Lambretta Showing 4 stroking

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A two stroke engine fires every time the piston comes up to the top of the cylinder. Firing at the top is one stroke, and reaching bottom is the second. When you get the engine set rich enough (too much fuel), it won’t fire every time so they call that 4 stroking.

When a Lambretta (2 cycle engine) is running rich the exhaust gas is traveling so slow that it can’t all leave the cylinder. This leaves exhaust in the cylinder contaminating the inlet charge for the next cycle resulting in what is known as oxygen deprivation. This contamination will not allow the cylinder to fire on the upcoming power stroke. The next inlet cycle is now “contaminated” and an unburned charge is pushed out by the next inlet charge. This inlet charge scavenges the diluted/contaminated charge enough that it will burn on the power stroke. In summary, the contamination gets so bad that the spark plug can not ignite it, so the engine misses one firing stroke, which results in 4 stroking. During the second (missed) stroke, the exhaust port opens and most of this charge escapes, allowing a fresh new charge the next time around. This cycle repeats itself, which becomes known as 4 stroking.

Note: Although unlikely and depending on what problems you experiencing, you may also have a build up in the exhaust itself, causing problems in this area.

Comments in general: If your working on new / replacement kit and still have the old kit, put the old kit back on to rule out if it’s a problem with the new kit. If all is fine, you can start playing with the new kit and rectify the problem. This may mean decreasing jet sizes or atomiser (if your installing a GP kit to an LI) or simply look at replacing the entire kit if it may be faulty / not suitable.

Jets and information: For further information on Lambretta Jets; Needles; Atomiser and information in this area, please visit this page https://lambretta.org.uk/ld-restores-and-rebuilds/carburettor/

Summary of parts:

Pilot Jet: The higher the number, the more fuel you will allow in.
Main Jet: This controls the fuel when you open up the throttle (supposedly between 50 and 100 percent power)
Atomiser – Fuel is regulated by the main jet and the atomiser outlet areas which is also impacted depending on how far the needle is located inside the atomiser. The more the throttle is open, the more mist / fuel is released in this area
Needle – Moves up and down the atomizer, when the throttle is open (between 20 and 80 or 15 to 60 percent of throttle). At idle, the needle sits inside the atomizer. As you use the throttle, the needle comes further out. When you’re at full throttle, you come off the main jet only.
Starter Jet – Used when choke is engaged. The throttle must not be used when starting a Lambretta up from cold. When the Choke Assemble is used, the Starter Jet is used.

Four-stroking normally occurs when a two-stroke engine fires every four strokes, rather than every two strokes. This firing may damage the engine over time and is often caused by poor idling in two stroke engines.

Four stroking may occur in a correctly adjusted two stroke engine, normally at full throttle and without load and when the air-fuel mixture becomes overly rich and prevents the engine from running faster. You can sometimes experience this event at a quarter to half open throttle and it may disappear when full throttle is used.

Example of Four Stroking videos (Excludes Lambretta at quarter / half throttle, but OK at full throttle)

Lambretta Four Strocking Example

Engine “4 Strokes” areas to check:

4 stroking is often referred to as fuel/air mixture being too rich. It normally occurs in the main jet is too big and at example full throttle. It can sometimes be experiences at a quarter or half throttle.

Factors to take into consideration include the jet sizes and the position of the needle, as well as the hole sizes in the atomiser. Adding a smaller main jet can sometimes aid in fixing the issue, or simply adjusting the needle position can sometimes help in this area (The higher the needle is, means more fuel is being released which may be causing a 4 stroking effect. Reminder: at full throttle, you’re using the main jet only, so if you have problems at half throttle, it may be that the atomiser is releasing too much fuel at this stage or the needle is too high?)

My personal views are that it depends on a number of factors in this area. If you are using the standard ones, review this page for specification (Click here for the standard ones and recommended sizes) or the link above for LC; LD models.

If you are fitting something like a variotonic or upgrading; replacing a MA18; MA19 carb with something different to a MA (Example LI carb with a SH1 / SH2), you may need to review a number of areas. Note: I only use standard and historic MA carbs and a few varitronic ignitions systems and am therefore not an authority in this area. The Facebook Group called lambrettashaftriders people may also aid in answering question in this area.

My comments:

4 Stroking in general and areas to check if your Lambretta is experience technical details similar to what may be interpreted as a 4 stocking effect and in no order:

Check that the choke is not stuck on / slightly on.
Check that the choke cable is not very tight (if applicable). Disconnect the cable temporarily and make sure you push the choke downwards, to ensure it’s off after start-up. Having it up a very small amount can impact this area.
Check that the jets are tightly secured / screwed in.
If you have an air scoop in place, make sure it’s not blocked in any way. Try removing it temporarily or check the air filter for blockages.
If you use a fuel filter, remove it while you’re looking into the fault.
Check guidance for how many turns you may need unscrew the mixture screw. Note on some carbs it says 0.5 of a turn and some say 1.5 or up to 2.5 (Scootopia info – Click Here). For MA carbs when screwed in all the way, in general you start with 1.5 turns outwards. For others (SH) Review links are (Review here link SH1/20 – Lambretta workshop book suggests 0.5 turns out as a starting point / setting, many dealers prefer 1.5 turns., Second link Review here link SH2/22 – no further info in this area)
Review the atomiser information above and on this website. Understand how the atomiser works and its purpose.
Some people say use a smaller pilot jet (Reduce amount of fuel being allowed in and in general), but keep the main jet (Used during throttle / open throttle) high. Again, review the page above and decide for yourself.
Ensure you are using the correct fuel to oil ratio. Normal oil and synthetic oil ratios are different. Review here for further info.
Ensure you are not using old fuel (Which goes stale!)
Consider cleaning your carb and jets for sticky fuel and potential debris.
The float can get stuck for a number of reasons (fuel to oil mix / left standing for a while) and could cause problems in this area. Cleaning your carb will help in this area. Frequently cleaning may be required if the fault is cured and comes back fast (check oil to fuel mix and if smoking is occurring out the exhaust).
If it’s a new carb, probably no need to check If old you may want to change the needle and check float needle seating
Review or swap out a potential and faulty HT lead.
Review the spark plug and its condition (Click Here for information). Consider replacing with a different brand. If you’re ordering from e-Bay, you may have purchased one that’s not genuine. Something as simple as the size of a spark plug washer can impact this area.
Fault diags only – Review fitting a hotter spark plug. If the current spark plug is getting very hot, it may pre ignite the charge without a spark causing a stroking feeling,
Review spark plug gap (0.020 / 0.5 mm)
A difficult one to check is for a potential and faulty stator plate. For C’ LC; D; LD’s this is an impossible task to check and that I will not cover in this post. If I get time at some stage, I will post a news post in this area. I do not own an LI, but this may help (Click Here – LI Stator Plate Review)
If your using a CDI, although and hopefully unlikely, they can fail. Unfortunately, the only proper way to check this is to replace it with a working one.
Unfortunately, you can also not rule out the coil. Another item that can prove difficult to fault diagnose!
If you have a spare exhaust (old or new), try fitting the old one back on and review. If the exhaust isn’t purged completely during the 2 stroke cycle, your may experience problems that show 4 stroking occurring. Fuel mixture is left in the exhaust that cannot be ignited until the following stroke occurs which then produces more mixture that again isn’t purged completely. The more mixture left in the exhaust, the more skipped strokes. At full throttle, the mixture may be purged out; hence it only occurs quarter / half throttle.
I would also remove the cylinder head and check the state of the piston and cylinder head. This is a good article to review in this area – Click Here article 1 and Click Here article 2
Check the timing (If your using varitronic, consider adding the original back on temporarily for fault diags work) For further information in this area, please read here: Wiki Article on 4 stroking and retarding ignition timing. Look at the area titles: “avoid four stroking”.
Check cables in and around the stator plate and that they are secure and tight and not worn away.
As part of my fault diags work, I would swap out the condenser.
As part of my fault diags work, I would swap out the coil (If applicable)
I need to mention this. Although unlikely I would hope, please check the flywheel and woodruff key, as it may be throwing the timing out if you have wear and tear in this area.