What is leak-down testing?


Leak-down is a simple method of checking an engine for air leaks by sealing and pressurising the engine. Leak testing is a wise precaution for every engine build. With the kit shown here it is cheap and easy to carry out.

I was first introduced to the concept by Charlie Edmonds of Performance Tuning, who used an expensive American kit for the job. After seeing it I developed my own home-brew testing kit. It has proved itself invaluable in finding issues that would other wise have been difficult to eradicate. Subsequent to that, a member of the LCGB forum showed an even cheaper and easier way to fabricate a test kit using an old cycle inner tube. There’s a video explaining that method at the end of this article…

What’s the point of leak-down testing my scooter engine?

Air leaks kill 2-strokes as effectively as Kryptonite puts Superman off his cornflakes. My very first Lambretta in the 1980s was a drops-and-rear-sets racer with a Stage 4 tuned top end. It blew up on the way home. I then fitted a Stage 5 Hemspeed top end which I could never get jetted right and proceeded to blow that up several times as well. It wasn’t until I fitted a TS1 kit that we noticed a hairline crack in the engine casing around one of the heavily-ported transfer cutaways. This crack was letting in air and affecting the carburetion.

 If your casings have a leak then you will always find it difficult to set up the carb and there’s a good chance that you’ll overheat any top end you fit to it. Leaks therefore, are the enemy.

Ebay is good for something. This type of sphygmomanometer with the gauge and bulb together as a unit is best for this job.
Ebay is good for something. This type of sphygmomanometer with the gauge and bulb together as a unit is best for this job.

What are air-leak symptoms?

For instance, if you have an engine that ticks over fine when it is cold, but ticks over faster when you shut off after a long stretch at full throttle, this can signify an air-leak. Like an auctioneer identifying a conveyor of Chinese vases, the engine will make a rapid “ming ming ming” sound, before slowly settling back to a more sensible tick-over. Hot engines seem to show more signs of air leaks as components expand and gaps get wider. Air leaks weaken the carburetion leading to overheating and this becomes a vicious cycle. If left unchecked, an air leak can quickly result in a seizure or a holed piston.

Leaks don’t have to come from cracks. They can occur anywhere that there is a joint between components such as a gasket face or metal-to-metal mating surface. On Lambrettas, particularly large capacity tuned ones, then cylinder base and head gasket surfaces are prone to leaks, as is the badly-designed 2-stud exhaust manifold joint. Vespas are less susceptible but inlet gaskets can still cause trouble, as can the head to cylinder joints on some larger-bore models.

Worn or damaged crankshaft oil seals are another potential cause of leaks. Drive side oil seal problems will often be accompanied by blackened spark plugs and a constant smoke trail as your motor sucks in and burns your gearbox oil. Worn flywheel-side seals have the same symptoms as any other air leak.

Sometimes however, these idling-speed symptoms are not anything to do with a leak and are simply due to poor carb settings or other engine problems. So how do you tell the difference?

This is where leak-down testing comes in.

Here’s one I made earlier…

The function of a leak-down testing kit is to seal the engine from inlet to exhaust and apply modest pressure. The pressurised engine can then be monitored to see if the pressure drops and where the leak is coming from.

The device I found to both apply and monitor the air pressure is an Aneroid Sphygmomanometer, or to you and I, a blood pressure testing kit. Mine came from eBay for the princely sum of £8.49 + P&P. This model was chosen for the fact that the gauge and pump were a single unit rather than being on separate hoses. The gauge is graduated in units of mmHg but the maximum reading of 300mmHg is equivalent to 5.8psi and therefore perfect for our needs.

Next you will need some way of applying the pressure via the sealed inlet port of the engine. We had a section of plastic rod machined to the same size as our carburettor spigot and this was drilled and tapped with an M7 x 1.0mm thread to accept a bleed nipple from a hydraulic brake caliper. If you have a rubber-mount carb then any piece of solid material that may be clamped into the carb rubber will do as an inlet bung as long as you can find a way to attach the pipe. Be inventive!

For the exhaust port the easiest way to seal it up is to block up the stub manifold or downpipe with an expanding rubber bung. Expanding bungs which tighten into the exhaust stub with a wing-nut are available from large plumbing suppliers. Alternatively a small piece of old inner tube rubber may be clamped tightly around the outlet with a good quality hose clip.

Finding a bad leak just by ear…
The grey area of the gasket is where it was audibly leaking in the video above.
The grey area of the gasket is where it was audibly leaking in the video above.

The pressure’s on

The engine I used for Coast 2 Coast a few years ago behaved like it had an air leak, but I could find no visible sign of leaks from any of the cylinder gaskets. As such it would be the perfect patient for my experiments.

With the inlet and exhaust suitably blocked it was just a matter to connect the pump and gauge to the bleed nipple on the inlet bung and pump away. If the motor is healthy then you should expect it to hold pressure, or more likely to leak very slowly. It has been suggested that the threshold of acceptability is losing less than 1 psi (or in this case 50mmHg) per minute.

Note: it is better to sit the piston at the bottom of its stroke prior to testing to allow pressure to equalise rapidly via the transfer ports. If the piston is at the top of the stroke it will take time for pressure to bleed past the piston rings to the combustion chamber.

The first step in testing should always be to check if there are any leaks at the bungs. Use an old spray bottle (e.g. window cleaning fluid) filled with water and washing up liquid to spray around the inlet and outlet bungs to look for leaks. If you find a leak it will form bubbles that either grow or multiply over time.

With my Rapido engine the needle plummeted back towards zero at a very rapid rate. After pumping frantically on the bulb it was possible to actually hear the air popping as it escaped from the vicinity of the flywheel. With the flywheel and stator removed I tested with the spray expecting the flywheel side oil seal to be worn out, but instead bubbles formed around one of the stator plate bolt holes.

With the mag housing removed the culprit was very obviously a leaking mag housing gasket.

Try Again

After fitting a new mag housing gasket – and this time using silicone sealant on it instead of grease – the motor was reassembled and re-tested. The needle still visibly descended, but much more slowly this time.

After more spraying with soapy water solution the leaks now proved to be from the exhaust manifold: not at the gasket but from porous welds between the flange and the tubular part of the exhaust stub. With those re-welded the rate of leakage was reduced to an acceptable level.


After rebuilding the motor following the pressure testing its behaviour on closed throttle after a thrash was noticeably improved. It’s safe to say that without the leak-down testing kit I’d not have found the leaking gasket until I rebuilt the motor. If you want to check the quality of an engine rebuild you’ve just done or test a motor that you are suspicious about then this is really a workshop essential. For less than £20 this home-brew version works just as well on a scooter as one ten times its price.

Leak-down testing is advisable on any engine you rebuild but some engines present difficulties such as Vespa PX motors with standard-style SI carburettors. One solution might be to make a flat plate and gasket to bolt down in place of the carburettor and to pressurise the engine via the exhaust port.

Words and images: Sticky

Further viewing:

The end section of this video from Casa Lambretta demonstrates how the guys at Rimini Lambretta Centre leak test an engine.

The cycle inner tube solution


LCGB forum members Grandpa and Scooterlam suggested using an old cycle inner tube to seal the engine, which is a genius idea. As long as you have a tubular inlet and exhaust manifold then you can clip a section of inner tube to them. Then all you do is remove the valve core and use the valve as a way of pressurising the system using the blood pressure pump.

In my tests it took a few goes to get the hose clip seated right to seal the inlet and exhaust, but once sorted then it is a very cheap and effective method…

The inner-tube method...
The inner-tube method…
The inner-tube method
Pressure testing finds the leaks that other methods don't.
Pressure testing finds the leaks that other methods don’t.