LD 150 Mark 3 fuel flow

[Fairburn]

Following Full rebuild including tank clean scooter covered 5 miles no problem. Next trip fuel starvation, no fuel in see through in line filter. Few minitues wait ok but holding back and stopping after each mile. Fitted fuel cap with hole. Stopping every quarter mile. Checked fuel flow both after and before in line filter. Rate one third of an imperial pint, per minute, same on reserve. Using dell orto fuel tap. Fuel drawn off in test clear. Carb has been stripped and cleaned during rebuild. Is fuel rate ok in which case any suggestions gratefully received.

[Steve J]

Take out the inline fuel filter - there's a fuel filter built into the fuel tap, a second one built into the carb - why do you think you need a third?

[Fairburn]

Used in line filter as fuel tap original but with a damaged tube shaped filter. Scooter not run since at least 2008 and so was wary of stray rust. Cleaned tank with acid plus finished off with bottle brush. Do not know where to source a suitable tap filter. Fuel running clear. Have a couple of SIL taps but none with filter. Could swop taps and check rate. Perhaps flow rate like rear hub torque is one of the lesser available pieces of information. As both main and reserve are putting out same third of a pint per minute then any tap blockage is likely to be in any shared part. Taking out in line filter is acceptable and it would also give a more direct route. It creates a sag which may not be helpful.
I think the fact that the scooter filter slowly refilled when I came to a halt suggests a tap issue.

[Nelly]

Check the breather in the tank cap is not partially blocked.

[Fairburn]

This LD has an internal air breather tube but when problem developed replaced it with earlier cap with drilled hole. Also when scooter stopped I loosened cap and drove with cap loose. That would have been my best bet. Scooter was holding back and third gear was only possible for a minute or two. Concerned that problem has got worse over only two or three days. The scooter has only had three outings but will tick over happily on drive. Beginning to wonder if carburretor has a problem.

[Steve J]

1/3 pint per minute doesn't sound a lot, but then an LD doesn't burn fuel like a TS1.....
I'm wondering if the problem might be ignition related - the capacitor/condenser on the stator plate has a nasty habit of failing when the engine gets warm, only to work again once it's cooled off. Also, an ignition coil with intermittent internal connections can give odd symptoms. I'd be inclined to run the engine until it stops, remove the spark plug & check there's still a good spark before taking the carb to bits.
If the ignition advance mechanism is stuck or seized in the fully retarded position, you'll be lucky to develop enough power to get as far as 3rd gear, unless you're going downhill. The engine will start normally, however. Ignition timing needs to be around 26 degrees fully advanced, which means holding the mechanism in the fully advanced position whilst setting the timing.
It's possible it might be a carb issue, but LD carbs are a bit agricultural in design, and provided they are put together correctly, there's not a lot to go wrong.

[Fairburn]

Nelly, did a fuel test with cap completely off same third of a pint.
Steve thanks for info re ignition. Do not have a coil to swop out and to run til it stops may only be possible on a street run ie under load. Will try it on drive.
Re carb noted slight weeping of fuel on end of main jet bolt on underside and down side of cup that holds filter. One other point is that slide I put in to replace scored original had a rectangular vertical slot through slide starting next to where needle is clipped in. The original did not. Could this be the cause? At the time I bought this I understood that it was the correct one. I have today looked at a slide in another LD and it has no slot! What would be effect of such a vertical cut out (approx half a centimetre by a quarter centimetre) from top to where slide has 45 degree slant.

Can you identify it. Is it from an earlier Dell orto.

[Steve J]

The correct throttle slide for the LD 150 Mk3 is an N75 slide, stamped '75', and it has a single slot for the throttle stop screw, which also acts as a guide to prevent the slide from rotating.
Series 1 & 2 Li125 & Li150 models have a similar slide, stamped either '50' or '65' - these may well have two slots, see this illustration on the SR website:- http://www.scooterrestorations.com/lamb ... s-1-and-2/
Although the throttle cutaway could be filed to match that of the LD, I've got a vague recollection that there's a reason why it might not work. It's years since I've seen one for an Li, maybe the slide doesn't have a throttle stop built in, as the Li slow running adjustment is on the lever on top of the carb.

[Fairburn]

Steve
I may have been unclear. The replacement slide NOS has single cut out for screw and is stamped 75 (mm dia) but the cut out I was referring to is like a vertices rectangular shaft/hole that is set in from edge and runs vertically through slide.
I have checked for photos but have none. If it would help could pull it out of scooter tomorrow and send photos of each. On reflection cannot see how what is a machined hole from top to bottom can be beneficial or a substitute. I thought one I stopped out was the one that is not original. One way to check it out is to replace old one back in and go for a run.
Thank you for your help which tripped my memory re possible slide issue.

[Steve J]

If the rectangular hole is in fact a vertical hole through the slide, not just a slot cut into the side of the slide, it's possible you have a slide designed for an earlier type of DellOrto carb, maybe similar to those fitted to the C/LC125 or early D/LD125 Mk1. On the early machines, there was no separate choke built into the carb, but only a 'strangler' device, which was a plunger, rectangular in cross-section, and which was pushed down through the throttle slide to restrict the airflow through the carb for starting. This was a fairly crude way of enriching the mixture, and was often combined with plunger on top of the float chamber (a.k.a. a 'tickler') - this was pushed down briefly to cause the float chamber to flood/overflow, increasing the amount of fuel swilling around in the carb's venturi. Restricted to the early open machines or early enclosed machines with the hinged cover on the right hand side panel.
I don't understand the '75' marking on the slide, which you believe is related to the diameter - AFAIK, the '75' relates to the height of the cutaway - maybe 7.5mm? I had a throttle slide from an LC125 with a smaller cutaway which I believe was stamped '55' - I re-profiled that (i.e. filed it down) to match the shape of the worn slide in my LD150. It worked well enough, and yes, that had a vertical hole for the original 'strangler' plunger.
If the slide you have is marked '75' and has the rectangular hole, my guess is that it may have come from a non-Lambretta application - maybe even an early V**pa? A photo would be useful.

[Fairburn]

Will post photos tomorrow. If it turns out to be for earlier model it may explain unexpected behaviour of scooter. I hope so as it is cold and embarrassing covering 400 yards at a time. This restoration has taken two years and I am wanting to move on to restoration of another LD.

[Fairburn]

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Photos attached

The photos show different slides the hole through one of them can be seen even though they are otherwise identical.

[Steve J]

OK - the one with the vertical slot was for an earlier carb with the 'strangler' type of choke - as fitted to the early 125 D/LD models - up to about 1954. I did a bit of digging around - according to the Vittorio Tessera book, a slide stamped '75' with the large rectangular slot was fitted to the MA18B3 variant of the carb on those 125s - it was never fitted to the 150 engines. I doubt that is the main cause of the problem, but I'd be inclined to temporarily swap the slide from the other carb in your images, and see if it makes any difference.
I've also had a quick look in my spares box, and I found a few good used (& correct) slides for the LD150 - all stamped '75', & none with a large slot through the middle! (I also discovered they all have slots for the throttle stop on both sides of the slide - looks like my memory failed me badly on that one - sorry! ) If you would like one of these, please PM me your address & I'll post it on to you.

[Fairburn]

Will try other carb slide tomorrow and do a run out.
Will PM my address as old slide is scored will do this after ride out.. What makes you discount slide as cause of problem?
If no joy on ride out will look to ignition. Will in any case post solution when I find it.
Thank you for help with thinking this through.

[Steve J]

The reasons I don't think the slide is the cause are that apart from the vertical slot, it's identical to the correct slide, so the cutaway at the front will be OK for use with the 150 engine, and the needle will be held in the correct position, so the mixture will be unchanged. It's possible a small amount of air may find its way down through the rectangular slot, but I would not expect it to be enough to upset the carburation - the area above the slide is covered by the cap formed by the carburettor top casting. Although this isn't airtight, the amount of air which could get in would be very small. Also, the symptoms you describe only seem to occur when the engine has run for a short while - these symptoms could possibly be due to a fuel starvation problem when the engine is put under load, or a high voltage breakdown once the engine has warmed up. If the slide was upsetting the carburation, it might cause the engine to run badly, but the effects wouldn't just appear after a period of running - they would be there all the time.
Finally, as I hinted earlier, I once used a modified throttle slide (with the large rectangular slot) from an LC125 Lambretta in my LD150 without problems, once I had filed the cutaway to match the profile of the worn slide removed from the LD150 carb.

[Fairburn]

Removed in line filter, shortened fuel pipe to aid gravity, cleaned carb jets, replaced slide with correct item. (Using 75mm main jet apparently for epicyclic kickstarts, D3 nos needle) checked gaskets and took bike for run. After a mile felt some very slight hesitation/holding back and then on a small slope it cut out. After a minute it kicked over at third attempt and I covered last mile home. Had no tools to check spark. Little more I can do on carburretor although could swop out complete item for one with 72 main jet and D1 needle. Mixture half a pint of SAE 30 to a gallon of fuel. Ie 16:1. The plug is B7HS. Timing is 26 degrees.

Remaining possibilities I can think of are coil, condenser and plug lead.
One comment was that because it kicked over and started almost immediately that could be more in favour of fuel than electrical.
Any thoughts anyone?

[Steve J]

If you've got a Mk3 150 LD, with a Mk3 cylinder barrel, then you need the 75 main jet and the D3 needle. The 72 main jet with the D1 needle was for early 150 LD engines, which had a barrel with a rectangular exhaust port, and a shallower inlet port giving extremely conservative inlet timings - not really worth trying that, IMHO.
I'd spend some time checking over the ignition system - it's possible the problem might be related to loss of magnetism from the flywheel magnets. If the engine cuts out when the lights are turned on, that can be an indication. The spark plug must be a short-reach type (1/2" long thread), in the Bosch 225 heat range - I've used Champion L5 plugs without problems, and the gap should be set to 0.6 mm to 0.7 mm (some people prefer to use 0.5mm). It might be worth reducing the spark plug gap down to 0.3mm (about 0.012") and see if the situation improves. If it does, it could be a pointer to weak flywheel magnets. Try changing the spark plug, ignition lead and plug cap - and use some proper 2-stroke oil. Using straight SAE30 motor oil was possibly considered OK in the 1950s, but at 16:1 ratio, it will smoke like crazy, and may also foul the spark plug. A semi-synthetic oil at around 20:1 to 25:1 would be a better idea - maybe start at 20:1 if the engine has just been rebored.
Ignition coils are generally not a problem on LD engines - I'm assuming you have a Marelli, Filso or Bosch TJ6/6 coil, as those were commonly used on the LD150. As I said before, the condenser/capacitor can give unexpected problems, and it can be difficult to diagnose - swapping it for a good one is sometimes the only way to discover whether it's the cause of the problem. Check all electrical connections for good continuity, and make sure the braided earth lead between frame and engine is fitted. A problem which can occur on magneto type ignition systems is with the cut-out switch and the wiring between the switch and the ignition coil - the insulation being faulty & the wire then touching the frame intermittently has been known to cause problems, not exclusively to LDs. It's even possible that the contact breaker is tight on its pivot - the pivot pin is brass, and the surface can wear, giving a rough surface for the contact breaker to pivot on.
One oddball thing to consider is that if water gets into the fuel, it will eventually collect at the bottom of the float chamber - that can give some interesting effects. I once had a friend with a Series 2 Li (similar type of carb), which would run just fine, but would cut out during hard braking - it turned out to be a layer of water under the fuel in the float chamber, which moved towards the main jet every time the brakes were applied.
It's always difficult to diagnose problems at a distance - just check everything thoroughly, make only one change at a time - it's unfortunately a slow process of elimination!

[Rob Miller]

I should think that Steve J's suggestion of a faulty condensor should be taken seriously. I had similar trials returning from the S type open day on my TV1 and just managed to limp home. It starts and runs ok but hesitates and cuts out when warm. I'm sure it is the condensor at fault on my bike and possibly on yours. The fuel flow on my D is a trickle but I have never suffered fuel starvation problems on it.

[ricey]

Think Steve and rob are on track with the condenser as the prob!!
Rob ! Have managed to get the kickstart pinion and other bits from Stevej and have now Finnished the d I got from ralf
Many thanks again Steve and hope to see you both soon

[Belplasca]

Just a comment on the fuel flow from the tank: 1/3 pint per minute = 1 pint every three minutes or a gallon every 24 minutes. At 60 MPH that would be 24 miles. For a gallon. These bikes are supposed to do 100 MPG, so your fule flow is about four times as fast as necessary.

Which suggests that the condenser may well be the problem.

Bob