MVVS 35 two-stroke
For me, the initial appeal of MVVS petrol engines is that theyíve been designed exclusively for use with model aircraft and model boats. Theyíre not a revamped leaf blower / chainsaw / general hand tool engine or a mishmash of parts sourced off the shelf, assembled and then presented as an aero engine.

For some reason the odd modeller will point to the carburettor used on an engine to indicate that itís a Ďconvertí engine, as hand tool engines all use this type of carby. Not so, actually - the carburettor is almost always out-sourced from quite a few specialist manufacturers. If your car has a carburettor thereís more than a good chance itís been sourced from a specialist manufacturer, as is the case for motorcycles, outboard engines and full-size aircraft engines. Itís a bit like spark plugs - these are certainly not manufactured by the company who manufactures the engine. The Walbro carburettor used on this engine is of a style manufactured by two or three other companies under their own name and all are available for whatever use the purchaser has in mind. Walbro are extremely popular, and theyíve a range to suit many applications. Iíve seen them on a variety of engines, from motorcycles to ultra light aircraft, as well as on outboard boat motors. So, do please note that the carburettor doesnít designate the use of the engine; instead, itís the type of engine that designates the carby it will need.

Based in the Czech Republic, MVVS have been manufacturing high performance engines since 1953, and some of those have claimed a number of European and world records in model competitions. Their current range extends from 2cc capacity to a monster 116cc twin thatíll swing a 30 x 10Ē prop at 6,100rpm. Apart from this great range of sizes you can choose between diesel, glow or petrol and, if you really want an eye opener at the field, try the 58cc liquid cooled engine. Catering for specific needs, several of the larger capacity engines are available in glow plug or spark ignition requiring either straight glow fuel (methanol / oil) or unleaded petrol mixed 50:1 with racing type two-stroke oil. Oh, and if you want to hedge your bets, so to speak, you can convert the engines either way with a couple of simple modifications.

MVVS also produce a range of exhaust systems for almost all the engines they manufacture - these include canister style mufflers, tuned mufflers, tuned pipes, Pitts style, in-cowl varieties, plus the extra long lightweight canister style, intended for fitting internally in the fuselage of the aircraft. If you canít find something you like in this range thereís also a great number of after-market mufflers on offer, but be careful of those that over-restrict the exhaust flow - theyíre engine cookers. In truth I doubt youíll need to go outside the MVVS range, theyíre designed for the engines and are of very robust construction.

Having had previous experience of MVVS products I knew roughly what to expect in terms of manufacturing quality and general performance, but I must say I really enjoyed running this petrol engine, and hereís why... Read the instructions on how to start the motor and youíll find the procedure so easy that once your engineís had a little time on the clock itíll give instant starts every time. Whilst I donít recommend it, I found that hand starting was most convenient. Even from cold, a few turns (about 5) with the choke closed, throttle down, ignition on and a reasonable flick gave a non-kick start within no more than 3 flicks. I found the most reliable starts were with the throttle closed down to low idle (adjusted with the pointed screw); allow around 30 seconds for the engine to stabilise then open the throttle. Take your time with the tuning, as set out in the instructions, and be very light handed - a little movement each time. The engine will exhibit quite a pronounced malfunction if the tuning is out, but once you have the correct setting, further fiddling just isnít necessary.

Different from a glow engine thatíll kick back violently when flooded, a Ďsparkyí will spit the dummy and sit there doing... nothing! Thereís no heat to dry out a wet plug so donít be overzealous with priming. Avoid problems by reading and understanding the simple instructions.

Ok then, Iíve had a good time running the engine and itís now in lots of bits on the bench. Weíll check it all out, then discuss the running and testing.

MAINCASE
This and the piston are the only cast components, all the other parts being CNC (Computer Numerical Control) machined. Leaving the piston for later, letís examine that Ďcase.

In short we have the crankcase section - in which the crank rotates - and the front housing, in which the crankshaft rotates supported by an open rear bearing and a front sealed (rubber seal) front bearing. Included in this front section is the intake manifold, where the adaptor mount is retained with opposed 4mm grub screws, and the mounting platform for the Hall Effect sensor (ignition instigator) with two 2.5mm caphead screws. Two washers are fitted to each screw with one each side, possibly required for setting the sensor-to-magnet gap of 0.5mm.

The crankcase section contains the mounting lugs, which are 8.75mm thick, 52.4mm long with 5mm clearance holes spaced 41mm apart - quite a good-size footprint. Rising from the crankcase is the finned barrel, which includes the exhaust manifold with 4mm tapped holes on 40.5mm centres; a gasket supplied with the muffler ensures a gas-tight joint. Below the finned area is a large screw-in nipple that taps into the engine pressure pulses for the purpose of driving the diaphragm in the fuel pump, incorporated in the carby. Neoprene tubing is suppled for the connection and this is generally a no-worry area, though it pays to check it now and again. The general casting work is of a very high quality and the material is a super-tough aluminium alloy.

The back cover is the common insert, 4-bolt style and this one is fully machined from stock aluminium alloy with a Ďflatí cut into the upper section; sealing is by an ĎOí ring. Fully machined from aluminium alloy bar stock, the head is a solid bit of work - deeply finned on top with two peripheral fins adjacent to the combustion chamber. Petrol-powered engines run a bit hotter than methanol burners, so the cooling potential is increased over that of a glow engine. The head-to-liner seal is complete with a 0.2mm soft aluminium gasket, and the 12mm thread Ďspecialí spark plug nestles in a cupped component that serves as the plug gasket and high-tension lead cap securing device. The latter by way of a 4mm grub screw. Be warned - failure to secure the plug cap on the plug and in the cup will almost certainly lead to a conflagration in the CDI unit... it will cook, burn and melt, all in one action!

As an indicator that the engine is for petrol use and a little bit of aesthetic appeal, the cylinder head and prop driver are anodised red (glow engines are blue).

PISTON, LINER & CONROD
The piston is a sturdy beast, being cast from a very tough aluminium alloy. Itís fitted with one ring (pinned in position), is surface coated on the skirt (area below the ring) with a wear resistant material, and two short-tail wire circlips (no bits to break off and do nasties in the liner) retain the hollow gudgeon pin. A nice bit of work with a long life potential. The connecting rod is very substantial, fully machined from stock material and fitted with caged needle roller bearings at both ends. This is the main reason you need only a small drop of oil in the fuel. The bearings are INA brand from Germany - a quality known for long and trouble-free service.

Of hard steel, the liner has Schneurle porting (no rear boost port - not needed) and a superfine finish all over. The combination of a cast iron ring in a steel liner has long been recognised as an excellent match of materials, maintaining a very good fit at all temperatures and exhibiting extremely long life, given reasonable and simple care. Use a carefully measured amount of good quality oil and this combination will give many hours of excellent service.

CRANKSHAFT
Here again a very substantial unit made from high tensile steel alloy with a 22mm diameter main journal (thickest part of the shaft). The counterweight is 14.5mm thick x 49mm diameter - thatís a lot of metal, spinning around and absorbing a lot of engine vibration. The forward section of the shaft is 12mm diameter and the crankpin is a reassuring 10mm. All these sections are machined from one billet (piece) of steel, and finely ground on all bearing sections.

The 44mm diameter, cross milled (teeth to secure the prop) propeller driver is machined from aluminium alloy stock, driven by a ĎDí- shape Woodruff key in the crankshaft, fitted with two timing magnets and anodised red. The prop washer and main nut are a homogenous (one piece) unit, and extra security is provided by a lock nut. The main nut is 19mm across the flats and the locknut is 17mm . I advise using ring spanners or sockets on these as theyíre quite narrow, and an open-ended spanner will easily slip and round the corners. A nice bit of work all round.

CARBURETTOR
The carburettor is a WT 561 Walbro that includes a fuel pump, automatic release choke, high and low mixture adjustment screws and an idle stop screw. Itís mounted on an adaptor manifold that incorporates an input nipple to feed the pressure pulses from the engine to drive the fuel pump diaphragm. These are incredibly reliable carburettors that, once set, will give hours of trouble-free service provided you use a fuel filter and donít fiddle with the needles! Due to the pump the position of the fuel tank isnít critical - in fact, it can be quite a distance from the carburettor and the fuel supply will still be reliable. About the only maintenance required would be to clean the built-in filters once in a while, especially if your fuel arrives in the tank a little dirty. If youíre filled with trepidation about this job (and you do need care), you can have it done at your local mower repair shop.

Incidentally, I urge you to not fiddle with the carby, and donít let friendly field fiddlers poke around inside it unless youíre really certain they fully understand what theyíre doing. It doesnít need any internal adjusting, and to do so will more than likely cause you lots of problems. If youíre going to Ďhangarí the engine for some time I suggest you run it with the fuel supply cut off in order to bleed the carby dry. If itís left with fuel inside the chamber you might have a problem with oil varnish if a very high quality oil isnít used. Even so, any residual petrol will eventually evaporate and leave a drop of oil that will slow the works a bit on the next start.

ELECTRONIC IGNITION
The EZM 1 TI ignition unit is fully electronic with a 16-bit microprocessor system. If this leaves you wondering, just consider it as a Ďuse and forgetí system. Itís fully self-contained and takes care of supplying a fat spark (18,000V) and controlling the ignition timing regardless of the speed of the engine. All you need to consider is that itís like a receiver and will greatly benefit from being secured well and protected from harsh vibration.

An interesting point is that the unit incorporates two very sophisticated fail-safe programs. If thereís no propeller activity (i.e. not being turned) for 90 seconds the circuit will turn itself off and lock into Ďstandbyí mode. The instructions donít include details of getting it back to working mode, however I found that by switching the battery supply off and then on again it became operative once again. The second fail-safe is a form of battery power detection. If the power supply battery is discharged to 4.4V (1.1V per cell with a 4.8V pack), the ignition timing is altered and the engine will not run beyond about 3,000rpm. If youíre flying your model and the engine goes off song your first thought should be that the power supply to the ignition is on the way out. My recommendation would be to use a 6V pack (5 cells), with a reasonably high capacity if you want to fly for long periods. The current consumption of the ignition unit is 500mAh at 9,000rpm, and this will serve as a good guide for the flying time potential. The 6V pack will also assist here as itís further away from the cut-off point of 4.4V.

Keep all the connections tight, donít turn the engine with the ignition on and the plug disconnected, and watch out for vibrations abrading the cables. Follow these simple precautions and you should have trouble-free service with this state-of-the-art ignition unit.

SHOWING OUT
The test was conducted with the temperature at 18įC and 73% humidity. I used an MVVS Pitts-style muffler that was well built, effective and much quieter than the thin steel chain saw-type mufflers. Fuel was unleaded petrol with 20ml (25ml for the run-in period) per litre of RS 7 high performance two-stroke oil for racing applications. Pay particular note to the instructions here, which state that the engine is to be run with racing grade two-stroke oils only.

During the test period I found the engine very enjoyable to use. Hand starting was best at very low idle, there was no kick back, the inactive period fail-safe was spot on and the idle was dead steady. Cooling down was rapid and no leaks, stains or marks were evident after the test period. Throttle response was also very good but donít get too vigorous on the noise lever until the engine warms up (good advice for all engines!). General running was very reliable, and itís a nice engine - I really like it.

A couple of notes for your info:
  • Two-stroke engines donít like stale petrol (past experience with chainsaws) so donít mix up large drums of fuel
  • The engine requires at least half an hour of running in before it will perform nicely for you