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Mainjet...ma18b3...runs superb with choke on..

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Mainjet...ma18b3...runs superb with choke on..

Postby jdevane5 » Wed Jun 16, 2021 6:00 pm

Had a chance to get my 125 LD running right as have varitronic and miller booster but would only run (really well) with choke on.
So bought some mainjets to improve the situation.
Had the carb restored by Eurocarb and they did a grand job.

So had a 78 mainjet fitted from the restoration and my logic was to get a selection of larger jets, from 82 upto 110.
I've tried a fair few and its making no difference, still sounds like a 4 stroke on WOT and only goes well with choke out.

Any ideas? I did spot the original mainjet size for original MA18B3 was a 68?
Anyone else with a similar set up? What mainjet did you use...

Will have a double check on mixture screw next as again my logic is it gets the right mix with choke out.

The joys of a 68 yr old scooter!
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Re: Mainjet...ma18b3...runs superb with choke on..

Postby jbcollier » Thu Jun 17, 2021 4:13 pm

This type of thing is not easy to diagnose over the phone.

Too rich: four strokes wide open

Too lean: goes "flat" wide open

Usually you increase the main jet until it just starts to 4T wide open. Go back down until it just clears and that will be good if a little on the rich side.

As far as I understand, the Miller booster, does not require much upsetting.
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Re: Mainjet...ma18b3...runs superb with choke on..

Postby jdevane5 » Thu Jun 17, 2021 7:11 pm

Thanks, it sure is a tricky one.

I have a stack of main jets so will work my way through the sizes.

And need to double check for air leaks once more now I have had a few ride outs.
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Re: Mainjet...ma18b3...runs superb with choke on..

Postby jdevane5 » Sun Jun 27, 2021 5:09 pm

So the running issue continues with my LD125.

Tried a load of jets today (going up in size from 78 main jet ) and they all sound 4T on WOT until the I got to 100 main jet and got a back fire!
They would be flat when opening throttle then sound 4T on WOT.

I had the carb restored and Eurorcarb had fitted a 78 main je...t -So I'm going to try a 68 -the original one for MA18 B3...

The saga continues

Testing for air leaks -What's the best way?
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Re: Mainjet...ma18b3...runs superb with choke on..

Postby jbcollier » Mon Jun 28, 2021 7:11 pm

With it 4T wide open, I doubt it has an air leak. Remove the carb and plug the intake. Remove the exhaust and plug it at the cylinder. On one of the plugs I fit a tire valve. Use either pressure (7 - 8 psi tops), or vacuum. Should hold steady for at least a few minutes, the longer the better.
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Re: Mainjet...ma18b3...runs superb with choke on..

Postby jdevane5 » Sun Jul 11, 2021 11:47 am

Think I finally found the issue -Went downwards with main jet size and settled on a 70.
Biggest issue was that I noticed some exhaust soot and have found a decent leak on the miller to the exhaust manifold.
It looks like the exhaust nut is bottoming out before the joint is sealed...

Need to give Mr Miller a call and try and see if I can fix it without cutting the exhaust up for a new nut!
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Re: Mainjet...ma18b3...runs superb with choke on..

Postby jbcollier » Sun Jul 11, 2021 10:08 pm

Thicker exhaust ring gasket would do the trick.
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Re: Mainjet...ma18b3...runs superb with choke on..

Postby rlpembridge » Wed Jan 05, 2022 7:28 pm

If you're still scratching your head about this issue, rather than proceed on the basis of a problem with jetting, I have one suggestion that may be worthwhile checking.

I haven't been using my Ld much, it stays packed away under a work bench at the back of my garage and only comes out in fine weather. Over the past five years or more I seemed to encounter problem after problem and could not get the machine idling smoothly 'off' choke. Even 'on' choke it would only idle once the engine was fully up to operating temperature.

Having (I think) resolved or checked and dismissed a host of other issues, I became convinced that there was an air leak somewhere in the fuel inlet system. I had timed the ignition with a strobe and was confident that there was no shorting wire. I took the carburettor on and off so many times, replacing floats, needles, gaskets and jets, cleaning everything and looking for any signs or distortion. In the end I gave-up and bought a replacement, modern Del'Orto carb. from Patch Hood at Scooter Surgery.

Of course, everything ran beautifully then. But it was only then that my eye was drawn to the four slits in the manifold of the original carb., where it clamps onto the inlet pipe to the cylinder.
What I saw was that, in the course of very many years of mounting and dismounting the carburettor for one job or another, the tightening clamp had distorted the alignment of those four slits.

With the unit dismounted it was evident that the slits nearest the point where the clamp nut was compressing the manifold sleeve had become dragged / stretched / distorted into an almost fully and permanently closed position. Of course, this meant that the slits on the other side would not become properly squeezed together when the clamp ring was tightened.

Now, I haven't tested this theory or measured the depth or engagement of the manifold over the inlet in comparison to the depth of those slits but I think it is highly likely that the distortion of those slits on the manifold has led to an air leak, sufficient to cause the engine to cut out when it should rest at normal idle speeds.

One of these days I shall do those measurements and then set about re-setting the slits on the manifold - anyone know how much heat that ally manifold can be safely subjected to without risk of cracking?
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Re: Mainjet...ma18b3...runs superb with choke on..

Postby jbcollier » Thu Jan 06, 2022 3:15 pm

Scooter carbs use very small idle jets. It is very important that the carb is run dry before storage. Today's fuels go off quite quickly and deposits build up in the jets. It doesn't take much to block, or partial block, the idle jet and have the scoot run poorly.

Regarding the slots used in clamping the carb to the manifold, these have no effect as long as the carb is fully seated on the manifold and the correct spacer is in position.
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