1955 Model 150D - Starting problem

[Big Al]

I am having trouble starting the scooter after re-assembly.
Everything seems to be connected properly, and a spark is visible at the plug.
With the fuel tap turned on, petroil in the fuel pipe, the choke knob pulled right up so that it stays up on its own and the throttle closed, I have tried to kick over the engine.
After a few kicks, the engine suddenly bursts into life, but dies after a few seconds and will not restart.
I have put in a new spark plug, gapped to 20 thou, and after the engine has started for a few seconds I have removed the plug, and it is not wet with petroil mixture.
I have also tried with no choke and the throttle slightly open, but this makes no difference.
The jets were cleaned before I put the carb back, but I have blown through them again just to make sure they are clear.
The needle clip is in the second of the three grooves in the needle.
Any suggestions please?

[jbcollier]

What brand of spark plugs are you using?

[jbcollier]

Keep the procedure you have but push the choke off after it first fires.

[Big Al]

I bought a couple of new NGK B6H S plugs, John, so I'm using one of those.

Will try closing the choke immediately the engine fires.

[jbcollier]

About 15 years ago we started having problems with standard NGK plugs. We'd fit a new set of plugs and the car would be towed back in a day or two as it wouldn't start. Take the plugs out, hold them against the block and they would spark. Put them in a special spark tester (pressurized chamber) and they wouldn't spark. We talked to the NGK rep and they admitted there was a problem. It depends on the engine and the local gas formulation which leads to the plugs fouling even though the insulator is still clear and white. He recommended fitting iridium plugs. We fit standard Bosch plugs and the problem went away. This may, or may not be an issue in your area.

In your case, I would try either another brand of spark plug (Bocsh W6AC), or an NGK iridium plug (BR6HIX). Unfortunately, all the iridium plugs have internal resistors.

[jbcollier]

Jeez, I'm getting old. I think it's more like 20 years ago...

[missing lynx]

About 15 years ago we started having problems with standard NGK plugs. We'd fit a new set of plugs and the car would be towed back in a day or two as it wouldn't start. Take the plugs out, hold them against the block and they would spark. Put them in a special spark tester (pressurized chamber) and they wouldn't spark. We talked to the NGK rep and they admitted there was a problem. It depends on the engine and the local gas formulation which leads to the plugs fouling even though the insulator is still clear and white. He recommended fitting iridium plugs. We fit standard Bosch plugs and the problem went away. This may, or may not be an issue in your area.

In your case, I would try either another brand of spark plug (Bocsh W6AC), or an NGK iridium plug (BR6HIX). Unfortunately, all the iridium plugs have internal resistors.

You would have thought NGK would have sorted that issue out by now!! But as with many things there are counterfeit plugs on the market so beware.

[coaster]

Has your model got the spring weightauto advance on the flywheel? I had a mate with that system on his 150D Sport which was missing the springs and he'd have to manually close the weights each time he stopped the engine. Stalling at the lights was a right palaver

[Big Al]

Sorry for the delay in replying but I'm afraid I have no idea.
I have not tampered at all with the timing or points.

Is the choke either "Off" or "On" as seems likely, or can it be partly opened?
If it's a hot day (some hope!) and the scooter has not previously been started that day, should I even be using the choke at all?

[jbcollier]

And, how did it work with disabling the choke after it first fires?

[Big Al]

John. I've just tried again to start the D150.
This time it started almost first kick, but the engine died when I disabled the choke.
I tried again and left the choke enabled. The engine continued to run.
After allowing the engine to warm up (with lots of blue exhaust smoke), I then disabled the choke, but the engine immediately died.
It seems not to want to run on the main jet. I will check this jet again, but any thoughts appreciated.

[Big Al]

On second thoughts, it must presumably still be trying to run on the pilot jet, since I have not opened the throttle.
Maybe it's the pilot jet that is blocked?

[Big Al]

I've just opened up the carb again, and taken out the small pilot jet. (I missed this when overhauling the carb.)
It looks completely blocked, so I've submerged it in vinegar to try to shift the blockage, which doesn't respond to blowing through.
Will leave it for a while, and hope it will clear. Then I'll have another try to get the engine running without choke after starting.

[jbcollier]

Definitely found your problem. If vinegar doesn't clear it, buy a new jet as they are very small, hard to clean but cheap to buy.

[Big Al]

The pilot jet cleared using the vinegar. I also cleaned the main and needle/atomiser jets again, and checked the filter bowl before re-assembling the carb and making sure it was tight on the inlet pipe. I screwed the horizontal idling mixture screw right in, and then unscrewed it one and a half turns as suggested in my Lambretta Serviceman's Book.
I turned on the petrol and choke, and the engine fired up pretty much straightaway. I quickly turned off the choke, opened up the throttle a little, and the engine continued to run, so I opened up the throttle a bit more.
After a couple of minutes with the engine running happily at higher revs, I began to close down the throttle, but the engine became hesitant on low revs and would not reach an idling speed without cutting out completely.
(The engine started up, ran and idled smoothly before I stripped down the scooter, and I have not changed the timing or points since then.)
What should I try first to get a low, even idling speed (and to keep the engine running at lower revs, but higher than an idling speed) ?

[jbcollier]

The idle mixture screw is adjustable and it is more accurately called an "idle air screw". You turn it in to make it richer. Recommended settings are starting points only. Screw it in a 1/2 turn and try again. Once warm, adjust the screw to get the best idle from the lean (out) side. Then turn it in a further 1/4 turn. Drive it. Works well? You're good. Bogs a bit off the line, turn the screw in a further 1/4 turn and try again.

[Big Al]

The idle air screw is now adjusted to 3/4 of a turn unscrewed from the fully in position, and the engine starts up and idles well once it's warmed up, with no hesitancy or cutting out.
Many thanks once again John.
Al