ignition kits.....
So this is what makes the issue so weird. I've been paying more close attention to my running and starting conditions and practices. I pulled the plug before trying to start it at all (fresh after a 2+ mile ride) and it looks normal. A bit black on one side and gray/chocolatey on the other side. When I'm experiencing starting problems, if I pull the plug it's black and wet. So it's not fuel delivery.

The odd thing to me is that if I pull the plug, it cures the problem instantly.

The next thing I'm going to try, now that I've determined the engine itself (when running) isn't fouling the plug, is experiment with turning the fuel on for a few seconds, then turning it off, see if that makes a difference.

I can't imagine it's a stuck float valve. To me, that would prevent fuel from reaching the engine. I also don't know how much I believe it's spark, because like I say, if I remove the plug and put it back in it fires up first kick no problem. How could that even be a timing issue?

Is there a chance I could just have a bad plug, like you say? One thing I will add about the spark plug is a few tanks of gas ago I overfilled it and had to dump some fuel off on the side of the road, and, long story short, I ended up running VERY oil rich for a while. It's been about 2 gallons since then, and my plug has probably burned most of that oil off, but it could have damaged the plug irreparably. Who knows.

I'm still learning how temperamental Lambrettas are, so please forgive me if some of my suggestions seem random.
Warkton Tornado No.1
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kurtminus wrote: Wed Jun 16, 2021 7:51 pm
Warkton Tornado No.1 wrote: Wed Jun 16, 2021 1:34 pm Just to add to my earlier suggestion (still relevant) I think your carburettor settings are not causing the issue

Float sticking aside, personally, I have had issues with NGK plugs & reverted to Champion. My understanding is that NGK get counterfeited. Perhaps an N4 might be worth a try.

The other thing you could do, as you are now so familiar with the process, is to whip out the plug even before trying the engine, rather than after. That may just confirm the state of play & give you a better idea of where the problem is. If the plug looks OKeh & yet (after reassembly!) the starting issue remains, it's likely to be fuel delivery.

Of course, the earth connections & wiring are all rock solid, presumably?

As I keep banging on about the unreliability of electronic ignitions, it is worth spinning the motor up via a good electric drill on the flywheel nut with plug out but earthed & a strobe connected. You'd be amazed @ how much the timing can dance around with brand new SIL ignition kits.....
So this is what makes the issue so weird. I've been paying more close attention to my running and starting conditions and practices. I pulled the plug before trying to start it at all (fresh after a 2+ mile ride) and it looks normal. A bit black on one side and gray/chocolatey on the other side. When I'm experiencing starting problems, if I pull the plug it's black and wet. So it's not fuel delivery.

The odd thing to me is that if I pull the plug, it cures the problem instantly.

The next thing I'm going to try, now that I've determined the engine itself (when running) isn't fouling the plug, is experiment with turning the fuel on for a few seconds, then turning it off, see if that makes a difference.

I can't imagine it's a stuck float valve. To me, that would prevent fuel from reaching the engine. I also don't know how much I believe it's spark, because like I say, if I remove the plug and put it back in it fires up first kick no problem. How could that even be a timing issue?

Is there a chance I could just have a bad plug, like you say? One thing I will add about the spark plug is a few tanks of gas ago I overfilled it and had to dump some fuel off on the side of the road, and, long story short, I ended up running VERY oil rich for a while. It's been about 2 gallons since then, and my plug has probably burned most of that oil off, but it could have damaged the plug irreparably. Who knows.

I'm still learning how temperamental Lambrettas are, so please forgive me if some of my suggestions seem random.
Your latest response put me in mind of the TV commercials we had herein the UK for combined shampoo & conditioner:

"Take two bottles into the shower?"

In 'our' case, many (besides me) would be thinking:

"Take your Lambretta out riding with just the one spark plug?"

I never ride without @ least one good spare (with plug cap) besides the plug spanner. That ethos of carrying spares for substitution will apply to many with a similar set up to you, though the majority will also carry spare CDI's & pick-ups.

I am not targeting NGK, but can excuse them to an extent as they are subject to counterfeiting. That said, I have bought genuine that have never sparked! Others have run intermittently. I have dozens of them in various grades, lengths, suppressed etc that are gathering dust. When we used to race, Ray Neal always swore by Champion & he has put his money where his mouth is by Rayspeed selling the brand, including the caps. Of course, me being a points type bloke (for the road) I bought from him the unsuppressed caps & they are excellent. I have to shop around for the fine wire plugs though on t'Internet, but worth it for me.

Excuse my attempt @ humour, but do try an alternative plug, & cap, preferably another brand, purely in the interests of claiming an open mind.

Fuel floats are SO easy to check! They can stick in any position, predominantly 'just enough' to mess you around.

What oils can you easily get? This is such an emotive subject, but I always run tuned engines with Avgas/Super unleaded with good quality fully synthetic no richer than 3 %. There are many good oils such as Rock etc but I'm a big fan of Exol as the only fully synthetic I am aware of specifically for air cooled race engines.

You really ought to be able to use 4 or even 5 % with a good quality oil, but the knock on effect is that to change will richen up your carburation. Which is 'safe' but bear that in mind, once your initial running problem is sorted out.
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